HomeMy WebLinkAbout5ALateDocument21361 7 Biological Effects of Carbon Dioxide Enrichment 7. Biological Effects of Carbon Dioxide Enrichment 7.1. Plant Productivity Responses 7.2. Water Use Efficiency 7.3. Amelioration
of Environmental Stresses 7.4. Acclimation 7.5. Competition 7.6. Respiration 7.7. Carbon Sequestration 7.8. Other Benefits 7.9. Greening of the Earth Introduction The Contribution of
Group 1 to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) hardly mentions the beneficial effects of earth’s rising atmospheric carbon dioxide (CO2)
concentration on the biosphere. In a chapter titled “Changes in Atmospheric Constituents and in Radiative Forcing” the authors say the following (IPCC, 2007–I, p. 186): Increased CO2
concentrations can also “fertilize” plants by stimulating photosynthesis, which models suggest has contributed to increased vegetation cover and leaf area over the 20th century (Cramer
et al., 2001). Increases in the Normalized Difference Vegetation Index, a remote sensing product indica?tive of leaf area, biomass and potential photosynthesis, have been observed (Zhou
et al., 2001), although other causes including climate change itself are also likely to have contributed. Increased vegetative cover and leaf area would decrease surface albedo, which
would act to oppose the increase in albedo due to deforestation. The RF due to this process has not been evaluated and there is a very low scientific understanding of these effects.
Later in that report, in a chapter titled “Couplings Between Changes in the Climate System and Biogeochemistry,” a single paragraph is devoted to the “effects of elevated carbon dioxide”
on plants. The paragraph concludes, “it is not yet clear how strong the CO2 fertilization effect actually is” (p. 527). Since CO2 fertilization could affect crop yields and how efficiently
plants use mineral nutritients and water, one would expect the subject to be addressed in the Contribution of Group 2, on “Impacts, Adaption and Vulnerability,” and indeed it is, in
a chapter titled titled “Food, Fibre and Forest Products” (IPCC, 2007-II). But that chapter belittles and largely ignores research on the benefits of enhanced CO2 while exaggerating
the possible negative effects of rapidly rising temperatures and extreme weather events predicted by computer models. The subject is not even mentioned in Chapter 8 of that report, on
“Human Health,” although even a modest effect on crops would have some effect on human health. (See Chapter 9 of the present report for our own, more complete, discussion of the health
effects of climate change.) The IPCC’s failure to report the beneficial effects of rising CO2 concentrations is surprising when literally thousands of peer-reviewed journal articles
exist on the subject. It is also a major defect of the IPCC report and one reason why it is not a reliable summary of the science of climate change. In this
Climate Change Reconsidered 362 chapter, we seek to provide the balance that eluded the IPCC. The chapter begins with a survey of the scientific literature on the productivity responses
of plants to higher CO2 concentrations, and then reviews research on the effect of enhanced CO2 on plant water-use efficiency, responsiveness to environmental stress, acclimation, competition
among species (e.g., crops versus weeds), respiration, and other effects. We end with a survey of literature showing the general “greening of the Earth” that has occurred during the
Current Warm Period. References Cramer, W., A. Bondeau, F. I. Woodward, I. C. Prentice, R. A. Betts, V. Brovkin, P. M. Cox, V. Fisher, J. Foley, A. D. Friend, C. Kucharik, M. R. Lomas,
N. Ramankutty, S. Sitch, B. Smith, A. White, and C. Young-Molling. 2001. Global response of terrestrial ecosystem structure and function to CO2 and climate change: Results from six dynamic
global vegetation models. Global Change Biol. 7: 357-373. IPCC. 2007-I. Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report
of the Intergovernmental Panel on Climate Change. Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller. (Eds.) Cambridge University Press, Cambridge,
UK. IPCC. 2007-II. Climate Change 2007: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate
Change. M.L. Parry, O.F. Canziani, J.P. Palutikof, P.J. van der Linden and Hanson, C.D. (Eds.) Cambridge University Press, Cambridge, UK. Zhou, L.M., Tucker, C.J., Kaufmann, R.K., Slayback,
D., Shabanov, N.V., and Myneni, R.B. 2001. Variations in northern vegetation activity inferred from satellite data of vegetation index during 1981 to 1999. Journal of Geophysical Research
106 (D17): 20069-20083. 7.1. Plant Productivity Responses Perhaps the best-known consequence of the rise in atmospheric CO2 is the stimulation of plant productivity. This growth enhancement
occurs because carbon dioxide is the primary raw material utilized by plants to produce the organic matter out of which they construct their tissues. Consequently, the more CO2 there
is in the air, the better plants grow. Over the past decade, the Center for the Study of Carbon Dioxide and Global Change, has archived thousands of results from hundreds of peer-reviewed
research studies conducted by hundreds of researchers demonstrating this fact. The archive is available free of charge at http://www.co2science.org/data/plantgrowth/plantgrowth.php.
The Center’s Web site lists the photosynthetic and dry weight responses of plants growing in CO2-enriched air, arranged by scientific or common plant name. It also provides the full
peer-reviewed journal references and experimental conditions in which each study was conducted for each record. We have summarized those results in two tables appearing in Appendix 2
and Appendix 3 to the current report. The first table, Table 7.1.1, indicates the mean biomass response of nearly 1,000 plants to a 300-ppm increase in atmospheric CO2 concentration.
The second table, Table 7.1.2, indicates the photosynthetic response to the same CO2 enrichment for a largely similar list of plants. In the rest of this section of Chapter 7, we provide
a review of research on a representative sample of herbaceous and woody plants, chosen with an eye toward their importance to agriculture and the forestry and papermaking industries,
followed by several acquatic plants. 7.1.1. Herbaceous Plants A 300 ppm increase in the air’s CO2 content typically raises the productivity of most herbaceous plants by about one-third
(Cure and Acock, 1986; Mortensen, 1987). This positive response occurs in plants that utilize all three of the major biochemical pathways (C3, C4, and crassulacean acid metabolism (CAM))
of photosynthesis (Poorter, 1993). Thus, with more CO2 in the air, the productivity of nearly all crops rises, as they produce more branches and tillers, more and thicker leaves, more
extensive root systems, and more flowers and fruit (Idso, 1989). On average, a 300 ppm increase in atmospheric CO2 enrichment leads to yield increases of 15 percent for CAM crops, 49
percent for C3 cereals, 20 percent for C4 cereals, 24 percent for fruits and melons, 44 percent for legumes, 48 percent for roots and tubers, and 37 percent for vegetables (Idso and
Idso, 2000). It should come as no surprise, therefore, that the father of modern research in this area—Sylvan H. Wittwer—has said “it should be considered good fortune that we are
Biological Effects of Carbon Dioxide Enrichment 363 living in a world of gradually increasing levels of atmospheric CO2,” and “the rising level of atmospheric CO2 is a universally free
premium, gaining in magnitude with time, on which we can all reckon for the future.” Additional information on this topic, including reviews of herbaceous plants not discussed here,
can be found at http://www.co2science.org/subject/a/subject_a.php under the heading Agriculture. References Cure, J.D., and Acock, B. (1986). Crop Responses to Carbon Dioxide Doubling:
A Literature Survey. Agric. For. Meteorol. 38, 127-145. Idso, C.D. and Idso, K.E. (2000) Forecasting world food supplies: The impact of rising atmospheric CO2 concentration. Technology
7 (suppl): 33-56. Idso, S.B. (1989) Carbon Dioxide: Friend or Foe? IBR Press, Tempe, AZ. Mortensen, L.M. (1987). Review: CO2 Enrichment in Greenhouses. Crop Responses. Sci. Hort. 33,
1-25. Poorter, H. (1993). Interspecific Variation in the Growth Response of Plants to an Elevated Ambient CO2 Concentration. Vegetatio 104/105 77-97. 7.1.1.1. Alfalfa Morgan et al. (2001)
grew the C3 legume alfalfa (Medicago sativa L.) for 20 days post-defoliation in growth chambers maintained at atmospheric CO2 concentrations of 355 and 700 ppm and low or high levels
of soil nitrogen to see how these factors affected plant regrowth. They determined that the plants in the elevated CO2 treatment attained total dry weights over the 20-day regrowth period
that were 62 percent greater than those reached by the plants grown in ambient air, irrespective of soil nitrogen concentration. De Luis et al. (1999) grew alfalfa plants in controlled
environment chambers in air of 400 and 700 ppm CO2 for two weeks before imposing a twoweek water treatment on them, wherein the soil in which half of the plants grew was maintained at
a moisture content approaching field capacity while the soil in which the other half grew was maintained at a moisture content that was only 30 percent of field capacity. Under these
conditions, the CO2-enriched water-stressed plants displayed an average water-use efficiency that was 2.6 and 4.1 times greater than that of the water-stressed and well-watered plants,
respectively, growing in ambient 400-ppm-CO2 air. In addition, under ambient CO2 conditions, the water stress treatment increased the mean plant root:shoot ratio by 108 percent, while
in the elevated CO2 treatment it increased it by 269 percent. As a result, the nodule biomass on the roots of the CO2-enriched water-stressed plants was 40 and 100 percent greater than
the nodule biomass on the roots of the wellwatered and water-stressed plants, respectively, growing in ambient air. Hence, the CO2-enriched water-stressed plants acquired 31 and 97 percent
more total plant nitrogen than the well-watered and waterstressed plants, respectively, growing in ambient air. The CO2-enriched water-stressed plants attained 2.6 and 2.3 times more
total biomass than the waterstressed and well-watered plants, respectively, grown at 400 ppm CO2. Luscher et al. (2000) grew effectively and ineffectively nodulating (good nitrogen-fixing
vs. poor nitrogen-fixing) alfalfa plants in large free-air CO2 enrichment (FACE) plots for multiple growing seasons at atmospheric CO2 concentrations of 350 and 600 ppm, while half of
the plants in each treatment received a high supply of soil nitrogen and the other half received only minimal amounts of this essential nutrient. The extra CO2 increased the yield of
effectively nodulating plants by about 50 percent, regardless of soil nitrogen supply; caused a 25 percent yield reduction in ineffectively nodulating plants subjected to low soil nitrogen;
and produced an intermediate yield stimulation of 11 percent for the same plants under conditions of high soil nitrogen, which suggests that the ability to symbiotically fix nitrogen
is an important factor in eliciting strong positive growth responses to elevated CO2 under conditions of low soil nitrogen supply. Sgherri et al. (1998) grew alfalfa in open-top chambers
at ambient (340 ppm) and enriched (600 ppm) CO2 concentrations for 25 five days, after which water was withheld for five additional days so they could investigate the interactive effects
of elevated CO2 and water stress on plant water status, leaf soluble protein and carbohydrate content, and chloroplast thylakoid membrane composition. They found that the plants grown
in elevated CO2 exhibited the best water status during the moisture deficit part of the study, as indicated by leaf water potentials that were approximately 30 percent higher (less negative)
than those observed in plants grown in ambient CO2. This beneficial adjustment was achieved by partial
Climate Change Reconsidered 364 closure of leaf stomata and by greater production of nonstructural carbohydrates (a CO2-induced enhancement of 50 percent was observed), both of which
phenomena can lead to decreases in transpirational water loss, the former by guard cells physically regulating stomatal apertures to directly control the exodus of water from leaves,
and the latter by nonstructural carbohydrates influencing the amount of water available for transpiration. This latter phenomenon occurs because many nonstructural carbohydrates are
osmotically active solutes that chemically associate with water through the formation of hydrogen bonds, thereby effectively reducing the amount of unbound water available for bulk flow
during transpiration. Under water-stressed conditions, however, the CO2-induced difference in total leaf nonstructural carbohydrates disappeared. This may have resulted from an increased
mobilization of nonstructural carbohydrates to roots in the elevated CO2 treatment, which would decrease the osmotic potential in that part of the plant, thereby causing an increased
influx of soil moisture into the roots. If this did indeed occur, it would also contribute to a better overall water status of CO2-enriched plants during drought conditions. The plants
grown at elevated CO2 also maintained greater leaf chlorophyll contents and lipid to protein ratios, especially under conditions of water stress. Leaf chlorophyll content, for example,
decreased by a mere 6 percent at 600 ppm CO2, while it plummeted by approximately 30 percent at 340 ppm, when water was withheld. Moreover, leaf lipid contents in plants grown with atmospheric
CO2 enrichment were about 22 and 83 percent higher than those measured in plants grown at ambient CO2 during periods of ample and insufficient soil moisture supply, respectively. Furthermore,
at elevated CO2 the average amounts of unsaturation for two of the most important lipids involved in thylakoid membrane composition were approximately 20 and 37 percent greater than
what what was measured in plants grown at 340 ppm during times of adequate and inadequate soil moisture, respectively. The greater lipid contents observed at elevated CO2, and their
increased amounts of unsaturation, may allow thylakoid membranes to maintain a more fluid and stable environment, which is critical during periods of water stress in enabling plants
to continue photosynthetic carbon uptake. Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/a/a
riculturealfalfa.php. References De Luis, J., Irigoyen, J.J. and Sanchez-Diaz, M. 1999. Elevated CO2 enhances plant growth in droughted N2-fixing alfalfa without improving water stress.
Physiologia Plantarum 107: 84-89. Luscher, A., Hartwig, U.A., Suter, D. and Nosberger, J. 2000. Direct evidence that symbiotic N2 fixation in fertile grassland is an important trait
for a strong response of plants to elevated atmospheric CO2. Global Change Biology 6: 655-662. Morgan, J.A., Skinner, R.H. and Hanson, J.D. 2001. Nitrogen and CO2 affect regrowth and
biomass partitioning differently in forages of three functional groups. Crop Science 41: 78-86. Sgherri, C.L.M., Quartacci, M.F., Menconi, M., Raschi, A. and Navari-Izzo, F. 1998. Interactions
between drought and elevated CO2 on alfalfa plants. Journal of Plant Physiology 152: 118-124. 7.1.1.2. Cotton As the CO2 content of the air increases, cotton (Gossypium hirsutum L.)
plants typically display enhanced rates of photosynthetic carbon uptake, as noted by Reddy et al. (1999), who reported that twice-ambient atmospheric CO2 concentrations boosted photosynthetic
rates of cotton by 137 to 190 percent at growth temperatures ranging from 2°C below ambient to 7°C above ambient. Elevated CO2 also enhances total plant biomass and harvestable yield.
Reddy et al. (1998), for example, reported that plant biomass at 700 ppm CO2 was enhanced by 31 to 78 percent at growth temperatures ranging from 20 to 40°C, while boll production was
increased by 40 percent. Similarly, Tischler et al. (2000) found that a doubling of the atmospheric CO2 concentration increased seedling biomass by at least 56 percent. These results
indicate that elevated CO2 concentrations tend to ameliorate the negative effects of heat stress on productivity and growth in cotton. In addition, Booker (2000) discovered that elevated
CO2 reduced the deleterious effects of elevated ozone on leaf biomass and starch production. Atmospheric CO2 enrichment also can induce changes in cotton leaf chemistry that tend to
increase carbon sequestration in plant biolitter and soils. Booker et al. (2000), for example, observed that biolitter produced from cotton plants grown at 720 ppm CO2 decomposed at
rates that were 10 to 14
Biological Effects of Carbon Dioxide Enrichment 365 percent slower than those displayed by ambiently grown plants; a after three years of exposure to air containing 550 ppm CO2, Leavitt
et al. (1994) reported that 10 percent of the organic carbon present in soils beneath CO2-enriched FACE plots resulted from the extra CO2 supplied to them. In summary, as the CO2 content
of the air increases, cotton plants will display greater rates of photosynthesis and biomass production, which should lead to greater boll production in this important fiber crop, even
under conditions of elevated air temperature and ozone concentration. In addition, carbon sequestration in fields planted to cotton should also increase with future increases in the
air’s CO2 content. Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/a/agriculturecotton.php.
References Booker, F.L. 2000. Influence of carbon dioxide enrichment, ozone and nitrogen fertilization on cotton (Gossypium hirsutum L.) leaf and root composition. Plant, Cell and Environment
23: 573-583. Booker, F.L., Shafer, S.R., Wei, C.-M. and Horton, S.J. 2000. Carbon dioxide enrichment and nitrogen fertilization effects on cotton (Gossypium hirsutum L.) plant residue
chemistry and decomposition. Plant and Soil 220: 89-98. Leavitt, S.W., Paul, E.A., Kimball, B.A., Hendrey, G.R., Mauney, J.R., Rauschkolb, R., Rogers, H., Lewin, K.F., Nagy, J., Pinter
Jr., P.J. and Johnson, H.B. 1994. Carbon isotope dynamics of free-air CO2-enriched cotton and soils. Agricultural and Forest Meteorology 70: 87-101. Reddy, K.K., Davidonis, G.H., Johnson,
A.S. and Vinyard, B.T. 1999. Temperature regime and carbon dioxide enrichment alter cotton boll development and fiber properties. Agronomy Journal 91: 851-858. Reddy, K.R., Robana, R.R.,
Hodges, H.F., Liu, X.J. and McKinion, J.M. 1998. Interactions of CO2 enrichment and temperature on cotton growth and leaf characteristics. Environmental and Experimental Botany 39: 117-129.
Tischler, C.R., Polley, H.W., Johnson, H.B. and Pennington, R.E. 2000. Seedling response to elevated CO2 in five epigeal species. International Journal of Plant Science 161: 779-783.
7.1.1.3. Maize Maroco et al. (1999) grew corn (Zea mays L.), or maize as it is often called, for 30 days in plexiglass chambers maintained at either ambient or tripleambient concentrations
of atmospheric CO2 to determine the effects of elevated CO2 on the growth of this important agricultural C4 species. This exercise revealed that elevated CO2 (1,100 ppm) increased maize
photosynthetic rates by about 15 percent relative to those measured in plants grown at 350 ppm CO2, in spite of the fact that both rubisco and PEPcarboxylase were down-regulated. This
increase in carbon fixation likely contributed to the 20 percent greater biomass accumulation observed in the CO2-enriched plants. In addition, leaves of CO2-enriched plants contained
approximately 10 percent fewer stomates per unit leaf area than leaves of control plants, and atmospheric CO2 enrichment reduced stomatal conductance by as much as 71 percent in elevated-CO2-grown
plants. As a result of these several different phenomena, the higher atmospheric CO2 concentration greatly increased the intrinsic water-use efficiency of the CO2-enriched plants. In
a study designed to examine the effects of elevated CO2 under real-world field conditions, Leakey et al. (2004) grew maize out-of-doors at the SoyFACE facility in the heart of the United
States Corn Belt, while exposing different sections of the field to atmospheric CO2 concentrations of either 354 or 549 ppm. The crop was grown, in the words of the researchers, using
cultural practices deemed “typical for this region of Illinois,” during a year that turned out to have experienced summer rainfall that was “very close to the 50-year average for this
site, indicating that the year was not atypical or a drought year.” Then, on five different days during the growing season (11 and 22 July, 9 and 21 August, and 5 September), they measured
diurnal patterns of photosynthesis, stomatal conductance, and microclimatic conditions. Contrary to what many people had long assumed would be the case for a C4 crop such as corn growing
under even the best of natural conditions, Leakey et al. found that “growth at elevated CO2 significantly increased leaf photosynthetic CO2 uptake rate by up to 41 percent.” The greatest
whole-day increase was 21 percent (11 July) followed by 11 percent (22 July), during a period of low rainfall. Thereafter, however, during a period of greater rainfall, there were no
significant differences between the photosynthetic rates of the plants in the two CO2 treatments, so that
Climate Change Reconsidered 366 over the entire growing season, the CO2-induced increase in leaf photosynthetic rate averaged 10 percent. Additionally, on all but the first day of measurements,
stomatal conductance was significantly lower (-23 percent on average) under elevated CO2 compared to ambient CO2, which led to reduced transpiration rates in the CO2-enriched plants
on those days as well. Since “low soil water availability and high evaporative demand can both generate water stress and inhibit leaf net CO2 assimilation in C4 plants,” they state that
the lower stomatal conductance and transpiration rate they observed under elevated CO2 “may have counteracted the development of water stress under elevated CO2 and prevented the inhibition
of leaf net CO2 assimilation observed under ambient CO2.” The implication of their research, in the words of Leakey et al., was that “contrary to expectations, this US Corn Belt summer
climate appeared to cause sufficient water stress under ambient CO2 to allow the ameliorating effects of elevated CO2 to significantly enhance leaf net CO2 assimilation.” They concluded
that “this response of Z. mays to elevated CO2 indicates the potential for greater future crop biomass and harvestable yield across the US Corn Belt.” Also germane to this subject and
supportive of the above conclusion are the effects of elevated CO2 on weeds associated with corn. Conway and Toenniessen (2003), for example, speak of maize in Africa being attacked
by the parasitic weed Striga hermonthica, which sucks vital nutrients from its roots, as well as from the roots of many other C4 crops of the semi-arid tropics, including sorghum, sugar
cane, and millet, plus the C3 crop rice, particularly throughout much of Africa, where Striga is one of the region’s most economically important parasitic weeds. Research shows how atmospheric
CO2 enrichment greatly reduces the damage done by this devastating weed (Watling and Press, 1997; Watling and Press, 2000). Baczek-Kwinta and Koscielniak (2003) studied another phenomenon
that is impacted by atmospheric CO2 enrichment and that can affect the productivity of maize. Noting the tropical origin of maize and that the crop “is extremely sensitive to chill (temperatures
0-15°C),” they report that it is nevertheless often grown in cooler temperate zones. In such circumstances, however, maize can experience a variety of maladies associated with exposure
to periods of low air temperature. To see if elevated CO2 either exacerbates or ameliorates this problem, they grew two hybrid genotypes—KOC 9431 (chill-resistant) and K103xK85 (chill-sensitive)—from
seed in air of either ambient (350 ppm) or elevated (700 ppm) CO2 concentration (AC or EC, respectively), after which they exposed the plants to air of 7°C for eleven days, whereupon
they let them recover for one day in ambient air of 20°C, all the while measuring several physiological and biochemical parameters pertaining to the plants’ third fully expanded leaves.
The two researchers’ protocol revealed that “EC inhibited chill-chill-induced depression of net photosynthetic rate (PN), especially in leaves of chillresistant genotype KOC 9431,” which
phenomenon “was distinct not only during chilling, but also during the recovery of plants at 20°C.” In fact, they found that “seedlings subjected to EC showed 4-fold higher PN when compared
to AC plants.” They also determined that “EC diminished the rate of superoxide radical formation in leaves in comparison to the AC control.” In addition, they found that leaf membrane
injury “was significantly lower in samples of plants subjected to EC than AC.” Last, they report that enrichment of the air with CO2 successfully inhibited the decrease in the maximal
quantum efficiency of photosystem 2, both after chilling and during the one-day recovery period. And in light of all of these positive effects of elevated CO2, they concluded that “the
increase in atmospheric CO2 concentration seems to be one of the protective factors for maize grown in cold temperate regions.” But what about the effects of climate change, both past
and possibly future, on corn production? For nine areas of contrasting environment within the Pampas region of Argentina, Magrin et al. (2005) evaluated changes in climate over the twentieth
century along with changes in the yields of the region’s chief crops. Then, after determining upward low-frequency trends in yield due to technological improvements in crop genetics
and management techniques, plus the aerial fertilization effect of the historical increase in the air’s CO2 concentration, annual yield anomalies and concomitant climatic anomalies were
calculated and used to develop relations describing the effects of changes in precipitation, temperature and solar radiation on crop yields, so that the effects of long-term changes
in these climatic parameters on Argentina agriculture could be determined. Noting that “technological improvements account for most of the observed changes in crop yields during the
second part of the twentieth century,” which totaled 110 percent for maize, Magrin et al. report that due to changes in climate between the
Biological Effects of Carbon Dioxide Enrichment 367 periods 1950-70 and 1970-99, maize yields increased by 18 percent. Much the same has been found to be true in Alberta, Canada, where
Shen et al. (2005) derived and analyzed long-term (1901-2002) temporal trends in the agroclimate of the region. They report, for example, that “an earlier last spring frost, a later
first fall frost, and a longer frost-free period are obvious all over the province.” They also found that May-August precipitation in Alberta increased 14 percent from 1901 to 2002,
and that annual precipitation exhibited a similar increasing trend, with most of the increase coming in the form of low-intensity events. In addition, the researchers note that “the
area with sufficient corn heat units for corn production, calculated according to the 1973-2002 normal, has extended to the north by about 200-300 km, when compared with the 1913-32
normal, and by about 50-100 km, when compared with the 1943-72 normal.” In light of these findings, Shen et al. conclude that “the changes of the agroclimatic parameters imply that Alberta
agriculture has benefited from the last century’s climate change,” emphasizing that “the potential exists to grow crops and raise livestock in more regions of Alberta than was possible
in the past.” They also note that the increase in the length of the frost-free period “can greatly reduce the frost risks to crops and bring economic benefits to Alberta agricultural
producers,” and that the northward extension of the corn heat unit boundary that is sufficient for corn production “implies that Alberta farmers now have a
larger variety of crops to choose from than were available previously.” Hence, they say “there is no hesitation for us to conclude that the warming climate and increased precipitation
benefit agriculture in Alberta.” With respect to the future, Bootsma et al. (2005) derived relationships between agroclimatic indices and average yields of major grain crops, including
corn, from field trials conducted in eastern Canada, after which they used them to estimate potential impacts of projected climate change scenarios on anticipated average yields for
the period 2040 to 2069. Based on a range of available heat units projected by multiple General Circulation Model (GCM) experiments, they determined that average yields achievable in
field trials could increase by 40 to 115 percent for corn, “not including the direct effect of increased atmospheric CO2 concentrations.” Adding expected CO2 increases to the mix, along
with gains in yield anticipated to be achieved through breeding and improved technology, these numbers rose to 114 to 186 percent. In light of their findings, Bootsma et al. predict
there will be a “switch to high-energy and highprotein-content crops (corn and soybeans) that are better adapted to the warmer climate.” In summary, as the air’s CO2 content continues
to rise, and even if the climate of the world changes in the ways suggested by GCM and IPCC calculations, maize plants will likely display greater rates of photosynthesis and biomass
production, as well as reduced transpirational water losses and increased water-use efficiencies, and more areas of the world will likely become suitable for growing this important crop.
Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/a/agriculturemaize.php. References
Baczek-Kwinta, R. and Koscielniak, J. 2003. Antioxidative effect of elevated CO2 concentration in the air on maize hybrids subjected to severe chill. Photosynthetica 41: 161-165. Bootsma,
A., Gameda, S. and McKenney, D.W. 2005. Potential impacts of climate change on corn, soybeans and barley yields in Atlantic Canada. Canadian Journal of Plant Science 85: 345-357. Conway,
G. and Toenniessen, G. 2003. Science for African food security. Science 299: 1187-1188. Leakey, A.D.B., Bernacchi, C.J., Dohleman, F.G., Ort, D.R. and Long, S.P. 2004. Will photosynthesis
of maize (Zea mays) in the US Corn Belt increase in future [CO2] rich atmospheres? An analysis of diurnal courses of CO2 uptake under free-air concentration enrichment (FACE). Global
Change Biology 10: 951-962. Magrin, G.O., Travasso, M.I. and Rodriguez, G.R. 2005. Changes in climate and crop production during the twentieth century in Argentina. Climatic Change 72:
229-249. Maroco, J.P., Edwards, G.E. and Ku, M.S.B. 1999. Photosynthetic acclimation of maize to growth under elevated levels of carbon dioxide. Planta 210: 115-125. Shen, S.S.P., Yin,
H., Cannon, K., Howard, A., Chetner, S. and Karl, T.R. 2005. Temporal and spatial changes of the agroclimate in Alberta, Canada, from 1901 to 2002. Journal of Applied Meteorology 44:
1090-1105.
Climate Change Reconsidered 368 Watling, J.R. and Press, M.C. 1997. How is the relationship between the C4 cereal Sorghum bicolor and the C3 root hemi-parasites Striga hermonthica and
Striga asiatica affected by elevated CO2? Plant, Cell and Environment 20: 1292-1300. Watling, J.R. and Press, M.C. 2000. Infection with the parasitic angiosperm Striga hermonthica influences
the response of the C3 cereal Oryza sativa to elevated CO2. Global Change Biology 6: 919-930. 7.1.1.4. Peanut Stanciel et al. (2000) grew peanuts (Arachis hypogaea L.) hydroponically
for 110 days in controlled environment chambers maintained at atmospheric CO2 concentrations of 400, 800 and 1200 ppm, finding that the net photosynthetic rates of plants grown at 800
ppm CO2 were 29 percent greater than those of plants grown at 400 ppm CO2, but that plants grown at 1200 ppm CO2 displayed photosynthetic rates that were 24 percent lower than those
exhibited by plants grown in 400-ppm CO2 air. Nevertheless, the number of pods, pod weight and seed dry weight per unit area were all greater at 1200 ppm than at 400 ppm CO2. Also, harvest
index, which is the ratio of seed dry weight to pod dry weight, was 19 and 31 percent greater at 800 and 1200 ppm CO2, respectively, than it was at 400 ppm CO2. In addition, as the atmospheric
CO2 concentration increased, stomatal conductance decreased, becoming 44 and 50 percent lower at 800 and 1200 ppm than it was at 400 ppm CO2. Thus, atmospheric CO2 enrichment also reduced
transpirational water loss, leading to a significant increase in plant water use efficiency. Prasad et al. (2003) grew Virginia Runner (Georgia Green) peanuts from seed to maturity in
sunlit growth chambers maintained at atmospheric CO2 concentrations of 350 and 700 ppm and daytimemaximum/nighttime-minimum air temperatures of 32/22, 36/26, 40/30 and 44/34°C, while
they assessed various aspects of vegetative and reproductive growth. In doing so, they found that leaf photosynthetic rates were unaffected by air temperature over the range investigated,
but they rose by 27 percent in response to the experimental doubling of the air’s CO2 content. Vegetative biomass, on the other hand, increased by 51 percent and 54 percent in the ambient
and CO2-enriched air, respectively, as air temperature rose from 32/22 to 40/30°C. A further air temperature increase to 44/34°C, however, caused moderate to slight decreases in vegetative
biomass in both the ambient and CO2-enriched air, so that the final biomass increase over the entire temperature range investigated was 27 percent in ambient air and 53 percent in CO2-enriched
air. When going from the lowest temperature ambient CO2 treatment to the highest temperature elevated CO2 treatment, however, there was a whopping 106 percent increase in vegetative
biomass. By contrast, seed yields in both the ambient and CO2-enriched air dropped dramatically with each of the three temperature increases studied, declining at the highest temperature
regime to but a small percentage of what they were at the lowest temperature regime. Nevertheless, Prasad et al. report that “seed yields at 36.4/26.4°C under elevated CO2 were similar
to those obtained at 32/22°C under ambient CO2,” the latter pair of which temperatures they describe as “present-day seasonal temperatures.” It would appear that a warming of 4.4°C above
present-day seasonal temperatures for peanut production would have essentially no effect on peanut seed yields, as long as the atmosphere’s CO2 concentration rose concurrently, by something
on the order of 350 ppm. It is also important to note, according to Prasad et al., that “maximum/minimum air temperatures of 32/22°C and higher are common in many peanut-producing countries
across the globe.” In fact, they note that “the Anantapur district in Andhra Pradesh, which is one of the largest peanutproducing regions in India, experiences season-long temperatures
considerably greater than 32/22°C from planting to maturity.” In light of these real-world observations, i.e., that some of the best peanut-producing regions in the world currently experience
air temperatures considerably greater than what Prasad et al. suggest is optimum for peanuts (something less than 32/22°C), it would appear that real-world declines in peanut seed yields
in response to a degree or two of warming, even in air of ambient CO2 concentration, must be very slight or even non-existent, for how else could the places that commonly experience
these considerably higher temperatures remain some of the best peanut-producing areas in the world? This in turn suggests that for more realistic values of CO2-induced global warming,
i.e., temperature increases on the order of 0.4°C for a doubling of the air’s CO2 content (Idso, 1998), there would likely be a significant increase in real-world peanut production.
In another pertinent study, Vu (2005) grew peanut plants from seed to maturity in greenhouses maintained at atmospheric CO2 concentrations of 360
Biological Effects of Carbon Dioxide Enrichment 369 and 720 ppm and at air temperatures that were 1.5 and 6.0°C above outdoor air temperatures, while he measured a number of parameters
related to the plants’ photosynthetic performance. His work revealed that although Rubisco protein content and activity were down-regulated by elevated CO2, the Rubisco photosynthetic
efficiency (the ratio of midday light-saturated carbon exchange rate to Rubisco initial or total activity) of the elevated-CO2 plants “was 1.3-to 1.9-fold greater than that of the ambient-CO2
plants at both growth temperatures.” He also determined that “leaf soluble sugars and starch of plants grown at elevated CO2 were 1.3-and 2-fold higher, respectively, than those of plants
grown at ambient CO2.” In addition, he discovered that the leaf transpiration of the elevated-CO2 plants relative to that of the ambient-CO2 plants was 12 percent less at near-ambient
temperatures and 17 percent less in the higher temperature regime, while the water use efficiency of the elevated-CO2 plants relative to the ambient-CO2 plants was 56 percent greater
at nearambient temperatures and 41 percent greater in the higher temperature environment. In commenting on his findings, Vu notes that because less Rubisco protein was required by the
elevated-CO2 plants, the subsequent redistribution of excess leaf nitrogen “would increase the efficiency of nitrogen use for peanut under elevated CO2,” just as the optimization of
inorganic carbon acquisition and greater accumulation of the primary photosynthetic products in the CO2-enriched plants “would be beneficial for peanut growth at elevated CO2.” Indeed,
in the absence of other stresses, Vu’s conclusion was that “peanut photosynthesis would perform well under rising atmospheric CO2 and temperature predicted for this century.” In a somewhat
different type of study, Alexandrov and Hoogenboom (2000) studied how year-to-year changes in temperature, precipitation and solar radiation had influenced the yields of peanuts over
a 30-year period in the southeastern United States, after which they used the results to predict future crop yields based on climate-change output from various global circulation models
(GCMs) of the atmosphere. At ambient CO2 concentrations, the GCM scenarios suggested a decrease in peanut yields by the year 2020, due in part to predicted changes in temperature and
precipitation. However, when the yield-enhancing effects of a doubling of the atmospheric CO2 concentration were included, a totally different result was obtained: a yield increase.
Although we have little faith in GCM scenarios, it is interesting to note that their climate change predictions often result in positive outcomes for agricultural productivity when the
direct effects of elevated CO2 on plant growth and development are included in the analyses. These results support the research reported later in this chapter describing the stress-ameliorating
effects of atmospheric CO2 enrichment on plant growth and development under unfavorable growing conditions characterized by high air temperatures and inadequate soil moisture. In conclusion,
it would appear that even if the climate changes that are typically predicted to result from anticipated increases in the air’s CO2 content were to materialize, the concurrent rise in
the air’s CO2 concentration should more than compensate for any deleterious effects those changes in climate might otherwise have had on the growth and yield of peanuts. Additional information
on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/a/peanut.php. References Alexandrov, V.A. and Hoogenboom,
G. 2000. Vulnerability and adaptation assessments of agricultural crops under climate change in the Southeastern USA. Theoretical and Applied Climatology 67: 45-63. Idso, S.B. 1998.
CO2-induced global warming: a skeptic’s view of potential climate change. Climate Research 10: 69-82. Prasad, P.V.V., Boote, K.J., Allen Jr., L.H. and Thomas, J.M.G. 2003. Super-optimal
temperatures are detrimental to peanut (Arachis hypogaea L.) reproductive processes and yield at both ambient and elevated carbon dioxide. Global Change Biology 9: 1775-1787. Stanciel,
K., Mortley, D.G., Hileman, D.R., Loretan, P.A., Bonsi, C.K. and Hill, W.A. 2000. Growth, pod and seed yield, and gas exchange of hydroponically grown peanut in response to CO2 enrichment.
HortScience 35: 49-52. Vu, J.C.V. 2005. Acclimation of peanut (Arachis hypogaea L.) leaf photosynthesis to elevated growth CO2 and temperature. Environmental and Experimental Botany
53: 85-95.
Climate Change Reconsidered 370 7.1.1.5. Potato In the study of Sicher and Bunce (1999), exposure to twice-ambient atmospheric CO2 concentrations enhanced rates of net photosynthesis
in potato plants (Solanum tuberosum L.) by 49 percent; while in the study of Schapendonk et al. (2000), a doubling of the air’s CO2 content led to an 80 percent increase in net photosynthesis.
In a study that additionally considered the role of the air’s vapor pressure deficit (VPD), Bunce (2003) found that exposure to twice-ambient atmospheric CO2 concentrations boosted net
photosynthesis by 36 percent at a VPD of 0.5 kPa (moist air) but by 70 percent at a VPD of 3.5 kPa (dry air). Yet another complexity was investigated by Olivo et al. (2002), who assessed
the effect of a doubling of the air’s CO2 content on the net photosynthetic rates of high-altitude (Solanum curtilobum) and low-altitude (S. tuberosum) and found the rate of the former
to be enhanced by 56 percent and that of the latter by 53 percent. In addition, although they did not directly report photosynthetic rates, Louche-Tessandier et al. (1999) noted that
photosynthetic acclimation was reduced in CO2-enriched plants that were inoculated with a fungal symbiont, which consequently allowed them to produce greater amounts of biomass than
noninoculated control plants grown in ambient air. Because elevated CO2 concentrations stimulate photosynthesis in potatoes, it is to be expected they would also increase potato biomass
production. Miglietta et al. (1998), for example, reported that potatoes grown at 660 ppm CO2 produced 40 percent more tuber biomass than control plants grown in ambient air. Such reports
are common, in fact, with twice-ambient atmospheric CO2 concentrations having been reported to produce yield increases of 25 percent (Lawson et al., 2001), 36 percent (Chen and Setter,
2003), 37 percent (Schapendonk et al., 2000), 40 percent (Olivo et al., 2002), 44 percent (Sicher and Bunce, 1999), 85 percent (Olivo et al., 2002) and 100 percent (Ludewig et al., 1998).
A few studies have been conducted at even higher atmospheric CO2 concentrations. Kauder et al. (2000), for example, grew plants for up to seven weeks in controlled environments receiving
an extra 600 ppm CO2, obtaining final tuber yields that were 30 percent greater than those of ambiently grown plants. Also, in a study of potato microcuttings grown for four weeks in
environmental chambers maintained at ambient air and air enriched with an extra 1200 ppm CO2, Pruski et al. (2002) found that the average number of nodes per stem was increased by 64
percent, the average stem dry weight by 92 percent, and the average shoot length by 131 percent. Atmospheric CO2 enrichment also leads to reductions in transpirational water loss by
potato plants. Magliulo et al. (2003), for example, grew potatoes in the field within FACE rings maintained at either ambient (370 ppm) or enriched (550 ppm) atmospheric CO2 concentrations
for two consecutive years, finding that the CO2-enriched plants used 12 percent less water than the the ambient-treatment plants, while they produced 47 percent more tuber biomass. Hence,
the CO2-enriched plants experienced a 68 percent increase in water use efficiency, or the amount of biomass produced per unit of water used in producing it. Likewise, Olivo et al. (2002)
found that a doubling of the air’s CO2 content increased the instantaneous water-use efficiencies of high-altitude and low-altitude potato species by 90 percent and 80 percent, respectively.
In the final phenomenon considered here, we review the findings of three studies that evaluated the ability of atmospheric CO2 enrichment to mitigate the deleterious effects of ozone
pollution on potato growth. Fangmeier and Bender (2002) determined the mean tuber yield of potato as a function of atmospheric CO2 concentration for conditions of ambient and high atmospheric
O3 concentrations, as derived from a trans-European set of experiments. At the mean ambient CO2 concentration of 380 ppm, the high O3 stress reduced mean tuber yield by approximately
9 percent. At CO2 concentrations of 540 and 680 ppm, however, the high O3 stress had no significant effect on tuber yield. Much the same results were obtained by Wolf and van Oijen (2002,
2003), who used the validated potato model LPOTCO to project future European tuber yields. Under two climate change scenarios that incorporated the effects of increased greenhouse gases
on climate (i.e., increased air temperature and reduced precipitation), the model generated increases in irrigated tuber production ranging from 2,000 to 4,000 kg of dry matter per hectare
across Europe, with significant reductions in the negative effects of O3 pollution. Additional information on this topic, including reviews of newer publications as they become available,
can be found at http://www.co2science.org/subject/a/agriculturepotato.php.
Biological Effects of Carbon Dioxide Enrichment 371 References Bunce, J.A. 2003. Effects of water vapor pressure difference on leaf gas exchange in potato and sorghum at ambient and
elevated carbon dioxide under field conditions. Field Crops Research 82: 37-47. Chen, C.-T. and Setter, T.L. 2003. Response of potato tuber cell division and growth to shade and elevated
CO2. Annals of Botany 91: 373-381. Fangmeier, A. and Bender, J. 2002. Air pollutant combinations—Significance for future impact assessments on vegetation. Phyton 42: 65-71. Kauder, F.,
Ludewig, F. and Heineke, D. 2000. Ontogenetic changes of potato plants during acclimation to elevated carbon dioxide. Journal of Experimental Botany 51: 429-437. Lawson, T., Craigon,
J., Black, C.R., Colls, J.J., Tulloch, A.-M. and Landon, G. 2001. Effects of elevated carbon dioxide and ozone on the growth and yield of potatoes (Solanum tuberosum) grown in open-top
chambers. Environmental Pollution 111: 479-491. Louche-Tessandier, D., Samson, G., Hernandez-Sebastia, C., Chagvardieff, P. and Desjardins, Y. 1999. Importance of light and CO2 on the
effects of endomycorrhizal colonization on growth and photosynthesis of potato plantlets (Solanum tuberosum) in an in vitro tripartite system. New Phytologist 142: 539-550. Ludewig,
F., Sonnewald, U., Kauder, F., Heineke, D., Geiger, M., Stitt, M., Muller-Rober, B.T., Gillissen, B., Kuhn, C. and Frommer, W.B. 1998. The role of transient starch in acclimation to
elevated atmospheric CO2. FEBS Letters 429: 147-151. Magliulo, V., Bindi, M. and Rana, G. 2003. Water use of irrigated potato (Solanum tuberosum L.) grown under free air carbon dioxide
enrichment in central Italy. Agriculture, Ecosystems and Environment 97: 65-80. Miglietta, F., Magliulo, V., Bindi, M., Cerio, L., Vaccari, F.P., Loduca, V. and Peressotti, A. 1998.
Free Air CO2 Enrichment of potato (Solanum tuberosum L.): development, growth and yield. Global Change Biology 4: 163-172. Olivo, N., Martinez, C.A. and Oliva, M.A. 2002. The photosynthetic
response to elevated CO2 in high altitude potato species (Solanum curtilobum). Photosynthetica 40: 309-313. Pruski, K., Astatkie, T., Mirza, M. and Nowak, J. 2002. Photoautotrophic micropropagation
of Russet Burbank potato. Plant, Cell and Environment 69: 197-200. Schapendonk, A.H.C.M., van Oijen, M., Dijkstra, P., Pot, C.S., Jordi, W.J.R.M. and Stoopen, G.M. 2000. Effects of elevated
CO2 concentration on photosynthetic acclimation and productivity of two potato cultivars grown in open-top chambers. Australian Journal of Plant Physiology 27: 1119-1130. Sicher, R.C.
and Bunce, J.A. 1999. Photosynthetic enhancement and conductance to water vapor of fieldgrown Solanum tuberosum (L.) in response to CO2 enrichment. Photosynthesis Research 62: 155-163.
Wolf, J. and van Oijen, M. 2002. Modelling the dependence of European potato yields on changes in climate and CO2. Agricultural and Forest Meteorology 112: 217-231. Wolf, J. and van
Oijen, M. 2003. Model simulation of effects of changes in climate and atmospheric CO2 and O3 on tuber yield potential of potato (cv. Bintje) in the European Union. Agriculture, Ecosystems
and Environment 94: 141-157. 7.1.1.6. Rice DeCosta et al. (2003a) grew two crops of rice (Oryza sativa L.) at the Rice Research and Development Institute of Sri Lanka from January to
March (the maha season) and from May to August (the yala season) in open-top chambers in air of either ambient or ambient plus 200 pppm CO2, determining that leaf net photosynthetic
rates were significantly higher in the CO2-enriched chambers than in the ambient-air chambers: 51-75 percent greater in the maha season and 22-33 percent greater in the yala season.
Likewise, in the study of Gesch et al. (2002), where one-month-old plants were maintained at either 350 ppm CO2 or switched to a concentration of 700 ppm for 10 additional days, the
plants switched to CO2-enriched air immediately displayed large increases in their photosynthetic rates that at the end of the experiment were still 31 percent greater than those exhibited
by unswitched control plants. With respect to the opposite of photosynthesis, Baker et al. (2000) reported that rates of carbon loss via dark respiration in rice plants decreased with
increasing nocturnal CO2 concentrations. As a result, it is not surprising that in the study of Weerakoon et al. (2000), rice plants exposed to an extra 300 ppm of atmospheric CO2 exhibited
a 35 percent increase in mean season-long radiation-use efficiency, defined as the amount of biomass produced per unit of solar radiation intercepted. In light of these several observations,
therefore, one would logically expect rice plants to routinely produce more biomass at elevated levels of atmospheric CO2.
Climate Change Reconsidered 372 In conjunction with the study of DeCosta et al. (2003a), DeCosta et al. (2003b) found that CO2-enriched rice plants produced more leaves per hill, more
tillers per hill, more total plant biomass, greater root dry weight, and more panicles per plant and had harvest indices that were increased by 4 percent and 2 percent, respectively,
in the maha and yala seasons, which suite of benefits led to grain yield increases of 24 percent and 39 percent in those two periods. In another study, Kim et al. (2003) grew rice crops
from the seedling stage to maturity at atmospheric CO2 concentrations of ambient and ambient plus 200 ppm using FACE technology and three levels of applied nitrogen—low (LN, 4 g N m-2),
medium (MN, 8 and 9 g N m-2), and high (HN, 15 g N m-2)—for three cropping seasons (1998-2000). They found that “the yield response to elevated CO2 in crops supplied with MN (+14.6 percent)
or HN (+15.2 percent) was about twice that of crops supplied with LN (+7.4 percent),” confirming the importance of N availability to the response of rice to atmospheric CO2 enrichment
that had previously been determined by Kim et al. (2001) and Kobaysahi et al. (2001). Various environmental stresses can significantly alter the effect of elevated CO2 on rice. In the
study of Tako et al. (2001), rice plants grown at twice-ambient CO2 concentrations and ambient temperatures displayed no significant increases in biomass production; but when air temperatures
were raised by 2°C, the CO2-enriched plants produced 22 percent more biomass than the plants grown in non-CO2-enriched air. By contrast, Ziska et al. (1997) reported that CO2-enriched
rice plants grown at elevated air temperatures displayed no significant increases in biomass; but when the plants were grown at ambient air temperatures, the additional 300 ppm of CO2
boosted their rate of biomass production by 40 percent. In light of these observations, rice growers should select cultivars that are most responsive to elevated CO2 concentrations at
the air temperatures likely to prevail in their locality in order to maximize their yield production in a future high-CO2 world. Water stress can also severely reduce rice production.
As an example, Widodo et al. (2003) grew rice plants in eight outdoor, sunlit, controlledenvironment chambers at daytime atmospheric CO2 concentrations of 350 and 700 ppm for an entire
season. In one set of chambers, the plants were continuously flooded. In another set, drought stress was imposed during panicle initiation. In another, it was imposed during anthesis;
and in the last set, drought stress was imposed at both stages. The resultant drought-induced effects, according to the scientists, “were more severe for plants grown at ambient than
at elevated CO2.” They report, for example, that “plants grown under elevated CO2 were able to maintain midday leaf photosynthesis, and to some extent other photosynthetic-related parameters,
longer into the drought period than plants grown at ambient CO2.” Recovery from the drought-induced induced water stress was also more rapid in the elevated CO2 treatment. At panicle
initiation, for example, Widodo et al. observed that “as water was added back following a drought induction, it took more than 24 days for the ambient CO2 [water]-stressed plants to
recuperate in midday leaf CER, compared with only 6-8 days for the elevated CO2 [water]-stressed plants.” Similarly, they report that “for the drought imposed during anthesis, midday
leaf CER of the elevated CO2 [water]-stressed plants were fully recovered after 16 days of re-watering, whereas those of the ambient CO2 [water]-stressed plants were still 21 percent
lagging behind their unstressed controls at that date.” Hence, they concluded that “rice grown under future rising atmospheric CO2 should be better able to tolerate drought situations.”
In a somewhat different type of study, Watling and Press (2000) found that rice plants growing in ambient air and infected with a root hemiparasitic angiosperm obtained final biomass
values that were only 35 percent of those obtained by uninfected plants. In air of 700 ppm CO2, however, the infected plants obtained biomass values that were 73 percent of those obtained
by uninfected plants. Thus, atmospheric CO2 enrichment significantly reduced the negative impact of this parasite on biomass production in rice. In summary, as the CO2 concentration
of the air continues to rise, rice plants will likely experience greater photosynthetic rates, produce more biomass, be less affected by root parasites, and better deal with environmental
stresses, all of which effects should lead to greater grain yields. Additional information on this topic, including reviews of newer publications as they become available, can be found
at http://www.co2science.org/subject/a/agriculturerice.php. References Baker, J.T., Allen Jr., L.H., Boote, K.J. and Pickering, N.B. 2000. Direct effects of atmospheric carbon dioxide
Biological Effects of Carbon Dioxide Enrichment 373 concentration on whole canopy dark respiration of rice. Global Change Biology 6: 275-286. De Costa, W.A.J.M., Weerakoon, W.M.W., Abeywardena,
R.M.I. and Herath, H.M.L.K. 2003a. Response of photosynthesis and water relations of rice (Oryza sativa) to elevated atmospheric carbon dioxide in the subhumid zone of Sri Lanka. Journal
of Agronomy and Crop Science 189: 71-82. De Costa, W.A.J.M., Weerakoon, W.M.W., Herath, H.M.L.K. and Abeywardena, R.M.I. 2003b. Response of growth and yield of rice (Oryza sativa) to
elevated atmospheric carbon dioxide in the subhumid zone of Sri Lanka. Journal of Agronomy and Crop Science 189: 83-95. Gesch, R.W., Vu, J.C., Boote, K.J., Allen Jr., L.H. and Bowes,
G. 2002. Sucrose-phosphate synthase activity in mature rice leaves following changes in growth CO2 is unrelated to sucrose pool size. New Phytologist 154: 77-84. Kim, H.-Y., Lieffering,
M., Kobayashi, K., Okada, M., Mitchell, M.W. and Gumpertz, M. 2003. Effects of free-air CO2 enrichment and nitrogen supply on the yield of temperate paddy rice crops. Field Crops Research
83: 261-270. Kim, H.-Y., Lieffering, M., Miura, S., Kobayashi, K. and Okada, M. 2001. Growth and nitrogen uptake of
CO2-enriched rice under field conditions. New Phytologist 150: 223-229. Kobayashi, K., Lieffering, M. and Kim, H.-Y. 2001. Growth and yield of paddy rice under free-air CO2 enrichment.
In: Shiyomi, M. and Koizumi, H. (Eds.) Structure and Function in Agroecosystem Design and Management. CRC Press, Boca Raton, FL, USA, pp. 371-395. Tako, Y., Arai, R., Otsubo, K. and
Nitta, K. 2001. Application of crop gas exchange and transpiration data obtained with CEEF to global change problem. Advances in Space Research 27: 1541-1545. Watling, J.R. and Press,
M.C. 2000. Infection with the parasitic angiosperm Striga hermonthica influences the response of the C3 cereal Oryza sativa to elevated CO2. Global Change Biology 6: 919-930. Weerakoon,
W.M.W., Ingram, K.T. and Moss, D.D. 2000. Atmospheric carbon dioxide and fertilizer nitrogen effects on radiation interception by rice. Plant and Soil 220: 99-106. Widodo, W., Vu, J.C.V.,
Boote, K.J., Baker, J.T. and Allen Jr., L.H. 2003. Elevated growth CO2 delays drought stress and accelerates recovery of rice leaf photosynthesis. Environmental and Experimental Botany
49: 259-272. Ziska, L.H., Namuco, O., Moya, T. and Quilang, J. 1997. Growth and yield response of field-grown tropical rice to increasing carbon dioxide and air temperature. Agronomy
Journal 89: 45-53. 7.1.1.7. Sorghum Many laboratory and field experiments have demonstrated a significant positive impact of elevated levels of atmospheric CO2 on total biomass and grain
production in the C4 crop sorghum (Sorghum bicolor (L.) Moench). Ottman et al. (2001) grew sorghum plants in a FACE experiment conducted near Phoenix, Arizona, USA, where plants were
fumigated with air containing either 360 or 560 ppm CO2 and where they were further subjected to irrigation regimes resulting in both adequate and inadequate levels of soil moisture.
Averaged over the two years of their study, the extra CO2 increased grain yield by only 4 percent in the plots receiving adequate levels of soil moisture but by 16 percent in the dry
soil moisture plots. Prior et al. (2005) grew sorghum in two different years in 7-meter-wide x 76-meter-long x 2-m-deep bins filled with a silt loam soil, upon which they constructed
a number of clear-plastic-wall open-top chambers they maintained at ambient CO2 concentrations and ambient concentrations plus 300 ppm. In the first of the two years, the extra CO2 increased
sorghum residue production by 14 percent, while in the second year it increased crop residue production by 24 percent and grain production by 22 percent. For a CO2 increase of 200 ppm
comparable to that employed in the study of Ottman et al., these figures translate to crop residue increases of 9 percent and 16 percent and a grain increase of 15 percent. In a review
of primary research papers describing results obtained from large-scale FACE experiments conducted over the prior 15 years, Ainsworth and Long (2005) determined that, in the mean, sorghum
grain yield was increased by approximately 7 percent in response to a 200-ppm increase in the atmosphere’s CO2 concentration. An experiment with a bit more complexity was carried out
several years earlier by Watling and Press (1997), who grew sorghum with and without infection by the parasitic C3 weeds Striga hermonthica and S. asiatica. The study lasted for about
two months and was conducted in controlled environment cabinets fumigated with air of either 350 or 700 ppm CO2. In the absence of parasite infection, the extra 350 ppm of CO2 boosted
plant biomass production by 35 percent, which adjusted downward to make it compatible with
Climate Change Reconsidered 374 the 200-ppm increase employed in most FACE studies corresponds to an increase of just under 21 percent. When infected with S. asiatica, the biomass stimulation
provided by the extra CO2 was about the same; but when infected with S. hermonthica, it was almost 80 percent, which corresponds to a similarly downward adjusted biomass increase of
45 percent. In light of these several observations, it would appear that although the CO2-induced increase in total biomass and grain yield of sorghum is rather modest, ranging from
4 to 16 percent under well-watered conditions, it can be on the high end of this range when the plants are stressed by a shortage of water (16 percent has been observed) and by parasitic
infection (45 percent has been observed). Consequently, elevated levels of atmospheric CO2 seem to help sorghum most when help is most needed. Additional information on this topic, including
reviews on sorghum not discussed here, can be found at http://www.co2science.org/subject/subject/a/subject_a.php under the main heading Agriculture, sub heading Sorghum. References Ainsworth,
E.A. and Long, S.P. 2005. What have we learned from 15 years of free-air CO2 enrichment (FACE)? A meta-analytic review of the responses of photosynthesis, canopy properties and plant
production to rising CO2. New Phytologist 165: 351-372. Ottman, M.J., Kimball, B.A., Pinter Jr., P.J., Wall, G.W., Vanderlip, R.L., Leavitt, S.W., LaMorte, R.L., Matthias, A.D. and Brooks,
T.J. 2001. Elevated CO2 increases sorghum biomass under drought conditions. New Phytologist 150: 261-273. Prior, S.A., Runion, G.B., Rogers, H.H., Torbert, H.A. and Reeves, D.W. 2005.
Elevated atmospheric CO2 effects on biomass production and soil carbon in conventional and conservation cropping systems. Global Change Biology 11: 657-665. Watling, J.R. and Press,
M.C. 1997. How is the relationship between the C4 cereal Sorghum bicolor and the C3 root hemi-parasites Striga hermonthica and Striga asiatica affected by elevated CO2? Plant, Cell and
Environment 20: 1292-1300. 7.1.1.8. Soybean Wittwer (1995) reports that the common soybean (Glycine max L.) “provides about two-thirds of the world’s protein concentrate for livestock
feeding, and is a valuable ingredient in formulated feeds for poultry and fish.” Bernacchi et al. (2005) characterize the soybean as “the world’s most important seed legume.” Consequently,
it is important to determine how soybeans will likely respond to rising atmospheric CO2 concentrations with and without concomitant increases in air temperature and under both well-watered
and water-stressed conditions. Rogers et al. (2004) grew soybeans from emergence to grain maturity in ambient and CO2-enriched air (372 and 552 ppm CO2, respectively) at the SoyFACE
facility of the University of Illinois at Urbana-Champaign, Illinois, USA, while CO2 uptake and transpiration measurements were made from predawn to post-sunset on seven days representative
of different developmental stages of the crop. Across the growing season, they found that the mean daily integral of leaf net photosynthesis rose by 24.6 percent in the elevated CO2
treatment, while mid-day stomatal conductance dropped by 21.9 percent, in response to the 48 percent increase in atmospheric CO2 employed in their study. With respect to photosynthesis,
they additionally report “there was no evidence of any loss of stimulation toward the end of the growing season,” noting that the largest stimulation actually occurred during late seed
filling. Nevertheless, they say that the photosynthetic stimulation they observed was only “about half the 44.5 percent theoretical maximum increase calculated from Rubisco kinetics.”
Thus, there is an opportunity for soybeans to perhaps become even more responsive to atmospheric CO2 enrichment than they are currently, which potential could well be realized via future
developments in the field of genetic engineering. Bunce (2005) grew soybeans in the field in opentop chambers maintained at atmospheric CO2 concentrations of ambient and ambient +350
ppm at the Beltsville Agricultural Research Center in Maryland, USA, where net CO2 exchange rate measurements were performed on a total of 16 days between 18 July and 11 September of
2000 and 2003, during flowering to early pod-filling. Over the course of this study, daytime net photosynthesis per unit leaf area was 48 percent greater in the plants growing in the
CO2-enriched air, while nighttime respiration per unit leaf area was not affected by elevated CO2.
Biological Effects of Carbon Dioxide Enrichment 375 However, because the elevated CO2 increased leaf dry mass per unit area by an average of 23 percent, respiration per unit mass was
significantly lower for the leaves of the soybeans growing in the CO2-enriched air, producing a sure recipe for accelerated growth and higher soybean seed yields. Working in Australia,
Japan, and the United States, Ziska et al. (2001b) observed a recurrent diurnal pattern of atmospheric CO2 concentration, whereby maximum values of 440-540 ppm occurred during a three-hour
pre-dawn period that was followed by a decrease to values of 350-400 ppm by mid-morning, after which there was a slow but steady increase in the late afternoon and early evening that
brought the air’s CO2 concentration back to its predawn maximum value. In an attempt to see if the predawn CO2 spikes they observed affected plant growth, they grew soybeans for one
month in controlledenvironment chambers under three different sets of conditions: a constant 24-hour hour exposure to 370 ppm CO2, a constant 370 ppm CO2 exposure during the day followed
by a constant 500 ppm CO2 exposure at night, and a CO2 exposure of 500 ppm from 2200 to 0900 followed by a decrease to 370 ppm by 1000, which was maintained until 2200, somewhat mimicking
the CO2 cycle they observed in nature. This program revealed that the 24-hour exposure to 370 ppm CO2 and the 370-ppm-day/500-ppm-night treatments produced essentially the same results
in terms of biomass production after 29 days. However, the CO2 treatment that mimicked the observed atmospheric CO2 pattern resulted in a plant biomass increase of 20 percent. In a study
that evaluated a whole range of atmospheric CO2 concentrations, from far below ambient levels to high above them, Allen et al. (1998) grew soybeans for an entire season in growth chambers
maintained at atmospheric CO2 concentrations of 160, 220, 280, 330, 660, and 990 ppm. In doing so, they observed a consistent increase in total nonstructural carbohydrates in all vegetative
components including roots, stems, petioles, and especially the leaves, as CO2 concentrations rose. There was, however, no overall significant effect of treatment CO2 concentration on
nonstructural carbohydrate accumulation in soybean reproductive components, including podwalls and seeds, which observations indicate that the higher yields reported in the literature
for soybeans exposed to elevated CO2 most likely result from increases in the number of pods produced per plant, and not from the production of larger individual pods or seeds. The increasing
amounts of total nonstructural carbohydrates that were produced with each additional increment of CO2 provided the raw materials to support greater biomass production at each CO2 level.
Although final biomass and yield data were not reported in this paper, the authors did present biomass data obtained at 66 days into the experiment. Relative to above-ground biomass
measured at 330 ppm CO2, the plants that were grown in sub-ambient CO2 concentrations of 280, 280, 220, and 160 ppm exhibited 12, 33, and 60 percent less biomass, respectively, while
plants grown in atmospheric CO2 concentrations of 660 and 990 ppm displayed 46 and 66 percent more biomass. In a study of two contrasting soybean cultivars, Ziska and Bunce (2000) grew
Ripley, which is semidwarf and determinate in growth, and Spencer, which is standard-size and indeterminate in growth, for two growing seasons in open-top chambers maintained at atmospheric
CO2 concentrations of ambient and ambient plus 300 ppm. Averaged over both years, the elevated CO2 treatment increased photosynthetic rates in the Ripley and Spencer varieties by 76
and 60 percent, respectively. However, Spencer showed a greater CO2-induced increase in vegetative biomass than Ripley (132 vs. 65 percent). Likewise, elevated CO2 enhanced seed yield
in Spencer by 60 percent but by only 35 percent in Ripley, suggesting that cultivar selection for favorable yield responses to atmospheric CO2 enrichment could have a big impact on future
farm farm productivity. In another study of contrasting types of soybeans, Nakamura et al. (1999) grew nodulated and nonnodulated plants in pots within controlledenvironmental cabinets
maintained at atmospheric CO2 concentrations of 360 and 700 ppm in combination with low and high soil nitrogen supply for three weeks. They found that at low nitrogen, elevated CO2 increased
total plant dry mass by approximately 40 and 80 percent in nodulated soybeans grown at low and high nitrogen supply, respectively, while non-nodulated plants exhibited no CO2-induced
growth response at low nitrogen but an approximate 60 percent growth enhancement at high nitrogen supply. Hence, it would appear that as the air’s CO2 content continues to rise, non-nodulated
soybeans will display increases in biomass only if they are grown in nitrogen-rich soils. Nodulated soybeans, however, should display increased growth in both nitrogen-rich and nitrogen-poor
soils, with their responses being about twice as large in high as in low soil nitrogen conditions.
Climate Change Reconsidered 376 In yet another study of soybeans with different genetic characteristics, Ziska et al. (2001a) grew one modern and eight ancestral soybean genotypes in
glasshouses maintained at atmospheric CO2 concentrations of 400 and 710 ppm, finding that the elevated CO2 increased photosynthetic rates in all cultivars by an average of 75 percent.
This photosynthetic enhancement led to CO2-induced increases in seed yield that averaged 40 percent, except for one of the ancestral varieties that exhibited an 80 percent increase in
seed yield. To get a glimpse of what might happen if future temperatures also continue to rise, Ziska (1998) grew soybeans for 21 days in controlled environments having atmospheric CO2
concentrations of approximately 360 (ambient) or 720 ppm and soil temperatures of 25° (ambient) or 30°C. He found that elevated CO2 significantly increased whole plant net photosynthesis
at both temperatures, with the greatest effect occurring at 30°C. As time progressed, however, this this photosynthetic stimulation dropped from 50 percent at 13 days into the experiment
to 30 percent at its conclusion eight days later; in spite of this partial acclimation, which was far from complete, atmospheric CO2 enrichment significantly enhanced total plant dry
weight at final harvest by 36 and 42 percent at 25° and 30°C, respectively. Studying the complicating effects of water stress were Serraj et al. (1999), who grew soybeans from seed in
pots within a glasshouse until they were four weeks old, after which half of the plants were subjected to an atmospheric CO2 concentration of 360 ppm, while the other half were exposed
to an elevated concentration of 700 ppm. In addition, half of the plants at each CO2 concentration were well-watered and half of them were allowed to experience water stress for a period
of 18 days. This protocol revealed that short-term (18-day) exposure of soybeans to elevated CO2 significantly decreased daily and cumulative transpirational water losses compared to
plants grown at 360 ppm CO2, regardless of water treatment. In fact, elevated CO2 reduced total water loss by 25 and 10 percent in well-watered and waterstressed plants, respectively.
Also, drought stress significantly reduced rates of net photosynthesis among plants of both CO2 treatments. However, plants grown in elevated CO2 consistently exhibited higher photosynthetic
rates than plants grown at ambient CO2, regardless of soil water status. At final harvest, the elevated CO2 treatment had little effect on the total dry weight of plants grown at optimal
soil moisture, but it increased the total dry weight of water-stressed plants by about 33 percent. Also, while root dry weight declined for plants grown under conditions of water stress
and ambient CO2 concentration, no such decline was exhibited by plants subjected to atmospheric CO2 enrichment and water stress. Studying both water and high-temperature stress were
Ferris et al. (1999), who grew soybeans in glasshouses maintained at atmospheric CO2 concentrations of 360 360 and 700 ppm for 52 days, before having various environmental stresses imposed
on them for eight days during early seed filling. For the eight-day stress period, some plants were subjected to air temperatures that were 15°C higher than those to which the control
plants were exposed, while some were subjected to a water stress treatment in which their soil moisture contents were maintained at 40 percent of that experienced by the control plants.
Averaged across all stress treatments and harvests, this protocol revealed that the high CO2 treatment increased total plant biomass by 41 percent. Both high-temperature and water-deficit
treatments, singly or in combination, reduced overall biomass by approximately the same degree, regardless of CO2 treatment. Thus, even when the greatest biomass reductions of 17 percent
occurred in the CO2-enriched and ambiently grown plants, in response to the combined stresses of high temperature and low soil moisture, plants grown in elevated CO2 still exhibited
an average biomass that was 24 percent greater than that displayed by plants grown in ambient CO2. Averaged across all stress treatments and harvests, elevated CO2 increased seed yield
by 32 percent. In addition, it tended to ameliorate the negative effects of environmental stresses. CO2-enriched plants that were water stressed, for example, had an average seed yield
that was 34 percent greater than that displayed by water-stressed controls grown at ambient CO2, while CO2-enriched plants exposed to high temperatures produced 38 percent more seed
than their respectively stressed counterparts. In fact, the greatest relative impact of elevated CO2 on seed yield occurred in response to the combined stresses of high temperature and
low soil moisture, with CO2-enriched plants exhibiting a seed yield that was 50 percent larger than that of similarly stressed plants grown in ambient CO2. In a predictive application
of this type of knowledge, but based on a different means of obtaining it, Alexandrov and Hoogenboom (2000) studied studied how temperature, precipitation, and solar radiation influenced
soybean yields over a 30-year
Biological Effects of Carbon Dioxide Enrichment 377 period in the southeastern United States, after which they used the results they obtained to predict future crop yields based on climate
output from various global circulation models of the atmosphere. At ambient CO2 concentrations, the model-derived scenarios pointed to a decrease in soybean yields by the year 2020,
due in part to predicted changes in temperature and precipitation. However, when the yield-enhancing effects of a doubling of the air’s CO2 concentration were included in the simulations,
a completely different projection was obtained: a yield increase. Shifting to the subject of soybean seed quality, Caldwell et al. (2005) write that “the beneficial effects of isoflavone-rich
foods have been the subject of numerous studies (Birt et al., 2001; Messina, 1999),” and that “foods derived from soybeans are generally considered to provide both specific and general
health benefits,” presumably via these substances. Hence, it is only natural they would wonder how the isoflavone content of soybean seeds may be affected by the ongoing rise in the
air’s CO2 content, and that they would conduct a set of experiments to find the answer. The scientists grew well-watered and fertilized soybean plants from seed to maturity in pots within
two controlled-environment chambers, one maintained at an atmospheric CO2 concentration of 400 ppm and one at 700 ppm. The chambers were initially kept at a constant air temperature
of 25°C. At the onset of seed fill, however, air temperature was reduced to 18°C until seed development was complete, in order to simulate average outdoor temperatures at this stage
of plant development. In a second experiment, this protocol was repeated, except the temperature during seed fill was maintained at 23°C, with and without drought (a third treatment),
while in a third experiment, seed-fill temperature was maintained at 28°C, with or without drought. In the first experiment, where air temperature during seed fill was 18°C, the elevated
CO2 treatment increased the total isoflavone content of the soybean seeds by 8 percent. In the second experiment, where air temperature during seed fill was 23°C, the extra CO2 increased
total seed isoflavone content by 104 percent, while in the third experiment, where air temperature during seed fill was 28°C, the CO2-induced isoflavone increase was 101 percent. Finally,
when drought-stress was added as a third environmental variable, the extra CO2 boosted total seed isoflavone content by 186 percent when seed-fill air temperature was 23°C, while at
a seed-fill temperature of 28°C, it increased isoflavone content by 38 percent. Under all environmental circumstances studied, enriching the air with an extra 300 ppm of CO2 increased
the total isoflavone content of soybean seeds. In addition, the percent increases measured under the stress situations investigated were always greater than the percent increase measured
under optimal growing conditions. Also writing on the subject of soybean seed quality, Thomas et al. (2003) say “the unique chemical composition of soybean has made it one of the most
valuable agronomic crops worldwide,” noting that “oil and protein comprise ~20 and 40%, respectively, of the dry weight of soybean seed.” Consequently, they explored the effects of elevated
CO2 plus temperature on soybeans that were grown to maturity in sunlit controlled-environment chambers with sinusoidally varying day/night max/min temperatures of 28/18°, 32/22°, 36/26°,
40/30°, and 44/34°C and atmospheric CO2 concentrations of 350 and 700 ppm. This work revealed that the effect of temperature on seed composition and gene expression was “pronounced,”
but that “there was no effect of CO2.” In this regard, however, they note that “Heagle et al. (1998) observed a positive significant effect of CO2 enrichment on soybean seed oil and
oleic acid concentration,” the latter of which parameters their own study found to rise with increasing temperature all the way from 28/18° to 44/34°C. In addition, they determined that
“32/22°C is optimum for producing the highest oil concentration in soybean seed,” that “the degree of fatty acid saturation in soybean oil was significantly increased by increasing temperature,”
and that crude protein concentration increased with temperature to 40/30°C. In commenting on these observations, Thomas et al. note that “the intrinsic value of soybean seed is in its
supply of essential fatty acids and amino acids in the oil and protein, respectively.” Hence, we conclude that the temperature-driven changes they identified in these parameters, as
well as the CO2 effect observed by Heagle et al., bode well for the future production of this important crop and its value to society in a CO2-enriched and warming world. Thomas et al.
note, however, that “temperatures during the soybeangrowing season in the southern USA are at, or slightly higher than, 32/22°C,” and that warming could negatively impact the soybean
oil industry in this region. For the world as a whole, however, warming would be a positive development for soybean production; while in the southern United States, shifts
Climate Change Reconsidered 378 in planting zones could readily accommodate changing weather patterns associated with this phenomenon. In conclusion, as the air’s CO2 content continues
to rise, soybeans will likely respond by displaying significant increases in growth and yield, with possible improvements in seed quality; these beneficial effects will likely persist
even if temperatures rise or soil moisture levels decline, regardless of their cause. Additional information on this topic, including reviews on sorghum not discussed here, can be found
at http://www.co2science.org/subject/a/subject_a.php under the main heading Agriculture, sub heading Soybean. References Alexandrov, V.A. and Hoogenboom, G. 2000. Vulnerability and adaptation
assessments of agricultural crops under climate change in the Southeastern USA. Theoretical and Applied Climatology 67: 45-63. Allen Jr., L.H., Bisbal, E.C. and Boote, K.J. 1998. Nonstructural
carbohydrates of soybean plants grown in subambient and superambient levels of CO2. Photosynthesis Research 56: 143-155. Bernacchi, C.J., Morgan, P.B., Ort, D.R. and Long, S.P. 2005.
The growth of soybean under free air [CO2] enrichment (FACE) stimulates photosynthesis while decreasing in vivo Rubisco capacity. Planta 220: 434-446. Birt, D.F., Hendrich, W. and Wang,
W. 2001. Dietary agents in cancer prevention: flavonoids and isoflavonoids. Pharmacology & Therapeutics 90: 157-177. Bunce, J.A. 2005. Response of respiration of soybean leaves grown
at ambient and elevated carbon dioxide concentrations to day-to-day variation in light and temperature under field conditions. Annals of Botany 95: 1059-1066. Caldwell, C.R., Britz,
S.J. and Mirecki, R.M. 2005. Effect of temperature, elevated carbon dioxide, and drought during seed development on the isoflavone content of dwarf soybean [Glycine max (L.) Merrill]
grown in controlled environments. Journal of Agricultural and Food Chemistry 53: 1125-1129. Ferris, R., Wheeler, T.R., Ellis, R.H. and Hadley, P. 1999. Seed yield after environmental
stress in soybean grown under elevated CO2. Crop Science 39: 710-718. Heagle, A.S., Miller, J.E. and Pursley, W.A. 1998. Influence of ozone stress on soybean response to carbon dioxide
enrichment: III. Yield and seed quality. Crop Science 38: 128-134. Messina, M.J. 1999. Legumes and soybeans: overview of their nutritional profiles and health effects. American Journal
of Clinical Nutrition 70(S): 439s-450s. Nakamura, T., Koike, T., Lei, T., Ohashi, K., Shinano, T. and Tadano, T. 1999. The effect of CO2 enrichment on the growth of nodulated and non-nodulated
isogenic types of soybean raised under two nitrogen concentrations. Photosynthetica 37: 61-70. Rogers, A., Allen, D.J., Davey, P.A., Morgan, P.B., Ainsworth, E.A., Bernacchi, C.J., Cornic,
G., Dermody, O., Dohleman, F.G., Heaton, E.A., Mahoney, J., Zhu, X.-G., DeLucia, E.H., Ort, D.R. and Long, S.P. 2004. Leaf photosynthesis and carbohydrate dynamics of soybeans grown
throughout their life-cycle under Free-Air Carbon dioxide Enrichment. Plant, Cell and Environment 27: 449-458. Serraj, R., Allen Jr., L.H., Sinclair, T.R. 1999. Soybean leaf growth and
gas exchange response to drought under carbon dioxide enrichment. Global Change Biology 5: 283-291. Thomas, J.M.G., Boote, K.J., Allen Jr., L.H., Gallo-Meagher, M. and Davis, J.M. 2003.
Elevated temperature and carbon dioxide effects on soybean seed composition and transcript abundance. Crop Science 43: 1548-1557. Wittwer, S.H. 1995. Food, Climate, and Carbon Dioxide:
The Global Environment and World Food Production. CRC Press, Boca Raton, FL. Ziska, L.H. 1998. The influence of root zone temperature on photosynthetic acclimation to elevated carbon
dioxide concentrations. Annals of Botany 81: 717-721. Ziska, L.W. and Bunce, J.A. 2000. Sensitivity of fieldgrown soybean to future atmospheric CO2: selection for improved productivity
in the 21st century. Australian Journal of Plant Physiology 27: 979-984. Ziska, L.H., Bunce, J.A. and Caulfield, F.A. 2001a. Rising atmospheric carbon dioxide and seed yields of soybean
genotypes. Crop Science 41: 385-391. Ziska, L.H., Ghannoum, O., Baker, J.T., Conroy, J., Bunce, J.A., Kobayashi, K. and Okada, M. 2001b. A global perspective of ground level, ‘ambient’
carbon dioxide for assessing the response of plants to atmospheric CO2. Global Change Biology 7: 789-796. 7.1.1.9. Strawberry In the open-top chamber study of Bunce (2001), strawberry
plants (Fragaria x ananassa) exposed to air containing an extra 300 and 600 ppm CO2
Biological Effects of Carbon Dioxide Enrichment 379 displayed photosynthetic rates that were 77 and 106 percent greater, respectively, than rates displayed by plants grown in ambient
air containing 350 ppm CO2. Similarly, Bushway and Pritts (2002) reported that strawberry plants grown at atmospheric CO2 concentrations between 700 and 1,000 ppm exhibited photosynthetic
rates that were consistently more than 50 percent greater than rates displayed by control plants. Because elevated CO2 stimulates rates
of photosynthesis in strawberry plants, it is expected that it would also increase biomass production in this important agricultural species. After growing plants in air containing
an additional 170 ppm CO2 above ambient concentrations, Deng and Woodward (1998) reported that total fresh fruit weights were 42 and 17 percent greater than weights displayed by control
plants receiving high and low soil nitrogen inputs, respectively. In addition, Bushway and Pritts (2002) reported that a two-to three-fold increase in the air’s CO2 content boosted strawberry
fruit yield by 62 percent. As the air’s CO2 content continues to rise, strawberry plants will likely exhibit enhanced rates of photosynthesis and biomass production, which should lead
to greater fruit yields. Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/a/agriculturestraw.p
p. References Bunce, J.A. 2001. Seasonal patterns of photosynthetic response and acclimation to elevated carbon dioxide in field-grown strawberry. Photosynthesis Research 68: 237-245.
Bushway, L.J. and Pritts, M.P. 2002. Enhancing early spring microclimate to increase carbon resources and productivity in June-bearing strawberry. Journal of the American Society for
Horticultural Science 127: 415-422. Deng, X. and Woodward, F.I. 1998. The growth and yield responses of Fragaria ananassa to elevated CO2 and N supply. Annals of Botany 81: 67-71. 7.1.1.10.
Sunflower As the CO2 content of the air increases, sunflower plants (Helianthus annus L.) will likely display enhanced rates of photosynthetic carbon uptake. In the study of Sims et
al. (1999), exposure to twiceambient atmospheric CO2 concentrations enhanced rates of net photosynthesis in individual upper-canopy sunflower leaves by approximately 50 percent. Similarly,
Luo et al. (2000) reported that sunflowers grown at 750 ppm CO2 displayed canopy carbon uptake rates that were fully 53 percent greater than those exhibited by plants grown at 400 ppm
CO2. The study of Zerihun et al. (2000) reported that twice-ambient CO2 concentrations increased whole plant biomass in sunflowers by 44, 13, and 115 percent when the plants were simultaneously
exposed to low, medium, and high levels of soil nitrogen, respectively. Additional information on this topic, including reviews of newer publications as they become available, can be
found at http://www.co2science.org/subject/a/agriculturesun.php. References Luo, Y., Hui, D., Cheng, W., Coleman, J.S., Johnson, D.W. and Sims, D.A. 2000. Canopy quantum yield in a mesocosm
study. Agricultural and Forest Meteorology 100: 35-48. Sims, D.A., Cheng, W., Luo, Y. and Seeman, J.R. 1999. Photosynthetic acclimation to elevated CO2 in a sunflower canopy. Journal
of Experimental Botany 50: 645-653. Zerihun, A., Gutschick, V.P. and BassiriRad, H. 2000. Compensatory roles of nitrogen uptake and photosynthetic N-use efficiency in determining plant
growth response to elevated CO2: Evaluation using a functional balance model. Annals of Botany 86: 723-730. 7.1.1.11. Tomato In the study of Ziska et al. (2001), tomato plants (Lycopersicon
esculentum Mill.) grown at a nocturnal atmospheric CO2 concentration of 500 ppm displayed total plant biomass values that were 10 percent greater than those exhibited by control plants
growing in air containing 370 ppm CO2. This result was likely the consequence of the elevated CO2 reducing the rate of nocturnal respiration in the plants, which would have allowed them
to utilize the retained carbon to to produce more biomass. This CO2-induced benefit, as well as a host of other positive effects of atmospheric CO2 enrichment, are also manifest under
unfavorable growing conditions. Jwa and Walling (2001), for example,
Climate Change Reconsidered 380 reported that fungal infection reduced plant biomass in tomatoes growing in normal air by about 30 percent. However, in fungal-infected plants grown at
twice-ambient atmospheric CO2 concentrations, the elevated CO2 completely ameliorated the growthreducing effects of the pathogen. In another stressful situation, Maggio et al. (2002)
reported that a 500-ppm increase in the air’s CO2 concentration increased the average value of the root-zone salinity threshold in tomato plants by about 60 percent. In addition, they
reported that the wateruse efficiency of the CO2-enriched plants was about twice that of the ambiently grown plants. As the CO2 content of the air increases, tomato plants will likely
display greater rates of photosynthesis and biomass production, which should consequently lead to greater fruit yields, even under stressful conditions of fungal infection and high soil
salinity. Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/a/agriculturetomato.php.
References Jwa, N.-S. and Walling, L.L. 2001. Influence of elevated CO2 concentration on disease development in tomato. New Phytologist 149: 509-518. Maggio, A., Dalton, F.N. and Piccinni,
G. 2002. The effects of elevated carbon dioxide on static and dynamic indices for tomato salt tolerance. European Journal of Agronomy 16: 197-206. Ziska, L.H., Ghannoum, O., Baker, J.T.,
Conroy, J., Bunce, J.A., Kobayashi, K. and Okada, M. 2001. A global perspective of ground level, ‘ambient’ carbon dioxide for assessing the response of plants to atmospheric CO2. Global
Change Biology 7: 789-796. 7.1.1.12. Wheat In one study, Dijkstra et al. (1999) grew winter wheat (Triticum aestivum L.) in open-top chambers and field-tracking sun-lit climatized enclosures
maintained at atmospheric CO2 concentrations of ambient and ambient plus 350 ppm CO2 for two years, determining that the elevated CO2 increased both final grain yield and total above-above-ground
biomass by 19 percent. In another study, Masle (2000) grew two varieties of wheat for close to a month in greenhouses maintained at atmospheric CO2 concentrations of 350 and 900 ppm,
finding that the CO2-enriched plants exhibited biomass increases of 52 to 93 percent, depending upon variety and vernalization treatment. Based on a plethora of experimental observations
of this nature, many scientists have developed yield prediction models for wheat. Using the output of several such models, Alexandrov and Hoogenboom (2000) estimated the impact of typically
predicted climate changes on wheat production in Bulgaria in the twenty-first century, finding that a doubling of the air’s CO2 concentration would likely enhance wheat yields there
between 12 and 49 percent in spite of a predicted 2.9° to 4.1°C increase in air temperature. Likewise, Eitzinger et al. (2001) employed the WOFOST crop model to estimate wheat production
in northeastern Austria in the year 2080. For a doubled atmospheric CO2 concentration with concomitant climate changes derived from five different general circulation models of the atmosphere,
they obtained simulated yield increases of 30 to 55 percent, even in the face of predicted changes in both temperature and precipitation. Southworth et al. (2002) used the CERES-Wheat
growth model to calculate winter wheat production during the period 2050-2059 for 10 representative farm locations in Indiana, Illinois, Ohio, Michigan, and Wisconsin, USA, for six future
climate scenarios. They report that some of the southern portions of this group of states would have exhibited climate-induced yield decreases had the aerial fertilization effect of
the CO2 increase that drove the predicted changes in climate not been included in the model. When they did include the increase in the air’s CO2 concentration (to a value of 555 ppm),
however, they note that “wheat yields increased 60 to 100% above current yields across the central and northern areas of the study region,” while in the southern areas “small increases
and small decreases were found.” The few minor decreases, however, were associated with the more extreme Hadley Center greenhouse run that presumed a 1 percent increase in greenhouse
gases per year and a doubled climate variability; hence, they would have to be considered highly unlikely. In discussing their findings, Southworth et al. note that other modeling studies
have obtained similar results for other areas. They report, for example, that Brown and Rosenberg (1999) found winter wheat yields across other parts of the United States to increase
“under all climate change scenarios modeled (1, 2.5, and 5°C temperature increases),” and that
Biological Effects of Carbon Dioxide Enrichment 381 Cuculeanu et al. (1999) found modeled yields of winter wheat in southern Romania to increase by 15 to 21 percent across five sites.
Also, they note that Harrison and Butterfield (1996) “found increased yields of winter wheat across Europe under all the climate change scenarios they modeled.” Van Ittersum et al. (2003)
performed a number of simulation experiments with the Agricultural Production Systems Simulator (APSIM)-Nwheat model in which they explored the implications of possible increases in
atmospheric CO2 concentration and near-surface air temperature for wheat production and deep drainage at three sites in Western Australia differing in precipitation, soil characteristics,
nitrogenous fertilizer application rates, and wheat cultivars. They first assessed the impact of the ongoing rise in the air’s CO2 content, finding that wheat grain yield increased linearly
at a rate of 10-16 percent for each 100-ppm increase in atmospheric CO2 concentration, with only a slight concomitant increase in deep drainage (a big win, small loss outcome). For a
likely future CO2 increase of 200 ppm, increases in grain yield varied between 3 and 17 percent for low nitrogen fertilizer application rates and between 21 and 34 percent for high rates
of nitrogen application, with the greatest relative yield response being found for the driest site studied. When potential warming was factored into the picture, the results proved even
better. The positive effects of the CO2 increase on wheat grain yield were enhanced an extra 3-8 percent when temperatures were increased by 3°C in the model simulations. These yield
increases were determined to result in an increased financial return to the typical Western Australian wheat farmer of 15-35 percent. In addition, the imposition of the simultaneous
temperature increase led to a significant decline in deep drainage, producing a truly win-win situation that enhanced the average farmer’s net income by an additional 10-20 percent.
Consequently, it was determined that the CO2-induced increase in temperature predicted by the IPCC could well increase the net profitability of Western Australian wheat farmers by anywhere
from 25-55 percent, while at the same time mitigating what van Ittersum et al. refer to as “one of Australia’s most severe land degradation problems.” In a wide variety of circumstances,
atmospheric CO2 enrichment significantly increases the biomass production and yield of wheat plants, thereby benefiting both wheat producers and consumers alike. Additional information
on this topic, including reviews on sorghum not discussed here, can be found at http://www.co2science.org/subject/a/subject_a.php under the main heading Agriculture, sub heading Wheat.
References Alexandrov, V.A. and Hoogenboom, G. 2000. The impact of climate variability and change on crop yield in Bulgaria. Agricultural and Forest Meteorology 104: 315-327. Brown,
R.A. and Rosenberg, N.J. 1999. Climate change impacts on the potential productivity of corn and winter wheat in their primary United States growing regions. Climatic Change 41: 73-107.
Cuculeanu, V., Marcia, A. and Simota, C. 1999. Climate change impact on agricultural crops and adaptation options in Romania. Climate Research 12: 153-160. Dijkstra, P., Schapendonk,
A.H.M.C., Groenwold, K., Jansen, M. and Van de Geijn, S.C. 1999. Seasonal changes in the response of winter wheat to elevated atmospheric CO2 concentration grown in open-top chambers
and field tracking enclosures. Global Change Biology 5: 563-576. Eitzinger, J., Zalud, Z., Alexandrov, V., van Diepen, C.A., Trnka, M., Dubrovsky, M., Semeradova, D. and Oberforster,
M. 2001. A local simulation study on the impact of climate change on winter wheat production in north-east Austria. Ecology and Economics 52: 199-212. Harrison, P.A. and Butterfield,
R.E. 1996. Effects of climate change on Europe-wide winter wheat and sunflower productivity. Climate Research 7: 225-241. Masle, J. 2000. The effects of elevated CO2 concentrations on
cell division rates, growth growth patterns, and blade anatomy in young wheat plants are modulated by factors related to leaf position, vernalization, and genotype. Plant Physiology
122: 1399-1415. Southworth, J., Pfeifer, R.A., Habeck, M., Randolph, J.C., Doering, O.C. and Rao, D.G. 2002. Sensitivity of winter wheat yields in the Midwestern United States to future
changes in climate, climate variability, and CO2 fertilization. Climate Research 22: 73-86. van Ittersum, M.K., Howden, S.M. and Asseng, S. 2003. Sensitivity of productivity and deep
drainage of wheat cropping systems in a Mediterranean environment to changes in CO2, temperature and precipitation. Agriculture, Ecosystems and Environment 97: 255-273.
Climate Change Reconsidered 382 7.1.2. Woody Plants The growth response of woody plants to atmospheric CO2 enrichment has also been extensively studied. Ceulemans and Mousseau (1994),
for example, tabulated the results of 95 separate experimental investigations related to this topic. The review of Poorter (1993) includes 41 additional sets of pertinent results, and
the two reviews of Wullschleger et al. (1995, 1997) contain 40 other sets of applicable data. When averaged together, these 176 individual woody plant experiments reveal a mean growth
enhancement on the order of 50 percent for an approximate doubling of the air’s CO2 content, which is about one-and-a-half times as much as the response of non-woody herbaceous plants.
It is possible, however, that this larger result is still an underestimate of the capacity of trees and shrubs to respond to atmospheric CO2 enrichment; for the mean duration of the
176 woody plant experiments described above was only five months, which may not have been sufficient for the long-term equilibrium effects of the CO2 enrichment of the air to be manifest.
In the world’s longest such experiment, for example, Kimball et al. (2007) observed a 70 percent sustained increase in biomass production over the entire last decade of a 17-year study
in response to a 75 percent increase in the air’s CO2 content employed throughout the experiment. Likewise, studies of Eldarica pine trees conducted at the same location have revealed
a similarly increasing growth response over the same length of time (Idso and Kimball, 1994). In the subsections that follow, we highlight the results of studies that have examined the
growth response of several woody plants to atmospheric CO2 enrichment. We end the section with discussions of the effect of CO2 enhancement on wood density and forest productivity and
carbon sequestration. For more information on this topic, see http://www.co2 science.org/data/plant_growth/plantgrowth.php. References Ceulemans, R. and Mousseau, M. (1994). Effects
of elevated atmospheric CO2 on woody plants. New Phytologist 127: 425-446. Idso, S.B. and Kimball, B.A. (1994). Effects of atmospheric CO2 enrichment on biomass accumulation and distribution
in Eldarica pine trees. Journal of Experimental Botany 45: 1669-1672. Kimball, B.A., Idso, S.B., Johnson, S. and Rillig, M.C. 2007. Seventeen years of carbon dioxide enrichment of sour
orange trees: final results. Global Change Biology 13: 2171-2183. Poorter, H. (1993). Interspecific Variation in the Growth Response of Plants to an Elevated Ambient CO2 Concentration.
Vegetatio 104/105 77-97. Wullschleger, S.D., Post, W.M. and King, A.W. (1995). On the potential for a CO2 fertilization effect in forests: Estimates of the biotic growth factor based
on 58 controlled-exposure studies. In: Woodwell, G.M. and Mackenzie, F.T. (Eds.) Biotic Feedbacks in the Global Climate System. Oxford University Press, Oxford, pp. 85-107. Wullschleger,
S.D., Norby, R.J. and Gunderson, C.A. (1997). Forest trees and their response to atmospheric CO2 enrichment: A A compilation of results. In: Allen Jr., L.H., Kirkham, M.B., Olszyk, D.M.
and Whitman, C.E. (Eds.) Advances in Carbon Dioxide Effects Research. American Society of Agronomy, Madison, WI, pp. 79-100. 7.1.2.1. Aspen-Poplar Several studies have documented the
effects of elevated levels of atmospheric CO2 on photosynthesis in various aspen clones (Populus tremuloides). In the short-term study of Kruger et al. (1998), aspen seedlings grown
for 70 days at atmospheric CO2 concentrations of 650 ppm exhibited photosynthetic rates that were approximately 10 percent greater than those displayed by seedlings maintained at ambient
CO2 concentrations. In the longer five-month study of Kubiske et al. (1998), atmospheric CO2 enrichment significantly increased photosynthetic rates in four aspen genotypes, regardless
of soil nitrogen status. In an even longer 2.5-year study, Wang and Curtis (2001) also observed significant CO2-induced photosynthetic increases in two male and two female aspen clones;
when six aspen genotypes genotypes were grown in open-top chambers for 2.5 years at atmospheric CO2 concentrations of 350 and 700 ppm, Curtis et al. (2000) reported that the elevated
CO2 concentrations increased rates of net photosynthesis by 128 and 31 percent at high and low soil nitrogen contents, respectively. In addition, in a study that looked only at air temperature
effects that was conducted at ambient CO2 concentrations, King et al. (1999) determined that increasing the air temperature from 13° to 29°C enhanced photosynthetic rates in four different
aspen clones by an average of 35 percent.
Biological Effects of Carbon Dioxide Enrichment 383 In a FACE study, where O3-sensitive and O3-tolerant clones were grown for six months in field plots receiving 360 and 560 ppm CO2
in combination with ambient and enriched (1.5 times ambient) O3 levels, Noormets et al. (2001) reported that CO2-induced increases in photosynthetic rates were at least maintained, and
sometimes even increased, when clones were simultaneously exposed to elevated O3. After an entire year of treatment exposure, in fact, Karnosky et al. (1999) noted that the powerful
ameliorating effect of elevated CO2 on ozone-induced damage was still operating strongly in this system. O3-induced foliar damages in O3-sensitive and O3-tolerant clones were reduced
from 55 and 17 percent, respectively, at ambient CO2, to 38 and 3 percent, respectively, at elevated CO2. With respect to biomass production, Pregitzer et al. (2000) reported that 2.5
years of exposure to twice-ambient concentrations of atmospheric CO2 increased fine-root biomass in six aspen genotypes by an average of 65 and 17 percent on nitrogen-rich and nitrogen-poor
soils, respectively. Using this same experimental system, Zak et al. (2000) determined that elevated CO2 enhanced total seedling biomass by 38 percent at high soil nitrogen and by 16
percent at low soil nitrogen. Similar results were reported in the two-year open-top chamber study of Mikan et al. (2000), who observed 50 and 25 percent CO2-induced increases in total
seedling biomass at high and low soil nitrogen levels, respectively. As the air’s CO2 content continues to increase, aspen seedlings will likely display enhanced rates of photosynthesis
and biomass production, regardless of genotype, gender, O3-sensitvity, and soil nitrogen status. Consequently, greater amounts of carbon will likely be sequestered in the tissues of
this most abundant of North American tree species and in the soils in which they are rooted in the years and decades ahead. Additional information on this topic, including reviews of
newer publications as they become available, can be found at http://www.co2science.org/subject/t/treesaspen.php. References Curtis, P.S., Vogel, C.S., Wang, X.Z., Pregitzer, K.S., Zak,
D.R., Lussenhop, J., Kubiske, M. and Teeri, J.A. 2000. Gas exchange, leaf nitrogen, and growth efficiency of Populus tremuloides in a CO2-enriched atmosphere. Ecological Applications
10: 3-17. Karnosky, D.F., Mankovska, B., Percy, K., Dickson, R.E., Podila, G.K., Sober, J., Noormets, A., Hendrey, G., Coleman, M.D., Kubiske, M., Pregitzer, K.S. and Isebrands, J.G.
1999. Effects of tropospheric O3 on trembling aspen and interaction with CO2: results from an O3-gradient and a FACE experiment. Water, Air, and Soil Pollution 116: 311-322. King, J.S.,
Pregitzer, K.S. and Zak, D.R. 1999. Clonal variation in above-and below-ground responses of Populus tremuloides Michaux: Influence of soil warming and nutrient availability. Plant and
Soil 217: 119-130. Kruger, E.L., Volin, J.C. and Lindroth, R.L. 1998. Influences of atmospheric CO2 enrichment on the responses of sugar maple and trembling aspen to defoliation. New
Phytologist 140: 85-94. Kubiske, M.E., Pregitzer, K.S., Zak, D.R. and Mikan, C.J. 1998. Growth and C allocation of Populus tremuloides genotypes in response to atmospheric CO2 and soil
N availability. New Phytologist 140: 251-260. Mikan, C.J., Zak, D.R., Kubiske, M.E. and Pregitzer, K.S. 2000. Combined effects of atmospheric CO2 and N availability on the belowground
carbon and nitrogen dynamics of aspen mesocosms. Oecologia 124: 432-445. Noormets, A., Sober, A., Pell, E.J., Dickson, R.E., Podila, G.K., Sober, J., Isebrands, J.G. and Karnosky, D.F.
2001. Stomatal and non-stomatal limitation to photosynthesis in two trembling aspen (Populus tremuloides Michx.) clones exposed to elevated CO2 and O3. Plant, Cell and Environment 24:
327-336. Pregitzer, K.S., Zak, D.R., Maziaasz, J., DeForest, J., Curtis, P.S. and Lussenhop, J. 2000. Interactive effects of atmospheric CO2 and soil-N availability on fine roots of
Populus tremuloides. Ecological Applications 10: 18-33. Wang, X. and Curtis, P.S. 2001. Gender-specific responses of Populus tremuloides to atmospheric CO2 enrichment. New Phytologist
150: 675-684. Zak, D.R., Pregitzer, K.S., Curtis, P.S., Vogel, C.S., Holmes, W.E. and Lussenhop, J. 2000. Atmospheric CO2, soil-N availability, and allocation of biomass and nitrogen
by Populus tremuloides. Ecological Applications 10: 34-46. 7.1.2.2. Beech Egli and Korner (1997) rooted eight beech saplings (genus Fagus) directly into calcareous or acidic soils in
open-top chambers and exposed them to atmospheric CO2 concentrations of either 370 or 570 ppm. Over the first year of their study, the saplings growing on calcareous soil in CO2-enriched
air
Climate Change Reconsidered 384 exhibited a 9 percent increase in stem diameter; they speculated that this initial small difference may “cumulate to higher ‘final’ tree biomass through
compounding interest.” At the end of three years of differential CO2 exposure, the trees in the CO2-enriched chambers were experiencing net ecosystem carbon exchange rates that were
58 percent greater than the rates of the trees in the ambient CO2 chambers, regardless of soil type; the stem dry mass of the CO2-enriched trees was increased by about 13 percent over
that observed in the ambient-air chambers (Maurer et al., 1999). In a similar but much shorter experiment, Dyckmans et al. (2000) grew three-year-old seedlings of beech for six weeks
in controlled environment chambers maintained at atmospheric CO2 concentrations of 350 and 700 ppm, finding that the doubling of the air’s CO2 content increased seedling carbon uptake
by 63 percent. They also noted that the majority of the assimilated carbon was allocated to the early development of leaves, which would be expected to subsequently lead to greater absolute
amounts of photosynthetic carbon fixation. In the two-year study of Grams et al. (1999), beech seedlings grown at ambient CO2 concentrations displayed large reductions in photosynthetic
rates when simultaneously exposed to twice-ambient levels of ozone. However, at twice-ambient CO2 concentrations, twice-ambient ozone concentrations had no negative effects on the trees’
photosynthetic rates. Thus, atmospheric CO2 enrichment completely ameliorated the negative effects of ozone on photosynthesis in this species. Similarly, Polle et al. (1997) reported
that beech seedlings grown at 700 ppm CO2 for two years displayed significantly reduced activities of catalase and superoxide dismutase, which are antioxidative enzymes responsible for
detoxifying highly reactive oxygenated compounds within cells. Their data imply that CO2-enriched atmospheres are conducive to less oxidative stress and, therefore, less production of
harmful oxygenated compounds than typically occurs in ambient air. Consequently, the seedlings growing in the CO2-enriched air were likely able to remobilize a portion of some of their
valuable raw materials away from the production of detoxifying enzymes and reinvest them into other processes required for facilitating optimal plant development and growth. With respect
to this concept of resource optimization, Duquesnay et al. (1998) studied the relative amounts of 12C and 13C in tree rings of beech growing for the past century in northeastern France
and determined that the intrinsic water-use efficiency of the trees had increased by approximately 33 percent over that time period, no doubt in response to the concomitant rise in the
air’s CO2 concentration over the past 100 years. In conclusion, as the CO2 content of the air increases, beech trees will likely display enhanced rates of photosynthesis and decreased
damage resulting from oxidative stress. Together, these phenomena should allow greater optimization of raw materials within beech, allowing them to produce greater amounts of biomass
ever more efficiently as the atmospheric CO2 concentration increases. Additional information on this topic, including reviews of newer publications as they become available, can be found
at http://www.co2science.org/subject/t/treesbeech.php. References Duquesnay, A., Breda, N., Stievenard, M. and Dupouey, J.L. 1998. Changes of tree-ring d13C and water-use efficiency
of beech (Fagus sylvatica L.) in north-eastern France during the past century. Plant, Cell and Environment 21: 565-572. Dyckmans, J., Flessa, H., Polle, A. and Beese, F. 2000. The effect
of elevated [CO2] on uptake and allocation of 13C and 15N in beech (Fagus sylvatica L.) during leafing. Plant Biology 2: 113-120. Egli, P. and Korner, C. 1997. Growth responses to elevated
CO2 and soil quality in beech-spruce model ecosystems. Acta Oecologica 18: 343-349. Grams, T.E.E., Anegg, S., Haberle, K.-H., Langebartels, C. and Matyssek, R. 1999. Interactions of
chronic exposure to elevated CO2 and O3 levels in the photosynthetic light and dark reactions of European beech (Fagus sylvatica). New Phytologist 144: 95-107. Maurer, S., Egli, P.,
Spinnler, D. and Korner, C. 1999. Carbon and water fluxes in beech-spruce model ecosystems in response to long-term exposure to atmospheric CO2 enrichment and increased nitrogen deposition.
Functional Ecology 13: 748-755. Polle, A., Eiblmeier, M., Sheppard, L. and Murray, M. 1997. Responses of antioxidative enzymes to elevated CO2 in leaves of beech (Fagus sylvatica L.)
seedlings grown under range of nutrient regimes. Plant, Cell and Environment 20: 1317-1321.
Biological Effects of Carbon Dioxide Enrichment 385 7.1.2.3. Birch In the relatively short-term study of Wayne et al. (1998), yellow birch seedlings (Betula pendula) grown for two months
at atmospheric CO2 concentrations of 800 ppm exhibited photosynthetic rates that were about 50 percent greater than those displayed by control seedlings fumigated with air containing
400 ppm CO2. Similarly, in the threemonth study of Tjoelker et al. (1998a), paper birch seedlings grown at 580 ppm CO2 displayed photosynthetic rates that were approximately 30 percent
greater than those exhibited by seedlings exposed to 370 ppm CO2. Likewise, Kellomaki and Wang (2001) reported that birch seedlings exposed to an atmospheric CO2 concentration of 700
ppm for five months displayed photosynthetic rates that were about 25 percent greater than seedlings grown at 350 ppm CO2. Finally, in the much longer four-year study conducted by Wang
et al. (1998), silver birch seedlings grown in open-top chambers receiving twice-ambient concentrations of atmospheric CO2 displayed photosynthetic rates that were fully 110 percent
greater than rates displayed by their ambiently grown counterparts. Thus, short-term photosynthetic
enhancements resulting from atmospheric CO2 enrichment appear to persist for several years or longer. Because elevated CO2 enhances photosynthetic rates in birch trees, it likely will
also lead to increased biomass production in these important deciduous trees, as it has in several experiments. In the threemonth study of Tjoelker et al. (1998b), for example, a 57
percent increase in the air’s CO2 content increased the biomass of paper birch seedlings by 50 percent. When similar seedlings were grown at 700 ppm CO2 for four months, Catovsky and
Bazzaz (1999) reported that elevated CO2 increased total seedling biomass by 27 and 130 percent under wet and dry soil moisture regimes, respectively. In the interesting study of Godbold
et al. (1997), paper birch seedlings grown at 700 ppm for six months not only increased their total biomass, but also increased the number of root tips per plant by more than 50 percent.
In the longer two-year study of Berntson and Bazzaz (1998), twice-ambient levels of CO2 increased the biomass of a mixed yellow and white birch mesocosm by 31 percent; and in another
two-year study, Wayne et al. (1998) reported that yellow birch seedlings grown at 800 ppm CO2 produced 60 and 227 percent more biomass than seedlings grown at 400 ppm CO2 at ambient
and elevated air temperatures, respectively. Finally, after exposing silver birch seedlings to twiceambient CO2 concentrations for four years, Wang et al. (1998) noted that CO2-enriched
seedlings produced 60 percent more biomass than ambiently grown seedlings. Hence, atmospheric CO2 enrichment clearly enhances birch biomass in both short-and medium-term experiments.
In some studies, elevated CO2 also reduced stomatal conductances in birch trees, thereby boosting their water-use efficiencies. Tjoelker et al. (1998a), for example, reported that paper
birch seedlings grown at 580 ppm CO2 for three months experienced 10-25 percent reductions in stomatal conductance, which contributed to 40-80 percent increases in water-use efficiency.
Similar CO2-induced reductions in stomatal conductance (21 percent) were reported in silver birch seedlings grown for four years at 700 ppm CO2 by Rey and Jarvis (1998). The results
of these several studies suggest that the ongoing rise in the air’s CO2 content will likely increase rates of photosynthesis and biomass production in birch trees, as well as improve
their water use efficiencies, irrespective of any concomitant changes in air temperature and/or soil moisture status that might occur. Consequently, rates of carbon sequestration by
this abundant temperate forest species should also increase in the years and decades ahead. Additional information on this topic, including reviews of newer publications as they become
available, can be found at http://www.co2science.org/subject/t/treesbirch.php. References Berntson, G.M. and Bazzaz, F.A. 1998. Regenerating temperate forest mesocosms in elevated CO2:
belowground growth and nitrogen cycling. Oecologia 113: 115-125. Catovsky, S. and Bazzaz, F.A. 1999. Elevated CO2 influences the responses of two birch species to soil moisture: implications
for forest community structure. Global Change Biology 5: 507-518. Godbold, D.L., Berntson, G.M. and Bazzaz, F.A. 1997. Growth and mycorrhizal colonization of three North American tree
species under elevated atmospheric CO2. New Phytologist 137: 433-440. Kellomaki, S. and Wang, K.-Y. 2001. Growth and resource use of birch seedlings under elevated carbon dioxide and
temperature. Annals of Botany 87: 669-682.
Climate Change Reconsidered 386 Rey, A. and Jarvis, P.G. 1998. Long-Term photosynthetic acclimation to increased atmospheric CO2 concentration in young birch (Betula pendula) trees.
Tree Physiology 18: 441-450. Tjoelker, M.G., Oleksyn, J. and Reich, P.B. 1998a. Seedlings of five boreal tree species differ in acclimation of net photosynthesis to elevated CO2 and
temperature. Tree Physiology 18: 715-726. Tjoelker, M.G., Oleksyn, J. and Reich, P.B. 1998b. Temperature and ontogeny mediate growth response to elevated CO2 in seedlings of five boreal
tree species. New Phytologist 140: 197-210. Wang, Y.-P., Rey, A. and Jarvis, P.G. 1998. Carbon balance of young birch trees grown in ambient and elevated atmospheric CO2 concentrations.
Global Change Biology 4: 797-807. Wayne, P.M., Reekie, E.G. and Bazzaz, F.A. 1998. Elevated CO2 ameliorates birch response to high temperature and frost stress: implications for modeling
climate-induced geographic range shifts. Oecologia 114: 335-342. 7.1.2.4. Citrus Trees How does atmospheric CO2 enrichment affect the growth and development of citrus trees and the fruit
they produce? In the study of Keutgen and Chen (2001), cuttings of Citrus madurensis grown for three months at 600 ppm CO2 displayed rates of photosynthesis that were more than 300 percent
greater than those measured on control cuttings grown at 300 ppm CO2. In addition, elevated CO2 concentrations have been shown to increase photosynthetic rates in mango (Schaffer et
al., 1997), mangosteen (Schaffer et al., 1999), and sweet orange (Jifon et al., 2002). In the study of Jifon et al., it was further reported that twice-ambient CO2 concentrations increased
photosynthetic rates in mycorrhizal-and non-mycorrhizal-treated sour orange seedlings by 118 and 18 percent, respectively. Such CO2-induced increases in photosynthesis should lead to
enhanced biomass production; and so they do. Idso and Kimball (2001), for example, have documented how a 75 percent increase in the air’s CO2 content has boosted the long-term production
of above-ground wood and fruit biomass in sour orange trees by 80 percent in a study that has been ongoing since November 1987. Furthermore, Idso et al. (2002) have additionally demonstrated
that the 300-ppm increase in the air’s CO2 content has increased the fresh weight of individual oranges by an average of 4 percent and the vitamin C content of their juice by an average
of 5 percent. In summary, these peer-reviewed studies suggest that as the air’s CO2 content slowly but steadily rises, citrus trees will respond by increasing their rates of photosynthesis
and biomass production. In addition, they may also increase the vitamin C content of their fruit, which may help to prevent an array of human health problems brought about by insufficient
intake of vitamin C. Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/t/treescitrus.php.
References Idso, S.B. and Kimball, B.A. 2001. CO2 enrichment of sour orange trees: 13 years and counting. Environmental and Experimental Botany 46: 147-153. Idso, S.B., Kimball, B.A.,
Shaw, P.E., Widmer, W., Vanderslice, J.T., Higgs, D.J., Montanari, A. and Clark, W.D. 2002. The effect of elevated atmospheric CO2 on the vitamin C concentration of (sour) orange juice.
Agriculture, Ecosystems and Environment 90: 1-7. Jifon, J.L., Graham, J.H., Drouillard, D.L. and Syvertsen, J.P. 2002. Growth depression of mycorrhizal Citrus seedlings grown at high
phosphorus supply is mitigated by elevated CO2. New Phytologist 153: 133-142. Keutgen, N. and Chen, K. 2001. Responses of citrus leaf photosynthesis, chlorophyll fluorescence, macronutrient
and carbohydrate contents to elevated CO2. Journal of Plant Physiology 158: 1307-1316. Schaffer, B., Whiley, A.W. and Searle, C. 1999. Atmospheric CO2 enrichment, root restriction, photosynthesis,
and dry-matter partitioning in subtropical and tropical fruit crops. HortScience 34: 1033-1037. Schaffer, B., Whiley, A.W., Searle, C. and Nissen, R.J. 1997. Leaf gas exchange, dry matter
partitioning, and mineral element concentrations in mango as influenced by elevated atmospheric carbon dioxide and root restriction. Journal of the American Society of Horticultural
Science 122: 849-855. 7.1.2.5. Eucalyptus In the eight-month study of Roden et al. (1999), Eucalyptus pauciflora seedlings growing at 700 ppm CO2 displayed seasonal rates of net photosynthesis
Biological Effects of Carbon Dioxide Enrichment 387 that were approximately 30 percent greater than those exhibited by their ambiently grown counterparts. In another eight-month study,
Palanisamy (1999) reported that well-watered Eucalyptus cladocalyx seedlings exposed to 800 ppm CO2 exhibited photosynthetic rates that were 120 percent higher than those observed in
control plants growing at 380 ppm CO2. Moreover, after a one-month period of water stress, photosynthetic rates of CO2-enriched seedlings were still 12 percent greater than rates displayed
by ambiently grown water-stressed seedlings. Because elevated CO2 enhances photosynthetic rates in eucalyptus species, this phenomenon should lead to increased biomass production in
these rapidly growing trees. And so it does. In the eight-month experiment of Gleadow et al. (1998), for example, Eucalyptus cladocalyx seedlings growing at 800 ppm CO2 displayed 134
and 98 percent more biomass than seedlings growing at 400 ppm CO2 at low and high soil nitrogen concentrations, respectively. Similarly, Eucalyptus pauciflora seedlings growing at twiceambient
CO2 concentrations for eight months produced 53 percent more biomass than control seedlings (Roden et al., 1999). In summary, as the CO2 content of the air increases, eucalyptus seedlings
will likely display enhanced rates of photosynthesis and biomass production, regardless of soil moisture and nutrient status. Consequently, greater amounts of carbon will likely be sequestered
by this rapidly growing tree species. Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/t/trees
uc.php. References Gleadow, R.M., Foley, W.J. and Woodrow, I.E. 1998. Enhanced CO2 alters the relationship between photosynthesis and defense in cyanogenic Eucalyptus cladocalyx F. Muell.
Plant, Cell and Environment 21: 12-22. Palanisamy, K. 1999. Interactions of elevated CO2 concentration and drought stress on photosynthesis in Eucalyptus cladocalyx F. Muell. Photosynthetica
36: 635-638. Roden, J.S., Egerton, J.J.G. and Ball, M.C. 1999. Effect of elevated [CO2] on photosynthesis and growth of snow gum (Eucalyptus pauciflora) seedlings during winter and spring.
Australian Journal of Plant Physiology 26: 37-46. 7.1.2.6. Fruit-Bearing Several studies have recently documented the effects of elevated atmospheric CO2 concentrations on photosynthesis
in various fruiting trees. In an eightday experiment, Pan et al. (1998) found that twiceambient CO2 concentrations increased rates of net photosynthesis in one-year-old apple seedlings
by 90 percent. In a longer three-month study, Keutgen and Chen (2001) noted that cuttings of Citrus madurensis exposed to 600 ppm CO2 displayed rates of photosynthesis that were more
than 300 percent greater than rates observed in control cuttings exposed to 300 ppm CO2. Likewise, in the review paper of Schaffer et al. (1997), it was noted that atmospheric CO2 enrichment
had previously been shown to enhance rates of net photosynthesis in various tropical and sub-tropical fruit trees, including avocado, banana, citrus, mango, and mangosteen. Finally in
the two-year study of Centritto et al. (1999a), cherry seedlings grown at 700 ppm CO2 exhibited photosynthetic rates that were 44 percent greater than those displayed by seedlings grown
in ambient air, independent of a concomitant soil moisture treatment. Because elevated CO2 enhances the photosynthetic rates of fruiting trees, it should also lead to increased biomass
production in them. In the two-year study of Centritto et al. (1999b), for example, well-watered and water-stressed seedlings growing at twice-ambient CO2 concentrations displayed basal
trunk areas that were 47 and 51 percent larger than their respective ambient controls. Similarly, in a study spanning more than 13 years, Idso and Kimball (2001) demonstrated that the
aboveground wood biomass of mature sour orange trees growing in air enriched with an additional 300 ppm of CO2 was 80 percent greater than that attained by control trees growing in ambient
air. As the CO2 content of the air increases, fruit trees will likely display enhanced rates of photosynthesis and biomass production, regardless of soil moisture conditions. Consequently,
greater amounts of carbon will likely be sequestered in the woody trunks and branches of such species. Moreover, fruit yields may increase as well. In the study of Idso and Kimball,
for example, fruit yields were stimulated to essentially the same degree as above-ground wood biomass; i.e., by 80 percent in response to a 75 percent increase in the air’s CO2 content.
Additional information on this topic, including reviews of newer publications as they become
Climate Change Reconsidered 388 available, can be found at http://www.co2science.org/subject/t/treesfruit.php. References Centritto, M., Magnani, F., Lee, H.S.J. and Jarvis, P.G. 1999a.
Interactive effects of elevated [CO2] and drought on cherry (Prunus avium) seedlings. II. Photosynthetic capacity and water relations. New Phytologist 141: 141-153. Centritto, M., Lee,
H.S.J. and Jarvis, P.G. 1999b. Interactive effects of elevated [CO2] and drought on cherry (Prunus avium) seedlings. I. Growth, whole-plant water use efficiency and water loss. New Phytologist
141: 129-140. Idso, S.B. and Kimball, B.A. 2001. CO2 enrichment of sour orange trees: 13 years and counting. Environmental and Experimental Botany 46: 147-153. Keutgen, N. and Chen,
K. 2001. Responses of citrus leaf photosynthesis, chlorophyll fluorescence, macronutrient and carbohydrate contents to elevated CO2. Journal of Plant Physiology 158: 1307-1316. Pan,
Q., Wang, Z. and Quebedeaux, B. 1998. Responses of the apple plant to CO2 enrichment: changes in photosynthesis, sorbitol, other soluble sugars, and starch. Australian Journal of Plant
Physiology 25: 293-297. Schaffer, B., Whiley, A.W., Searle, C. and Nissen, R.J. 1997. Leaf gas exchange, dry matter partitioning, and mineral element concentrations in mango as influenced
by elevated atmospheric carbon dioxide and root restriction. Journal of the American Society of Horticultural Science 122: 849-855. 7.1.2.7. Nitrogen-Fixing Trees In the six-week study
of Schortemeyer et al. (1999), seedlings of Australian blackwood (Acacia melanoxylon) grown at twice-ambient atmospheric CO2 concentrations displayed photosynthetic rates that were 22
percent greater than those of ambiently grown seedlings. In addition, the CO2-enriched seedlings exhibited biomass values that were twice as large as those displayed by control seedlings
grown in air of 350 ppm CO2. Likewise, Polley et al. (1999) reported that a doubling of the atmospheric CO2 concentration for three months increased honey mesquite (Prosopis glandulosa)
seedling root and shoot biomass by 37 and 46 percent, respectively. Several studies have investigated the effects of elevated CO2 on black locust (Robinia pseudoacacia) seedlings. Uselman
et al. (2000), grew seedlings for three months at 700 ppm CO2 and reported that this treatment increased the root exudation of organic carbon compounds by 20 percent, while Uselman et
al. (1999) reported no CO2-induced increases in the root exudation of organic nitrogen compounds. Nonetheless, elevated CO2 enhanced total seedling biomass by 14 percent (Uselman et
al., 2000). In the study of Olesniewicz and Thomas (1999), black locust seedlings grown at twice-ambient CO2 concentrations for two months exhibited a 69 percent increase in their average
rate of nitrogen-fixation when they were not inoculated with an arbuscular mycorrhizal fungal species. It was further determined that the amount of seedling nitrogen derived from nitrogen-fixation
increased in CO2-enriched plants by 212 and 90 percent in non-inoculated and inoculated seedlings, respectively. Elevated CO2 enhanced total plant biomass by 180 and 51 percent in noninoculated
and inoculated seedlings, respectively. As the CO2 content of the air increases, nitrogenfixing trees respond by exhibiting enhanced rates of photosynthesis and biomass production, as
well as enhanced rates of nitrogen fixation. Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/
/treesnitrofix.php. References Olesniewicz, K.S. and Thomas, R.B. 1999. Effects of mycorrhizal colonization on biomass production and nitrogen fixation of black locust (Robinia pseudoacacia)
seedlings grown under elevated atmospheric carbon dioxide. New Phytologist 142: 133-140. Polley, H.W., Tischler, C.R., Johnson, H.B. and Pennington, R.E. 1999. Growth, water relations,
and survival of drought-exposed seedlings from six maternal families of honey mesquite (Prosopis glandulosa): responses to CO2 enrichment. Tree Physiology 19: 359-366. Schortemeyer,
M., Atkin, O.K., McFarlane, N. and Evans, J.R. 1999. The impact of elevated atmospheric CO2 and nitrate supply on growth, biomass allocation, nitrogen partitioning and N2 fixation of
Acacia melanoxylon. Australian Journal of Plant Physiology 26: 737-774. Uselman, S.M., Qualls, R.G. and Thomas, R.B. 1999. A test of a potential short cut in the nitrogen cycle: The
role of exudation of symbiotically fixed nitrogen from the roots of a N-fixing tree and the effects of increased atmospheric CO2 and temperature. Plant and Soil 210: 21-32.
Biological Effects of Carbon Dioxide Enrichment 389 Uselman, S.M., Qualls, R.G. and Thomas, R.B. 2000. Effects of increased atmospheric CO2, temperature, and soil N availability on root
exudation of dissolved organic carbon by a N-fixing tree (Robinia pseudoacacia L.). Plant and Soil 222: 191-202. 7.1.2.8. Oak How do oak (genus Quercus) trees respond to atmospheric
CO2 enrichment? In the two-month study of Anderson and Tomlinson (1998), northern red oak seedlings exposed to 700 ppm CO2 displayed photosynthetic rates that were 34 and 69 percent
greater than those displayed by control plants growing under well-watered and water-stressed conditions, respectively. Similarly, in the four-month study of Li et al. (2000), Quercus
myrtifolia seedlings growing at twice-ambient CO2 concentrations exhibited rates of photosynthesis at the onset of senescence that were 97 percent greater than those displayed by ambiently
growing seedlings. In the year-long study of Staudt et al. (2001), Quercus ilex seedlings grown at 700 ppm CO2 displayed trunk and branch biomasses that were 90 percent greater than
those measured on seedlings growing at 350 ppm CO2. Also, in the eight-month inter-generational study performed by Polle et al. (2001), seedlings produced from acorns collected from
ambient and CO2-enriched mother trees and germinated in air of either ambient or twice-ambient atmospheric CO2 concentration displayed whole-plant biomass values that were 158 and 246
percent greater, respectively, than those exhibited by their respective control seedlings growing in ambient air. In another study, Schulte et al. (1998) grew oak seedlings for 15 weeks
at twice-ambient CO2 concentrations, finding that elevated CO2 enhanced seedling biomass by 92 and 128 percent under wellwatered and water-stressed conditions, respectively. In a similar
study conducted by Tomlinson and Anderson (1998), water-stressed seedlings growing at 700 ppm CO2 displayed biomass values that were similar to those exhibited by well-watered plants
growing in ambient air. Thus, atmospheric CO2 enrichment continues to benefit oak trees even under water-stressed conditions. Additional studies have demonstrated that oak seedlings
also respond positively to atmospheric CO2 enrichment when they are faced with other environmental stresses and resource limitations. When pedunculate oak seedlings were subjected to
two different soil nutrient regimes, for example, Maillard et al. (2001) reported that a doubling of the atmospheric CO2 concentration enhanced seedling biomass by 140 and 30 percent
under high and low soil nitrogen conditions, respectively. And in the study of Usami et al. (2001), saplings of Quercus myrsinaefolia that were grown at 700 ppm CO2 displayed biomass
increases that were 110 and 140 percent greater than their ambiently grown counterparts when they were simultaneously subjected to air temperatures that were 3° and 5°C greater than
ambient temperature, respectively. Thus, elevated CO2 concentrations tend to ameliorate some of the negative effects caused by growth-reducing stresses in oaks. In fact, when Schwanz
and Polle (1998) reported that elevated CO2 exposure caused reductions in the amounts of several foliar antioxidative enzymes in mature oak trees, they suggested that this phenomenon
was the result of atmospheric CO2 enrichment causing the trees to experience less oxidative stress and, therefore, they had less need for antioxidative enzymes. In some studies, elevated
CO2 has been shown to reduce stomatal conductances in oak trees, thus contributing to greater tree water-use efficiencies. Tognetti et al. (1998a), for example, reported that oak seedlings
growing near a natural CO2-emitting spring exhibited less water loss and more favorable turgor pressures than trees growing further away from the spring. The resulting improvement in
water-use efficiency was so significant that Tognetti et al. (1998b) stated, “such marked increases in water-use efficiency under elevated CO2 might be of great importance in Mediterranean
environments in the perspective of global climate change.” In summary, it is clear that as the CO2 content of the air increases, oak seedlings will likely display enhanced rates of photosynthesis
and biomass production, regardless of air temperature, soil moisture, and soil nutrient status. Consequently, greater amounts of carbon will likely be removed from the atmosphere by
the trees of this abundant genus and stored in their tissues and the soils in which they are rooted. Additional information on this topic, including reviews of newer publications as
they become available, can be found at http://www.co2science.org/subject/t/treesoak.php.
Climate Change Reconsidered 390 References Anderson, P.D. and Tomlinson, P.T. 1998. Ontogeny affects response of northern red oak seedlings to elevated CO2 and water stress. I. Carbon
assimilation and biomass production. New Phytologist 140: 477-491. Li, J.-H., Dijkstra, P., Hymus, G.J., Wheeler, R.M., Piastuchi, W.C., Hinkle, C.R. and Drake, B.G. 2000. Leaf senescence
of Quercus myrtifolia as affected by long-term CO2 enrichment in its native environment. Global Change Biology 6: 727-733. Maillard, P., Guehl, J.-M., Muller, J.-F. and Gross, P. 2001.
Interactive effects of elevated CO2 concentration and nitrogen supply on partitioning of newly fixed 13C and 15N between shoot and roots of pedunculate oak seedlings (Quercus robur L.).
Tree Physiology 21: 163-172. Polle, A., McKee, I. and Blaschke, L. 2001. Altered physiological and growth responses to elevated [CO2] in offspring from holm oak (Quercus ilex L.) mother
trees with lifetime exposure to naturally elevated [CO2]. Plant, Cell and Environment 24: 1075-1083. Schwanz, P. and Polle, A. 1998. Antioxidative systems, pigment and protein contents
in leaves of adult mediterranean oak species (Quercus pubescens and Q. ilex) with lifetime exposure to elevated CO2. New Phytologist 140: 411-423. Schulte, M., Herschbach, C. and Rennenberg,
H. 1998. Interactive effects of elevated atmospheric CO2, mycorrhization and drought on long-distance transport of reduced sulphur in young pedunculate oak trees (Quercus robur L.).
Plant, Cell and Environment 21: 917-926. Staudt, M., Joffre, R., Rambal, S. and Kesselmeier, J. 2001. Effect of elevated CO2 on monoterpene emission of young Quercus ilex trees and its
relation to structural and ecophysiological parameters. Tree Physiology 21: 437-445. Tognetti, R., Longobucco, A., Miglietta, F. and Raschi, A. 1998a. Transpiration and stomatal behaviour
of Quercus ilex plants during the summer in a Mediterranean carbon dioxide spring. Plant, Cell and Environment 21: 613-622. Tognetti, R., Johnson, J.D., Michelozzi, M. and Raschi, A.
1998b. Response of foliar metabolism in mature trees of Quercus pubescens and Quercus ilex to long-term elevated CO2. Environmental and Experimental Botany 39: 233-245. Tomlinson, P.T.
and Anderson, P.D. 1998. Ontogeny affects response of northern red oak seedlings to elevated CO2 and water stress. II. Recent photosynthate distribution and growth. New Phytologist 140:
493-504. Usami, T., Lee, J. and Oikawa, T. 2001. Interactive effects of increased temperature and CO2 on the growth of Quercus myrsinaefolia saplings. Plant, Cell and Environment 24:
1007-1019. 7.1.2.9. Pine 7.1.2.9.1. Loblolly Tissue et al. (1997) grew seedlings of loblolly pine trees (Pinus taeda L.) for a period of four years in open-top chambers maintained at
atmospheric CO2 concentrations of either 350 or 650 ppm in a study of the long-term effects of elevated CO2 on the growth of this abundant pine species. This experiment indicated there
was a mean biomass accumulation in the seedlings grown in CO2-enriched air that was 90 percent greater than that attained by the seedlings grown in ambient air. Johnson et al. (1998)
reviewed 11 of their previously published papers, describing the results of a series of greenhouse and open-top chamber studies of the growth responses of loblolly pine seedlings to
a range of atmospheric CO2 and soil nitrogen concentrations. This work indicated that when soil nitrogen levels were so low as to be extremely deficient, or so high as to be toxic, growth
responses to atmospheric CO2 enrichment were negligible. For moderate soil nitrogen deficiencies, however, a doubling of the air’s CO2 content sometimes boosted growth by as much as
1,000 percent. Consequently, since the nitrogen status of most of earth’s ecosystems falls somewhere between extreme deficiency and toxicity, these results suggest that loblolly pine
trees may experience large increases in growth as the air’s CO2 content continues to climb. Naidu and DeLucia (1999) described the results of working one full year in 30-meter-diameter
circular FACE plots maintained at atmospheric CO2 concentrations of either 350 or 560 ppm in an originally 13-year-old loblolly pine plantation in North Carolina, USA, where they determined
the effects of the elevated CO2 treatment on the productivity of the trees, which were growing in soil that was characteristically low in nitrogen and phosphorus. After the first year
of atmospheric CO2 enrichment in this Duke Forest Face Study, the growth rate of the CO2-enriched trees was about 24 percent greater than that of the trees exposed to ambient CO2, in
spite of the likelihood of soil nutrient limitations and a severe summer drought (rainfall in August 1997 was about 90 percent below the 50-year average).
Biological Effects of Carbon Dioxide Enrichment 391 After four years of work at the Duke Forest Face Site, Finzi et al. (2002) reported that the extra 200 or so ppm of CO2 had increased
the average yearly dry matter production of the CO2-enriched trees by 32 percent, while at the eight-year point of the experiment Moore et al. (2006) reported there had been a sustained
increase in trunk basal area increment that varied between 13 and 27 percent with variations in weather and the timing of growth. What is more, they say “there was no evidence of a decline
in the relative enhancement of tree growth by elevated CO2 as might be expected if soil nutrients were becoming progressively more limiting,” which many people had expected would occur
in light of the site’s low soil nitrogen and phosphorus content. In addition, at the six-year point of the study Pritchard et al. (2008) determined that the extra CO2 had increased the
average standing crop of fine roots by 23 percent. Gavazzi et al. (2000) grew one-year-old loblolly pine seedlings for about four months in pots placed within growth chambers maintained
at atmospheric CO2 concentrations of either 360 or 660 ppm and adequate or inadequate levels of soil moisture, while the pots were seeded with a variety of C3 and C4 weeds. In the course
of this experiment, they found that total seedling biomass was always greater under well-watered as opposed to water-stressed conditions, and that elevated CO2 increased total seedling
biomass by 22 percent in both water treatments. In the elevated CO2 and water-stressed treatment, however, they also found that seedling root-to-shoot ratios were about 80 percent greater
than they were in the elevated CO2 and well-watered treatment, due to a 63 percent increase in root
biomass. In the case of the weeds, total biomass was also always greater under well-watered compared to water-stressed conditions. However, the elevated CO2 did not increase weed biomass;
in fact, it reduced it by approximately 22 percent. Consequently, in assessing the effects of elevated CO2 on competition between loblolly pine seedlings and weeds, the seedlings were
definitely the winners, with the researchers concluding that the CO2-induced increase in root-to-shoot ratio under water-stressed conditions may “contribute to an improved ability of
loblolly pine to compete against weeds on dry sites.” Working with data obtained from stands of loblolly pine plantations at 94 locations scattered throughout the southeastern United
States, Westfall and Amateis (2003) employed mean height measurements made at three-year intervals over a period of 15 years to calculate a site index related to mean growth rate for
each of the five three-year periods, which index would be expected to increase monotonically if growth rates were being enhanced above “normal” by some monotonically increasing growth-promoting
factor. This protocol indicated, in their words, that “mean site index over the 94 plots consistently increased at each remeasurement period,” which would suggest, as they phrase it,
that “loblolly pine plantations are realizing greater than expected growth rates,” and, we would add, that the growth rate increases are growing larger and larger with each succeeding
three-year period. As for what might be causing the monotonically increasing growth rates of loblolly pine trees over the entire southeastern region of the United States, the two researchers
say that in addition to rising atmospheric CO2 concentrations, “two other likely factors that could affect growth are temperature and precipitation.” However, they report that a review
of annual precipitation amounts and mean ground surface temperatures showed no trends in these factors over the period of their study. They also suggest that if increased nitrogen deposition
were the cause, “such a factor would have to be acting on a regional scale to produce growth increases over the range of study plots.” Hence, they are partial to the aerial fertilization
effect of atmospheric CO2 enrichment as the explanation. What is more, they note that “similar results were reported by Boyer (2001) for natural stands of longleaf pine, where increases
in dominant stand height are occurring over generations on the same site.” The studies reported here indicate that as the CO2 content of the air continues to rise, loblolly pine trees
will likely experience significant increases in biomass production, even on nutrient-poor soils, during times of drought, and in competition with weeds. Additional information on this
topic, including reviews loblolly pine trees not discussed here, can be found at http://www.co2science.org/subject/t/subject _t.php, under the heading Trees, Types, Pine, Loblolly. References
Boyer, W.D. 2001. A generational change in site index for naturally established longleaf pine on a south Alabama coastal plain site. Southern Journal of Applied Forestry 25: 88-92.
Climate Change Reconsidered 392 Finzi, A.C., DeLucia, E.H., Hamilton, J.G., Richter, D.D. and Schlesinger, W.H. 2002. The nitrogen budget of a pine forest under free air CO2 enrichment.
Oecologia 132: 567-578. Gavazzi, M., Seiler, J., Aust, W. and Zedaker, S. 2000. The influence of elevated carbon dioxide and water availability on herbaceous weed development and growth
of transplanted loblolly pine (Pinus taeda). Environmental and Experimental Botany 44: 185-194. Johnson, D.W., Thomas, R.B., Griffin, K.L., Tissue, D.T., Ball, J.T., Strain, B.R. and
Walker, R.F. 1998. Effects of carbon dioxide and nitrogen on growth and nitrogen uptake in ponderosa and loblolly pine. Journal of Environmental Quality 27: 414-425. Moore, D.J.P., Aref,
S., Ho, R.M., Pippen, J.S., Hamilton, J.G. and DeLucia, E.H. 2006. Annual basal area increment and growth duration of Pinus taeda in response to eight years of free-air carbon dioxide
enrichment. Global Change Biology 12: 1367-1377. Naidu, S.L. and DeLucia, E.H. 1999. First-year growth response of trees in an intact forest exposed to elevated CO2. Global Change Biology
5: 609-613. Pritchard, S.G., Strand, A.E., McCormack, M.L., Davis, M.A., Finzi, A.C., Jackson, R.B., Matamala, R., Rogers, H.H. and Oren, R. 2008. Fine root dynamics in a loblolly pine
forest are influenced by free-air-CO2-enrichment: a six-year-minirhizotron study. Global Change Biology 14: 588-602. Tissue, D.T., Thomas, R.B. and Strain, B.R. 1997. Atmospheric CO2
enrichment increases growth and photosynthesis of Pinus taeda: a 4-year experiment in the field. Plant, Cell and Environment 20: 1123-1134. Westfall, J.A. and Amateis, R.L. 2003. A model
to account for potential correlations between growth of loblolly pine and changing ambient carbon dioxide concentrations. Southern Journal of Applied Forestry 27: 279-284. 7.1.2.9.2.
Ponderosa Walker et al. (1998b) grew seedlings of Ponderosa pine (Pinus ponderosa Dougl. ex P. Laws & C. Laws) for an entire year in controlled environment chambers with atmospheric
CO2 concentrations of either 350 (ambient), 525, or 700 ppm. In addition, low or high levels of nitrogen and phosphorus were supplied to determine the main and interactive effects of
atmospheric CO2 enrichment and soil nutrition on seedling growth and fungal colonization of the seedlings’ roots. After 12 months, they found that phosphorus supply had little impact
on overall seedling growth, while high nitrogen increased nearly every parameter measured, including root, shoot, and total biomass, as did atmospheric CO2 enrichment. Averaged over
all nitrogen and phosphate treatments, total root dry weights at 525 and 700 ppm CO2 were 92 and 49 percent greater, respectively, than those observed at ambient CO2, while shoot dry
weights were 83 and 26 percent greater. Consequently, seedlings grown at 525 and 700 ppm CO2 had total dry weights that were 86 and 35 percent greater, respectively, than those measured
at ambient CO2. In addition, elevated CO2 increased the total number of ectomycorrhizal fungi on roots by 170 percent percent at 525 ppm CO2 and 85 percent at 700 ppm CO2 relative to
the number observed at ambient CO2. Walker et al. (1998a) grew Ponderosa pine seedlings for two growing seasons out-of-doors in open-top chambers having atmospheric CO2 concentrations
of 350, 525, and 700 ppm on soils of low, medium, and high nitrogen content to determine the interactive effects of these variables on juvenile tree growth. The elevated CO2 concentrations
had little effect on most growth parameters after the first growing season, with the one exception of belowground biomass, which increased with both CO2 and soil nitrogen. After two
growing seasons, however, elevated CO2 significantly increased all growth parameters, including tree height, stem diameter, shoot weight, stem volume, and root volume, with the greatest
responses typically occurring at the highest CO2 concentration in the highest soil nitrogen treatment. Root volume at 700 ppm CO2 and high soil nitrogen, for example, exceeded that of
all other treatments by at least least 45 percent, as did shoot volume by 42 percent. Similarly, at high CO2 and soil nitrogen coarse root and shoot weights exceeded those at ambient
CO2 and high nitrogen by 80 and 88 percent, respectively. Johnson et al. (1998) reviewed 11 of their previously published papers (including the two discussed above) in which they describe
the results of a series of greenhouse and open-top chamber studies of the growth responses of Ponderosa pine seedlings to a range of atmospheric CO2 and soil nitrogen concentrations.
These studies indicated that when soil nitrogen levels were so low as to be extremely deficient, or so high as to be toxic, growth responses to atmospheric CO2 enrichment were negligible.
For moderate soil nitrogen deficiencies, however, a doubling of the air’s CO2 content sometimes boosted growth by as much as 1,000 percent. In addition, atmospheric CO2 enrichment mitigated
the negative
Biological Effects of Carbon Dioxide Enrichment 393 growth response of ponderosa pine to extremely high soil nitrogen in two separate studies. Maherali and DeLucia (2000) grew Ponderosa
pine seedlings for six months in controlled environment chambers maintained at atmospheric CO2 concentrations ranging from 350 to 1,100 ppm, while they were subjected to either low (15/25°C
night/day) or high (20/30°C night/day) temperatures. This study revealed that although elevated CO2 had no significant effect on stomatal conductance, seedlings grown in the high temperature
treatment exhibited a 15 percent increase in this parameter relative to seedlings grown in the low temperature treatment. Similarly, specific hydraulic conductivity, which is a measure
of the amount of water moving through a plant relative to its leaf or needle area, also increased in the seedlings exposed to the high temperature treatment. In addition, biomass production
rose by 42 percent in the low temperature treatment and 62 percent in the high temperature treatment when the atmospheric CO2 concentration was raised from 350 to 1,100 ppm. Tingey et
al. (2005) studied the effects of atmospheric CO2 enrichment (to approximately 350 ppm above ambient) on the fine-root architecture of Ponderosa pine seedlings growing in open-top chambers
via minirhizotron tubes over a period of four years. This experiment showed that “elevated CO2 increased both fine root extensity (degree of soil exploration) and intensity (extent that
roots use explored areas) but had no effect on mycorrhizae,” the latter of which observations was presumed to be due to the fact that soil nitrogen was not limiting to growth in this
study. More specifically, they report that “extensity increased 1.5-to 2-fold in elevated CO2 while intensity increased only 20 percent or less,” noting that similar extensity results
had been obtained over shorter periods of four months to two years by Arnone (1997), Berntson and Bazzaz (1998), DeLucia et al. (1997) and Runion et al. (1997), while similar intensity
results had been obtained by Berntson (1994). Last, Soule and Knapp (2006) studied Ponderosa pine trees growing naturally at eight sites within the Pacific Northwest of the United States,
in order to see how they may have responded to the increase in the atmosphere’s CO2 concentration that occurred after 1950. In selecting these sites, they chose locations that “fit several
criteria designed to limit potential confounding influences associated with anthropogenic disturbance.” They also say they selected locations with “a variety of climatic and topoedaphic
conditions, ranging from extremely water-limiting environments ... to areas where soil moisture should be a limiting factor for growth only during extreme drought years,” additionally
noting that all sites were located in areas “where ozone concentrations and nitrogen deposition are typically low.” At each of the eight sites that met all of these criteria, Soule and
Knapp obtained core samples from about 40 mature trees that included “the potentially oldest trees on each site,” so that their results would indicate, as they put it, “the response
of mature, naturally occurring ponderosa pine trees that germinated before anthropogenically elevated CO2 levels, but where growth, particularly post-1950, has occurred under increasing
and substantially higher atmospheric CO2 concentrations.” Utilizing meteorological evaluations of the Palmer Drought Severity Index, they thus compared ponderosa pine radial growth rates
during matched wet and dry years pre-and post-1950. So what did they find? Overall, the two researchers report finding a post-1950 radial growth enhancement that was “more pronounced
during drought years compared with wet years, and the greatest response occurred at the most stressed site.” As for the magnitude of the response, they determined that “the relative
change in growth [was] upward at seven of our [eight] sites, ranging from 11 to 133%.” With respect to the meaning and significance of their observations, Soule and Knapp say their results
“showing that radial growth has increased in the post-1950s period ... while climatic conditions have generally been unchanged, suggest that nonclimatic driving forces are operative.”
In addition, they say that “these radial growth responses are generally consistent with what has been shown in long-term open-top chamber (Idso and Kimball, 2001) and FACE studies (Ainsworth
and Long, 2005).” Hence, they say their findings suggest that “elevated levels of atmospheric CO2 are acting as a driving force for increased radial growth of ponderosa pine, but that
the overall influence of this effect may be enhanced, reduced or obviated by site-specific conditions.” Summarizing their findings Soule and Knapp recount how they had “hypothesized
that ponderosa pine ... would respond to gradual increases in atmospheric CO2 over the past 50 years, and that these effects would be most apparent during drought stress and on environmentally
harsh sites,” and they state in their very next sentence that their results “support these hypotheses.” Hence, they conclude
Climate Change Reconsidered 394 their paper by stating it is likely that “an atmospheric CO2-driven growth-enhancement effect exists for ponderosa pine growing under specific natural
conditions within the [USA’s] interior Pacific Northwest.” Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2scienc
.org/subject/t/treesponderosa.php. References Ainsworth, E.A. and Long, S.P. 2005. What have we learned from 15 years of free-air CO2 enrichment (FACE)? A meta-analytic review of the
responses of photosynthesis, canopy properties and plant production to rising CO2. New Phytologist 165: 351-372. Arnone, J.A. 1997. Temporal responses of community fine root populations
to long-term elevated atmospheric CO2 and soil nutrient patches in model tropical ecosystems. Acta Oecologia 18: 367-376. Berntson, G.M. 1994. Modeling root architecture: are there tradeoffs
between efficiency and potential of resource acquisition? New Phytologist 127: 483-493. Berntson, G.M. and Bazzaz, F.A. 1998. Regenerating temperate forest mesocosms in elevated CO2:
belowground growth and nitrogen cycling. Oecologia 113: 115-125. DeLucia, E.H., Callaway, R.M., Thomas, E.M. and Schlesinger, W.H. 1997. Mechanisms of phosphorus acquisition for ponderosa
pine seedlings under high CO2 and temperature. Annals of Botany 79: 111-120. Idso, S.B. and Kimball, B.A. 2001. CO2 enrichment of sour orange trees: 13 years and counting. Environmental
and Experimental Botany 46: 147-153. Johnson, D.W., Thomas, R.B., Griffin, K.L., Tissue, D.T., Ball, J.T., Strain, B.R. and Walker, R.F. 1998. Effects of carbon dioxide and nitrogen
on growth and nitrogen uptake in ponderosa and loblolly pine. Journal of Environmental Quality 27: 414-425. Maherali, H. and DeLucia, E.H. 2000. Interactive effects of elevated CO2 and
temperature on water transport in ponderosa pine. American Journal of Botany 87: 243-249. Runion, G.B., Mitchell, R.J., Rogers, H.H., Prior, S.A. and Counts, T.K. 1997. Effects of nitrogen
and water limitation and elevated atmospheric CO2 on ectomycorrhiza of longleaf pine. New Phytologist 137: 681-689. Soule, P.T. and Knapp, P.A. 2006. Radial growth rate increases in
naturally occurring ponderosa pine trees: a late-twentieth century CO2 fertilization effect? New Phytologist: 10.1111/j.1469-8137.2006.01746.x. Tingey, D.T., Johnson, M.G. and Phillips,
D.L. 2005. Independent and contrasting effects of elevated CO2 and Nfertilization on root architecture in Pinus ponderosa. Trees 19: 43-50. Walker, R.F., Geisinger, D.R., Johnson, D.W.
and Ball, J.T. 1998a. Atmospheric CO2 enrichment and soil N fertility effects on juvenile ponderosa pine: Growth, ectomycorrhizal development, and xylem water potential. Forest Ecology
and Management 102: 33-44. Walker, R.F., Johnson, D.W., Geisinger, D.R. and Ball, J.T. 1998b. Growth and ectomycorrhizal colonization of ponderosa pine seedlings supplied different levels
of atmospheric CO2 and soil N and P. Forest Ecology and Management 109: 9-20. 7.1.2.9.3. Scots Rouhier and Read (1998) grew seedlings of Scots pine (Pinus sylvestris L.) for four months
in growth cabinets maintained at atmospheric CO2 concentrations of either 350 or 700 ppm. In addition, one-third of the seedlings were inoculated with one species of mycorrhizal fungi,
one-third were inoculated with another species, and one-third were not inoculated at all, in order to determine the effects of elevated CO2 on mycorrhizal fungi and their interactive
effects on seedling growth. These procedures resulted in the doubled atmospheric CO2 content increasing seedling dry mass by an average of 45 percent regardless of fungal inoculation.
In addition, the extra CO2 increased the number of hyphal tips associated with seedling roots by about 62 percent for both fungal species. Hyphal growth was also accelerated by elevated
CO2; after 55 days of treatment, the mycorrhizal network produced by one of the fungal symbionts occupied 444 percent more area than its counterpart exposed to ambient CO2. These results
suggest that as the air’s CO2 content continues to rise, fungal symbionts of Scots pine will likely receive greater allocations of carbon from their host. This carbon can be used to
increase their mycorrhizal networks, which would enable the fungi to explore greater volumes of soil in search of minerals and nutrients to benefit the growth of its host. In addition,
by receiving greater allocations of carbon, fungal symbionts may keep photosynthetic down regulation from occurring, as they provide an additional sink for leaf-produced carbohydrates.
Biological Effects of Carbon Dioxide Enrichment 395 Janssens et al. (1998) grew three-year-old Scots pine seedlings in open-top chambers kept at ambient and 700 ppm atmospheric CO2 concentrations
for six months while they studied the effects of elevated CO2 on root growth and respiration. In doing so, they learned that the elevated CO2 treatment significantly increased total
root length by 122 percent and dry mass by 135 percent relative to the roots of seedlings grown in ambient-CO2 air. In addition, although starch accumulation in the CO2-enriched roots
was nearly 90 percent greater than that observed in the roots produced in the ambient-CO2 treatment, the carbon-to-nitrogen ratio of the CO2-enriched roots was significantly lower than
that of the control-plant roots, indicative of the fact that they contained an even greater relative abundance of nitrogen. The most important implication of this study, therefore, was
that Scots pine seedlings will likely be able to find the nitrogen they need to sustain large growth responses to atmospheric CO2 enrichment with the huge root systems they typically
produce in CO2-enriched air. Kainulainen et al. (1998) constructed open-top chambers around Scots pine trees that were about 20 years old and fumigated them with combinations of ambient
or CO2-enriched air (645 ppm) and ambient or twice-ambient (20 to 40 ppb) ozone-enriched air for three growing seasons to study the interactive effects of these gases on starch and secondary
metabolite production. In doing so, they determined that elevated CO2 and O3 (ozone) had no significant impact on starch production in Scots pine, even after two years of treatment exposure.
However, near the end of the third year, the elevated CO2 alone significantly enhanced starch production in currentyear needles, although neither extra CO2, extra O3, nor combinations
thereof had any significant effects on the concentrations of secondary metabolites they investigated. Kellomaki and Wang (1998) constructed closedtop chambers around 30-year-old Scots
pine trees, which they fumigated with air containing either 350 or 700 ppm CO2 at ambient and elevated (ambient plus 4°C) air temperatures for one full year, after which they assessed
tree water-use by measuring cumulative sap flow for 32 additional days. This protocol revealed that the CO2-enriched air reduced cumulative sap flow by 14 percent at ambient air temperatures,
but that sap flow was unaffected by atmospheric CO2 concentration in the trees growing at the elevated air temperatures. These findings suggest that cumulative water-use by Scotts pine
trees in a CO2-enriched world of the future will likely be less than or equal to—but no more than—what it is today. Seven years later, Wang et al. (2005) published a report of a study
in which they measured sap flow, crown structure, and microclimatic parameters in order to calculate the transpiration rates of individual 30-year-old Scots pine trees that were maintained
for a period of three years in ambient air and air enriched with an extra 350 ppm of CO2 and/or warmed by 2° to 6°C in closed-top chambers constructed within a naturally seeded stand
of the trees. As they describe it, the results of this experiment indicated that “(i) elevated CO2 significantly enhanced whole-tree transpiration rate during the first measuring year
[by 14%] due to a large increase in whole-tree foliage area, 1998, but reduced it in the subsequent years of 1999 and 2000 [by 13% and 16%, respectively] as a consequence of a greater
decrease in crown conductance which off-set the increase in foliage area per tree; (ii) trees growing in elevated temperature always had higher sap flow rates throughout three measuring
years [by 54%, 45% and 57%, respectively]; and (iii) the response of sap flow to the combination of elevated temperature and CO2 was similar to that of elevated temperature alone, indicating
a dominant role for temperature and a lack of interaction between elevated CO2 and temperature.” These observations suggest that as the air’s CO2 content continues to rise, we probably
can expect to see a decrease in evaporative water loss rates from naturally occurring stands of Scots pine trees ... unless there is a large concurrent increase in air temperature. Also
working with closed-top chambers that were constructed around 20-year-old Scots pines and fumigated with air containing 350 and 700 ppm CO2 at ambient and elevated (ambient plus 4°C)
air temperatures for a period of three years were Peltola et al. (2002), who studied the effects of elevated CO2 and air temperature on stem growth in this coniferous species when it
was growing on a soil low in nitrogen. After three years of treatment, they found that cumulative stem diameter growth in the CO2-enriched trees growing at ambient air temperatures was
57 percent greater than that displayed by control trees growing at ambient CO2 and ambient air temperatures, while the trees exposed to elevated CO2 and elevated air temperature exhibited
cumulative stem-diameter growth that was 67 percent greater than that displayed by trees exposed to ambient-CO2 air and ambient air temperatures. Consequently, as the air’s CO2 content
continues to rise, Scots pine trees
Climate Change Reconsidered 396 will likely respond by increasing stem-diameter growth, even if growing on soils low in nitrogen, and even if air temperatures rise by as much as 4°C.
In a somewhat different type of study, Kainulainen et al. (2003) collected needle litter beneath 22-year-old Scots pines that had been growing for the prior three years in open-top chambers
that had been maintained at atmospheric CO2 concentrations of 350 and 600 ppm in combination with ambient and elevated (approximately 1.4 x ambient) ozone concentrations to determine
the impacts of these variables on the subsequent decomposition of senesced needles. This they did by enclosing the needles in litterbags and placing the bags within a native litter layer
in a Scots pine forest, where decomposition rates were assessed by measuring accumulated litterbag mass loss over a period of 19 months. Interestingly, the three researchers found that
exposure to elevated CO2 during growth did not affect subsequent rates of needle decomposition, nor did elevated O3 exposure affect decomposition, nor did exposure to elevated concentrations
of the two gases together affect it. Finally, Bergh et al. (2003) used a boreal version of the process-based BIOMASS simulation model to quantify the individual and combined effects
of elevated air temperature (2° and 4°C above ambient) and CO2 concentration (350 ppm above ambient) on the net primary production (NPP) of Scots pine forests growing in Denmark, Finland,
Iceland, Norway, and Sweden. This work revealed that air temperature increases of 2° and 4°C led to mean NPP increases of 11 and 20 percent, respectively. However, when the air’s CO2
concentration was simultaneously increased from 350 to 700 ppm, the corresponding mean NPP increases rose to 41 and 55 percent. Last, when the air’s CO2 content was doubled at the prevailing
ambient temperature, the mean value of the NPP rose by 27 percent. Consequently, as the air’s CO2 content continues to rise, Ponderosa pines of Denmark, Finland, Iceland, Norway, and
and Sweden should grow ever more productively; and if air temperature also rises, they will likely grow better still. Given the above results, as the air’s CO2 content continues to rise,
we can expect to see the root systems of Scots pines significantly enhanced, together with the mycorrhizal fungal networks that live in close association with them and help secure the
nutrients the trees need to sustain large CO2-induced increases in biomass production. Concurrently, we can expect to see much smaller changes in total evaporative water loss, which
means that whole-tree water use efficiency should also be significantly enhanced. Additional information on this topic, including reviews of newer publications as they become available,
can be found at http://www.co2science.org/subject/t/treesscots.php. References Bergh, J., Freeman, M., Sigurdsson, B., Kellomaki, S., Laitinen, K., Niinisto, S., Peltola, H. and Linder,
S. 2003. Modelling the short-term effects of climate change on the productivity of selected tree species in Nordic countries. Forest Ecology and Management 183: 327-340. Janssens, I.A.,
Crookshanks, M., Taylor, G. and Ceulemans, R. 1998. Elevated atmospheric CO2 increases fine root production, respiration, rhizosphere respiration and soil CO2 efflux in Scots pine seedlings.
Global Change Biology 4: 871-878. Kainulainen, P., Holopainen, J.K. and Holopainen, T. 1998. The influence of elevated CO2 and O3 concentrations on Scots pine needles: Changes in starch
and secondary metabolites over three exposure years. Oecologia 114: 455-460. Kainulainen, P., Holopainen, T. and Holopainen, J.K. 2003. Decomposition of secondary compounds from needle
litter of Scots pine grown under elevated CO2 and O3. Global Change Biology 9: 295-304. Kellomaki, S. and Wang, K.-Y. 1998. Sap flow in Scots pines growing under conditions of year-round
carbon dioxide enrichment and temperature elevation. Plant, Cell and Environment 21: 969-981. Peltola, H., Kilpelainen, A. and Kellomaki, S. 2002. Diameter growth of Scots pine (Pinus
sylvestris) trees grown at elevated temperature and carbon dioxide concentration under boreal conditions. Tree Physiology 22: 963-972. Rouhier, H. and Read, D.J. 1998. Plant and fungal
responses to elevated atmospheric carbon dioxide in mycorrhizal seedlings of Pinus sylvestris. Environmental and Experimental Botany 40: 237-246. Wang, K.-Y., Kellomaki, S., Zha, T.
and Peltola, H. 2005. Annual and seasonal variation of sap flow and conductance of pine trees grown in elevated carbon dioxide and temperature. Journal of Experimental Botany 56: 155-165.
Biological Effects of Carbon Dioxide Enrichment 397 7.1.2.10. Spruce Several studies have recently documented the effects of elevated CO2 on photosynthesis in various varieties of spruce
(genus Picea). In the relatively short-term study of Tjoelker et al. (1998a), black spruce (Picea mariana) seedlings grown for three months at atmospheric CO2 concentrations of 580 ppm
exhibited photosynthetic rates that were about 28 percent greater than those displayed by control seedlings fumigated with air containing 370 ppm CO2. Similarly, Egli et al. (1998) reported
that Norway spruce (Picea abies) seedlings grown at 570 ppm CO2 displayed photosynthetic rates that were 35 percent greater than those exhibited by seedlings grown at 370 ppm. In two
branch bag studies conducted on mature trees, it was demonstrated that twice-ambient levels of atmospheric CO2 enhanced rates
of photosynthesis in current-year needles by 50 percent in Norway spruce (Roberntz and Stockfors, 1998) and 100 percent in Sitka spruce (Picea sitchensis) (Barton and Jarvis, 1999).
Finally, in the four-year open-top chamber study of Murray et al. (2000), the authors reported that Sitka spruce seedlings growing at 700 ppm CO2 exhibited photosynthetic rates that
were 19 and 33 percent greater than those observed in control trees growing in ambient air and receiving low and high amounts of nitrogen fertilization, respectively. Because elevated
CO2 enhances photosynthetic rates in spruce species, this phenomenon should lead to increased biomass production in these important coniferous trees. In the short-term three-month study
of Tjoelker et al. (1998b), for example, black spruce seedlings receiving an extra 210 ppm CO2 displayed final dry weights that were about 20 percent greater than those of seedlings
growing at ambient CO2. Similarly, after growing Sitka spruce for three years in open-top chambers, Centritto et al. (1999) reported that a doubling of the atmospheric CO2 concentration
enhanced sapling dry mass by 42 percent. In summary, it is clear that as as the CO2 content of the air increases, spruce trees will likely display enhanced rates of photosynthesis and
biomass production, regardless of soil nutrient status. Consequently, rates of carbon sequestration by this abundant coniferous forest species will likely be enhanced. Additional information
on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/t/treesspruce.php. References Barton, C.V.M. and Jarvis,
P.G. 1999. Growth response of branches of Picea sitchensis to four years exposure to elevated atmospheric carbon dioxide concentration. New Phytologist 144: 233-243. Centritto, M., Lee,
H.S.J. and Jarvis, P.G. 1999. Long-term effects of elevated carbon dioxide concentration and provenance on four clones of Sitka spruce (Picea sitchensis). I. Plant growth, allocation
and ontogeny. Tree Physiology 19: 799-806. Egli, P., Maurer, S., Gunthardt-Goerg, M.S. and Korner, C. 1998. Effects of elevated CO2 and soil quality on leaf gas exchange and aboveground
growth in beech-spruce model ecosystems. New Phytologist 140: 185-196. Murray, M.B., Smith, R.I., Friend, A. and Jarvis, P.G. 2000. Effect of elevated [CO2] and varying nutrient application
rates on physiology and biomass accumulation of Sitka spruce (Picea sitchensis). Tree Physiology 20: 421-434. Roberntz, P. and Stockfors, J. 1998. Effects of elevated CO2 concentration
and nutrition on net photosynthesis, stomatal conductance and needle respiration of field-grown Norway spruce trees. Tree Physiology 18: 233-241. Tjoelker, M.G., Oleksyn, J. and Reich,
P.B. 1998a. Seedlings of five boreal tree species differ in acclimation of net photosynthesis to elevated CO2 and temperature. Tree Physiology 18: 715-726. Tjoelker, M.G., Oleksyn, J.
and Reich, P.B. 1998b. Temperature and ontogeny mediate growth response to elevated CO2 in seedlings of five boreal tree species. New Phytologist 140: 197-210. 7.1.2.11. Tropical Several
studies have recently documented the effects of elevated atmospheric CO2 concentrations on photosynthesis in various tropical and sub-tropical trees. In the relatively short-term study
of Lovelock et al. (1999a), for example, seedlings of the tropical tree Copaifera aromatica that were grown for two months at an atmospheric CO2 concentration of 860 ppm exhibited photosynthetic
rates that were consistently 50-100 percent greater than those displayed by control seedlings fumigated with air containing 390 ppm CO2. Similarly, Lovelock et al. (1999b) reported that
a 10-month 390-ppm increase in the air’s CO2 content boosted rates of net photosynthesis in 30-m tall Luehea seemannii trees by 30 percent. Likewise, in the review paper of Schaffer
et al. (1999), it was
Climate Change Reconsidered 398 noted that atmospheric CO2 enrichment had previously been shown to enhance rates of net photosynthesis in a number of tropical and subtropical fruit trees,
including avocado, banana, citrus, mango, and mangosteen. Even at the ecosystem level, Lin et al. (1998) found that a 1,700-m2 synthetic rainforest mesocosm displayed a 79 percent enhancement
in net ecosystem carbon exchange rate in response to a 72 percent increase in the air’s CO2 content. Because elevated CO2 enhances photosynthetic rates in tropical and sub-tropical trees,
it should also lead to increased carbohydrate and biomass production in these species. At a tropical forest research site in Panama, twice-ambient CO2 concentrations enhanced foliar
sugar concentrations by up to 30 percent (Wurth et al., 1998), while doubling the foliar concentrations of starch (Lovelock et al., 1998) in a number of tree species. Also, in the study
of Hoffmann et al. (2000), elevated CO2 (700 ppm) enhanced dry weights of an “uncut” Brazilian savannah tree species (Keilmeyera coriacea) by about 50 percent, while it enhanced the
dry weight of the same “cut” species by nearly 300 percent. Although not specifically quantified, Schaffer et al. (1997) noted that twice-ambient CO2 exposure for one year obviously
enhanced dry mass production in two mango ecotypes. Finally, in the six-month study of Sheu et al. (1999), a doubling of the atmospheric CO2 concentration increased seedling dry weight
in Schima superba by 14 and 49 percent when grown at ambient and elevated (5°C above ambient) air temperatures, respectively. It is clear that as the air’s CO2 content rises, tropical
and sub-tropical trees will likely display enhanced rates of photosynthesis and biomass production, even under conditions of herbivory and elevated air temperature. Consequently, greater
carbon sequestration will also likely occur within earth’s tropical and sub-tropical forests as ever more CO2 accumulates in the atmosphere. Additional information on this topic, including
reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/t/treestropical.php. References Hoffmann, W.A., Bazzaz, F.A., Chatterton, N.J.,
Harrison, P.A. and Jackson, R.B. 2000. Elevated CO2 enhances resprouting of a tropical savanna tree. Oecologia 123: 312-317. Lin, G., Marino, B.D.V., Wei, Y., Adams, J., Tubiello, F.
and Berry, J.A. 1998. An experimental and modeling study of responses in ecosystems carbon exchanges to increasing CO2 concentrations using a tropical rainforest mesocosm. Australian
Journal of Plant Physiology 25: 547-556. Lovelock, C.E., Posada, J. and Winter, K. 1999a. Effects of elevated CO2 and defoliation on compensatory growth and photosynthesis of seedlings
in a tropical tree, Copaifera aromatica. Biotropica 31: 279-287. Lovelock, C.E., Virgo, A., Popp, M. and Winter, K. 1999b. Effects of elevated CO2 concentrations on photosynthesis, growth
and reproduction of branches of the tropical canopy trees species, Luehea seemannii Tr. & Planch. Plant, Cell and Environment 22: 49-59. Lovelock, C.E., Winter, K., Mersits, R. and Popp,
M. 1998. Responses of communities of tropical tree species to elevated CO2 in a forest clearing. Oecologia 116: 207-218. Schaffer, B., Whiley, A.W. and Searle, C. 1999. Atmospheric CO2
enrichment, root restriction, photosynthesis, and dry-matter partitioning in subtropical and tropical fruit crops. HortScience 34: 1033-1037. Schaffer, B., Whiley, A.W., Searle, C. and
Nissen, R.J. 1997. Leaf gas exchange, dry matter partitioning, and mineral element concentrations in mango as influenced by elevated atmospheric carbon dioxide and root restriction.
Journal of the American Society of Horticultural Science 122: 849-855. Sheu, B.-H. and Lin, C.-K. 1999. Photosynthetic response of seedlings of the sub-tropical tree Schima superba with
exposure to elevated carbon dioxide and temperature. Environmental and Experimental Botany 41: 57-65. Wurth, M.K.R., Winter, K. and Korner, C. 1998. Leaf carbohydrate responses to CO2
enrichment at the top of a tropical forest. Oecologia 116: 18-25. 7.1.2.12. Wood Density Numerous experiments have demonstrated that trees grown in air enriched with CO2 nearly always
sequester more biomass in their trunks and branches than do trees grown in ambient air. Several studies also have looked at the effects of elevated CO2 on the density of that sequestered
biomass. Rogers et al. (1983) observed no difference in the wood density of loblolly pine (Pinus taeda) trees grown at 340 and 718 ppm CO2 for 10 weeks; but they found a 33 percent CO2-induced
increase in the wood density of sweetgum (Liquidambar styraciflua) trees that were grown at these concentrations for only eight weeks. Doyle (1987) and Telewski and Strain
Biological Effects of Carbon Dioxide Enrichment 399 (1987) studied the same two tree species over three growing seasons in air of 350 and 650 ppm CO2, finding no effect of atmospheric
CO2 enrichment on the stem density of sweetgum, but a mean increase of 9 percent in the stem density of loblolly pine. Conroy et al. (1990) grew seedlings of two Pinus radiata families
at 340 and 660 ppm CO2 for 114 weeks, finding CO2-induced trunk density increases for the two families of 5.4 and 5.6 percent when soil phosphorus was less than adequate and increases
of 5.6 and 1.2 percent when it was non-limiting. In a similar study, Hattenschwiler et al. (1996) grew six genotypes of clonally propagated four-year-old Norway spruce (Picea abies)
for three years at CO2 concentrations of 280, 420, and 560 ppm at three different rates of wet nitrogen deposition. On average, they found that wood density was 12 percent greater in
the trees grown at the two higher CO2 concentrations than it was in the trees grown at 280 ppm. Norby et et al. (1996) grew yellow poplar or “tulip” trees (Liriodendron tulipifera) at
ambient and ambient plus 300 ppm CO2 for three years, during which time the wood density of the trees increased by approximately 7 percent. Tognetti et al. (1998) studied two species
of oak tree—one deciduous (Quercus pubescens) and one evergreen (Quercus ilex)—growing in the vicinity of CO2 springs in central Italy that raised the CO2 concentration of the surrounding
air by approximately 385 ppm. This increase in the air’s CO2 content increased the wood density of the deciduous oaks by 4.2 percent and that of the evergreen oaks by 6.4 percent. Telewski
et al. (1999) grew loblolly pine trees for four years at ambient and ambient plus 300 ppm CO2. In their study, wood density determined directly from mass and volume measurements was
increased by 15 percent by the extra CO2; average ring density determined by X-ray densitometry was increased by 4.5 percent. Beismann et al. (2002) grew different genotypes of spruce
and beech (Fagus sylvatica) seedlings for four years in open-top chambers maintained at atmospheric CO2 concentrations of 370 and 590 ppm in combination with low and high levels of wet
nitrogen application on both rich calcareous and poor acidic soils to study the effects of these factors on seedling toughness (fracture characteristics) and rigidity (bending characteristics
such as modulus of elasticity). They found that some genotypes of each species were sensitive to elevated CO2, while others were not. Similarly, some were responsive to elevated nitrogen
deposition, while others were not. Moreover, such responses were often dependent upon soil type. Averaged across all tested genotypes, however, atmospheric CO2 enrichment increased wood
toughness in spruce seedlings grown on acidic soils by 12 and 18 percent at low and high levels of nitrogen deposition, respectively. In addition, atmospheric CO2 enrichment increased
this same wood property in spruce seedlings grown on calcareous soils by about 17 and 14 percent with low and high levels of nitrogen deposition, respectively. By contrast, elevated
CO2 had no significant effects on the mechanical wood properties of beech seedlings, regardless of soil type. Finally, Kilpelainen et al. (2003) erected 16 opentop chambers within a
15-year-old stand of Scots pines growing on a nutrient-poor sandy soil of low nitrogen content near the Mekrijarvi Research Station of the University of Joensuu, Finland. Over the next
three years they maintained the trees within these chambers in a well-watered condition, while they enriched the air in half of the chambers to a mean daytime CO2 concentration of approximately
580 ppm and maintained the air in half of each of the two CO2 treatments at 2°C above ambient. In the ambient temperature treatment the 60 percent increase in the air’s CO2 concentration
increased latewood density by 27 percent and maximum wood density by 11 percent, while in the elevated-temperature treatment it increased latewood density by 25 percent and maximum wood
density by 15 15 percent. These changes led to mean overall CO2-induced wood density increases of 2.8 percent in the ambient-temperature treatment and 5.6 percent in the elevated-temperature
treatment. In light of these several observations, it is clear that different species of trees respond differently to atmospheric CO2 enrichment, and that they respond with still greater
variety under different sets of environmental conditions. In general, however, atmospheric CO2 enrichment tends to increase wood density in both seedlings and mature trees more often
than not, thereby also increasing a number of strength properties of their branches and trunks. Additional information on this topic, including reviews of newer publications as they
become available, can be found at http://www.co2science.org/subject/w/wooddensity.php.
Climate Change Reconsidered 400 References Beismann, H., Schweingruber, F., Speck, T. and Korner, C. 2002. Mechanical properties of spruce and beech wood grown in elevated CO2. Trees
16: 511-518. Conroy, J.P., Milham, P.J., Mazur, M., Barlow, E.W.R. 1990. Growth, dry weight partitioning and wood properties of Pinus radiata D. Don after 2 years of CO2 enrichment.
Plant, Cell and Environment 13: 329-337. Doyle, T.W. 1987. Seedling response to CO2 enrichment under stressed and non-stressed conditions. In: Jacoby Jr., G.C. and Hornbeck, J.W. (Eds.)
Proceedings of the International Symposium on Ecological Aspects of Tree-Ring Analysis. National Technical Information Service, Springfield, VA, pp. 501-510. Hattenschwiler, S., Schweingruber,
F.H., Korner, C. 1996. Tree ring responses to elevated CO2 and increased N deposition in Picea abies. Plant, Cell and Environment 19: 1369-1378. Kilpelainen A., Peltola, H., Ryyppo,
A., Sauvala, K., Laitinen, K. and Kellomaki, S. 2003. Wood properties of Scots pines (Pinus sylvestris) grown at elevated temperature and carbon dioxide concentration. Tree Physiology
23: 889-897. Norby, R.J., Wullschleger, S.D., Gunderson, C.A. 1996. Tree responses to elevated CO2 and implications for forests. In: Koch, G.W. and Mooney, H.A. (Eds.) Carbon Dioxide
and Terrestrial Ecosystems. Academic Press, New York, NY, pp. 1-21. Rogers, H.H., Bingham, G.E., Cure, J.D., Smith, J.M. and Surano, K.A. 1983. Responses of selected plant species to
elevated carbon dioxide in the field. Journal of Environmental Quality 12: 569-574. Telewski, F.W. and Strain, B.R. 1987. Densitometric and ring width analysis of 3-year-old Pinus taeda
L. and Liquidambar styraciflua L. grown under three levels of CO2 and two water regimes. In: Jacoby Jr., G.C. and Hornbeck, J.W. (Eds.) Proceedings of the International Symposium on
Ecological Aspects of Tree-Ring Analysis. National Technical Information Service, Springfield, VA, pp. 494-500. Telewski, F.W., Swanson, R.T., Strain, B.R. and Burns, J.M. 1999. Wood
properties and ring width responses to long-term atmospheric CO2 enrichment in field-grown loblolly pine (Pinus taeda L.). Plant, Cell and Environment 22: 213-219. Tognetti, R., Johnson,
J.D., Michelozzi, M. and Raschi, A. 1998. Response of foliar metabolism in mature trees of Quercus pubescens and Quercus ilex to long-term elevated CO2. Environmental and Experimental
Botany 39: 233-245. 7.1.2.13. Forests Forests contain perennial trees that remove CO2 from the atmosphere during photosynthesis and store its carbon within their woody tissues for decades
to periods sometimes in excess of a thousand years. Thus, it is important to understand how increases in the air’s CO2 content affect forest productivity and carbon sequestration, which
has a great effect on the rate of rise of the air’s CO2 concentration. In this summary, we review several recent scientific publications pertaining to these subjects. By examining various
properties of tree rings, researchers can deduce how historical increases in the air’s CO2 concentration have already affected tree productivity and water use efficiency. Duquesnay et
al. (1998), for example, analyzed the relative amounts of 12C and 13C present in yearly growth rings of beech trees raised in silviculture regimes in northeastern France, determining
that their intrinsic water use efficiencies rose by approximately 33 percent during the past century, as the air’s CO2 concentration rose from approximately 280 to 360 ppm. In another
case, Rathgeber et al. (2000) used tree-ring density data to create a historical productivity baseline for forest stands of Pinus halepensis in southeastern France, from which they determined
that the net productivity of such forests would increase by 8 to 55 percent with a doubling of the air’s CO2 content. Finally, when running a forest growth model based on empirical observations
reported in the literature, Lloyd (1999) determined that the rise in the atmospheric CO2 concentration since the onset of the Industrial Revolution likely increased the net primary productivity
of mature temperate deciduous forests by about 7 percent. In addition, he determined that a proportional increase in anthropogenic nitrogen deposition likely increased forest net primary
productivity by 25 percent. And when he combined the two effects, the net primary productivity stimulation rose to 40 percent, which is more than the sum of the individual growth enhancements
resulting from the increases in CO2 and nitrogen. The results of these studies demonstrate that historic increases in the air’s CO2 content have already conferred great benefits upon
earth’s forests. But will future increases in the air’s CO2 concentration continue to do so? Several research teams have embarked on long-term studies of various forest communities in
an attempt to address this important question. What follows are some important
Biological Effects of Carbon Dioxide Enrichment 401 observations that have been made from their mostly ongoing CO2-enrichment studies. In 1996, circular FACE plots (30-m diameter) receiving
atmospheric CO2 concentrations of 360 and 560 ppm were established in a 15-year-old loblolly pine (Pinus taeda) plantation in North Carolina, USA, to study the effects of elevated CO2
on the growth and productivity of this particular forest community, which also had several hardwood species present in the understory beneath the primary coniferous canopy. Using this
experimental set-up as a platform for several experiments, Hymus et al. (1999) reported that net photosynthetic rates of CO2-enriched loblolly pines trees were 65 percent greater than
rates observed in control trees exposed to ambient air. These greater rates of carbon fixation contributed to the 24 percent greater growth rates observed in the CO2-enriched pine trees
in the first year of this longterm study (Naidu and DeLucia 1999). In addition, DeLucia and Thomas (2000) reported that the elevated CO2 increased rates of net photosynthesis by 50 to
160 percent in four subdominant hardwood species present in the forest understory. Moreover, for one species—sweetgum (Liquidambar styraciflua)— the extra CO2 enhanced rates of net photosynthesis
in sun and shade leaves by 166 and 68 percent, respectively, even when the trees were naturally subjected to summer seasonal stresses imposed by high temperature and low soil water availability.
Consequently, after two years of atmospheric CO2 enrichment, total ecosystem net primary productivity in the CO2-enriched plots was 25 percent greater than that measured in control plots
fumigated with ambient air. In a similar large-scale study, circular (25-m diameter) FACE plots receiving atmospheric CO2 concentrations of 400 and 530 ppm were constructed within a
10-year-old sweetgum plantation in Tennessee, USA, to study the effects of elevated CO2 on the growth and productivity of this forest community. After two years of treatment, Norby et
al. (2001) reported that the modest 35 percent increase in the air’s CO2 content boosted tree biomass production by an average of 24 percent. In addition, Wullschleger and Norby (2001)
noted that CO2-enriched trees displayed rates of transpirational water loss that were approximately 10 percent lower than those exhibited by control trees grown in ambient air. Consequently,
elevated CO2 enhanced seasonal water use efficiencies of these mature sweetgum trees by 28 to 35 percent. On a smaller scale, Pritchard et al. (2001) constructed idealized ecosystems
(containing five different species) representative of regenerating longleaf pine (Pinus palustris Mill.) communities of the southeastern USA, fumigating them for 18 months with air containing
365 and 720 ppm CO2 to study the effects of elevated CO2 on this forest community. They reported that elevated CO2 increased the aboveand below-ground biomass of the dominant longleaf
pine individuals by 20 and 62 percent, respectively. At the ecosystem level, elevated CO2 stimulated total above-ground biomass production by an average of 35 percent. Similar results
for regenerating temperate forest communities have been reported by Berntson and Bazzaz (1998), who documented a 31 percent increase in Transition Hardwood-White Pine-Hemlock forest
mesocosm biomass in response to two years of fumigation with twice-ambient concentrations of atmospheric CO2. It is clear that as the air’s CO2 concentration continues to rise, forests
will likely respond by exhibiting significant increases in total primary productivity and biomass production. Consequently, forests will likely grow much more robustly and significantly
expand their ranges, as has already been documented in many parts of the world, including gallery forest in Kansas, USA (Knight et al., 1994) and the Budal and Sjodal valleys in Norway
(Olsson et al., 2000). Such CO2-induced increases in growth and range expansion should result in large increases in global carbon sequestration within forests. Additional information
on this topic, including reviews on forests not discussed here, can be found at http://www.co2science.org/subject/f/subject_f.php under the heading Forests. References Berntson, G.M.
and Bazzaz, F.A. 1998. Regenerating temperate forest mesocosms in elevated CO2: belowground growth and nitrogen cycling. Oecologia 113: 115-125. DeLucia, E.H. and Thomas, R.B. 2000.
Photosynthetic responses to CO2 enrichment of four hardwood species in a forest understory. Oecologia 122: 11-19. Duquesnay, A., Breda, N., Stievenard, M. and Dupouey, J.L. 1998. Changes
of tree-ring ð13C and water-use efficiency of beech (Fagus sylvatica L.) in north-eastern France during the past century. Plant, Cell and Environment 21: 565-572. Hymus, G.J., Ellsworth,
D.S., Baker, N.R. and Long, S.P. 1999. Does free-air carbon dioxide enrichment affect
Climate Change Reconsidered 402 photochemical energy use by evergreen trees in different seasons? A chlorophyll fluorescence study of mature loblolly pine. Plant Physiology 120: 1183-1191.
Knight, C.L., Briggs, J.M. and Nellis, M.D. 1994. Expansion of gallery forest on Konza Prairie Research Natural Area, Kansas, USA. Landscape Ecology 9: 117-125. Lloyd, J. 1999. The CO2
dependence of photosynthesis, plant growth responses to elevated CO2 concentrations and their interaction with soil nutrient status, II. Temperate and boreal forest productivity and
the combined effects of increasing CO2 concentrations and increased nitrogen deposition at a global scale. Functional Ecology 13: 439-459. Naidu, S.L. and DeLucia, E.H. 1999. First-year
growth response of trees in an intact forest exposed to elevated CO2. Global Change Biology 5: 609-613. Norby, R.J., Todd, D.E., Fults, J. and Johnson, D.W. 2001. Allometric determination
of tree growth in a CO2-enriched sweetgum stand. New Phytologist 150: 477-487. Olsson, E.G.A., Austrheim, G. and Grenne, S.N. 2000. Landscape change patterns in mountains, land use and
environmental diversity, Mid-Norway 1960-1993. Landscape Ecology 15: 155-170. Pritchard, S.G., Davis, M.A., Mitchell, R.J., Prior, A.S., Boykin, D.L., Rogers, H.H. and Runion, G.B. 2001.
Root dynamics in an artificially constructed regenerating longleaf pine ecosystem are affected by atmospheric CO2 enrichment. Environmental and Experimental Botany 46: 35-69. Rathgeber,
C., Nicault, A., Guiot, J., Keller, T., Guibal, F. and Roche, P. 2000. Simulated responses of Pinus halepensis forest productivity to climatic change and CO2 increase using a statistical
model. Global and Planetary Change 26: 405-421. Wullschleger, S.D. and Norby, R.J. 2001. Sap velocity and canopy transpiration in a sweetgum stand exposed to freeair CO2 enrichment (FACE).
New Phytologist 150: 489-498. 7.1.3. Aquatic Plants We have shown how atmospheric CO2 enrichment typically enhances the growth and productivity of nearly all terrestrial plants. But
But what about aquatic plants? In this section we seek to answer that question. 7.1.3.1. Freshwater Algae How do freshwater algae respond to increases in the air’s CO2 content? The subject
has not been thoroughly researched, but the results of the studies discussed below provide a glimpse of what the future may hold as the atmosphere’s CO2 concentration continues its upward
course. Working with cells of the freshwater alga Chlorella pyrenoidosa, Xia and Gao (2003) cultured them in Bristol’s solution within controlled environment chambers maintained at low
and high light levels (50 and 200 µmol/m²/s) during 12-hour light periods that were followed by 12-hour dark periods for a total of 13 days, while the solutions in which the cells grew
were continuously aerated with air of either 350 or 700 ppm CO2. When the cells were harvested (in the exponential growth phase) at the conclusion of this period, the biomass (cell density)
of the twice-ambient CO2 treatment was found to be 10.9 percent and 8.3 percent greater than that of the ambient-air treatment in the low-and highlight regimes, respectively, although
only the highlight result was statistically significant. The two scientists concluded from these observations that a “doubled atmospheric CO2 concentration would affect the growth of
C. pyrenoidosa when it grows under bright solar radiation, and such an effect would increase by a great extent when the cell density becomes high.” Their data also suggest the same may
well be true when the alga grows under not-so-bright conditions. Working on a much larger scale “in the field” with six 1.5-m-diameter flexible plastic cylinders placed in the littoral
zone of Lake Hampen in central Jutland, Denmark (three maintained at the ambient CO2 concentration of the air and three enriched to 10 times the ambient CO2 concentration), Andersen
and Andersen (2006) measured the CO2-induced growth response of a mixture of several species of filamentous freshwater algae dominated by Zygnema species, but containing some Mougeotia
and Spirogyra. After one full growing season (May to November), they determined that the biomass of the microalgal mixture in the CO2-enriched cylinders was increased by 220 percent
in early July, by 90 percent in mid-August, and by a whopping 3,750 percent in mid-November. In another study of the subject, Schippers et al. (2004a) say “it is usually thought that
unlike terrestrial plants, phytoplankton will not show a significant response to an increase of atmospheric
Biological Effects of Carbon Dioxide Enrichment 403 CO2,” but they note, in this regard, that “most analyses have not examined the full dynamic interaction between phytoplankton production
and assimilation, carbon-chemistry and the air-water flux of CO2,” and that “the effect of photosynthesis on pH and the dissociation of carbon (C) species have been neglected in most
studies.” In an attempt to rectify this situation, Schippers et al. developed “an integrated model of phytoplankton growth, air-water exchange and C chemistry to analyze the potential
increase of phytoplankton productivity due to an atmospheric CO2 elevation,” and as a test of their model, they let the freshwater
alga Chlamydomonas reinhardtii grow in 300-ml bottles filled with 150 ml of a nutrient-rich medium at enclosed atmospheric CO2 concentrations of 350 and 700 ppm that they maintained
at two air-water exchange rates characterized by CO2 exchange coefficients of 2.1 and 5.1 m day-1, as described by Shippers et al. (2004b), while periodically measuring the biovolume
of the solutions by means of an electronic particle counter. The results of this effort, as they describe it, “confirm the theoretical prediction that if algal effects on C chemistry
are strong, increased phytoplankton productivity because of atmospheric CO2 elevation should become proportional to the increased atmospheric CO2,” which suggests that algal productivity
“would double at the predicted increase of atmospheric CO2 to 700 ppm.” Although they note that “strong algal effects (resulting in high pH levels) at which this occurs are rare under
natural conditions,” they still predict that effects on algal production in freshwater systems could be such that a “doubling of atmospheric CO2 may result in an increase of the productivity
of more than 50%.” In the last of the few papers we have reviewed in this area, Logothetis et al. (2004) note that “the function and structure of the photosynthetic apparatus of many
algal species resembles that of higher plants (Plumley and Smidt, Smidt, 1984; Brown, 1988; Plumley et al., 1993),” and that “unicellular green algae demonstrate responses to increased
CO2 similar to those of higher plants in terms of biomass increases (Muller et al., 1993).” However, they also note that “little is known about the changes to their photosynthetic apparatus
during exposure to high CO2,” which deficiency they began to correct via a new experiment, wherein batches of the unicellular green alga Scenedesmus obliquus (wild type strain D3) were
grown autotrophically in liquid culture medium for several days in a temperature-controlled water bath of 30°C at low (55 µmol m-2 s-1) and high (235 µmol m-2 s-1) light intensity while
they were continuously aerated with air of either 300 or 100,000 ppm CO2. This protocol revealed that exposure to the latter high CO2 concentration produces, in their words, a “reorganization
of the photosynthetic apparatus” that “leads to enhanced photosynthetic rates, which ... leads to an immense increase of biomass.” After five days under low light conditions, for example,
the CO2-induced increase in biomass was approximately 300 percent, while under high light conditions it was approximately 600 percent. Based on these few observations, it is not possible
to draw any sweeping conclusions about the subject. However, they do indicate there may be a real potential for the ongoing rise in the air’s CO2 content to significantly stimulate the
productivity of this freshwater contingent of earth’s plants. Additional information on this topic, including reviews of newer publications as they become available, can be found at
http://www.co2science.org/subject/a/aquaticplants.php. References Andersen, T. and Andersen, F.O. 2006. Effects of CO2 concentration on growth of filamentous algae and Littorella uniflora
in a Danish softwater lake. Aquatic Botany 84: 267-271. Brown, J.S. 1988. Photosynthetic pigment organization in diatoms (Bacillariophyceae). Journal of Phycology 24: 96-102. Logothetis,
K., Dakanali, S., Ioannidis, N. and Kotzabasis, K. 2004. 2004. The impact of high CO2 concentrations on the structure and function of the photosynthetic apparatus and the role of polyamines.
Journal of Plant Physiology 161: 715-724. Muller, C., Reuter, W. and Wehrmeyer, W. 1993. Adaptation of the photosynthetic apparatus of Anacystis nidulans to irradiance and CO2-concentration.
Botanica Acta 106: 480-487. Plumley, F.G., Marinson, T.A., Herrin, D.L., Ideuchi, M. and Schmidt, G.W. 1993. Structural relationships of the photosystem I and photosystem II chlorophyll
a/b and a/c light-harvesting apoproteins of plants and algae. Photochemistry and Photobiology 57: 143-151. Plumley, F.G. and Smidt, G.W. 1984. Immunochemical characterization of families
of light-harvesting pigmentprotein complexes in several groups of algae. Journal of Phycology 20: 10.
Climate Change Reconsidered 404 Schippers, P., Lurling, M. and Scheffer, M. 2004a. Increase of atmospheric CO2 promotes phytoplankton productivity. Ecology Letters 7: 446-451. Schippers,
P., Vermaat, J.E., de Klein, J. and Mooij, W.M. 2004b. The effect of atmospheric carbon dioxide elevation on plant growth in freshwater ecosystems. Ecosystems 7: 63-74. Xia, J. and Gao,
K. 2003. Effects of doubled atmospheric CO2 concentration on the photosynthesis and growth of Chlorella pyrenoidosa cultured at varied levels of light. Fisheries Science 69: 767-771.
7.1.3.2. Freshwater Macrophytes In this section we discuss the findings of papers that investigate the influence of CO2 concentrations as it applies to submersed, floating, and emergent
freshwater macrophytes, beginning with studies of aquatic plants that live their lives totally submersed in freshwater environments. For several multi-week periods, Idso (1997) grew
specimens of corkscrew vallisneria (Vallisneria tortifolia) in several 10-and 29-gallon glass tanks (containing 10-cm bottom-layers of common aquarium gravel) that were filled with tap
water maintained within 0.5°C of either 18.2°C or 24.5°C, while the semi-sealed air spaces above these “Poor Man’s Biospheres,” as he christened them, were maintained at a number of
different CO2 concentrations. With the harvesting of plants at the end of the study, this protocol revealed that the CO2-induced growth enhancement of the plants was linear (in contrast
to the gradually declining CO2-induced growth enhancements typically exhibited by most terrestrial plants as the air’s CO2 content climbs ever higher), and that the linear relationship
extended to the highest atmospheric CO2 concentration studied: 2,100 ppm. In addition, he found that the CO2-induced growth increase experienced by the plants in the higher of the two
water temperature treatments (a 128 percent increase in going from an atmospheric CO2 concentration of 365 ppm to one of 2,100 ppm) was 3.5 times greater than that of the plants in the
lower water temperature treatment. Although this response may seem rather dramatic, it is not unique; Idso reports that Titus et al. (1990), who studied the closely related Vallisneria
americana, “observed that the biomass of their experimental plants also rose linearly with the CO2 content of the air above the water within which they grew, and that [it] did so from
the value of the [then] current global mean (365 ppm) to a concentration fully ten times larger.” In another study of a closely allied species, Yan et al. (2006) collected turions of
Vallisneria spinulosa from Liangzi Lake, Hubei Province, China, and planted them in tanks containing 15-cm-deep layers of fertile lake sediments, topped with 40 cm of lake water, that
were placed in two glasshouses—one maintained at the ambient atmospheric CO2 concentration of 390 ppm and the other maintained at an elevated concentration of 1,000 ppm—where the plants
grew for a period of 120 days, after which they were harvested and the dry weights of their various organs determined. As they describe it, this work indicated that the “total biomass
accumulation of plants grown in the elevated CO2 was 2.3 times that of plants grown in ambient CO2, with biomass of leaves, roots and rhizomes increasing by 106%, 183% and 67%, respectively.”
Most spectacularly of all, they report that “turion biomass increased 4.5-fold,” because “the mean turion numbers per ramet and mean biomass per turion in elevated CO2 were 1.7-4.3 and
1.9-3.4 times those in ambient CO2.” In Denmark, in a study of small slow-growing evergreen perennials called isoetids that live submersed along the shores of numerous freshwater lakes,
Andersen et al. (2006) grew specimens of Littorella uniflora in sediment cores removed from Lake Hampen in 75-liter tanks with 10-cm overburdens of filtered lake water for a period of
53 days, while measuring various plant, water, and sediment properties, after which they destructively harvested the plants and measured their biomass. Throughout this period, half of
the tanks had ambient air bubbled through their waters, while the other half were similarly exposed to a mixture of ambient air and pure CO2 that produced a 10-fold increase in the air’s
CO2 concentration. This ultra-CO2-enrichment led to a 30 percent increase in plant biomass, as well as “higher O2 release to the sediment which is important for the cycling and retention
of nutrients in sediments of oligotrophic softwater lakes.” And when the ultra-CO2-enrichment was maintained for an entire growing season (May-November), Andersen and Andersen (2006)
report that the 10-fold increase in aquatic CO2 concentration enhanced the biomass production of Littorella uniflora by a much larger 78 percent. In a study of an “in-between” type of
plant that has submersed roots and rhizomes that are anchored in water-body sediments, but which has floating leaves on the surface of the water and emergent
Biological Effects of Carbon Dioxide Enrichment 405 flowers that protrude above the water surface, Idso et al. (1990) grew water lilies (Nymphaea marliac) for two consecutive years in
sunken metal stock tanks located out-of-doors at Phoenix, Arizona (USA) and enclosed within clear-plastic-wall open-top chambers through which air of either 350 or 650 ppm CO2 was continuously
circulated. This work revealed that in addition to the leaves of the plants being larger in the CO2-enriched treatment, there were 75 percent more of them than there were in the ambient-air
tanks at the conclusion of the initial five-month-long growing season. Each of the plants in the high-CO2 tanks also produced twice as many flowers as the plants growing in ambient air;
and the flowers that blossomed in the CO2-enriched air were more substantial than those that bloomed in the air of ambient CO2 concentration: they had more petals, the petals were longer,
and they had a greater percent dry matter content, such that each flower consequently weighed about 50 percent more than each flower in the ambient-air treatment. In addition, the stems
that supported the flowers were slightly longer in the CO2-enriched tanks, and the percent dry matter contents of both the flower and leaf stems were greater, so that the total dry matter
in the flower and leaf stems in the CO2-enriched tanks exceeded that of the flower and leaf stems in the ambient-air tanks by approximately 60 percent. Just above the surface of the
soil that covered the bottoms of the tanks, there were also noticeable differences. Plants in the CO2-enriched tanks had more and bigger basal rosette leaves, which were attached to
longer stems of greater percent dry matter content, which led to the total biomass of these portions of the plants being 2.9 times greater than the total biomass of the corresponding
portions of the plants in the ambient-air tanks. In addition, plants in the CO2-enriched tanks had more than twice as many unopened basal rosette leaves. The greatest differences of
all, however, were hidden within the soil that covered the bottoms of the stock tanks. When half of the plants were harvested at the conclusion of the first growing season, for example,
the number of new rhizomes produced over that period was discovered to be 2.4 times greater in the CO2-enriched tanks than it was in the ambient-air tanks; the number of major roots
produced there was found to be 3.2 times greater. As with all other plant parts, the percent dry matter contents of the new roots and rhizomes were also greater in the CO2-enriched tanks.
Overall, therefore, the total dry matter production within the submerged soils of the water lily ecosystems was 4.3 times greater in the CO2-enriched tanks than it was in the ambient-air
tanks; the total dry matter production of all plant parts— those in the submerged soil, those in the free water, and those in the air above—was 3.7 times greater in the high-CO2 enclosures.
Over the second growing season, the growth enhancement in the high-CO2 tanks was somewhat less; but the plants in those tanks were so far ahead of the plants in the ambient-air tanks
that in their first five months of growth, they produced what it took the plants in the ambient-air tanks fully 21 months to produce. Moving on to plants that are exclusively floating
freshwater macrophytes, Idso (1997) grew many batches of the common water fern (Azolla pinnata) over a wide range of atmospheric CO2 concentrations at two different water temperatures
(18.2°C and 24.5°C) in Poor Man’s Biospheres for periods of several weeks. This work revealed that a 900-ppm increase in the CO2 concentration of the air above the tanks led to only
a 19 percent increase in the biomass production of the plants floating in the cooler water, but that it led to a 66 percent biomass increase in the plants floating in the warmer water.
In an earlier study of Azolla pinnata, Idso et al. (1989) conducted three separate two-to three-month experiments wherein they grew batches of the floating fern out-of-doors in adequately
fertilized water contained in sunken metal stock tanks located within clear-plastic-wall open-top chambers that were continuously maintained at atmospheric CO2 concentrations of either
340 or 640 ppm, during which time the plants were briefly removed from the water and weighed at weekly intervals, while their photosynthetic rates were measured at hourly intervals from
dawn to dusk on selected cloudless days. As a result of this protocol, they found the photosynthetic and growth rates of the plants growing in ambient air “first decreased, then stagnated,
and finally became negative when mean air temperature rose above 30°C.” In the high CO2 treatment, on the other hand, they found that “the debilitating effects of high temperatures were
reduced: in one case to a much less severe negative growth rate, in another case to merely a short period of zero growth rate, and in a third case to no discernible ill effects whatsoever—in
spite of the fact that the ambient treatment plants in this instance all died.” Last, in a study of an emergent freshwater macrophyte, Ojala et al. (2002) grew water horsetail (Equisetum
fluviatile) plants at ambient and doubleambient atmospheric CO2 concentrations and ambient
Climate Change Reconsidered 406 and ambient + 3°C air temperatures for three years, although the plants were subjected to the doubleambient CO2 condition for only approximately five
months of each year. This work revealed that the increase in air temperature boosted maximum shoot biomass by 60 percent, but the elevated CO2 had no effect on this aspect of plant growth.
However, elevated CO2 and temperature—both singly and in combination—positively affected root growth, which was enhanced by 10, 15, and 25 percent by elevated air temperature, CO2, and
the two factors together, respectively. In light of the several experimental findings discussed above, we conclude that the ongoing rise in the air’s CO2 content will likely have significant
positive impacts on most freshwater macrophytes, including submersed, floating, and emergent species. Additional information on this topic, including reviews of newer publications as
they become available, can be found at http://www.co2science.org/subject/a/aquaticmacrophytes.php. References Andersen, T. and Andersen, F.O. 2006. Effects of CO2 concentration on growth
of filamentous algae and Littorella uniflora in a Danish softwater lake. Aquatic Botany 84: 267-271. Andersen, T., Andersen, F.O. and Pedersen, O. 2006. Increased CO2 in the water around
Littorella uniflora raises the sediment O2 concentration. Aquatic Botany 84: 294-300. Idso, S.B. 1997. The Poor Man’s Biosphere, including simple techniques for conducting CO2 enrichment
and depletion experiments on aquatic and terrestrial plants. Environmental and Experimental Botany 38: 15-38. Idso, S.B., Allen, S.G., Anderson, M.G. and Kimball, B.A. 1989. Atmospheric
CO2 enrichment enhances survival of Azolla at high temperatures. Environmental and Experimental Botany 29: 337-341. Idso, S.B., Allen, S.G. and Kimball, B.A. 1990. Growth response of
water lily to atmospheric CO2 enrichment. Aquatic Botany 37: 87-92. Ojala, A., Kankaala, P. and Tulonen, T. 2002. Growth response of Equisetum fluviatile to elevated CO2 and temperature.
Environmental and Experimental Botany 47: 157-171. Titus, J.E., Feldman, R.S. and Grise, D. 1990. Submersed macrophyte growth at low pH. I. CO2 enrichment effects with fertile sediment.
Oecologia 84: 307-313. Yan, X., Yu, D. and Li, Y.-K. 2006. The effects of elevated CO2 on clonal growth and nutrient content of submerged plant Vallisneria spinulosa. Chemosphere 62:
595-601. 7.1.3.3. Marine Macroaglae How do marine macroalgae respond to increases in the air’s CO2 content? The results of the studies discussed below provide a glimpse of what the future
may hold in this regard, as the atmosphere’s CO2 concentration continues its upward climb. Gao et al. (1993) grew cultures of the red macroalgae Gracilaria sp. and G. chilensis in vessels
enriched with nitrogen and phosphorus that were continuously aerated with normal air containing 350 ppm CO2, air enriched with an extra 650 ppm CO2, and air enriched with an extra 1,250
ppm CO2 for a period of 19 days. Compared to the control treatments, the relative growth enhancements in the + 650-ppm and +1250-ppm CO2 treatments were 20 percent and 60 percent, respectively,
for G. chilensis, and 130 percent and 190 percent, respectively, for the Gracilaria sp. With respect to these findings, the researchers comment that “in their natural habitats, photosynthesis
and growth of Gracilaria species are likely to be CO2-limited, especially when the population density is high and water movement is slow.” Hence, as the air’s CO2 content continues to
rise, these marine macroalgae should grow ever better in the years ahead. Such should also be the case with many other macroalgae, for Gao et al. note that “photosynthesis by most macroalgae
is probably limited by inorganic carbon sources in natural seawater,” citing the studies of Surif and Raven (1989), Maberly (1990), Gao et al. (1991), and Levavasseur et al. (1991) as
evidence for this statement. In a subsequent study, Kubler et al. (1999) grew Lomentaria articulata, a red seaweed common to the Northeast Atlantic intertidal zone, for for three weeks
in hydroponic cultures subjected to various atmospheric CO2 and O2 concentrations. In doing so, they found that oxygen concentrations ranging from 10 to 200 percent of ambient had no
significant effect on either the seaweed’s daily net carbon gain or its total wet biomass production rate. By contrast, CO2 concentrations ranging from 67 to 500 percent of ambient had
highly significant effects on these parameters. At twice the ambient CO2 concentration, for example, daily net carbon gain and total wet
Biological Effects of Carbon Dioxide Enrichment 407 biomass production rates were 52 and 314 percent greater than they were at ambient CO2. More recently, Zou (2005) collected specimens
of the brown seaweed Hizikia fusiforme from intertidal rocks along the coast of Nanao Island, Shantou, China, and maintained them in glass aquariums that contained filtered seawater
enriched with 60 µM NaNO3 and 6.0 µM NaH2PO4, while continuously aerating the aquariums with air of either 360 or 700 ppm CO2 and periodically measuring seaweed growth and nitrogen assimilation
rates, as well as nitrate reductase activities. By these means they determined that the slightly less than a doubling of the air’s CO2 concentration increased the seaweed’s mean relative
growth rate by about 50 percent, its mean rate of nitrate uptake during the study’s 12-hour light periods by some 200 percent, and its nitrate reductase activity by approximately 20
percent over a wide range of substrate nitrate concentrations. As a subsidiary aspect of the study, Zou notes that “the extract of H. fusiforme has an immunomodulating activity on humans
and this ability might be used for clinical application to treat several diseases such as tumors (Suetsuna, 1998; Shan et al., 1999).” He also reports that the alga “has been used as
a food delicacy and an herbal ingredient in China, Japan and Korea.” In fact, he says that it “is now becoming one of the most important species for seaweed mariculture in China, owing
to its high commercial value and increasing market demand.” The ongoing rise in the air’s CO2 content bodes well for all of these applications. In addition, Zou notes that “the intensive
cultivation of H. fusiforme would remove nutrients more efficiently with the future elevation of CO2 levels in seawater, which could be a possible solution to the problem of ongoing
coastal eutrophication,” suggesting that rising CO2 levels may also assist in the amelioration of this environmental problem. In light of these several observations, the ongoing rise
in the air’s CO2 content should help marine macroalgae to become more productive with the passage of time. Additional information on this topic, including reviews of newer publications
as they become available, can be found at http://www.co2science.org/subject/a/aquaticmacroalgae.php. References Gao, K., Aruga, Y., Asada, K., Ishihara, T., Akano, T. and Kiyohara, M.
1991. Enhanced growth of the red alga Porphyra yezoensis Ueda in high CO2 concentrations. Journal of Applied Phycology 3: 355-362. Gao, K., Aruga, Y., Asada, K. and Kiyohara, M. 1993.
Influence of enhanced CO2 on growth and photosynthesis of the red algae Gracilaria sp. and G. chilensis. Journal of Applied Phycology 5: 563-571. Kubler, J.E., Johnston, A.M. and Raven,
J.A. 1999. The effects of reduced and elevated CO2 and O2 on the seaweed Lomentaria articulata. Plant, Cell and Environment 22: 1303-1310. Levavasseur, G., Edwards, G.E., Osmond, C.B.
and Ramus, J. 1991. Inorganic carbon limitation of photosynthesis in Ulva rotundata (Chlorophyta). Journal of Phycology 27: 667-672. Maberly, S.C. 1990. Exogenous sources of inorganic
carbon for photosynthesis by marine macroalgae. Journal of Phycology 26: 439-449. Shan, B.E., Yoshida, Y., Kuroda, E. and Yamashita, U. 1999. Immunomodulating activity of seaweed extract
on human lymphocytes in vitro. International Journal of Immunopharmacology 21: 59-70. Suetsuna, K. 1998. Separation and identification of angiotensin I-converting enzyme inhibitory peptides
from peptic digest of Hizikia fusiformis protein. Nippon Suisan Gakkaishi 64: 862-866. Surif, M.B. and Raven, J.A. 1989. Exogenous inorganic carbon sources for photosynthesis in seawater
by members of the Fucales and the Laminariales (Phaeophyta): ecological and taxonomic implications. Oecologia 78: 97-103. Zou, D. 2005. Effects of elevated atmospheric CO2 on growth,
photosynthesis and nitrogen metabolism in the economic brown seaweed, Hizikia fusiforme (Sargassaceae, Phaeophyta). Aquaculture 250: 726-735. 7.1.3.4. Marine Microalgae How do marine
microalgae respond to increases in the air’s CO2 content? Based on the late twentieth century work of Riebesell et al. (1993), Hein and Sand-Jensen (1997), and Wolf-Gladrow et al., (1999),
it would appear that the productivity of earth’s marine microalgae may be significantly enhanced by elevated concentrations of atmospheric CO2. More recent work by other researchers
suggests the same.
Climate Change Reconsidered 408 In a study of the unicellular marine diatom Skeletonema costatus, which is widely distributed in coastal waters throughout the world and is a major component
of most natural assemblages of marine phytoplankton, Chen and Gao (2004) grew cell cultures of the species in filtered nutrient-enriched seawater maintained at 20°C under a light/dark
cycle of 12/12 hours at a light intensity of 200 µmol m-2 s-1, while continuously aerating the culture solutions with air of either 350 or 1,000 ppm CO2 and measuring a number of physiological
parameters related to the diatom’s photosynthetic activity. They report that cell numbers of the alga “increased steadily throughout the light period and they were 1.6 and 2.1 times
higher after the 12 h light period for the alga grown at 350 and 1000 ppm CO2, respectively.” They also say that chlorophyll a concentrations “increased 4.4-and 5.4-fold during the middle
8 h of the light period for the alga grown at 350 and 1000 ppm CO2, respectively,” and that “the contents of cellular chlorophyll a were higher for the alga grown at 1000 ppm CO2 than
that at 350 ppm CO2.” In addition, they note that the initial slope of the light saturation curve of photosynthesis and the photochemical efficiency of photosystem II “increased with
increasing CO2, indicating that the efficiency of light-harvesting and energy conversion in photosynthesis were increased.” The end result of these several responses, in the words of
Chen and Gao, was that “S. costatum benefited from CO2 enrichment.” In another report of a study of marine microalgae that would appear to have enormous implications, Gordillo et al.
(2003) begin by noting that “one of the main queries for depicting future scenarios of evolution of atmospheric composition and temperature is whether an atmospheric CO2 increase stimulates
primary production, especially in aquatic plants.” Why do they say that? They say it because, as they put it, “aquatic primary producers account for about 50 percent of the total carbon
carbon fixation in the biosphere (Falkowski and Raven, 1997).” Although the question addressed by Gordillo et al. sounds simple enough, its answer is not straightforward. In many phytoplankton,
both freshwater and marine, photosynthesis appears to be saturated under current environmental conditions. Raven (1991), however, has suggested that those very same species, many of
which employ carbonconcentrating mechanisms, could well decrease the amount of energy they expend in this latter activity in a CO2-enriched world, which metabolic readjustment would
leave a larger proportion of their captured energy available for fueling enhanced growth. To explore this possibility, the four researchers studied various aspects of the growth response
of the microalgal chlorophyte Dunaliella viridis (which possesses a carbon concentrating mechanism and has been used as a model species for the study of inorganic carbon uptake) to atmospheric
CO2 enrichment. Specifically, they batch-cultured the chlorophyte, which is one of the most ubiquitous eukaryotic organisms in hypersaline environments, in 250-ml Perspex cylinders under
controlled laboratory conditions at high (5 mM) and low (0.5 mM) nitrate concentrations, while continuously aerating half of the cultures with ambient air of approximately 350 ppm CO2
and the other half with air of approximately 10,000 ppm CO2. In doing so, they discovered that atmospheric CO2 enrichment had little effect on dark respiration in both N treatments.
Likewise, it had little effect on photosynthesis in the low-N treatment. In the high-N treatment, the extra CO2 increased photosynthesis by 114 percent. In the case of biomass production,
the results were even more extreme: in the low-N treatment elevated CO2 had no effect at all, while in the high-N treatment it nearly tripled the cell density of the culture solution.
In discussing their findings, Gordillo et al. note that “it has long been debated whether phytoplankton species are growth-limited by current levels of CO2 in aquatic systems, i.e. whether
an increase in atmospheric CO2 could stimulate growth (Riebesell et al., 1993).” Their results clearly indicate that it can, as long as sufficient nitrogen is available. But that was
not all that Gordillo et al. learned. In the high-N treatment, where elevated CO2 greatly stimulated photosynthesis and biomass production, once the logarithmic growth phase had run
its course and equilibrium growth was attained, approximately 70 percent of the carbon assimilated by the chlorophyte was released to the water, while in the low-CO2 treatment only 35
percent was released. With respect to this suite of observations, Gordillo et al. say “the release of organic carbon to the external medium has been proposed as a mechanism for maintaining
the metabolic integrity of the cell (Ormerod, 1983),” and that “according to Wood and Van Valen (1990), organic carbon release would be a sink mechanism protecting the photosynthetic
apparatus from an overload of products that cannot be invested in growth or stored.” They additionally state that stores of photosynthetic products “are reduced to
avoid overload and produce
Biological Effects of Carbon Dioxide Enrichment 409 a high demand for photosynthates.” Under these conditions, they conclude that “the process would then divert assimilated C to either
the production of new biomass, or the release to the external medium once the culture conditions do not allow further exponential growth.” A second consequence of enhanced organic carbon
release in the face of atmospheric CO2 enrichment and sufficient N availability is that the internal C:N balance of the phytoplankton is maintained within a rather tight range. This
phenomenon has also been observed in the green seaweed Ulva rigida (Gordillo et al., 2001) and the cyanobacterium Spirulina platensis (Gordillo et al., 1999). Hence, what the study of
Gordillo et al. implies about the response of Dunaliella viridis to atmospheric CO2 enrichment may well be widely applicable to many, if not most, aquatic plants, not the least of which
may be the zooxanthellae that by this means (enhanced organic carbon release) could provide their coral hosts with the source of extra energy they need to continue building their skeletons
at a non-reduced rate in the face of the negative calcification pressure produced by the changes in seawater chemistry that have been predicted to result from the ongoing rise in the
air’s CO2 concentration. In light of these several observations, there would appear to be ample reason to be optimistic about the response of earth’s marine macroalgae to the ongoing
rise in the air’s CO2 content. Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/a/aquaticmicro
lgae.php. References Chen, X. and Gao, K. 2004. Characterization of diurnal photosynthetic rhythms in the marine diatom Skeletonema costatum grown in synchronous culture under ambient
and elevated CO2. Functional Plant Biology 31: 399-404. Falkowski, P.G. and Raven, J.A. 1997. Aquatic Photosynthesis. Blackwell Science, Massachusetts, USA. Gordillo, F.J.L., Jimenez,
C., Figueroa, F.L. and Niell, F.X. 1999. Effects of increased atmospheric CO2 and N supply on photosynthesis, growth and cell composition of the cyanobacterium Spirulina platensis (Arthrospira).
Journal of Applied Phycology 10: 461-469. Gordillo, F.J.L., Jimenez, C., Figueroa, F.L. and Niell, F.X. 2003. Influence of elevated CO2 and nitrogen supply on the carbon assimilation
performance and cell composition of the unicellular alga Dunaliella viridis. Physiologia Plantarum 119: 513-518. Gordillo, F.J.L., Niell, F.X. and Figueroa, F.L. 2001. Nonphotosynthetic
enhancement of growth by high CO2 level in the nitrophilic seaweed Ulva rigida C. Agardh (Chlorophyta). Planta 213: 64-70. Hein, M. and Sand-Jensen, K. 1997. CO2 increases oceanic primary
production. Nature 388: 988-990. Ormerod, J.G. 1983. The carbon cycle in aquatic ecosystems. In: Slater, J.H., Whittenbury, R. and Wimpeny, J.W.T. (Eds.) Microbes in Their Natural Environment.
Cambridge University Press, Cambridge, UK, pp. 463-482. Raven, J.A. 1991. Physiology of inorganic carbon acquisition and implications for resource use efficiency by marine phytoplankton:
Relation to increased CO2 and temperature. Plant, Cell and Environment 14: 774-794. Riebesell, U., Wolf-Gladrow, D.A. and Smetacek, V. 1993. Carbon dioxide limitation of marine phytoplankton
growth rates. Nature 361: 249-251. Wood, A.M. and Van Valen, L.M. 1990. Paradox lost? On the release of energy rich compounds by phytoplankton. Marine Microbial Food Webs 4: 103-116.
Wolf-Gladrow, D.A., Riebesell, U., Burkhardt, S. and Bijma, J. 1999. Direct effects of CO2 concentration on growth and isotopic composition of marine plankton. Tellus 51B: 461-476. 7.2.
Water Use Efficiency Another major consequence of atmospheric CO2 enrichment is that plants exposed to elevated levels of atmospheric CO2 generally do not open their leaf stomatal pores—through
which they take in carbon dioxide and give off water vapor—as wide as they do at lower CO2 concentrations and tend to produce fewer of these pores per unit area of leaf surface. Both
changes tend to reduce most plants’ rates of water loss by transpiration. The amount of carbon they gain per unit of water lost—or water-use efficiency— therefore typically rises, increasing
their ability to withstand drought. In this section, we explore the phenomena of water use efficiency as it pertains to agricultural, grassland, and woody species. Additional information
on this topic, including reviews water use efficiency not discussed here, can be found at http://www.co2science.org/subject/w/subject_w.php under the heading Water Use Efficiency.
Climate Change Reconsidered 410 7.2.1. Agricultural Species In the study of Serraj et al. (1999), soybeans grown at 700 ppm CO2 displayed 10 to 25 percent reductions in total water loss
while simultaneously exhibiting increases in dry weight of as much as 33 percent. Thus, elevated CO2 significantly increased the wateruse efficiencies of the studied plants. Likewise,
Garcia et al. (1998) determined that spring wheat grown at 550 ppm CO2 exhibited a water-use efficiency that was about one-third greater than that exhibited by plants grown at 370 ppm
CO2. Similarly, Hakala et al. (1999) reported that twice-ambient CO2 concentrations increased the water-use efficiency of spring wheat by 70 to 100 percent, depending on experimental
air temperature. In addition, Hunsaker et al. (2000) reported CO2-induced increases in wateruse efficiency for field-grown wheat that were 20 and 10 percent higher than those displayed
by ambiently grown wheat subjected to high and low soil nitrogen regimes, respectively. Also, pea plants grown for two months in growth chambers receiving atmospheric CO2 concentrations
of 700 ppm displayed an average water-use efficiency that was 27 percent greater than that exhibited by ambiently grown control plants (Gavito et al., 2000). In some cases, the water-use
efficiency increases caused by atmospheric CO2 enrichment are spectacularly high. De Luis et al. (1999), for example, demonstrated that alfalfa plants subjected to atmospheric CO2 concentrations
of 700 ppm had water-use efficiencies that were 2.6 and 4.1 times greater than those displayed by control plants growing at 400 ppm CO2 under water-stressed and wellwatered conditions,
respectively. Also, when grown at an atmospheric CO2 concentration of 700 ppm, a 2.7-fold increase in water-use efficiency was reported by Malmstrom and Field (1997) for oats infected
with the barley yellow dwarf virus. In addition to enhancing the water-use efficiencies of agricultural C3 crops, as reported in the preceding paragraphs, elevated CO2 also enhances
the water-use efficiencies of crops possessing alternate carbon fixation pathways. Maroco et al. (1999), for example, demonstrated that maize—a C4 crop— grown for 30 days at an atmospheric
CO2 concentration of 1,100 ppm exhibited an intrinsic water-use efficiency that was 225 percent higher than that of plants grown at 350 ppm CO2. In addition, Conley et al. (2001) reported
that a 200-ppm increase in the air’s CO2 content boosted the water-use efficiency of field-grown sorghum by 9 and 19 percent under well-watered and water-stressed conditions, respectively.
Also, Zhu et al. (1999) reported that pineapple—a CAM plant—grown at 700 ppm CO2 exhibited water-use efficiencies that were always significantly greater than those displayed by control
plants grown at 350 ppm CO2 over a range of growth temperatures. It is clear from the studies above that as the CO2 content of the air continues to rise, earth’s agricultural species
will respond favorably by exhibiting increases in water-use efficiency. It is likely that food and fiber production will increase on a worldwide basis, even in areas where productivity
is severely restricted due to limited availability of soil moisture. Additional information on this topic, including reviews of newer publications as they become available, can be found
at http://www.co2science.org/subject/w/wateruseag.php. References Conley, M.M., Kimball, B.A., Brooks, T.J., Pinter Jr., P.J., Hunsaker, D.J., Wall, G.W., Adams, N.R., LaMorte, R.L.,
Matthias, A.D., Thompson, T.L., Leavitt, S.W., Ottman, M.J., Cousins, A.B. and Triggs, J.M. 2001. CO2 enrichment increases water-use efficiency in sorghum. New Phytologist 151: 407-412.
De Luis, J., Irigoyen, J.J. and Sanchez-Diaz, M. 1999. Elevated CO2 enhances plant growth in droughted N2-fixing alfalfa without improving water stress. Physiologia Plantarum 107: 84-89.
Garcia, R.L., Long, S.P., Wall, G.W., Osborne, C.P., Kimball, B.A., Nie, G.Y., Pinter Jr., P.J., LaMorte, R.L. and Wechsung, F. 1998. Photosynthesis and conductance of spring-wheat leaves:
field response to continuous freeair atmospheric CO2 enrichment. Plant, Cell and Environment 21: 659-669. Gavito, M.E., Curtis, P.S., Mikkelsen, T.N. and Jakobsen, I. 2000. Atmospheric
CO2 and mycorrhiza effects on biomass allocation and nutrient uptake of nodulated pea (Pisum sativum L.) plants. Journal of Experimental Botany 52: 1931-1938. Hakala, K., Helio, R.,
Tuhkanen, E. and Kaukoranta, T. 1999. Photosynthesis and Rubisco kinetics in spring wheat and meadow fescue under conditions of simulated climate change with elevated CO2 and increased
temperatures. Agricultural and Food Science in Finland 8: 441-457. Hunsaker, D.J., Kimball. B.A., Pinter Jr., P.J., Wall, G.W., LaMorte, R.L., Adamsen, F.J., Leavitt, S.W., Thompson,
T.L., Matthias, A.D. and Brooks, T.J. 2000. CO2 enrichment and soil nitrogen effects on wheat
Biological Effects of Carbon Dioxide Enrichment 411 evapotranspiration and water use efficiency. Agricultural and Forest Meteorology 104: 85-105. Malmstrom, C.M. and Field, C.B. 1997.
Virus-induced differences in the response of oat plants to elevated carbon dioxide. Plant, Cell and Environment 20: 178-188. Maroco, J.P., Edwards, G.E. and Ku, M.S.B. 1999. Photosynthetic
acclimation of maize to growth under elevated levels of carbon dioxide. Planta 210: 115-125. Serraj, R., Allen Jr., L.H. and Sinclair, T.R. 1999. Soybean leaf growth and gas exchange
response to drought under carbon dioxide enrichment. Global Change Biology 5: 283-291. Zhu, J., Goldstein, G. and Bartholomew, D.P. 1999. Gas exchange and carbon isotope composition
of Ananas comosus in response to elevated CO2 and temperature. Plant, Cell and Environment 22: 999-1007. 7.2.2. Grassland Species In the study of Grunzweig and Korner (2001), model grasslands
representative of the semi-arid Negev of Israel, which were grown for five months at atmospheric CO2 concentrations of 440 and 600 ppm, exhibited cumulative water-use efficiencies that
were 17 and 28 percent greater, respectively, than control communities grown at 280 ppm CO2. Similarly, Szente et al. (1998) reported a doubling of the atmospheric CO2 concentration
increased the wateruse efficiency of two C3 grasses and two broad-leaved species common to the loess grasslands of Budapest by 72 and 266 percent, respectively. In addition, Leymarie
et al. (1999) calculated that twice-ambient CO2 concentrations increased the water-use efficiency of the herbaceous weedy species Arabidopsis thaliana by 41 and 120 percent under well-watered
and waterstressed conditions, respectively. Other CO2-induced increases in C3 plant water-use efficiency have been documented by Clark et al. (1999) for several New Zealand pasture species
and Roumet et al. (2000) for various Mediterranean herbs. Elevated CO2 also has been shown to substantially increase the water-use efficiency of C4 grassland species. Adams et al. (2000),
for example, reported that twice-ambient CO2 concentrations enhanced the daily water-use efficiency of a C4 tallgrass prairie in Kansas, USA, dominated by Andropogon gerardii. LeCain
and Morgan (1998) also documented enhanced water-use efficiencies for six different C4 grasses grown with twice-ambient CO2 concentrations. Likewise, Seneweera et al. (1998) reported
that a 650-ppm increase in the air’s CO2 content dramatically increased the water-use efficiency of the perennial C4 grass Panicum coloratum. As the air’s CO2 content continues to rise,
nearly all of earth’s grassland species—including both C3 and C4 plants—will likely experience increases in water-use efficiency. Concomitantly, the productivity of the world’s grasslands
should increase, even if available moisture decreases in certain areas. Moreover, such CO2-induced increases in water-use efficiency will likely allow grassland species to expand their
ranges into desert areas where they previously could not survive due to lack of sufficient moisture. Additional information on this topic, including reviews of newer publications as
they become available, can be found at http://www.co2science.org/subject/w/waterusegrass.php. References Adams, N.R., Owensby, C.E. and Ham, J.M. 2000. The effect of CO2 enrichment on
leaf photosynthetic rates and instantaneous water use efficiency of Andropogon gerardii in the tallgrass prairie. Photosynthesis Research 65: 121-129. Clark, H., Newton, P.C.D. and Barker,
D.J. 1999. Physiological and morphological responses to elevated CO2 and a soil moisture deficit of temperate pasture species growing in an established plant community. Journal of Experimental
Botany 50: 233-242. Grunzweig, J.M. and Korner, C. 2001. Growth, water and nitrogen relations in grassland model ecosystems of the semi-arid Negev of Israel exposed to elevated CO2.
Oecologia 128: 251-262. LeCain, D.R. and Morgan, J.A. 1998. Growth, gas exchange, leaf nitrogen and carbohydrate concentrations in NAD-ME and NADP-ME C4 grasses grown in elevated CO2.
Physiologia Plantarum 102: 297-306. Leymarie, J., Lasceve, G. and Vavasseur, A. 1999. Elevated CO2 enhances stomatal responses to osmotic stress and abscisic acid in Arabidopsis thaliana.
Plant, Cell and Environment 22: 301-308. Roumet, C., Garnier, E., Suzor, H., Salager, J.-L. and Roy, J. 2000. Short and long-term responses of whole-plant gas exchange to elevated CO2
in four herbaceous species. Environmental and Experimental Botany 43: 155-169. Seneweera, S.P., Ghannoum, O. and Conroy, J. 1998. High vapor pressure deficit and low soil water availability
Climate Change Reconsidered 412 enhance shoot growth responses of a C4 grass (Panicum coloratum cv. Bambatsi) to CO2 enrichment. Australian Journal of Plant Physiology 25: 287-292. Szente,
K., Nagy, Z. and Tuba, Z. 1998. Enhanced water use efficiency in dry loess grassland species grown at elevated air CO2 concentration. Photosynthetica 35: 637-640. 7.2.3. Woody Species
The effect of elevated atmospheric CO2 concentrations on the water-use efficiencies of trees is clearly positive, having been documented in a number of different single-species studies
of longleaf pine (Runion et al., 1999), red oak (Anderson and Tomlinson, 1998), scrub oak (Lodge et al., 2001), silver birch (Rey and Jarvis, 1998), beech (Bucher-Wallin et al., 2000;
Egli et al., 1998), sweetgum (Gunderson et al., 2002; Wullschleger and Norby, 2001), and spruce (Roberntz and Stockfors, 1998). Likewise, in a multi-species study performed by Tjoelker
et al. (1998), seedlings of quaking aspen, paper birch, tamarack, black spruce, and jack pine, which were grown at 580 ppm CO2 for three months, displayed water-use efficiencies that
were 40 to 80 percent larger than those exhibited by their respective controls grown at 370 ppm CO2. Similar results are also obtained when trees are exposed to different environmental
stresses. In a study conducted by Centritto et al. (1999), for example, cherry seedlings grown at twice-ambient levels of atmospheric CO2 displayed water-use efficiencies that were 50
percent greater than their ambient controls, regardless of soil moisture status. And in the study of Wayne et al. (1998), yellow birch seedlings grown at 800 ppm CO2 had water-use efficiencies
that were 52 and 94 percent greater than their respective controls, while simultaneously subjected to uncharacteristically low and high air temperature regimes. In some parts of the
world, perennial woody species have been exposed to elevated atmospheric CO2 concentrations for decades, due to their proximity to CO2-emitting springs and vents in the earth, allowing
scientists to assess the long-term effects of this phenomenon. In Venezuela, for example, the water-use efficiency of a common tree exposed to a lifetime atmospheric CO2 concentration
of approximately 1,000 ppm rose 2-fold and 19-fold during the local wet and dry seasons, respectively (Fernandez et al., 1998). Similarly, Bartak et al. (1999) reported that 30-year-old
Arbutus unedo trees growing in central Italy at a lifetime atmospheric CO2 concentration around 465 ppm exhibited water-use efficiencies that were 100 percent greater than control trees
growing at a lifetime CO2 concentration of 355 ppm. In addition, two species of oaks in central Italy that had been growing for 15 to 25 years at an atmospheric CO2 concentration ranging
from 500 to 1,000 ppm displayed “such marked increases in water-use efficiency under elevated CO2,” in the words of the scientists who studied them, that this phenomenon “might be of
great importance in Mediterranean environments in the perspective of global climate change” (Blaschke et al., 2001; Tognetti et al., 1998). Thus, the long-term effects of elevated CO2
concentrations on water-use efficiency are likely to persist and increase with increasing atmospheric CO2 concentrations. In some cases, scientists have looked to the past and determined
the positive impact the historic rise in the air’s CO2 content has already had on plant wateruse efficiency. Duquesnay et al. (1998), for example, used tree-ring data derived from beech
trees to determine that over the past century the water-use efficiency of such trees in north-eastern France increased by approximately 33 percent. Similarly, Feng (1999) used tree-ring
chronologies derived from a number of trees in western North America to calculate a 10 to 25 percent increase in tree water-use efficiency from 1750 to 1970, during which time the atmospheric
CO2 concentration rose by approximately 16 percent. In another study, Knapp et al. (2001) developed tree-ring chronologies from western juniper stands located in Oregon, USA, for the
past century, determining that growth recovery from drought was much greater in the latter third of their chronologies (1964-1998) than it was in the first third (1896-1930). In this
case, the authors suggested that the greater atmospheric CO2 concentrations of the latter period allowed the trees to more quickly recover from water stress. Finally, Beerling et al.
(1998) grew Gingko saplings at 350 and 650 ppm CO2 for three years, finding that elevated atmospheric CO2 concentrations reduced leaf stomatal densities to values comparable to those
measured on fossilized Gingko leaves dating back to the Triassic and Jurassic periods, implying greater water-use efficiencies for those times too. On another note, Prince et al. (1998)
demonstrated that rain-use efficiency, which is similar to water-use efficiency, slowly increased in the African Sahel from 1982 to 1990, while Nicholson et
Biological Effects of Carbon Dioxide Enrichment 413 al. (1998) observed neither an increase nor a decrease in this parameter from 1980 to 1995 for the central and western Sahel. In summary,
it is clear that as the CO2 content of the air continues to rise, nearly all of earth’s trees will respond favorably by exhibiting increases in wateruse efficiency. It is thus likely
that as time progresses, earth’s woody species will expand into areas where they previously could not exist due to limiting amounts of available moisture. Therefore, one can expect the
earth to become a greener biospheric body with greater carbon sequestering capacity as the atmospheric CO2 concentration continues to rise. Additional information on this topic, including
reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/w/waterusetrees.php. References Anderson, P.D. and Tomlinson, P.T. 1998. Ontogeny
affects response of northern red oak seedlings to elevated CO2 and water stress. I. Carbon assimilation and biomass production. New Phytologist 140: 477-491. Bartak, M., Raschi, A. and
Tognetti, R. 1999. Photosynthetic characteristics of sun and shade leaves in the canopy of Arbutus unedo L. trees exposed to in situ long-term elevated CO2. Photosynthetica 37: 1-16.
Beerling, D.J., McElwain, J.C. and Osborne, C.P. 1998. Stomatal responses of the ‘living fossil’ Ginkgo biloba L. to changes in atmospheric CO2 concentrations. Journal of Experimental
Botany 49: 1603-1607. Blaschke, L., Schulte, M., Raschi, A., Slee, N., Rennenberg, H. and Polle, A. 2001. Photosynthesis, soluble and structural carbon compounds in two Mediterranean
oak species (Quercus pubescens and Q. ilex) after lifetime growth at naturally elevated CO2 concentrations. Plant Biology 3: 288-298. Bucher-Wallin, I.K., Sonnleitner, M.A., Egli, P.,
Gunthardt-Goerg, M.S., Tarjan, D., Schulin, R. and Bucher, J.B. 2000. Effects of elevated CO2, increased nitrogen deposition and soil on evapotranspiration and water use efficiency of
spruce-beech model ecosystems. Phyton 40: 49-60. Centritto, M., Lee, H.S.J. and Jarvis, P.G. 1999. Interactive effects of elevated [CO2] and drought on cherry (Prunus avium) seedlings.
I. Growth, whole-plant water use efficiency and water loss. New Phytologist 141: 129-140. Duquesnay, A., Breda, N., Stievenard, M. and Dupouey, J.L. 1998. Changes of tree-ring d13C and
water-use efficiency of beech (Fagus sylvatica L.) in north-eastern France during the past century. Plant, Cell and Environment 21: 565-572. Egli, P., Maurer, S., Gunthardt-Goerg, M.S.
and Korner, C. 1998. Effects of elevated CO2 and soil quality on leaf gas exchange and aboveground growth in beech-spruce model ecosystems. New Phytologist 140: 185-196. Feng, X. 1999.
Trends in intrinsic water-use efficiency of natural trees for the past 100-200 years: A response to atmospheric CO2 concentration. Geochimica et Cosmochimica Acta 63: 1891-1903. Fernandez,
M.D., Pieters, A., Donoso, C., Tezara, W., Azuke, M., Herrera, C., Rengifo, E. and Herrera, A. 1998. Effects of a natural source of very high CO2 concentration on the leaf gas exchange,
xylem water potential and stomatal characteristics of plants of Spatiphylum cannifolium and Bauhinia multinervia. New Phytologist 138: 689-697. Gunderson, C.A., Sholtis, J.D., Wullschleger,
S.D., Tissue, D.T., Hanson, P.J. and Norby, R.J. 2002. Environmental and stomatal control of photosynthetic enhancement in the canopy of a sweetgum (Liquidambar styraciflua L.) plantation
during 3 years of CO2 enrichment. Plant, Cell and Environment 25: 379-393. Knapp, P.A., Soule, P.T. and Grissino-Mayer, H.D. 2001. Post-drought growth responses of western juniper (Junipers
occidentalis var. occidentalis) in central Oregon. Geophysical Research Letters 28: 2657-2660. Lodge, R.J., Dijkstra, P., Drake, B.G. and Morison, J.I.L. 2001. Stomatal acclimation to
increased CO2 concentration in a Florida scrub oak species Quercus myrtifolia Willd. Plant, Cell and Environment 24: 77-88. Nicholson, S.E., Tucker, C.J. and Ba, M.B. 1998. Desertification,
drought, and surface vegetation: An example from the West African Sahel. Bulletin of the American Meteorological Society 79: 815-829. Prince, S.D., Brown De Colstoun, E. and Kravitz,
L.L. 1998. Evidence from rain-use efficiencies does not indicate extensive Sahelian desertification. Global Change Biology 4: 359-374. Rey, A. and Jarvis, P.G. 1998. Long-term photosynthetic
acclimation to increased atmospheric CO2 concentration in young birch (Betula pendula) trees. Tree Physiology 18: 441-450. Roberntz, P. and Stockfors, J. 1998. Effects of elevated CO2
concentration and nutrition on net photosynthesis, stomatal conductance and needle respiration of field-grown Norway spruce trees. Tree Physiology 18: 233-241. Runion, G.B., Mitchell,
R.J., Green, T.H., Prior, S.A., Rogers, H.H. and Gjerstad, D.H. 1999. Longleaf pine
Climate Change Reconsidered 414 photosynthetic response to soil resource availability and elevated atmospheric carbon dioxide. Journal of Environmental Quality 28: 880-887. Tjoelker,
M.G., Oleksyn, J. and Reich, P.B. 1998. Seedlings of five boreal tree species differ in acclimation of net photosynthesis to elevated CO2 and temperature. Tree Physiology 18: 715-726.
Tognetti, R., Johnson, J.D., Michelozzi, M. and Raschi, A. 1998. Response of foliar metabolism in mature trees of Quercus pubescens and Quercus ilex to long-term elevated CO2. Environmental
and Experimental Botany 39: 233-245. Wayne, P.M., Reekie, E.G. and Bazzaz, F.A. 1998. Elevated CO2 ameliorates birch response to high temperature and frost stress: implications for modeling
climate-induced geographic range shifts. Oecologia 114: 335-342. Wullschleger, S.D. and Norby, R.J. 2001. Sap velocity and canopy transpiration in a sweetgum stand exposed to freeair
CO2 enrichment (FACE). New Phytologist 150: 489-498. 7.3. Amelioration of Environmental Stresses Atmospheric CO2 enrichment has been shown to help ameliorate the detrimental effects
of several environmental stresses on plant growth and development, including disease, herbivory (predation by insects), shade (caused by increased cloudiness), ozone (a common air pollutant),
low temperatures, nitrogen deficiency, UV-B radiation, and water stress. In this section we survey research on each of these types of stress. Additional information on this topic, including
reviews on stresses not discussed here, can be found at http://www.co2science.org/subject/g/subject_g.php under the heading Growth Response to CO2 with Other Variables. 7.3.1. Disease
According to the IPCC, CO2-induced global warming will increase the risk of plant disease outbreaks, resulting in negative consequences for food, fiber, and forestry across all world
regions (IPCC, 2007-II). But it appears the IPCC has omitted the results of realworld observations that contradict this forecast. Chakraborty and Datta (2003) note there are a number
of CO2-induced changes in plant physiology, anatomy and morphology that have been implicated in increased plant resistance to disease and that “can potentially enhance host resistance
at elevated CO2.” Among these phenomena they list “increased net photosynthesis allowing mobilization of resources into host resistance (Hibberd et al., 1996a.); reduced stomatal density
and conductance (Hibberd et al., 1996b); greater accumulation of carbohydrates in leaves; more waxes, extra layers of epidermal cells and increased fibre content (Owensby, 1994); production
of papillae and accumulation of silicon at penetration sites (Hibberd et al., 1996a); greater number of mesophyll cells (Bowes, 1993); and increased biosynthesis of phenolics (Hartley
et al., 2000), among others.” Malmstrom and Field (1997) grew individual oat plants for two months in pots placed within phytocells maintained at atmospheric CO2 concentrations of 350
and 700 ppm, while they infected one-third of the plants with the barley yellow dwarf virus (BYDV), which plagues more than 150 plant species worldwide, including all major cereal crops.
Over the course of their study, they found that elevated CO2 stimulated rates of net photosynthesis in all plants, regardless of pathogen infection. However, the greatest percentage
increase occurred in diseased individuals (48 percent vs. 34 percent). Moreover, atmospheric CO2 enrichment decreased stomatal conductance by 50 percent in infected plants but by only
34 percent in healthy ones, which led to a CO2-induced doubling of the instantaneous water-use efficiency of the healthy plants, but an increase of fully 2.7-fold in the diseased plants.
Last, after 60 days of growth under these conditions, they determined that the extra CO2 increased total plant biomass by 36 percent in infected plants, but by only 12 percent in healthy
plants. In addition, while elevated CO2 had little effect on root growth in the healthy plants, it increased root biomass in the infected plants by up to 60 percent. Consequently, it
can be appreciated that as the CO2 content of the air continues to rise, its many positive effects will likely offset some, if not most, of the negative effects of the destructive BYDV.
Quoting Malmstrom and Field with respect to two specific examples, they say in their concluding remarks that CO2 enrichment “may reduce losses of infected plants to drought” and “may
enable diseased plants to compete better with healthy neighbors.”
Biological Effects of Carbon Dioxide Enrichment
415 Tiedemann and Firsching (2000) grew spring wheat plants from germination to maturity in controlled-environment chambers maintained at ambient (377 ppm) and elevated (612 ppm) concentrations
of atmospheric CO2 and at ambient (20 ppb) and elevated (61 ppb) concentrations of ozone (and combinations thereof), the latter of which gases is typically toxic to most plants. In addition,
half of the plants in each treatment were inoculated with a leaf rust-causing fungus. Under these conditions, the elevated CO2 increased the photosynthetic rates of the diseased plants
by 20 and 42 percent at the ambient and elevated ozone concentrations, respectively. It also enhanced the yield of the infected plants, increasing it by 57 percent, even in the presence
of high ozone concentrations. Jwa and Walling (2001) grew tomato plants hydroponically for eight weeks in controlledenvironment chambers maintained at atmospheric CO2 concentrations
of 350 and 700 ppm. In addition, at week five of the study, half of all plants growing in each CO2 concentration were infected with a fungal pathogen that attacks plant roots and induces
a water stress that decreases growth and yield. At the end of the study, they found that the pathogenic infection had reduced total plant biomass by nearly 30 percent at both atmospheric
CO2 concentrations. However, the elevated CO2 had increased the total biomass of the healthy and diseased plants by the same amount (+30 percent), with the result that the infected tomato
plants grown at 700 ppm CO2 had biomass values that were essentially identical to those of the healthy tomato plants grown at 350 ppm CO2. Thus, the extra CO2 completely counterbalanced
the negative effect of the pathogenic infection on overall plant productivity. Chakraborty and Datta (2003) studied the aggressiveness of the fungal anthracnose pathogen Colletotrichum
gloeosporioides by inoculating two isolates of the pathogen onto two cultivars of the tropical pasture legume Stylosanthes scabra (Fitzroy, which is susceptible to the fungal pathogen,
and Seca, which is more resistant) over 25 sequential infection cycles in controlled-environment chambers filled with air of either 350 or 700 ppm CO2. By these means they determined
that the aggressiveness of the pathogen was reduced at the twice-ambient level of atmospheric CO2, where aggressiveness is defined as “a property of the pathogen reflecting the relative
amount of damage caused to the host without regard to resistance genes (Shaner et al., 1992).” As they describe it, “at twice-ambient CO2 the overall level of aggressiveness of the two
[pathogen] isolates was significantly reduced on both cultivars.” Simultaneously, however, pathogen fecundity was found to increase at twice-ambient CO2. Of this finding, Chakraborty
and Datta report that their results “concur with the handful of studies that have demonstrated increased pathogen fecundity at elevated CO2 (Hibberd et al., 1996a; Klironomos et al.,
1997; Chakraborty et al., 2000).” How this happened in the situation they investigated, according to Chakraborty and Datta, is that the overall increase in fecundity at high CO2 “is
a reflection of the altered canopy environment,” wherein “the 30% larger S. scabra plants at high CO2 (Chakraborty et al., 2000) makes the canopy microclimate more conducive to anthracnose
development.” In view of these opposing changes in pathogen behavior at elevated levels of atmospheric CO2, it is difficult to know the outcome of atmospheric CO2 enrichment for this
specific pathogen-host relationship. More research, especially under realistic field conditions, will be needed to clarify the situation; and, of course, different results are likely
to be observed for different pathogen-host associations. What is more, results could also differ under different climatic conditions. Nevertheless, the large number of ways in which
elevated CO2 has been demonstrated to increase plant resistance to pathogen attack gives us reason to believe that plants will gain the advantage as as the air’s CO2 content continues
to climb in the years ahead. Another study that fuels this optimism was conducted by Parsons et al. (2003), who grew twoyear-old saplings of paper birch and three-year-old saplings of
sugar maple in well-watered and fertilized pots from early May until late August in glasshouse rooms maintained at either 400 or 700 ppm CO2. In these circumstances, the whole-plant
biomass of paper birch was increased by 55 percent in the CO2-enriched portions of the glasshouse, while that of sugar maple was increased by 30 percent. Also, concentrations of condensed
tannins were increased by 27 percent in the paper birch (but not the sugar maple) saplings grown in the CO2-enriched air; in light of this finding, Parsons et al. conclude that “the
higher condensed tannin concentrations that were present in the birch fine roots may offer these tissues greater protection against soil-borne pathogens and herbivores.” Within this
context, it is interesting to note that Parsons et al. report that CO2-induced increases in fine root concentrations of total phenolics and
Climate Change Reconsidered 416 condensed tannins have also been observed in warm temperate conifers by King et al. (1997), Entry et al. (1998), Gebauer et al. (1998), and Runion et
al. (1999), as well as in cotton by Booker (2000). In another intriguing study, Gamper et al. (2004) begin by noting that arbuscular mycorrhizal fungi (AMF) are expected to modulate
plant responses to elevated CO2 by “increasing resistance/tolerance of plants against an array of environmental stressors (Smith and Read, 1997).” In investigating this subject in a
set of experiments conducted over a seven-year period of free-air CO2-enrichment on two of the world’s most extensively grown cool-season forage crops (Lolium perenne and Trifolium repens)
at the Swiss free-air CO2 enrichment (FACE) facility near Zurich, they determined that “at elevated CO2 and under [two] N treatments, AMF root colonization of both host plant species
was increased,” and that “colonization levels of all three measured intraradical AMF structures (hyphae, hyphae, arbuscules and vesicles) tended to be higher.” Hence, they concluded
that these CO2-induced benefits may lead to “increased protection against pathogens and/or herbivores.” Pangga et al. (2004) grew well-watered and fertilized seedlings of a cultivar
(Fitzroy) of the pencilflower (Stylosanthes scabra)—an important legume crop that is susceptible to anthracnose disease caused by Colletotrichum gloeosporioides (Penz.) Penz. & Sacc.—within
a controlled-environment facility maintained at atmospheric CO2 concentrations of either 350 or 700 ppm, where they inoculated six-, nine-and 12-week-old plants with conidia of C. gloeosporioides.
Then, 10 days after inoculation, they counted the anthracnose lesions on the plants and classified them as either resistant or susceptible. Adherence to this protocol revealed, in their
words, that “the mean number of susceptible, resistant, and total lesions per leaf averaged over the three plant ages was significantly (P<0.05) greater at 350 ppm than at 700 ppm CO2,
reflecting the development of a level of resistance in susceptible cv. Fitzroy at high CO2.” In fact, with respect to the plants inoculated at 12 weeks of age, they say that those grown
“at 350 ppm had 60 and 75% more susceptible and resistant lesions per leaf, respectively, than those [grown] at 700 ppm CO2.” In terms of infection efficiency (IE), the Australian scientists
say their work “clearly shows that at 350 ppm overall susceptibility of the canopy increases with increasing age because more young leaves are produced on secondary and tertiary branches
of the more advanced plants.” However, they report that “at 700 ppm CO2, IE did not increase with increasing plant age despite the presence of many more young leaves in the enlarged
canopy,” which finding, in their words, “points to reduced pathogen efficiency or an induced partial resistance to anthracnose in Fitzroy at 700 ppm CO2.” Consequently, as the air’s
CO2 content continues to rise, it would appear that the Fitzroy cultivar of the pasture legume Stylosanthes scabra will acquire a greater intrinsic resistance to the devastating anthracnose
disease. McElrone et al. (2005) “assessed how elevated CO2 affects a foliar fungal pathogen, Phyllosticta minima, of Acer rubrum [red maple] growing in the understory at the Duke Forest
free-air CO2 enrichment experiment in Durham, North Carolina, USA ... in the 6th, 7th, and 8th years of the CO2 exposure.” Surveys conducted in those years, in their words, “revealed
that elevated CO2 [to 200 ppm above ambient] significantly reduced disease incidence, with 22%, 27% and 8% fewer saplings and 14%, 4%, and 5% fewer leaves infected per plant in the three
consecutive years, respectively.” In addition, they report that the elevated CO2 “also significantly reduced disease severity in infected plants in all years (e.g. mean lesion area reduced
35%, 50%, and 10% in 2002, 2003, and 2004, respectively).” What underlying mechanism or mechanisms produced these beneficent consequences? Thinking it could have been a direct deleterious
effect of elevated CO2 on the fungal pathogen, McElrone et al. performed some side experiments in controlledenvironment chambers. However, they found that the elevated CO2 benefited
the fungal pathogen as well as the red maple saplings, observing that “exponential growth rates of P. minima were 17% greater under elevated CO2.” And they obtained similar results when
they repeated the in vitro growth analysis two additional times in different growth chambers. Taking another tack when “scanning electron micrographs verified that conidia germ tubes
of P. minima infect A. rubrum leaves by entering through the stomata,” the researchers turned their attention to the pathogen’s mode of entry into the saplings’ foliage. In this investigation
they found that both stomatal size and density were unaffected by atmospheric CO2 enrichment, but that “stomatal conductance was reduced by 21-36% under elevated CO2, providing smaller
openings for infecting germ tubes.” In addition, they concluded that reduced disease severity under elevated CO2 was also likely due to altered leaf chemistry, as elevated CO2
Biological Effects of Carbon Dioxide Enrichment 417 increased total leaf phenolic concentrations by 15 percent and tannin concentrations by 14 percent. Because the phenomena they found
to be important in reducing the amount and severity of fungal pathogen infection (leaf spot disease) of red maple have been demonstrated to be operative in most other plants as well,
McElrone et al. say these CO2-enhanced leaf defensive mechanisms “may be prevalent in many plant pathosystems where the pathogen targets the stomata.” Indeed, they state that their results
“provide concrete evidence for a potentially generalizable mechanism to predict disease outcomes in other pathosystems under future climatic conditions.” Matros et al. (2006) grew tobacco
plants (Nicotiana tabacum L.) in 16-cm-diameter pots filled with quartz sand in controlled-climate chambers maintained at either 350 or 1,000 ppm CO2 for a period of eight weeks, where
they were irrigated daily with a complete nutrient solution containing either 5 or 8 mM NH4NO3. In addition, some of the plants in each treatment were mechanically infected with the
potato virus Y (PVY) when they were six weeks old. Then, at the end of the study, the plants were harvested and a number of their chemical constitutes identified and quantified. This
work revealed, in the researchers words, that “plants grown at elevated CO2 and 5 mM NH4NO3 showed a marked and significant decrease in content of nicotine in leaves as well as in roots,”
while at 8 mM NH4NO3 the same was found to be true of upper leaves but not of lower leaves and roots. With respect to the PVY part of the study, they further report that the “plants
grown at high CO2 showed a markedly decreased spread of virus.” Both of these findings would likely be considered beneficial by most people, as potato virus Y is an economically important
virus that infects many crops and ornamental plants throughout the world, while nicotine is nearly universally acknowledged to have significant negative impacts on human health (Topliss
et al., 2002). Braga et al. (2006) conducted three independent experiments where they grew well-watered soybean (Glycine max (L.) Merr) plants of two cultivars (IAC-14, susceptible to
stem canker disease, and IAC-18, resistant to stem canker disease) from seed through the cotyledon stage in five-liter pots placed within open-top chambers maintained at atmospheric
CO2 concentrations of either 360 or 720 ppm in a glasshouse, while they measured various plant properties and processes, concentrating on the production of glyceollins (the major phytoalexins,
or anti-microbial compounds, produced in soybeans) in response to the application of ß-glucan elicitor (derived from mycelial walls of Phythophthora sojae) to carefully created and replicated
wounds in the surfaces of several soybean cotyledons. In doing so, they found that the IAC-14 cultivar did not exhibit a CO2-induced change in glyceollin production in response to elicitation—as
Braga et al. had hypothesized would be the case, since this cultivar is susceptible susceptible to stem canker disease—but they found that the IAC-18 cultivar (which has the potential
to resist the disease to varying degrees) experienced a 100 percent CO2-induced increase in the amount of glyceollins produced after elicitation, a response the researchers described
as remarkable. As for its significance, Braga et al. say the CO2-induced response they observed “may increase the potential of the soybean defense since infection at early stages of
plant development, followed by a long incubation period before symptoms appear, is characteristic of the stem canker disease cycle caused by Dpm [Diaporthe phaseolorum (Cooke & Ellis)
Sacc. f. sp. meridionalis Morgan-Jones].” They say the response they observed “indicates that raised CO2 levels forecasted for next decades may have a real impact on the defensive chemistry
of the cultivars.” Last, in a study conducted within the BioCON (Biodiversity, Carbon dioxide, and Nitrogen effects on ecosystem functioning) FACE facility located at the Cedar Creek
Natural History Area in east-central Minnesota, USA, Strengbom and Reich (2006) evaluated the effects of an approximate 190-ppm increase in the air’s daytime CO2 concentration on leaf
photosynthetic rates of stiff goldenrod (Solidago rigida) growing in monoculture for two full seasons, together with its concomitant effects on the incidence and severity of leaf spot
disease. Although they found that elevated CO2 had no significant effect on plant photosynthetic rate in their study, they report that “both disease incidence and severity were lower
on plants grown under elevated CO2.” More specifically, they found that “disease incidence was on average more than twice as high [our italics] under ambient as under elevated CO2,”
and that “disease severity (proportion of leaf area with lesions) was on average 67% lower under elevated CO2 compared to ambient conditions.” In discussing their results, Strengbom
and Reich say the “indirect effects from elevated CO2, i.e., lower disease incidence, had a stronger effect on realized photosynthetic rate than the direct effect of higher CO2,” which
as noted above was negligible in their
Climate Change Reconsidered 418 study. They conclude “it may be necessary to consider potential changes in susceptibility to foliar diseases to correctly estimate the effects on plant
photosynthetic rates of elevated CO2.” In addition, they note that the plants grown in CO2-enriched air had lower leaf nitrogen concentrations than the plants grown in ambient air, as
is often observed in studies of this type; and they say that their results “are, thus, also in accordance with other studies that have found reduced pathogen performance following reduced
nitrogen concentration in plants grown under elevated CO2 (Thompson and Drake, 1994).” What is more, they conclude that their results are “also in accordance with studies that have found
increased [disease] susceptibility following increased nitrogen concentration of host plants (Huber and Watson, 1974; Nordin et al., 1998; Strengbom et al., 2002).” It is possible, therefore,
that the ongoing rise in the air’s CO2 content may help many plants of the future reduce the deleterious impacts of various pathogenic fungal diseases that currently beset them, thereby
enabling them to increase their productivities above and beyond what is typically provided by the more direct growth stimulation resulting from the aerial fertilization effect of elevated
atmospheric CO2 concentrations. In summation, the bulk of the available data shows atmospheric CO2 enrichment asserts its greatest positive influence on infected as opposed to healthy
plants. Moreover, it would appear that elevated CO2 has the ability to significantly ameliorate the deleterious effects of various stresses imposed upon plants by numerous pathogenic
invaders. Consequently, as the atmosphere’s CO2 concentration continues its upward climb, earth’s vegetation should be increasingly better equipped to successfully deal with pathogenic
organisms and the damage they have traditionally done to mankind’s crops, as well as to the plants that sustain the rest of the planet’s animal life. Additional information on this topic,
including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/g/disease.php References Booker, F.L. 2000. Influence of carbon dioxide
enrichment, ozone and nitrogen fertilization on cotton (Gossypium hirsutum L.) leaf and root composition. Plant, Cell and Environment 23: 573-583. Bowes, G. 1993. Facing the inevitable:
Plants and increasing atmospheric CO2. Annual Review of Plant Physiology and Plant Molecular Biology 44: 309-332. Braga, M.R., Aidar, M.P.M., Marabesi, M.A. and de Godoy, J.R.L. 2006.
Effects of elevated CO2 on the phytoalexin production of two soybean cultivars differing in the resistance to stem canker disease. Environmental and Experimental Botany 58: 85-92. Chakraborty,
S. and Datta, S. 2003. How will plant pathogens adapt to host plant resistance at elevated CO2 under a changing climate? New Phytologist 159: 733-742. Chakraborty, S., Pangga, I.B.,
Lupton, J., Hart, L., Room, P.M. and Yates, D. 2000. Production and dispersal of Colletotrichum gloeosporioides spores on Stylosanthes scabra under elevated CO2. Environmental Pollution
108: 381-387. Entry, J.A., Runion, G.B., Prior, S.A., Mitchell, R.J. and Rogers, H.H. 1998. Influence of CO2 enrichment and nitrogen fertilization on tissue chemistry and carbon allocation
in longleaf pine seedlings. Plant and Soil 200: 3-11. Gamper, H., Peter, M., Jansa, J., Luscher, A., Hartwig, U.A. and Leuchtmann, A. 2004. Arbuscular mycorrhizal fungi benefit from
7 years of free air CO2 enrichment in well-fertilized grass and legume monocultures. Global Change Biology 10: 189-199. Gebauer, R.L., Strain, B.R. and Reynolds, J.F. 1998. The effect
of elevated CO2 and N availability on tissue concentrations and whole plant pools of carbon-based secondary compounds in loblolly pine. Oecologia 113: 29-36. Hartley, S.E., Jones, C.G.
and Couper, G.C. 2000. Biosynthesis of plant phenolic compounds in elevated atmospheric CO2. Global Change Biology 6: 497-506. Hibberd, J.M., Whitbread, R. and Farrar, J.F. 1996a. Effect
of elevated concentrations of CO2 on infection of barley by Erysiphe graminis. Physiological and Molecular Plant Pathology 48: 37-53. Hibberd, J.M., Whitbread, R. and Farrar, J.F. 1996b.
Effect of 700 µmol per mol CO2 and infection of powdery mildew on the growth and partitioning of barley. New Phytologist 134: 309-345. Huber, D.M. and Watson, R.D. 1974. Nitrogen form
and plant disease. Annual Reviews of Phytopathology 12: 139-155. IPCC. 2007-II. Climate Change 2007: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Fourth
Assessment Report of the Intergovernmental Panel on Climate Change, M.L. Parry, O.F. Canziani, J.P.
Biological Effects of Carbon Dioxide Enrichment 419 Palutikof, P.J. van der Linden and C.E. Hanson (Eds.) Cambridge University Press, Cambridge, UK. Jwa, N.-S. and Walling, L.L. 2001.
Influence of elevated CO2 concentration on disease development in tomato. New Phytologist 149: 509-518. King, J.S., Thomas, R.B. and Strain, B.R. 1997. Morphology and tissue quality
of seedling root systems of Pinus taeda and Pinus ponderosa as affected by varying CO2, temperature, and nitrogen. Plant and Soil 195: 107-119. Klironomos, J.N., Rillig, M.C., Allen,
M.F., Zak, D.R., Kubiske, M. and Pregitzer, K.S. 1997. Soil fungalarthropod responses to Populus tremuloides grown under enriched atmospheric CO2 under field conditions. Global Change
Biology 3: 473-478. Malmstrom, C.M. and Field, C.B. 1997. Virus-induced differences in the response of oat plants to elevated carbon dioxide. Plant, Cell and Environment 20: 178-188.
Matros, A., Amme, S., Kettig, B., Buck-Sorlin, G.H., Sonnewald, U. and Mock, H.-P. 2006. Growth at elevated CO2 concentrations leads to modified profiles of secondary metabolites in
tobacco cv. SamsunNN and to increased resistance against infection with potato virus Y. Plant, Cell and Environment 29: 126-137. McElrone, A.J., Reid, C.D., Hoye, K.A., Hart, E. and
Jackson, R.B. 2005. Elevated CO2 reduces disease incidence and severity of a red maple fungal pathogen via changes in host physiology and leaf chemistry. Global Change Biology 11: 1828-1836.
Nordin, A., Nasholm, T. and Ericson, L. 1998. Effects of simulated N deposition on understorey vegetation of a boreal coniferous forest. Functional Ecology 12: 691-699. Owensby, C.E.
1994. Climate change and grasslands: ecosystem-level responses to elevated carbon dioxide. Proceedings of the XVII International Grassland Congress. Palmerston North, New Zealand: New
Zealand Grassland Association, pp. 1119-1124. Pangga, I.B., Chakraborty, S. and Yates, D. 2004. Canopy size and induced resistance in Stylosanthes scabra determine anthracnose severity
at high CO2. Phytopathology 94: 221-227. Parsons, W.F.J., Kopper, B.J. and Lindroth, R.L. 2003. Altered growth and fine root chemistry of Betula papyrifera and Acer saccharum under elevated
CO2. Canadian Journal of Forest Research 33: 842-846. Runion, G.B., Entry, J.A., Prior, S.A., Mitchell, R.J. and Rogers, H.H. 1999. Tissue chemistry and carbon allocation in seedlings
of Pinus palustris subjected to elevated atmospheric CO2 and water stress. Tree Physiology 19: 329-335. Shaner, G., Stromberg, E.L., Lacy, G.H., Barker, K.R. and Pirone, T.P. 1992. Nomenclature
and concepts of aggressiveness and virulence. Annual Review of Phytopathology 30: 47-66. Smith, S.E. and Read, D.J. 1997. Mycorrhizal Symbioses. Academic Press, London, UK. Strengbom,
J. and Reich, P.B. 2006. Elevated [CO2] and increased N supply reduce leaf disease and related photosynthetic impacts on Solidago rigida. Oecologia 149: 519-525. Strengbom, J., Nordin,
A., Nasholm, T. and Ericson, L. 2002. Parasitic fungus mediates change in nitrogenexposed boreal forest vegetation. Journal of Ecology 90: 61-67. Thompson, G.B. and Drake, B.G. 1994.
Insect and fungi on a C3 sedge and a C4 grass exposed to elevated atmospheric CO2 concentrations in open-top chambers in the field. Plant, Cell and Environment 17: 1161-1167. Tiedemann,
A.V. and Firsching, K.H. 2000. Interactive effects of elevated ozone and carbon dioxide on growth and yield of leaf rust-infected versus non-infected wheat. Environmental Pollution 108:
357-363. Topliss, J.G., Clark, A.M., Ernst, E. et al. 2002. Natural and synthetic substances related to human health. Pure and Applied Chemistry 74: 1957-1985. 7.3.2. Herbivory Insect
pests have greatly vexed earth’s plants in the past and will likely continue to do so in the future. It is possible, however, that the ongoing rise in the atmosphere’s CO2 content may
affect this phenomenon, for better or for worse. In this section we review the results of several studies that have addressed this subject as it applies to herbaceous and woody plants.
Additional information on this topic, including reviews on herbivory not discussed here, can be found at http://www.co2science.org/subject/h/subject_h.php under the heading Herbivory.
7.3.2.1. Herbaceous Plants Kerslake et al. (1998) grew five-year-old heather (Calluna vulgaris) plants collected from a Scottish moor in open-top chambers maintained at atmospheric CO2
concentrations of 350 and 600 ppm. At two different times during the study, larvae of the
Climate Change Reconsidered 420 destructive winter moth Operophtera brumata— whose outbreaks periodically cause extensive damage to heather moorland—were allowed to feed upon current-year
shoots. Feeding upon the high-CO2-grown foliage did not affect larval growth rates, development, or final pupal weights; neither was moth survivorship significantly altered. The authors
concluded that their study “provides no evidence that increasing atmospheric CO2 concentrations will affect the potential for outbreak of Operophtera brumata on this host.” What it did
show, however, was a significant CO2-induced increase in heather water use efficiency. Newman et al. (1999) inoculated tall fescue (Festuca arundinacea) plants growing in open-top chambers
maintained at atmospheric CO2 concentrations of 350 and 700 ppm with bird cherryoat aphids (Rhopalosiphum padi). After nine weeks, the plants growing in the CO2-enriched air had experienced
a 37 percent increase in productivity and were covered with many fewer aphids than the plants growing in ambient air. Goverde et al. (1999) collected four genotypes of Lotus corniculatus
near Paris and grew them in controlled environment chambers kept at atmospheric CO2 concentrations of 350 and 700 ppm. Larvae of the Common Blue Butterfly (Polyommatus icarus) that were
allowed to feed upon the foliage produced in the CO2-enriched air ate more, grew larger, and experienced shorter development times than larvae feeding on the foliage produced in the
ambient-air treatment, suggesting that this butterfly species will likely become more robust and plentiful as the air’s CO2 content continues to rise. Brooks and Whittaker (1999) removed
grassland monoliths containing eggs of the xylem-feeding spittlebug Neophilaenus lineatus from the UK’s Great Dun Fell in Cumbria and placed them in glasshouses maintained at atmospheric
CO2 concentrations of 350 and 600 ppm for two years. Survival of the spittlebug’s nymphal states was reduced by 24 percent in both of the generations produced in their experiment, suggesting
that this particular insect will likely cause less tissue damage to the plants of this species-poor grassland in a CO2-enriched world of the future. Joutei et al. (2000) grew bean (Phaseolus
vulgaris) plants in controlled environments kept at atmospheric CO2 concentrations of 350 and 700 ppm, to which they introduced the destructive agricultural mite Tetranychus urticae,
observing that female mites produced 34 percent and 49 percent less offspring in the CO2-enriched chambers in their first and second generations, respectively. This CO2-induced reduction
in the reproductive success of this invasive insect, which negatively affects more than 150 crop species worldwide, bodes well for mankind’s ability to grow the food we will need to
feed our growing numbers in the years ahead. Peters et al. (2000) fed foliage derived from FACE plots of calcareous grasslands of Switzerland (maintained at 350 and 650 ppm CO2) to terrestrial
slugs, finding they exhibited no preference with respect to the CO2 treatment from which the foliage was derived. Also, in a study that targeted no specific insect pest, Castells et
al. (2002) found that a doubling of the air’s CO2 content enhanced the total phenolic concentrations of two Mediterranean perennial grasses (Dactylis glomerata and Bromus erectus) by
15 percent and 87 percent, respectively, which compounds tend to enhance plant defensive and resistance mechanisms to attacks by both herbivores and pathogens. Coviella and Trumbel (2000)
determined that toxins produced by Bacillus thuringiensis (Bt), which are applied to crop plants by spraying as a means of combating various crop pests, were “more efficacious” in cotton
grown in an elevated CO2 environment than in ambient air, which is a big plus for modern agriculture. In addition, Coviella et al. (2000) determined that
“elevated CO2 appears to eliminate differences between transgenic [Btcontaining] and nontransgenic plants for some key insect developmental/fitness variables including length of the
larval stage and pupal weight.” In summary, the majority of evidence that has been accumulated to date suggests that rising atmospheric CO2 concentrations may reduce the frequency and
severity of pest outbreaks that are detrimental to agriculture, while not seriously impacting herbivorous organisms found in natural ecosystems that are normally viewed in a more favorable
light. Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/h/herbivoresherbplants.php.
References Brooks, G.L. and Whittaker, J.B. 1999. Responses of three generations of a xylem-feeding insect, Neophilaenus lineatus (Homoptera), to elevated CO2. Global Change Biology
5: 395-401.
Biological Effects of Carbon Dioxide Enrichment 421 Castells, E., Roumet, C., Penuelas, J. and Roy, J. 2002. Intraspecific variability of phenolic concentrations and their responses
to elevated CO2 in two mediterranean perennial grasses. Environmental and Experimental Botany 47: 205-216. Coviella, C.E. and Trumble, J.T. 2000. Effect of elevated atmospheric carbon
dioxide on the use of foliar application of Bacillus thuringiensis. BioControl 45: 325-336. Coviella, C.E., Morgan, D.J.W. and Trumble, J.T. 2000. Interactions of elevated CO2 and nitrogen
fertilization: Effects on production of Bacillus thuringiensis toxins in transgenic plants. Environmental Entomology 29: 781-787. Goverde, M., Bazin, A., Shykoff, J.A. and Erhardt, A.
1999. Influence of leaf chemistry of Lotus corniculatus (Fabaceae) on larval development of Polyommatus icarus (Lepidoptera, Lycaenidae): effects of elevated CO2 and plant genotype.
Functional Ecology 13: 801-810. Joutei, A.B., Roy, J., Van Impe, G. and Lebrun, P. 2000. Effect of elevated CO2 on the demography of a leafsucking mite feeding on bean. Oecologia 123:
75-81. Kerslake, J.E., Woodin, S.J. and Hartley, S.E. 1998. Effects of carbon dioxide and nitrogen enrichment on a plant-insect interaction: the quality of Calluna vulgaris as a host
for Operophtera brumata. New Phytologist 140: 43-53. Newman, J.A., Gibson, D.J., Hickam, E., Lorenz, M., Adams, E., Bybee, L. and Thompson, R. 1999. Elevated carbon dioxide results in
smaller populations of the bird cherry-oat aphid Rhopalosiphum padi. Ecological Entomology 24: 486-489. Peters, H.A., Baur, B., Bazzaz, F. and Korner, C. 2000. Consumption rates and
food preferences of slugs in a calcareous grassland under current and future CO2 conditions. Oecologia 125: 72-81. 7.3.2.2. Woody Plants 7.3.2.2.1. Maple Working with Acer rubrum saplings
beginning their fourth year of growth in open-top chambers maintained at four different atmospheric CO2/temperature conditions—(1) ambient temperature, ambient CO2, (2) ambient temperature,
elevated CO2 ((ambient + 300 ppm), (3) elevated temperature (ambient + 3.5°C), ambient CO2, and (4) elevated temperature, elevated CO2—Williams et al. (2003) bagged first instar gypsy
moth larvae on various branches of the trees and observed their behavior. The data they obtained demonstrated, in their words, “that larvae feeding on CO2-enriched foliage ate a comparably
poorer food source than those feeding on ambient CO2-grown plants, irrespective of temperature,” and that there was a minor reduction in leaf water content due to CO2 enrichment. Nevertheless,
they found the “CO2-induced reductions in foliage quality (e.g. nitrogen and water) were unrelated [our italics] to insect mortality, development rate and pupal weight,” and that these
and any other phytochemical changes that may have occurred “resulted in no negative effects on gypsy moth performance.” They also found that “irrespective of CO2 concentration, on average,
male larvae pupated 7.5 days earlier and female larvae 8 days earlier at elevated temperature,” and noting that anything that prolongs the various development stages of insects potentially
exposes them to greater predation and parasitism risk, they concluded that the observed temperature-induced hastening of the insects’ development would likely expose them to less predation
and parasitism risk. One year later, Hamilton et al. (2004) began the report of their study of this important subject by noting that many single-species investigations have suggested
that increases in atmospheric CO2 will increase herbivory (Bezemer and Jones, 1998; Cannon, 1998; Coviella and Trumble, 1999; Hunter, 2001; Lincoln et al., 1993; Whittaker, 1999). However,
because there are so many feedbacks and complex interactions among the numerous components of real-world ecosystems, they warned that one ought not put too much faith in these predictions
until relevant real-world ecosystem-level experiments have been completed. In one such study they conducted at the Duke Forest FACE facility near Chapel Hill, North Carolina, USA, Hamilton
et al. “measured the amount of leaf tissue damaged by insects and other herbivorous arthropods during two growing seasons in a deciduous forest understory continuously exposed to ambient
(360 ppm) and elevated (560 ppm) CO2 conditions.” This forest is dominated by loblolly pine trees that account for fully 92 percent of the ecosystem’s total woody biomass. In addition,
it contains 48 species of other woody plants (trees, shrubs, and vines) that have naturally established themselves in the forest’s understory. In their study of this ecosystem, Hamilton
et al. quantified the loss of foliage due to herbivory that was experienced by three deciduous tree species, one of which was Acer rubrum.
Climate Change Reconsidered 422 As Hamilton et al. describe it, “we found that elevated CO2 led to a trend toward reduced herbivory [our italics] in [the] deciduous understory in a situation
that included the full complement of naturally occurring plant and insect species.” In 1999, for example, they determined that “elevated CO2 reduced overall herbivory by more than 40
percent,” while in 2000 they say they observed “the same pattern and magnitude of reduction.” With respect to changes in foliage properties that might have been expected to lead to increases
in herbivory, Hamilton et al. report they “found no evidence for significant changes in leaf nitrogen, C/N ratio, sugar, starch or total leaf phenolics in either year of [the] study,”
noting that these findings agree with those of “another study performed at the Duke Forest FACE site that also found no effect of elevated CO2 on the chemical composition of leaves of
understory trees (Finzi and Schlesinger, 2002).” Hamilton et al. thus concluded their landmark paper by emphasizing that “despite the large number of studies that predict increased herbivory,
particularly from leaf chewers, under elevated CO2, our study found a trend toward reduced herbivory two years in a row.” In addition, they note that their realworld results “agree with
the only other large-scale field experiment that quantified herbivory for a community exposed to elevated CO2 (Stiling et al., 2003).” Consequently, and in spite of the predictions of
increased destruction of natural ecosystems by insects and other herbivorous arthropods in a CO2-enriched world of the future, just the opposite would appear to be the more likely outcome;
i.e., greater plant productivity plus less foliage consumption by herbivores, “whether expressed on an absolute or a percent basis,” as Hamilton et al. found to be the case in their
study. In another study conducted at the same site, Knepp et al. (2005) quantified leaf damage by chewing insects on saplings of seven species (including Acer rubrum) in 2001, 2002,
and 2003, while five additional species (including Acer barbatum) were included in 2001 and 2003. This work revealed, in their words, that “across the seven species that were measured
in each of the three years, elevated CO2 caused a reduction in the percentage of leaf area removed by chewing insects,” which was such that “the percentage of leaf tissue damaged by
insect herbivores was 3.8 percent per leaf under ambient CO2 and 3.3 percent per leaf under elevated CO2.” Greatest effects were observed in 2001, when they report that “across 12 species
the average damage per leaf under ambient CO2 was 3.1 percent compared with 1.7 percent for plants under elevated CO2,” which was “indicative of a 46 percent decrease in the total area
and total mass of leaf tissue damaged by chewing insects in the elevated CO2 plots.” What was responsible for these welcome results? Knepp et al. say that “given the consistent reduction
in herbivory under high CO2 across species in 2001, it appears that some universal feature of chemistry or structure that affected leaf suitability was altered by the treatment.” Another
possibility they discuss is that “forest herbivory may decrease under elevated CO2 because of a decline in the abundance of chewing insects,” citing the observations of Stiling et al.
(2002) to this effect and noting that “slower rates of development under elevated CO2 prolongs the time that insect herbivores are susceptible to natural enemies, which may be abundant
in open-top chambers and FACE experiments but absent from greenhouse experiments.” In addition, they suggest that “decreased foliar quality and increased per capita consumption under
elevated CO2 may increase exposure to toxins and insect mortality,” also noting that “CO2-induced changes in host plant quality directly decrease insect fecundity,” citing the work of
Coviella and Trumble (1999) and Awmack and Leather (2002). So what’s the bottom line with respect to the outlook for earth’s forests, and especially its maple trees, in a high-CO2 world
of the future? In their concluding paragraph, Knepp et al. say that “By contrast to the view that herbivore damage will increase under elevated CO2 as a result of compensatory feeding
on lower quality foliage, our results and those of Stiling et al. (2002) and Hamilton et al. (2004) in open experimental systems suggest that damage to trees may decrease.” But what
if herbivore-induced damage in fact increases in a future CO2-enriched world? The likely answer is provided by the work of Kruger et al. (1998), who grew well-watered and fertilized
oneyear-old Acer saccharum saplings in glasshouses maintained at atmospheric CO2 concentrations of either 356 or 645 ppm for 70 days to determine the effects of elevated CO2 on photosynthesis
and growth. In addition, on the 49th day of differential CO2 exposure, 50 percent of the saplings’ leaf area was removed from half of the trees in order to study the impact of simulated
herbivory. This protocol revealed that the 70-day CO2 enrichment treatment increased the total dry weight weight of the non-defoliated seedlings by
Biological Effects of Carbon Dioxide Enrichment 423 about 10 percent. When the trees were stressed by simulated herbivory, however, the CO2-enriched maples produced 28 percent more dry
weight over the final phase of the study than the maples in the ambient-air treatment did. This result thus led Kruger et al. to conclude that in a high-CO2 world of the future “sugar
maple might be more capable of tolerating severe defoliation events which in the past have been implicated in widespread maple declines.” It appears that earth’s maple trees—and probably
many, if not most, other trees—will fare much better in the future with respect to the periodic assaults of leaf-damaging herbivores, as the air’s CO2 content continues its upward climb.
Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/h/herbivoresmaple.php. References
Awmack, C.S. and Leather, S.R. 2002. Host plant quality and fecundity in herbivorous insects. Annual Review of Entomology 47: 817-844. Bezemer, T.M. and Jones, T.H. 1998. Plant-insect
herbivore interactions in elevated atmospheric CO2: quantitative analyses and guild effects. Oikos 82: 212-222. Cannon, R.J. 1998. The implications of predicted climate change for insect
pests in the UK, with emphasis on nonindigenous species. Global Change Biology 4: 785-796. Coviella, C.E. and Trumble, J.T. 1999. Effects of elevated atmospheric carbon dioxide on insect-plant
interactions. Conservation Biology 13: 700-712. Finzi, A.C. and Schlesinger, W.H. 2002. Species control variation in litter decomposition in a pine forest exposed to elevated CO2. Global
Change Biology 8: 1217-1229. Hamilton, J.G., Zangerl, A.R., Berenbaum, M.R., Pippen, J., Aldea, M. and DeLucia, E.H. 2004. Insect herbivory in an intact forest understory under experimental
CO2 enrichment. Oecologia 138: 10.1007/s00442-003-1463-5. Hunter, M.D. 2001. Effects of elevated atmospheric carbon dioxide on insect-plant interactions. Agricultural and Forest Entomology
3: 153-159. Knepp, R.G., Hamilton, J.G., Mohan, J.E., Zangerl, A.R., Berenbaum, M.R. and DeLucia, E.H. 2005. Elevated CO2 reduces leaf damage by insect herbivores in a forest community.
New Phytologist 167: 207-218. Kruger, E.L., Volin, J.C. and Lindroth, R.L. 1998. Influences of atmospheric CO2 enrichment on the responses of sugar maple and trembling aspen to defoliation.
New Phytologist 140: 85-94. Lincoln, D.E., Fajer, E.D. and Johnson, R.H. 1993. Plantinsect herbivore interactions in elevated CO2 environments. Trends in Ecology and Evolution 8: 64-68.
Stiling, P., Cattell, M., Moon, D.C., Rossu, A., Hungate, B.A., Hymuss, G. and Drake, B. 2002. Elevated atmospheric CO2 lowers herbivore abundance, but increases leaf abscission rates.
Global Change Biology 8: 658-667. Stiling, P., Moon, D.C., Hunter, M.D., Colson, J., Rossi, A.M., Hymus, G.J. and Drake, B.G. 2003. Elevated CO2 lowers relative and absolute herbivore
density across all species of a scrub-oak forest. Oecologia 134: 82-87. Whittaker, J.B. 1999. Impacts and responses at population level of herbivorous insects to elevated CO2. European
Journal of Entomology 96: 149-156. Williams, R.S., Lincoln, D.E. and Norby, R.J. 2003. Development of gypsy moth larvae feeding on red maple saplings at elevated CO2 and temperature.
Oecologia 137: 114-122. 7.3.2.2.2. Oak Dury et al. (1998) grew four-year-old Quercus robur seedlings in pots in greenhouses maintained at ambient and twice-ambient atmospheric CO2 concentrations
in combination with ambient and elevated (ambient plus 3°C) air temperatures for approximately one year to study the interactive effects of elevated CO2 and temperature on leaf nutritional
quality. In doing so, they found that the elevated air temperature treatment significantly reduced leaf palatability, and that leaf toughness increased as a consequence of temperature-induced
increases in condensed tannin concentrations. In addition, the higher temperatures significantly reduced leaf nitrogen content, while elevated CO2 caused a a temporary increase in leaf
phenolic concentrations and a decrease in leaf nitrogen content. In one of the first attempts to move outside the laboratory/greenhouse and study the effects of atmospheric CO2 enrichment
on trophic food webs in a natural ecosystem, Stiling et al. (1999) enclosed portions of a native scrub-oak community in Florida (USA) within 3.6-m-diameter open-top chambers and fumigated
them with air having CO2 concentrations of either 350 or 700 ppm for approximately one year, in order to see if elevated CO2 would impact leaf miner
Climate Change Reconsidered 424 densities, feeding rates, and mortality in this nutrientpoor ecosystem. Adherence to this protocol led to the finding that total leaf miner densities
were 38 percent less on the foliage of trees growing in CO2-enriched air than on the foliage of trees growing in ambient air. In addition, atmospheric CO2 enrichment consistently reduced
the absolute numbers of the study’s six leaf miner species. At the same time, however, the elevated CO2 treatment increased the leaf area consumed by the less abundant herbivore miners
by approximately 40 percent relative to the areas mined by the more abundant herbivores present on the foliage exposed to ambient air; but in spite of this increase in feeding, the leaf
miners in the CO2-enriched chambers experienced significantly greater mortality than those in the ambient-air chambers. Although CO2-induced reductions in leaf nitrogen content played
a role in this phenomenon, the greatest factor contributing to increased herbivore mortality was a a four-fold increase in parasitization by various wasps, which could more readily detect
the moreexposed leaf miners on the CO2-enriched foliage. If extended to agricultural ecosystems, these findings suggest that crops may experience less damage from such herbivores in
a high-CO2 world of the future, thus increasing potential harvest and economic gains. In addition, with reduced numbers of leaf miners in CO2-enriched air, farmers could reduce their
dependency upon chemical pesticides to control them. In another study conducted on five scrub-oak forest species at the same experimental facility, Stiling et al. (2003) investigated
the effects of an approximate doubling of the air’s CO2 concentration on a number of characteristics of several insect herbivores. As before, they found that the “relative levels of
damage by the two most common herbivore guilds, leaf-mining moths and leaf-chewers (primarily larval lepidopterans and grasshoppers), were significantly lower in elevated CO2 than in
ambient CO2, for all all five plant species,” and they found that “the response to elevated CO2 was the same across all plant species.” In addition, they report that “more host-plant
induced mortality was found for all miners on all plants in elevated CO2 than in ambient CO2.” These effects were so powerful that in addition to the relative densities of insect herbivores
being reduced in the CO2-enriched chambers, and “even though there were more leaves of most plant species in the elevated CO2 chambers,” the total densities of leaf miners in the high-CO2
chambers were also lower for all plant species. Consequently, it would appear that in a high-CO2 world of the future, many of earth’s plants may be able to better withstand the onslaughts
of various insect pests that have plagued them in the past. Another intriguing implication of this finding, as Stiling et al. note, is that “reductions in herbivore loads in elevated
CO2 could boost plant growth beyond what might be expected based on pure plant responses to elevated CO2.” Continuing to investigate the same ecosystem, which is dominated by two species
of scrub oak (Quercus geminata and Q. myrtifolia) that account for more than 90 percent of the ecosystem’s biomass, and focusing on the abundance of a guild of lepidopteran leafminers
that attack the leaves of Q. myrtifolia, as well as various leaf chewers that also like to munch on this species, Rossi et al. (2004) followed 100 marked leaves in each of 16 open-top
chambers (half exposed to ambient air and half exposed to air containing an extra 350 ppm of CO2) for a total of nine months, after which, in their words, “differences in mean percent
of leaves with leafminers and chewed leaves on trees from ambient and elevated chambers were assessed using paired t-tests.” In reporting their findings the researchers wrote that “both
the abundance of the guild of leafmining lepidopterans and damage caused by leaf chewing insects attacking myrtle oak were depressed in elevated CO2.” Specifically, they found that leafminer
abundance was 44 percent lower (P = 0.096) in the CO2-enriched chambers compared to the ambient-air chambers, and that the abundance of leaves suffering chewing damage was 37 percent
lower (P = 0.072) in the CO2-enriched air. The implications of these findings are obvious: Myrtle oak trees growing in their natural habitat will likely suffer less damage from both
leaf miners and leaf chewers as the air’s CO2 concentration continues to rise in the years and decades ahead. Still concentrating on the same ecosystem, where atmospheric enrichment
with an extra 350 ppm of CO2 was begun in May 1996, Hall et al. (2005b) studied the four species that dominate the community and are present in every experimental chamber: the three
oaks (Quercus myrtifolia, Q. chapmanii and Q. geminata) plus the nitrogen-fixing legume Galactia elliottii. At three-month intervals from May 2001 to May 2003, undamaged leaves were
removed from each of these species in all chambers and analyzed for various chemical constituents, while 200 randomly selected leaves of each species in each chamber were
Biological Effects of Carbon Dioxide Enrichment 425 scored for the presence of six types of herbivore damage. Throughout the study there were no significant differences between the CO2-enriched
and ambienttreatment leaves of any single species in terms of either condensed tannins, hydrolyzable tannins, total phenolics, or lignin. However, in all four species together there
were always greater concentrations of all four leaf constituents in the CO2-enriched leaves, with accross-species mean increases of 6.8 percent for condensed tannins, 6.1 percent for
hydrolyzable tannins, 5.1 percent for total phenolics, and 4.3 percent for lignin. In addition, there were large and often significant CO2-induced decreases in all leaf damage categories
among all species: chewing (-48 percent, P < 0.001), mines (-37 percent, P = 0.001), eye spot gall (-45 percent, P < 0.001), leaf tier (-52 percent, P = 0.012), leaf mite (-23 percent,
P = 0.477), and leaf gall (-16 percent, P = 0.480). Hall et al. thus concluded that the changes they observed in leaf chemical constituents and herbivore damage “suggest that damage
to plants may decline as atmospheric CO2 levels continue to rise.” In one additional study to come out of the Florida scrub-oak ecosystem, Hall et al. (2005a) studied the effects of
an extra 350 ppm of CO2 on litter quality, herbivore activity and their interactions. Over the three years of this experiment (2000, 2001, 2002), they determined that “changes in litter
chemistry from year to year were far larger than effects of CO2 or insect damage, suggesting that these may have only minor effects on litter decomposition.” The one exception to this
finding, in their words, was that “condensed tannin concentrations increased under elevated CO2 regardless of species, herbivore damage, or growing season,” rising by 11 percent in 2000,
18 percent in 2001, and 41 percent in 2002 as a result of atmospheric CO2 enrichment, as best we can determine from their bar graphs. Also, the five researchers report that “lepidopteran
larvae larvae can exhibit slower growth rates when feeding on elevated CO2 plants (Fajer et al., 1991) and become more susceptible to pathogens, parasitoids, and predators (Lindroth,
1996; Stiling et al., 1999),” noting further that at their field site, “which hosts the longest continuous study of the effects of elevated CO2 on insects, herbivore populations decline[d]
markedly under elevated CO2 (Stiling et al., 1999, 2002, 2003; Hall et al., 2005b).” In conclusion, from the evidence accumulated to date with respect to herbivory in oak trees, it would
appear that ever less damage will be done to such trees by various insect pests as the air’s CO2 concentration continues to climb ever higher. Additional information on this topic, including
reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/h/herbivoreswoodoak.php. References Dury, S.J., Good, J.E.G., Perrins, C.M.,
Buse, A. and Kaye, T. 1998. The effects of increasing CO2 and temperature on oak leaf palatability and the implications for herbivorous insects. Global Change Biology 4: 55-61. Fajer,
E.D., Bowers, M.D. and Bazzaz, F.A. 1991. The effects of enriched CO2 atmospheres on the buckeye butterfly, Junonia coenia. Ecology 72: 751-754. Hall, M.C., Stiling, P., Hungate, B.A.,
Drake, B.G. and Hunter, M.D. 2005a. Effects of elevated CO2 and herbivore damage on litter quality in a scrub oak ecosystem. Journal of Chemical Ecology 31: 2343-2356. Hall, M.C., Stiling,
P., Moon, D.C., Drake, B.G. and Hunter, M.D. 2005b. Effects of elevated CO2 on foliar quality and herbivore damage in a scrub oak ecosystem. Journal of Chemical Ecology 31: 267-285.
Lindroth, R.L. 1996. CO2-mediated changes in tree chemistry and tree-Lepidoptera interactions. In: Koch, G.W. and Mooney, H.A. (Eds.) Carbon Dioxide and Terrestrial Ecosystems. Academic
Press, San Diego, California, USA, pp. 105-120. Rossi, A.M., Stiling, P., Moon, D.C., Cattell, M.V. and Drake, B.G. 2004. Induced defensive response of myrtle oak to foliar insect herbivory
in ambient and elevated CO2. Journal of Chemical Ecology 30: 1143-1152. Stiling, P., Cattell, M., Moon, D.C., Rossi, A., Hungate, B.A., Hymus, G. and Drake, B.G. 2002. Elevated atmospheric
CO2 lowers herbivore abundance, but increases leaf abscission rates. Global Change Biology 8: 658-667. Stiling, P., Moon, D.C., Hunter, M.D., Colson, J., Rossi, A.M., Hymus, G.J. and
Drake, B.G. 2003. Elevated CO2 lowers relative and absolute herbivore density across all species of a scrub-oak forest. Oecologia 134: 82-87. Stiling, P., Rossi, A.M., Hungate, B., Dijkstra,
P., Hinkle, C.R., Knot III, W.M., and Drake, B. 1999. Decreased leafminer abundance in elevated CO2: Reduced leaf quality and increased parasitoid attack. Ecological Applications 9:
240-244.
Climate Change Reconsidered 426 7.3.2.2.3. Other Stiling et al. (1999) enclosed portions of a Florida scrub-oak community in open-top chambers and maintained them at atmospheric CO2
concentrations of 350 and 700 ppm for approximately one year, while they studied the effects of this treatment on destructive leaf miners. Among their many findings, the researchers
noted that the individual areas consumed by leaf miners munching on leaves in the CO2-enriched chambers were larger than those created by leaf miners dining on leaves in the ambient-air
chambers. As a result, there was a fourfold increase in parasitization by various wasps that could more readily detect the more-exposed leaf miners on the CO2-enriched foliage. Consequently,
leaf miners in the elevated CO2 chambers suffered significantly greater mortality than those in the control chambers. In a subsequent and much expanded study of the same ecosystem, Stiling
et al. (2002) investigated several characteristics of a number of insect herbivores found on on the five species of plants that accounted for more than 98 percent of the total plant
biomass within the chambers. As they describe their results, the “relative levels of damage by the two most common herbivore guilds, leaf-mining moths and leaf-chewers (primarily larval
lepidopterans and grasshoppers), were significantly lower in elevated CO2 than in ambient CO2, for all five plant species.” In another study that did not involve herbivores, Gleadow
et al. (1998) grew eucalyptus seedlings in glasshouses maintained at 400 and 800 ppm CO2 for a period of six months, observing biomass increases of 98 percent and 134 percent in high
and low nitrogen treatments, respectively. They also studied a sugarbased compound called prunasin, which produces cyanide in response to tissue damage caused by foraging herbivores.
Although elevated CO2 caused no significant change in leaf prunasin content, it was determined that the proportion of nitrogen allocated to prunasin increased by approximately 20 percent
in the CO2-enriched saplings, suggestive of a potential for increased prunasin production had the saplings been under attack by herbivores. Lovelock et al. (1999) grew seedlings of the
tropical tree Copaifera aromatica for 50 days in pots placed within open-top chambers maintained at atmospheric CO2 concentrations of 390 and 860 ppm. At the 14-day point of the experiment,
half of the seedlings in each treatment had about 40 percent of their total leaf area removed. In this case, none of the defoliated trees of either CO2 treatment fully recovered from
this manipulation, but at the end of the experiment, the total plant biomass of the defoliated trees in the CO2-enriched treatment was 15 percent greater than that of the defoliated
trees in the ambient-CO2 treatment, again attesting to the benefits of atmospheric CO2 enrichment in helping trees to deal with herbivory. Docherty et al. (1997) grew beech and sycamore
saplings in glasshouses maintained at atmospheric CO2 concentrations of 350 and 600 ppm, while groups of three sap-feeding aphid species and two sap-feeding leafhopper species were allowed
to feed on them. Overall, elevated CO2 had few
significant effects on the performance of the insects, although there was a non-significant tendency for elevated CO2 to reduce the individual weights and population sizes of the aphids.
Finally, Hattenschwiler and Schafellner (1999) grew seven-year-old spruce (Picea abies) trees at atmospheric CO2 concentrations of 280, 420, and 560 ppm and various nitrogen deposition
treatments for three years, allowing nun moth larvae to feed on current-year needles for a period of 12 days. Larvae placed upon the CO2-enriched foliage consumed less needle biomass
than larvae placed upon the ambiently grown foliage, regardless of nitrogen treatment. This effect was so pronounced that the larvae feeding on needles produced by the CO2-enriched trees
attained an average final biomass that was only two-thirds of that attained by the larvae that fed on needles produced at 280 ppm CO2. Since the nun moth is a deadly defoliator that
resides in most parts of Europe and East Asia between 40° and 60°N latitude and is commonly regarded as the coniferous counterpart of its close relative the gypsy moth, which feeds primarily
on deciduous trees, the results of this study suggest that the ongoing rise in the air’s CO2 content will likely lead to significant reductions in damage to spruce and other coniferous
trees by this voracious insect pest in the years and decades ahead. In light of these several observations, the balance of evidence seems to suggest that earth’s woody plants will be
better able to deal with the challenges provided by herbivorus insects as the air’s CO2 content continues to rise. Additional information on this topic, including reviews of newer publications
as they become available, can be found at http://www.co2science.org/subject/h/herbivoreswoodyplants.php.
Biological Effects of Carbon Dioxide Enrichment 427 References Docherty, M., Wade, F.A., Hurst, D.K., Whittaker, J.B. and Lea, P.J. 1997. Responses of tree sap-feeding herbivores to
elevated CO2. Global Change Biology 3: 51-59. Gleadow, R.M., Foley, W.J. and Woodrow, I.E. 1998. Enhanced CO2 alters the relationship between photosynthesis and defense in cyanogenic
Eucalyptus cladocalyx F. Muell. Plant, Cell and Environment 21: 12-22. Hattenschwiler, S. and Schafellner, C. 1999. Opposing effects of elevated CO2 and N deposition on Lymantria monacha
larvae feeding on spruce trees. Oecologia 118: 210-217. Lovelock, C.E., Posada, J. and Winter, K. 1999. Effects of elevated CO2 and defoliation on compensatory growth and photosynthesis
of seedlings in a tropical tree, Copaifera aromatica. Biotropica 31: 279-287. Stiling, P., Moon, D.C., Hunter, M.D., Colson, J., Rossi, A.M., Hymus, G.J. and Drake, B.G. 2002. Elevated
CO2 lowers relative and absolute herbivore density across all species of a scrub-oak forest. Oecologia 10.1007/s00442-002-1075-5. Stiling, P., Rossi, A.M., Hungate, B., Dijkstra, P.,
Hinkle, C.R., Knot III, W.M., and Drake, B. 1999. Decreased leafminer abundance in elevated CO2: Reduced leaf quality and increased parasitoid attack. Ecological Applications 9: 240-244.
7.3.3. Insects As the atmosphere’s CO2 concentration climbs ever higher, it is important to determine how this phenomenon will affect the delicate balance that exists between earth’s
plants and the insects that feed on them. In this section we thus review what has been learned about this subject with respect to aphids, moths, and other insects. Additional information
on this topic, including reviews on insects not discussed here, can be found at http://www.co2science.org/subject/i/subject_i.php under the heading Insects. 7.3.3.1. Aphids Docherty
et al. (1997) grew beech and sycamore saplings in glasshouses maintained at atmospheric CO2 concentrations of 350 and 600 ppm, while groups of three sap-feeding aphid species were allowed
to feed on the saplings. Overall, the elevated CO2 had few significant effects on aphid feeding and performance. There was, however, a non-significant tendency for elevated CO2 to reduce
the individual weights and population sizes of the aphids, suggesting that future increases in the air’s CO2 content might reduce aphid feeding pressures on beech and sycamore saplings,
and possibly other plants as well. Whittaker (1999) reviewed the scientific literature dealing with population responses of herbivorous insects to atmospheric CO2 enrichment, concentrating
on papers resulting from relatively long-term studies. Based on all pertinent research reports available at that time, the only herbivorous insects that exhibited population increases
in response to elevated CO2 exposure were those classified as phloem feeders, specifically, aphids. Although this finding appeared to favor aphids over plants, additional studies would
complicate the issue and swing the pendulum back the other way. Newman et al. (1999) grew tall fescue plants for two weeks in open-top chambers maintained at atmospheric CO2 concentrations
of 350 and 700 ppm before inoculating them with aphids (Rhopalosiphum padi). After nine additional weeks of differential CO2 exposure, the plants were harvested and their associated
aphids counted. Although elevated CO2 increased plant dry matter production by 37 percent, the plants grown in air of elevated CO2 concentration contained fewer aphids than the plants
grown in ambient air. Percy et al. (2002) grew the most widely distributed tree species in all of North America— trembling aspen—in twelve 30-m-diameter FACE rings in air maintained
at (1) ambient CO2 and O3 concentrations, (2) ambient O3 and elevated CO2 (560 ppm during daylight hours), (3) ambient CO2 and elevated O3 (46.4-55.5 ppb during daylight hours), and
(4) elevated CO2 and O3 over each growing season from 1998 through 2001. Throughout their experiment they assessed a number of the young trees’ growth characteristics, as well as the
responses of the sap-feeding aphid Chaitophorus stevensis, which they say “infests aspen throughout its range.” This experiment revealed that, by itself, elevated CO2 did not affect
aphid abundance, but it increased the densities of natural enemies of the aphids, which over the long term would tend to reduce aphid numbers. Also, by itself, elevated O3 did not affect
aphid abundance, but it had a strong negative effect on natural enemies of aphids, which over the long term would tend to increase aphid numbers. When both
Climate Change Reconsidered 428 trace gases were applied together, elevated CO2 completely counteracted the reduction in the abundance of natural enemies of aphids caused by elevated
O3. Hence, elevated CO2 tended to reduce the negative impact of aphids on trembling aspen in this comprehensive study. At about the same time, Holopainen (2002) reviewed the scientific
literature dealing with the joint effects of elevated concentrations of atmospheric O3 and CO2 on aphid-plant interactions. After compiling the results of 26 pertinent studies, it was
found that atmospheric CO2 enrichment increased aphid performance in six studies, decreased it in six studies, and had no significant impact on it in the remaining 14 studies. Similar
results were found for aphid-plant interactions in the presence of elevated O3 concentrations. Newman (2003) reviewed what was known and not known about aphid responses to concurrent
increases in atmospheric CO2 and air temperature, while also investigating the subject via the aphid population model of Newman et al. (2003). This literature review and model analysis
led him to conclude that when the air’s CO2 concentration and temperature are both elevated, “aphid population dynamics will be more similar to current ambient conditions than expected
from the results of experiments studying either factor alone.” We can draw only the general conclusion, according to Newman, that “insect responses to CO2 are unlikely to all be in the
same direction.” Nevertheless, he says that “the lack of a simple common phenomenon does not deny that there is some overriding generality in the responses by the system.” It’s just
that we did not at that time know what that overriding generality was, which is why experimental work on the subject has continued apace. Concentrating on thermal effects alone, Ma et
al. (2004) conducted detailed experiments on the effects of high temperature, period of exposure, and developmental stage on the survival of the aphid Metopolophium dirhodum, which they
say “is the most abundant of the three cereal aphid species in Germany and central European countries.” This protocol revealed, in their words, that “temperatures over 29°C for 8 hours
significantly reduced survival, which decreased generally as the temperature increased.” They also determined that “exposing aphids to 32.5°C for 4 hours or longer significantly reduced
survival,” and that “mature aphids had a lower tolerance of high temperatures than nymphs.” In light of what they observed, therefore, as well as what a number of other scientists had
observed, Ma et al. concluded that “global warming may play a role in the long-term changes in the population abundance of M. dirhodum.” Specifically, they say that “an increase in TX
[daily average temperature] of 1°C and MaxT [maximum daily temperature] of 1.3°C during the main period of the aphid population increase would result in a 33 percent reduction in peak
population size,” while “an increase in TX of 2°C and MaxT of 2.6°C would result in an early population collapse (74 percent reduction of population size).” It would appear that a little
global warming could greatly decrease aphid infestations of cereal crops grown throughout Germany and Central Europe. Returning to the subject of joint CO2 and O3 effects on aphids,
Awmack et al. (2004) conducted a two-year study at the Aspen FACE site near Rhinelander, Wisconsin, USA, of the individual and combined effects of elevated CO2 (+200 ppm) and O3 (1.5
x ambient) on the performance of Cepegillettea betulaefoliae aphids feeding on paper birch trees in what they call “the first investigation of the long-term effects of elevated CO2 and
O3 atmospheres on natural insect herbivore populations.” At the individual scale, they report that “elevated CO2 and O3 did not significantly affect [aphid] growth rates, potential fecundity
(embryo number) or offspring quality.” At the population scale, on the other hand, they found that “elevated O3 had a strong positive effect,” but that “elevated CO2 did not significantly
affect aphid populations.” populations.” In comparing their results with those of prior related studies, the three scientists report that “the responses of other aphid species to elevated
CO2 or O3 are also complex.” In particular, they note that “tree-feeding aphids show few significant responses to elevated CO2 (Docherty et al., 1997), while cropfeeding species may
respond positively (Awmack et al., 1997; Bezemer et al., 1998; Hughes and Bazzaz, 2001; Zhang et al., 2001; Stacey and Fellowes, 2002), negatively (Newman et al., 1999), or not at all
(Hughes and Bazzaz, 2001), and the same species may show different responses on different host plant species (Awmack et al., 1997; Bezemer et al., 1999).” In summarizing their observations,
they stated that “aphid individual performance did not predict population responses to CO2 and O3,” and they concluded that “elevated CO2 and O3 atmospheres are unlikely to affect C.
betulaefoliae populations in the presence of natural enemy communities.” In a study of a different aphid (Chaitophorus stevensis) conducted at the same FACE site, Mondor
Biological Effects of Carbon Dioxide Enrichment 429 et al. (2004) focused on the subject of pheromones, which they say “are utilized by insects for several purposes, including alarm
signaling,” and which in the case of phloem-feeding aphids induces highdensity groups of them on exposed leaves of trembling aspen trees to disperse and move to areas of lower predation
risk. In this experiment the four treatments were: control (367 ppm CO2, 38 ppb O3), elevated CO2 (537 ppm), elevated O3 (51 ppb), and elevated CO2 and O3 (537 ppm CO2, 51 ppb O3). Within
each treatment, several aspen leaves containing a single aphid colony of 25 ± 2 individuals were treated in one of two different ways: (1) an aphid was prodded lightly on the thorax
so as to not produce a visible pheromone droplet, or (2) an aphid was prodded more heavily on the thorax and induced to emit a visible pheromone droplet, after which, in the words of
the scientists, “aphids exhibiting any dispersal reactions in response to pheromone emission as well as those exhibiting the most extreme dispersal response, walking down the petiole
and off the leaf, were recorded over 5 min.” Mondor et al.’s observations were striking. They found that the aphids they studied “have diminished escape responses in enriched carbon
dioxide environments, while those in enriched ozone have augmented escape responses, to alarm pheromone.” In fact, they report that “0 percent of adults dispersed from the leaf under
elevated CO2, while 100 percent dispersed under elevated O3,” indicating that the effects of elevated CO2 and elevated O3 on aphid response to pheromone alarm signaling are diametrically
opposed to each other, with elevated O3 (which is detrimental to vegetation) helping aphids to escape predation and therefore live to do further harm to the leaves they infest, but with
elevated CO2 (which is beneficial to vegetation) making it more difficult for aphids to escape predation and thereby providing yet an additional benefit to plant foliage. Within this
context, therefore, ozone may be seen to be doubly bad for plants, while carbon dioxide may be seen to be doubly good. In addition, Mondor et al. state that this phenomenon may be of
broader scope than what is revealed by their specific study, noting that other reports suggest that “parasitoids and predators are more abundant and/or efficacious under elevated CO2
levels (Stiling et al., 1999; Percy et al., 2002), but are negatively affected by elevated O3 (Gate et al., 1995; Percy et al., 2002).” In another intriguing study, Chen et al. (2004)
grew spring wheat from seed to maturity in highfertility well-watered pots out-of-doors in open-top chambers (OTCs) maintained at atmospheric CO2 concentrations of 370, 550, and 750
ppm. Approximately two months after seeding, 20 apterous adult aphids (Sitobion avenae) from an adjacent field were placed upon the wheat plants of each of 25 pots in each OTC, while
15 pots were left as controls; and at subsequent 5-day intervals, both apterous and alate aphids were counted. Then, about one month later, 10 alate morph fourth instar nymphs were introduced
onto the plants of each of nine control pots; for the next two weeks the number of offspring laid on those plants were recorded and removed daily to measure reproductive activity. At
the end of the study, the wheat plants were harvested and their various growth responses determined. Adherence to these protocols revealed that the introduced aphid populations increased
after infestation, peaked during the grain-filling stage, and declined a bit as the wheat matured. On the final day of measurement, aphids in the 550-ppm CO2 treatment were 32 percent
more numerous than those in ambient air, while aphids in the 750-ppm treatment were 50 percent more numerous. Alate aphids also produced more offspring on host plants grown in elevated
CO2: 13 percent more in the 550-ppm treatment and 19 percent more in the 750-ppm treatment. As for the wheat plants, Chen et al. report that “elevated CO2 generally enhanced plant height,
aboveground biomass, ear length, length, and number of and dry weight of grains per ear, consistent with most other studies.” With respect to above-ground biomass, for example, the 550-ppm
treatment displayed an increase of 36 percent, while the 750-ppm treatment displayed an increase of 50 percent, in the case of both aphid-infested and non-infested plants. In commenting
on their findings, Chen et al. report that “aphid infestation caused negative effects on all the plant traits measured ... but the negative effects were smaller than the positive effects
of elevated CO2 on the plant traits.” Hence, they concluded that “the increased productivity occurring in plants exposed to higher levels of CO2 more than compensate for the increased
capacity of the aphids to cause damage.” In this experiment, therefore, we have a situation where both the plant and the insect that feeds on it were simultaneously benefited by the
applied increases in atmospheric CO2 concentration. Last, in a study that investigated a number of plant-aphid-predator relationships, Chen et al. (2005) grew transgenic cotton plants
for 30 days in well watered and fertilized sand/vermiculite mixtures in pots set in controlled-environment chambers
Climate Change Reconsidered 430 maintained at atmospheric CO2 concentrations of 370, 700, and 1050 ppm. A subset of aphid-infected plants was additionally supplied with predatory ladybugs,
while three generations of cotton aphids (Aphis gossypii) were subsequently allowed to feed on some of the plants. Based on measurements made throughout this complex set of operations,
Chen et al. found that (1) “plant height, biomass, leaf area, and carbon:nitrogen ratios were significantly higher in plants exposed to elevated CO2 levels,” (2) “more dry matter and
fat content and less soluble protein were found in A. gossypii in elevated CO2,” (3) “cotton aphid fecundity significantly increased ... through successive generations reared on plants
grown under elevated CO2,” (4) “significantly higher mean relative growth rates were observed in lady beetle larvae under elevated CO2,” and (5) “the larval and pupal durations of the
lady beetle were significantly shorter and [their] consumption rates increased when fed A. gossypii from elevated CO2 treatments.” In commenting on the significance of their findings,
Chen et al. say their study “provides the first empirical evidence that changes in prey quality mediated by elevated CO2 can alter the prey preference of their natural enemies,” and
in this particular case, they found that this phenomenon could “enhance the biological control of aphids by lady beetle.” In considering the totality of these many experimental findings,
it would appear that the ongoing rise in the air’s CO2 content will likely not have a major impact, one way or the other, on aphidplant interactions, although the scales do appear to
be slightly tipped in favor of plants over aphids. Yet a third possibility is that both plants and aphids will be benefited by atmospheric CO2 enrichment, but with plants benefiting
more. Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/i/insectsaphids.php. References
Awmack, C.C.S., Harrington, R. and Leather, S.R. 1997. Host plant effects on the performance of the aphid Aulacorthum solani (Homoptera: Aphididae) at ambient and elevated CO2. Global
Change Biology 3: 545-549. Awmack, C.S., Harrington, R. and Lindroth, R.L. 2004. Aphid individual performance may not predict population responses to elevated CO2 or O3. Global Change
Biology 10: 1414-1423. Bezemer, T.M., Jones, T.H. and Knight, K.J. 1998. Longterm effects of elevated CO2 and temperature on populations of the peach potato aphid Myzus persicae and
its parasitoid Aphidius matricariae. Oecologia 116: 128-135. Bezemer, T.M., Knight, K.J., Newington, J.E. et al. 1999. How general are aphid responses to elevated atmospheric CO2? Annals
of the Entomological Society of America 92: 724-730. Chen, F., Ge, F., and Parajulee, M.N. 2005. Impact of elevated CO2 on tri-trophic interaction of Gossypium hirsutum, Aphis gossypii,
and Leis axyridis. Environmental Entomology 34: 37-46. Chen, F.J., Wu, G. and Ge, F. 2004. Impacts of elevated CO2 on the population abundance and reproductive activity of aphid Sitobion
avenae Fabricius feeding on spring wheat. JEN 128: 723-730. Docherty, M., Wade, F.A., Hurst, D.K., Whittaker, J.B. and Lea, P.J. 1997. Responses of tree sap-feeding herbivores to elevated
CO2. Global Change Biology 3: 51-59. Gate, I.M., McNeill, S. and Ashmore, M.R. 1995. Effects of air pollution on the searching behaviour of an insect parasitoid. Water, Air and Soil
Pollution 85: 1425-1430. Holopainen, J.K. 2002. Aphid response to elevated ozone and CO2. Entomologia Experimentalis et Applicata 104: 137-142. Hughes, L. and Bazzaz, F.A. 2001. Effects
of elevated CO2 on five plant-aphid interactions. Entomologia Experimentalis et Applicata 99: 87-96. Ma, C.S., Hau, B. and Poehling, M.-M. 2004. The effect of heat stress on the survival
of the rose grain aphid, Metopolophium dirhodum (Hemiptera: Aphididae). European Journal of Entomology 101: 327-331. Mondor, E.B., Tremblay, M.N., Awmack, C.S. and Lindroth, R.L. 2004.
Divergent pheromone-mediated insect behavior under global atmospheric change. Global Change Biology 10: 1820-1824. Newman, J.A. 2003. Climate change and cereal aphids: the relative effects
of increasing CO2 and temperature on aphid population dynamics. Global Change Biology 10: 5-15. Newman, J.A., Gibson, D.J., Hickam, E., Lorenz, M., Adams, E., Bybee, L. and Thompson,
R. 1999. Elevated carbon dioxide results in smaller populations of the bird cherry-oat aphid Rhopalosiphum padi. Ecological Entomology 24: 486-489. Newman, J.A., Gibson, D.J., Parsons,
A.J. and Thornley, J.H.M. 2003. How predictable are aphid population
Biological Effects of Carbon Dioxide Enrichment 431 responses to elevated CO2? Journal of Animal Ecology 72: 556-566. Percy, K.E., Awmack, C.S., Lindroth, R.L., Kubiske, M.E., Kopper,
B.J., Isebrands, J.G., Pregitzer, K.S., Hendrey, G.R., Dickson, R.E., Zak, D.R., Oksanen, E., Sober, J., Harrington, R. and Karnosky, D.F. 2002. Altered performance of forest pests under
atmospheres enriched by CO2 and O3. Nature 420: 403-407. Stacey, D. and Fellowes, M. 2002. Influence of elevated CO2 on interspecific interactions at higher trophic levels. Global Change
Biology 8: 668-678. Stiling, P., Rossi, A.M., Hungate, B. et al. 1999. Decreased leaf-miner abundance in elevated CO2: reduced leaf quality and increased parasitoid attack. Ecological
Applications 9: 240-244. Whittaker, J.B. 1999. Impacts and responses at population level of herbivorous insects to elevated CO2. European Journal of Entomology 96: 149-156. Zhang, J.,
Liu, J., Wang, G. et al. 2001. Effect of elevated atmospheric CO2 concentration on Rhopalsiphum padi population under different soil water levels. Yingyong Shengtai Xuebao 12: 253-256.
7.3.3.2. Butterflies How will earth’s butterflies respond to atmospheric CO2 enrichment and global warming? We here explore what has been learned about the question over the past few
years, beginning with a review of studies that focus on carbon dioxide and concluding with studies that focus on temperature. In a study of Lotus corniculatus (a cyanogenic plant that
produces foliar cyanoglycosides to deter against herbivory by insects) and the Common Blue Butterfly (Polyommatus icarus, which regularly feeds upon L. corniculatus because it possesses
an enzyme that detoxifies cyanide-containing defensive compounds), Goverde et al. (1999) collected four genotypes of L. corniculatus differing in their concentrations of cyanoglycosides
and tannins (another group of defensive compounds) near Paris, France. They then grew them in controlledenvironment chambers maintained at atmospheric CO2 concentrations of 350 and 700
ppm, after after which they determined the effects of the doubled CO2 concentration on leaf quality and allowed the larvae of the Common Blue Butterfly to feed upon the plants’ leaves.
This work revealed that elevated CO2 significantly increased leaf tannin and starch contents in a genotypically dependent and -independent manner, respectively, while decreasing leaf
cyanoglycoside contents independent of genotype. These CO2-induced changes in leaf chemistry increased leaf palatability, as indicated by greater dry weight consumption of CO2-enriched
leaves by butterfly larvae. In addition, the increased consumption of CO2-enriched leaves led to greater larval biomass and shorter larval development times, positively influencing the
larvae of the Common Blue Butterfly. Hence, it is not surprising that larval mortality was lower when feeding upon CO2-enriched as opposed to ambiently grown leaves. Goverde et al. (2004)
grew L. corniculatus plants once again, this time from seed in tubes recessed into the ground under natural natural conditions in a nutrient-poor calcareous grassland, where an extra
232 ppm of CO2 was supplied to them via a Screen-Aided CO2 Control (SACC) system (Leadley et al., 1997, 1999), and where insect larvae were allowed to feed on the plants (half of which
received extra phosphorus fertilizer) for the final month of the experiment. The atmospheric CO2 enrichment employed in this experiment increased the total dry weight of plants growing
on the unfertilized soil by 21.5 percent and that of plants growing on the phosphorus-enriched soil by 36.3 percent. However, the elevated CO2 treatment had no effect on pupal and adult
insect mass, although Goverde et al. report there were “genotype-specific responses in the development time of P. icarus to elevated CO2 conditions,” with larvae originating from different
mothers developing better under either elevated CO2 or ambient CO2, while for still others the air’s CO2 concentration had no effect on development. In another study by some of the same
researchers, Goverde et al. (2002) raised larvae of the satyrid butterfly (Coenonympha pamphilus) in semi-natural undisturbed calcareous grassland plots exposed to atmospheric CO2 concentrations
of 370 and 600 ppm for five growing seasons. In doing so, they found that the elevated CO2 concentration increased foliar concentrations of total nonstructural carbohydrates and condensed
tannins in the grassland plants; but in what is often considered a negative impact, they found that it also decreased foliar nitrogen concentrations. Nevertheless, this phenomenon had
no discernible effect on butterfly growth and performance. Larval development time, for example, was not affected by elevated CO2, nor was adult dry mass. In fact, the elevated CO2 increased
lipid concentrations in adult male butterflies by nearly 14 percent, while it marginally increased the number of
Climate Change Reconsidered 432 eggs in female butterflies. The former of these responses is especially important, because lipids are used as energy resources in these and other butterflies,
while increased egg numbers in females also suggests an increase in fitness. Turning to the study of temperature effects on butterflies, Parmesan et al. (1999) analyzed distributional
changes over the past century of nonmigratory species whose northern boundaries were in northern Europe (52 species) and whose southern boundaries were in southern Europe or northern
Africa (40 species). This work revealed that the northern boundaries of the first group shifted northward for 65 percent of them, remained stable for 34 percent, and shifted southward
for 2 percent, while the southern boundaries of the second group shifted northward for 22 percent of them, remained stable for 72 percent, and shifted southward for 5 percent, such that
“nearly all northward shifts involved extensions at the northern boundary with the southern boundary remaining stable.” This behavior is precisely what we would expect to see if the
butterflies were responding to shifts in the ranges of the plants upon which they depend for their sustenance, because increases in atmospheric CO2 concentration tend to ameliorate the
effects of heat stress in plants and induce an upward shift in the temperature at which they function optimally. These phenomena tend to cancel the impetus for poleward migration at
the warm edge of a plant’s territorial range, yet they continue to provide the opportunity for poleward expansion at the cold edge of its range. Hence, it is possible that the observed
changes in butterfly ranges over the past century of concomitant warming and rising atmospheric CO2 concentration are related to matching changes in the ranges of the plants upon which
they feed. Or, this similarity could be due to some more complex phenomenon, possibly even some direct physiological effect of temperature and atmospheric CO2 concentration on the butterflies
themselves. In any event, and in the face of the 0.8°C of global warming that occurred in Europe over the twentieth century, the consequences for European butterflies were primarily
beneficial because, as Parmesan et al.
describe the situation, “most species effectively expanded the size of their range when shifting northwards,” since “nearly all northward shifts involved extensions at the northern
boundary with the southern boundary remaining stable.” Across the Atlantic in America, Fleishman et al. (2001) used comprehensive data on butterfly distributions from six mountain ranges
in the U.S. Great Basin to study how butterfly assemblages of that region may respond to IPCC-projected climate change. Whereas prior, more-simplistic analyses have routinely predicted
the extirpation of great percentages of the butterfly species in this region in response to model-predicted increases in air temperature, Fleishman et al.’s study revealed that “few
if any species of montane butterflies are likely to be extirpated from the entire Great Basin (i.e., lost from the region as a whole).” In further discussing their results, the three
researchers note that “during the Middle Holocene, approximately 8000-5000 years ago, temperatures in the Great Basin were several degrees warmer than today.” Thus, they go on say that
“we might expect that most of the montane species—including butterflies—that currently inhabit the Great Basin would be able to tolerate the magnitude of climatic warming forecast over
the next several centuries.” Consequently, it would appear that even if the global warming projections of the IPCC were true, the predictions of butterfly extinctions associated with
those projections are almost certainly false. Returning to the British Isles, Thomas et al. (2001) documented an unusually rapid expansion of the ranges of two butterfly species (the
silver-spotted skipper butterfly and the brown argus butterfly) along with two cricket species (the long-winged cone-head and Roesel’s bush cricket). They write that the warming-warming-induced
“increased habitat breadth and dispersal tendencies have resulted in about 3-to 15-fold increases in expansion rates.” In commenting on these findings, Pimm (2001) truly states the obvious
when he says the geographical ranges of these insects are “expanding faster than expected,” and that the synergies involved in the many intricacies of the range expansion processes are
also “unexpected.” Crozier (2004) writes that “Atalopedes campestris, the sachem skipper butterfly, expanded its range from northern California into western Oregon in 1967, and into
southwestern Washington in 1990,” where she reports that temperatures rose by 2-4°C over the prior half-century. Thus intrigued, and in an attempt to assess the importance of this regional
warming for the persistence of A. campestris in the recently colonized areas, Crozier “compared population dynamics at two locations (the butterfly’s current range edge and just inside
the range) that differ by 2-3°C.” Then, to determine the role of over
Biological Effects of Carbon Dioxide Enrichment 433 winter larval survivorship, she “transplanted larvae over winter to both sites.” This work revealed, in her words, that “combined
results from population and larval transplant analyses indicate that winter temperatures directly affect the persistence of A. campestris at its northern range edge, and that winter
warming was a prerequisite for this butterfly’s range expansion.” Noting that “populations are more likely to go extinct in colder climates,” Crozier says “the good news about rapid
climate change [of the warming type] is that new areas may be available for the introduction of endangered species.” Her work also demonstrates that the species she studied has responded
to regional warming by extending its northern range boundary and thereby expanding its range. Two years later, Davies et al. (2006) introduced their study of the silver-spotted skipper
butterfly (Hesperia comma L.) by noting that during the twentieth century it “became increasingly rare in Britain [as] a result of the widespread reduction of sparse, short-turfed calcareous
grassland containing the species’ sole larval host plant, sheep’s fescue grass [Festuca ovina L].” As a result, they describe the “refuge” colonies of 1982 as but a “remnant” of what
once had been. The four researchers analyzed population density data together with estimates of the percentage bare ground and the percentage of sheep’s fescue available to the butterflies,
based on surveys conducted in Surrey in the chalk hills of the North Downs, south of London, in 1982 (Thomas et al., 1986), 1991 (Thomas and Jones, 1993), 2000 (Thomas et al., 2001;
Davies et al., 2005), and 2001 (R.J. Wilson, unpublished data). In addition, they assessed egg-laying rates in different microhabitats, as well as the effects of ambient and oviposition
site temperatures on egg laying, and the effects of sward composition on egg location. This work revealed, in their words, that “in 1982, 45 habitat patches were occupied by H. comma
[but] in the the subsequent 18-year period, the species expanded and, by 2000, a further 29 patches were colonized within the habitat network.” In addition, they found that “the mean
egglaying rate of H. comma females increased with rising ambient temperatures,” and that “a wider range of conditions have become available for egg-laying.” In discussing their findings,
Davies et al. state that “climate warming has been an important driving force in the recovery of H. comma in Britain [as] the rise in ambient temperature experienced by the butterfly
will have aided the metapopulation reexpansion in a number of ways.” First, they suggest that “greater temperatures should increase the potential fecundity of H. comma females,” and
that “if this results in larger populations, for which there is some evidence (e.g. 32 of the 45 habitat patches occupied in the Surrey network experienced site-level increases in population
density between 1982 and 2000), they will be less prone to extinction,” with “larger numbers of dispersing migrant individuals being available to colonize unoccupied habitat patches
and establish new populations.” Second, they state that “the wider range of thermal and physical microhabitats used for egg-laying increased the potential resource density within each
grassland habitat fragment,” and that “this may increase local population sizes.” Third, they argue that “colonization rates are likely to be greater as a result of the broadening of
the species realized niche, [because] as a larger proportion of the calcareous grassland within the species’ distribution becomes thermally suitable, the relative size and connectivity
of habitat patches within the landscape increases.” Fourth, they note that “higher temperatures may directly increase flight (dispersal) capacity, and the greater fecundity of immigrants
may improve the likelihood of successful population establishment.” Consequently, Davies et al. conclude that “the warmer summers predicted as a consequence of climate warming are likely
to be beneficial to H. comma within Britain,” and they suggest that “warmer winter temperatures could also allow survival in a wider range of microhabitats.” In a concurrent study, Menendez
et al. (2006) provided what they call “the first assessment, at a geographical scale, of how species richness has changed in response to climate change,” concentrating on British butterflies.
This they did by testing “whether average species richness of resident British butterfly species has increased in recent decades, whether these changes are as great as would be expected
given the amount of warming that has taken place, and whether the composition of butterfly communities is changing towards a dominance by generalist species.” By these means they determined
that “average species richness of the British butterfly fauna at 20 x 20 km grid resolution has increased since 1970-82, during a period when climate warming would lead us to expect
increases.” They also found, as expected, that “southerly habitat generalists increased more than specialists,” which require a specific type of habitat that is sometimes difficult for
them to find, especially in the modern world where habitat destruction is commonplace. In addition, they were able to determine that observed species richness
Climate Change Reconsidered 434 increases lagged behind those expected on the basis of climate change. These results “confirm,” according to the nine UK researchers, “that the average
species richness of British butterflies has increased since 1970-82.” However, some of the range shifts responsible for the increase in species richness take more time to occur than
those of other species; they say their results imply “it may be decades or centuries before the species richness and composition of biological communities adjusts to the current climate.”
Also working in Britain, Hughes et al. (2007) examined evolutionary changes in adult flight morphology in six populations of the speckled wood butterfly—Pararge aegeria L. (Satyrinae)—along
a transect from its distribution core to its warminginduced northward expanding range margin. The results of this exercise were then compared with the output of an individual-based spatially
explicit model that was developed “to investigate impacts of habitat availability on the evolution of dispersal in expanding populations.” This work indicated that the empirical data
the researchers gathered “were in agreement with model output,” and that they “showed increased dispersal ability with increasing distance from the distribution core,” which included
favorable changes in thorax shape, abdomen mass, and wing aspect ratio for both males and females, as well as thorax mass and wing loading for females. In addition, they say that “increased
dispersal ability was evident in populations from areas colonized >30 years previously.” In discussing their findings, Hughes et al. suggest that “evolutionary increases in dispersal
ability in expanding populations may help species track future climate changes and counteract impacts of habitat fragmentation by promoting colonization.” However, they report that in
the specific situation they investigated, “at the highest levels of habitat loss, increased dispersal was less evident during expansion and reduced dispersal was observed at equilibrium,
indicating that for many species, continued habitat fragmentation is likely to outweigh any benefits from dispersal.” Put another way, it would appear that global warming is proving
not to be an insurmountable problem for the speckled wood butterfly, which is evolving physical characteristics that allow it to better keep up with the poleward migration of its current
environmental niche, but that the direct destructive assaults of humanity upon its natural habitat could still end up driving it to extinction. Analyzing data pertaining to the general
abundance of Lepidoptera in Britain over the period 1864-1952, based on information assembled by Beirne (1955) via his examination of “several thousand papers in entomological journals
describing annual abundances of moths and butterflies,” were Dennis and Sparks (2007), who report that “abundances of British Lepidoptera were significantly positively correlated with
Central England temperatures in the current year for each month from May to September and November,” November,” and that “increased overall abundance in Lepidoptera coincided significantly
with increased numbers of migrants,” which latter data were derived from the work of Williams (1965). In addition, they report that Pollard (1988) subsequently found much the same thing
for 31 butterfly species over the period 1976-1986, and that Roy et al. (2001) extended the latter investigation to 1997, finding “strong associations between weather and population
fluctuations and trends in 28 of 31 species which confirmed Pollard’s (1988) findings,” all of which observations indicate that the warming-driven increase in Lepidopteran species and
numbers in Britain has been an ongoing phenomenon ever since the end of the Little Ice Age. Returning to North America for one final study, White and Kerr (2006), as they describe it,
“report butterfly species’ range shifts across Canada between 1900 and 1990 and develop spatially explicit tests of the degree to which observed shifts result from climate or human population
density,” the latter of which factors they describe as “a reasonable proxy for land use change,” within which broad category they include such things as “habitat loss, pesticide use,
and habitat fragmentation,” all of which anthropogenicdriven factors have been tied to declines of various butterfly species. In addition, they say that to their knowledge, “this is
the broadest scale, longest term dataset yet assembled to quantify global change impacts on patterns of species richness.” This exercise led White and Kerr to discover that butterfly
species richness “generally increased over the study period, a result of range expansion among the study species,” and they further found that this increase “from the early to late part
of the twentieth century was positively correlated with temperature change,” which had to have been the cause of the change, for they also found that species richness was “negatively
correlated with human population density change.” Contrary to the doom-and-gloom prognostications of some experts, the supposedly unprecedented global
Biological Effects of Carbon Dioxide Enrichment 435 warming of the twentieth century has been beneficial for the butterfly species that inhabit Canada, Britain, and the United States,
as their ranges have expanded and greater numbers of species are now being encountered in each country. Additional information on this topic, including reviews of newer publications
as they become available, can be found at http://www.co2science.org/subject/i/summaries/butterflies.php. References Beirne, B.P. 1955. Natural fluctuations in abundance of British Lepidoptera.
Entomologist’s Gazette 6: 21-52. Crozier, L. 2004. Warmer winters drive butterfly range expansion by increasing survivorship. Ecology 85: 231-241. Davies, Z.G., Wilson, R.J., Brereton,
T.M. and Thomas, C.D. 2005. The re-expansion and improving status of the silver-spotted skipper butterfly (Hesperia comma) in Britain: a metapopulation success story. Biological Conservation
124: 189-198. Davies, Z.G., Wilson, R.J., Coles, S. and Thomas, C.D. 2006. Changing habitat associations of a thermally constrained species, the silver-spotted skipper butterfly, in
response to climate warming. Journal of Animal Ecology 75: 247-256. Dennis, R.L.H. and Sparks, T.H. 2007. Climate signals are reflected in an 89 year series of British Lepidoptera records.
European Journal of Entomology 104: 763-767. Fleishman, E., Austin, G.T. and Murphy, D.D. 2001. Biogeography of Great Basin butterflies: revisiting patterns, paradigms, and climate change
scenarios. Biological Journal of the Linnean Society 74: 501-515. Goverde, M., Bazin, A., Shykoff, J.A. and Erhardt, A. 1999. Influence of leaf chemistry of Lotus corniculatus (Fabaceae)
on larval development of Polyommatus icarus (Lepidoptera, Lycaenidae): effects of elevated CO2 and plant genotype. Functional Ecology 13: 801-810. Goverde, M., Erhardt, A. and Niklaus,
P.A. 2002. In situ development of a satyrid butterfly on calcareous grassland exposed to elevated carbon dioxide. Ecology 83: 1399-1411. Goverde, M., Erhardt, A. and Stocklin, J. 2004.
Genotypespecific response of a lycaenid herbivore to elevated carbon dioxide and phosphorus availability in calcareous grassland. Oecologia 139: 383-391. Hughes, C.L., Dytham, C. and
Hill, J.K. 2007. Modelling and analyzing evolution of dispersal in populations at expanding range boundaries. Ecological Entomology 32: 437-445. Leadley, P.W., Niklaus, P., Stocker,
R. and Korner, C. 1997. Screen-aided CO2 control (SACC): a middle-ground between FACE and open-top chamber. Acta Oecologia 18: 207-219. Leadley, P.W., Niklaus, P.A., Stocker, R. and
Korner, C. 1999. A field study of the effects of elevated CO2 on plant biomass and community structure in a calcareous grassland. Oecologia 118: 39-49. Menendez, R., Gonzalez-Megias,
A., Hill, J.K., Braschler, B., Willis, S.G., Collingham, Y., Fox, R., Roy, D.B. and Thomas, C.D. 2006. Species richness changes lag behind climate change. Proceedings of the Royal Society
B 273: 1465-1470. Parmesan, C., Ryrholm, N., Stefanescu, C., Hill, J.K., Thomas, C.D., Descimon, H., Huntley, B., Kaila, L., Kullberg, J., Tammaru, T., Tennent, W.J., Thomas, J.A. and
Warren, M. 1999. Poleward shifts in geographical ranges of butterfly species associated with regional warming. Nature 399: 579-583. Pimm, S.L. 2001. Entrepreneurial insects. Nature 411:
531-532. Pollard, E. 1988. Temperature, rainfall and butterfly numbers. Journal of Applied Ecology 25: 819-828. Roy, D.B., Rothery, P., Moss, D., Pollard, E. and Thomas, J.A. 2001. Butterfly
numbers and weather: predicting historical trends in abundance and the future effects of climate change. Journal of Animal Ecology 70: 201-217. Thomas, C.D., Bodsworth, E.J., Wilson,
R.J., Simmons, A.D., Davies, Z.G., Musche, M. and Conradt, L. 2001. Ecological and evolutionary processes at expanding range margins. Nature 411: 577-581. Thomas, C.D. and Jones, T.M.
1993. Partial recovery of a skipper butterfly (Hesperia comma) from population refuges: lessons for conservation in a fragmented landscape. Journal of Animal Ecology 62: 472-481. Thomas,
C.D. and Lennon, J.J. 1999. Birds extend their ranges northwards. Nature 399: 213. Thomas, J.A., Thomas, C.D., Simcox, D.J. and Clarke, R.T. 1986. Ecology and declining status of the
silverspotted skipper butterfly (Hesperia comma) in Britain. Journal of Applied Ecology 23: 365-380. White, P. and Kerr, J.T. 2006. Contrasting spatial and temporal global change impacts
on butterfly species richness during the twentieth century. Ecography 29: 908-918. Williams, C.B. 1965. Insect Migration. Collins, London, UK, 237 pp.
Climate Change Reconsidered 436 7.3.3.3. Moths Kerslake et al. (1998) collected five-year-old heather plants from a Scottish moor and grew them in opentop chambers maintained at atmospheric
CO2 concentrations of 350 and 600 ppm for 20 months, with and without soil nitrogen fertilization. At two different times during the study, larvae of Operophtera brumata, a voracious
winter moth whose outbreaks have caused extensive damage to heather moorland in recent years, were allowed to feed upon current-year shoots for up to one month. The results obtained
from this experiment revealed that the survivorship of larvae placed on CO2-enriched foliage was not significantly different from that of larvae placed on foliage produced in ambient
air, regardless of nitrogen treatment. In addition, feeding upon CO2-enriched foliage did not affect larval growth rate, development, or final pupal weight. Consequently, Kerslake et
al. concluded that their study “provides no evidence that increasing atmospheric CO2 concentrations will affect the potential for outbreak of Operophtera brumata on this host.” Hattenschwiler
and Schafellner (1999) grew seven-year-old spruce trees at atmospheric CO2 concentrations of 280, 420, and 560 ppm in various nitrogen deposition treatments for three years, after which
they performed needle quality assessments and allowed nun moth (Lymantria monacha) larvae to feed upon current-year needles for 12 days. This moth is an especially voracious defoliator
that resides in most parts of Europe and East Asia between 40 and 60° N latitude, and it is commonly regarded as the “coniferous counterpart” of its close relative the gypsy moth, which
feeds primarily upon deciduous trees. The two scientists determined from their observations that elevated CO2 significantly enhanced needle starch, tannin, and phenolic concentrations,
while significantly decreasing needle water and nitrogen contents. Thus, atmospheric CO2 enrichment reduced overall needle quality from the perspective of this foliage-consuming moth,
as nitrogen content is the primary factor associated with leaf quality. Increasing nitrogen deposition, on the other hand, tended to enhance needle quality, for it lowered starch, tannin,
and phenolic concentrations while boosting needle nitrogen content. Nevertheless, the positive influence of nitrogen deposition on needle quality was not large enough to completely offset
the quality reduction caused by elevated CO2. In light of these observations, it was no surprise that larvae placed on CO2-enriched foliage consumed less needle biomass than larvae placed
on low-CO2-grown foliage, regardless of nitrogen treatment, and that the larvae feeding on CO2-enriched foliage exhibited reduced relative growth rates and attained an average biomass
that was only two-thirds of that attained by larvae consuming foliage produced at 280 ppm CO2. As a result, Hattenschwiler and Schafellner concluded that “altered needle quality in response
to elevated CO2 will impair the growth and development of Lymantria monacha larvae,” which should lead to reductions in the degree of spruce tree destruction caused by this voracious
defoliator. Stiling et al. (2002) studied the effects of an approximate doubling of the air’s CO2 concentration on a number of characteristics of several insect herbivores feeding on
plants native to a scrub-oak forest ecosystem at the Kennedy Space Center, Florida, USA, in eight ambient and eight CO2-enriched open-top chambers. They say that the “relative levels
of damage by the two most common herbivore guilds, leaf-mining moths and leaf-chewers (primarily larval lepidopterans and grasshoppers), were significantly lower in elevated CO2 than
in ambient CO2,” and that “the response to elevated CO2 was the same across all plant species.” In a follow-up study to that of Stiling et al., which was conducted at the same facilities,
Rossi et al. (2004), focused on the abundance of a guild of lepidopteran leafminers that attack the leaves of myrtle oak, as well as various leaf chewers that also like to munch on this
species. species. Specifically, they periodically examined 100 marked leaves in each of the 16 open-top chambers for a total of nine months, after which, in their words, “differences
in mean percent of leaves with leafminers and chewed leaves on trees from ambient and elevated chambers were assessed using paired t-tests.” This protocol revealed, in their words, that
“both the abundance of the guild of leafmining lepidopterans and damage caused by leaf chewing insects attacking myrtle oak were depressed in elevated CO2.” Leafminer abundance was 44
percent lower (P = 0.096) in the CO2-enriched chambers compared to the ambient-air chambers, while the abundance of leaves suffering chewing damage was 37 percent lower (P = 0.072) in
the CO2-enriched air. Working with red maple saplings, Williams et al. (2003) bagged first instar gypsy moth larvae on branches of trees that were entering their fourth year of growth
within open-top chambers maintained at
Biological Effects of Carbon Dioxide Enrichment 437 four sets of CO2/temperature conditions: (1) ambient temperature, ambient CO2, (2) ambient temperature, elevated CO2 (ambient + 300
ppm), (3) elevated temperature (ambient + 3.5°C), ambient CO2, and (4) elevated temperature, elevated CO2. For these conditions they measured several parameters that were required to
test their hypothesis that a CO2-enriched atmosphere would lead to reductions in foliar nitrogen concentrations and increases in defensive phenolics that would in turn lead to increases
in insect mortality. The results they obtained indicated, in their words, “that larvae feeding on CO2-enriched foliage ate a comparably poorer food source than those feeding on ambient
CO2-grown plants, irrespective of temperature.” Nevertheless, they determined that “CO2-induced reductions in foliage quality were unrelated to insect mortality, development rate and
pupal weight.” As a result, they were forced to conclude that “phytochemical changes resulted in no negative effects on gypsy moth performance,” but neither did they help them. Noting
that increases in the atmosphere’s CO2 concentration typically lead to greater decreases in the concentrations of nitrogen in the foliage of C3 as opposed to C4 grasses, Barbehenn et
al. (2004) say “it has been predicted that insect herbivores will increase their feeding damage on C3 plants to a greater extent than on C4 plants (Lincoln et al., 1984, 1986; Lambers,
1993). To test this hypothesis, they grew Lolium multiflorum (Italian ryegrass, a common C3 pasture grass) and Bouteloua curtipendula (sideoats gramma, a native C4 rangeland grass) in
chambers maintained at either the ambient atmospheric CO2 concentration of 370 ppm or the doubled CO2 concentration of 740 ppm for two months, after which newly molted sixth-instar larvae
of Pseudaletia unipuncta (a grass-specialist noctuid) and Spodoptera frugiperda (a generalist noctuid) were allowed to feed upon the two grasses. As expected, Barbehenn et al. found
that foliage protein concentration decreased by 20 percent in the C3 grass, but by only 1 percent in the C4 grass, when they were grown in CO2-enriched air; and they say that “to the
extent that protein is the most limiting of the macronutrients examined, these changes represent a decline in the nutritional quality of the C3 grass.” However, and “contrary to our
expectations,” in the words of Barbehenn et al., “neither caterpillar species significantly increased its consumption rate to compensate for the lower concentration of protein in [the]
C3 grass,” and they note that “this result does not support the hypothesis that C3 plants will be subject to greater rates of herbivory relative to C4 plants in future [high-CO2] atmospheric
conditions (Lincoln et al., 1984).” In addition, and “despite significant changes in the nutritional quality of L. multiflorum under elevated CO2,” they note that “no effect on the relative
growth rate of either caterpillar species on either grass species resulted,” and that there were “no significant differences in insect performance between CO2 levels.” By way of explanation
of these results, they suggest that “post-ingestive mechanisms could provide a sufficient means of compensation for the lower nutritional quality of C3 plants grown under elevated CO2.”
In light of these observations, Barbehenn et al. suggest “there will not be a single pattern that characterizes all grass feeders” with respect to their feeding preferences and developmental
responses in a world where certain C3 plants may experience foliar protein concentrations that are lower than those they exhibit today, nor will the various changes that may occur necessarily
be detrimental to herbivore development or to the health and vigor of their host plants. Nevertheless, subsequent studies continue to suggest that various moth species will likely be
negatively affected by the ongoing rise in the air’s CO2 content. A case in point is the study of Chen et al. (2005), who grew well watered and fertilized cotton plants of two varieties
(one expressing Bacillus thurigiensis toxin genes and one a non-transgenic cultivar from the same recurrent parent) in pots placed within opentop chambers maintained at either 376 or
754 ppm CO2 in Sanhe County, Hebei Province, China, from planting in mid-May to harvest in October, while immature bolls were periodically collected and analyzed for various chemical
characteristics and others were stored under refrigerated conditions for later feeding to larvae of the cotton bollworm. By these means they found that the elevated CO2 treatment increased
immature boll concentrations of condensed tannins by approximately 22 percent and 26 percent in transgenic and non-transgenetic cotton, respectively, and that it slightly decreased the
body biomass of the cotton bollworm and reduced moth fecundity. The Bt treatment was even more effective in this regard; and in the combined Bt-high-CO2 treatment, the negative cotton
bollworm responses were expressed most strongly of all. Bidart-Bouzat et al. (2005) grew three genotypes genotypes of mouse-ear cress (Arabidopsis thaliana) from seed in pots within
controlled-environment chambers
Climate Change Reconsidered 438 maintained at either ambient CO2 (360 ppm) or elevated CO2 (720 ppm). On each of half of the plants (the herbivory treatment) in each of these CO2 treatments,
they placed two second-instar larvae of the diamondback moth (Plutella xylostella) at bolting time and removed them at pupation, which resulted in an average of 20 percent of each plant’s
total leaf area in the herbivory treatment being removed. Then, each pupa was placed in a gelatin capsule until adult emergence
and ultimate death, after which insect gender was determined and the pupa’s weight recorded. At the end of this herbivory trial, the leaves of the control and larvae-infested plants
were analyzed for concentrations of individual glucosinolates—a group of plant-derived chemicals that can act as herbivore deterrents (Maruicio and Rausher, 1997)—while total glucosinolate
production was determined by summation of the individual glucosinolate assays. Last, influences of elevated CO2 on moth performance and its association with plant defense-related traits
were evaluated. Overall, it was determined by these means that herbivory by larvae of the diamondback moth did not induce any increase in the production of glucosinolates in the mouse-ear
cress in the ambient CO2 treatment. However, the three scientists report that “herbivory-induced increases in glucosinolate contents, ranging from 28% to 62% above basal levels, were
found under elevated CO2 in two out of the three genotypes studied.” In addition, they determined that “elevated CO2 decreased the overall performance of diamondback moths.” And because
“induced defenses can increase plant fitness by reducing subsequent herbivore attacks (Agrawal, 1999; Kessler and Baldwin, 2004),” according to Bidart-Bouzat et al., they suggest that
“the pronounced increase in glucosinolate levels under CO2 enrichment may pose a threat not only for insect generalists that are likely to be more influenced by rapid changes in the
concentration of these chemicals, but also for other insect specialists more susceptible than diamondback moths to high glucosinolate levels (Stowe, 1998; Kliebenstein et al., 2002).”
In a study of a major crop species, Wu et al. (2006) grew spring wheat (Triticum aestivum L.) from seed to maturity in pots placed within open-top chambers maintained at either 370 or
750 ppm CO2 in Sanhe County, Hebei Province, China, after which they reared three generations of cotton bollworms (Helicoverpa armigera Hubner) on the milky grains of the wheat, while
monitoring a number of different bollworm developmental characteristics. In doing so, as they describe it, “significantly lower pupal weights were observed in the first, second and third
generations,” and “the fecundity of H. armigera decreased by 10% in the first generation, 13% in the second generation and 21% in the third generation,” resulting in a “potential population
decrease in cotton bollworm by 9% in the second generation and 24% in the third generation.” In addition, they say that “population consumption was significantly reduced by 14% in the
second generation and 24% in the third generation,” and that the efficiency of conversion of ingested food was reduced “by 18% in the first generation, 23% in the second generation and
30% in the third generation.” As a result, they concluded that the “net damage of cotton bollworm on wheat will be less under elevated atmospheric CO2,” while noting that “at the same
time, gross wheat production is expected to increase by 63% under elevated CO2.” In another report of their work, Wu et al. (2007) write that “significant decreases in the protein, total
amino acid, water and nitrogen content by 15.8%, 17.7%, 9.1% and 20.6% and increases in free fatty acid by 16.1% were observed in cotton bolls grown under elevated CO2.” And when fed
with these cotton bolls, they say that the larval survival rate of H. armigera “decreased by 7.35% in the first generation, 9.52% in the second generation and 11.48% in the third generation
under elevated CO2 compared with ambient CO2.” In addition, they observed that “the fecundity of H. armigera decreased by 7.74% in the first generation, 14.23% in the second generation
and 16.85% in the third generation,” while noting that “fecundity capacity is likely to be reduced even further in the next generation.” The synergistic effects of these several phenomena,
in the words of Wu et al., “resulted in a potential population decrease in cotton bollworm by 18.1% in the second generation and 52.2% in the third generation under elevated CO2,” with
the result that “the potential population consumption of cotton bollworm decreased by 18.0% in the second generation and 55.6% in the third generation ... under elevated CO2 compared
with ambient CO2.” And in light of these several findings, they concluded that “the potential population dynamics and potential population consumption of cotton bollworm will alleviate
the harm to [cotton] plants in the future rising-CO2 atmosphere.” In a different type of study, Esper et al. (2007) reconstructed an annually resolved history history of population cycles
of a foliage-feeding Lepidopteran commonly known as the larch budmoth (Zeiraphera
Biological Effects of Carbon Dioxide Enrichment 439 diniana Gn.)—or LBM for short—within the European Alps in the southern part of Switzerland. As is typical of many such insect pests,
they note that “during peak activity, populations may reach very high densities over large areas,” resulting in “episodes of massive defoliation and/or tree mortality” that could be
of great ecological and economic significance. The first thing the team of Swiss and US researchers thus did in this regard was develop a history of LBM outbreaks over the 1,173-year
period AD 832-2004, which they describe as “the longest continuous time period over which any population cycle has ever been documented.” They accomplished this feat using radiodensitometric
techniques to characterize the treering density profiles of 180 larch (Larix deciduas Mill.) samples, where “LBM outbreaks were identified based upon characteristic maximum latewood
density (MXD) patterns in wood samples, and verified using more traditional techniques of comparison with tree-ring chronologies from non-host species,” i.e., fir and spruce. Then, they
developed a matching temperature history for the same area, which was accomplished by combining “a tree-ring width-based reconstruction from AD 951 to 2002 integrating 1527 pine and
larch samples (Buntgen et al., 2005) and a MXD-based reconstruction from AD 755 to 2004 based upon the same 180 larch samples used in the current study for LBM signal detection (Buntgen
et al., 2006).” Over almost the entire period studied, from its start in AD 832 to 1981, there were a total of 123 LBM outbreaks with a mean reoccurrence time of 9.3 years. In addition,
the researchers say “there was never a gap that lasted longer than two decades.” From 1981 to the end of their study in 2004, however, there were no LBM outbreaks; since there had never
before (within their record) been such a long outbreak hiatus, they concluded that “the absence of mass outbreaks since the 1980s is truly exceptional.” To what do Esper et al. attribute
this unprecedented recent development? Noting that “conditions during the late twentieth century represent the warmest period of the past millennium”—as per their temperature reconstruction
for the region of the Swiss Alps within which they worked—they point to “the role of extraordinary climatic conditions as the cause of outbreak failure,” and they discuss what they refer
to as the “probable hypothesis” of Baltensweiler (1993), who described a scenario by which local warmth may lead to reduced LBM populations. Such may well be the case, but we hasten
to add that atmospheric CO2 concentrations since 1980 have also been unprecedented over the 1,173-year period of Esper et al.’s study. Hence, the suppression of LBM outbreaks over the
past quarter-century may have been the result of some synergistic consequence of the two factors (temperature and CO2) acting in unison, while a third possibility may involve only the
increase in the air’s CO2 content. Esper et al. say their findings highlight the “vulnerability of an otherwise stable ecological system in a warming environment,” in what would appear
to be an attempt to attach an undesirable connotation to the observed outcome. This wording seems strange indeed, for it is clear that the “recent disruption of a major disturbance regime,”
as Esper et al. refer to the suppression of LBM outbreaks elsewhere in their paper, would be considered by most people to be a positive outcome, and something to actually be welcomed.
Working with Antheraea polyphemus—a leafchewing generalist lepidopteran herbivore that represents the most abundant feeding guild in the hardwood trees that grow beneath the canopy of
the unmanaged loblolly pine plantation that hosts the Forest Atmosphere Carbon Transfer and Storage (FACTS-1) research site in the Piedmont region of North Carolina, USA, where the leaf-chewer
can consume 2-15 percent of the forest’s net primary production in any given year—Knepp et al. (2007) focused their attention on two species of oak tree— Quercus alba L. (white oak)
and Quercus velutina Lam. (black oak)—examining host plant preference and larval performance of A. polyphemus when fed foliage of the two tree species that had been grown in either ambient
or CO2-enriched air (to 200 ppm above ambient) in this long-running FACE experiment. In doing so, they determined that “growth under elevated CO2 reduced the food quality of oak leaves
for caterpillars,” while “consuming leaves of either oak species grown under elevated CO2 slowed the rate of development of A. polyphemus larvae.” In addition, they found that feeding
on foliage of Q. velutina that had been grown under elevated CO2 led to reduced consumption by the larvae and greater mortality. As a result, they concluded that “reduced consumption,
slower growth rates, and increased mortality of insect larvae may explain [the] lower total leaf damage observed previously in plots of this forest exposed to elevated CO2,” as documented
by Hamilton et al.
Climate Change Reconsidered 440 (2004) and Knepp et al. (2005), which finding bodes well indeed for the growth and vitality of such forests in the years and decades ahead, as the air’s
CO2 content continues to rise. Kampichler et al. (2008) also worked with oak trees. Noting, however, that “systems studied so far have not included mature trees,” they attempted to remedy
this situation by determining “the abundance of dominant leaf-galls (spangle-galls induced by the cynipid wasps Neuroterus quercusbaccarum and N. numismalis) and leaf-mines (caused by
the larvae of the moth Tischeria ekebladella) on freely colonized large oaks in a mixed forest in Switzerland, which received CO2 enrichment [540 ppm vs. 375 ppm during daylight hours]
from 2001 to 2004” via “the Swiss Canopy Crane (SCC) and a new CO2 enrichment technique (web-FACE)” in a forest that they say “is 80-120 years old with a canopy height of 32-38 m, consisting
of seven deciduous and four coniferous species.” This work allowed the German, Mexican, and Swiss researchers to discover that although elevated CO2 reduced various leaf parameters (water
content, proteins, non-structural carbohydrates, tannins, etc.) at the SCC site, “on the long term, their load with cynipid spangle-galls and leaf-mines of T. ekebladella was not distinguishable
from that in oaks exposed to ambient CO2 after 4 years of treatment.” Kampichler et al. concluded that in the situation they investigated, “CO2 enrichment had no lasting effect in all
three [animal] taxa, despite the substantial and consistent change in leaf chemistry of oak due to growth in elevated CO2.” In conclusion, therefore, and considering the results of all
of the studies reviewed in this section, it would appear that the ongoing rise in the air’s CO2 content will not result in greater damage to earth’s vegetation by the larvae of the many
moths that inhabit the planet, and could reduce the damage they cause. Additional information on this topic, including reviews of newer publications as they become available, available,
can be found at http://www.co2science.org/subject/i/summaries/moths.php. References Agrawal, A.A. 1999. Induced-responses to herbivory in wild radish: effects on several herbivores and
plant fitness. Ecology 80: 1713-1723. Baltensweiler, W. 1993. Why the larch bud moth cycle collapsed in the subalpine larch-cembran pine forests in the year 1990 for the first time since
1850. Oecologia 94: 62-66. Barbehenn, R.V., Karowe, D.N. and Spickard, A. 2004. Effects of elevated atmospheric CO2 on the nutritional ecology of C3 and C4 grass-feeding caterpillars.
Oecologia 140: 86-95. Bidart-Bouzat, M.G., Mithen, R. and Berenbaum, M.R. 2005. Elevated CO2 influences herbivory-induced defense responses of Arabidopsis thaliana. Oecologia 145: 415-424.
Buntgen, U., Esper, J., Frank, D.C., Nicolussi, K. and Schmidhalter, M. 2005. A 1052-year tree-ring proxy for alpine summer temperatures. Climate Dynamics 25: 141-153. Buntgen, U., Frank,
D.C., Nievergelt, D. and Esper, J. 2006. Alpine summer temperature variations, variations, AD 755-2004. Journal of Climate 19: 5606-5623. Chen, F., Wu, G., Ge, F., Parajulee, M.N. and
Shrestha, R.B. 2005. Effects of elevated CO2 and transgenic Bt cotton on plant chemistry, performance, and feeding of an insect herbivore, the cotton bollworm. Entomologia Experimentalis
et Applicata 115: 341-350. Esper, J., Buntgen, U., Frank, D.C., Nievergelt, D. and Liebhold, A. 2007. 1200 years of regular outbreaks in alpine insects. Proceedings of the Royal Society
B 274: 671-679. Hamilton, J.G., Zangerl, A.R., Berenbaum, M.R., Pippen, J.S., Aldea, M. and DeLucia, E.H. 2004. Insect herbivory in an intact forest understory under experimental CO2
enrichment. Oecologia 138: 566-573. Hattenschwiler, S. and Schafellner, C. 1999. Opposing effects of elevated CO2 and N deposition on Lymantria monacha larvae feeding on spruce trees.
Oecologia 118: 210-217. Kampichler, C., Teschner, M., Klein, S. and Korner, C. 2008. Effects of 4 years of CO2 enrichment on the abundance of leaf-galls and leaf-mines in mature mature
oaks. Acta Oecologica 34: 139-146. Kerslake, J.E., Woodin, S.J. and Hartley, S.E. 1998. Effects of carbon dioxide and nitrogen enrichment on a plant-insect interaction: the quality of
Calluna vulgaris as a host for Operophtera brumata. New Phytologist 140: 43-53. Kessler, A. and Baldwin, I.T. 2004. Herbivore-induced plant vaccination. Part I. The orchestration of
plant defenses in nature and their fitness consequences in the wild tobacco, Nicotiana attenuata. Plant Journal 38: 639-649. Kliebenstein, D., Pedersen, D., Barker, B. and Mitchell-Olds,
T. 2002. Comparative analysis of quantitative trait
Biological Effects of Carbon Dioxide Enrichment 441 loci controlling glucosinolates, myrosinase and insect resistance in Arabidopsis thaliana. Genetics 161: 325-332. Knepp, R.G., Hamilton,
J.G., Mohan, J.E., Zangerl, A.R., Berenbaum, M.R. and DeLucia, E.H. 2005. Elevated CO2 reduces leaf damage by insect herbivores in a forest community. New Phytologist 167: 207-218. Knepp,
R.G., Hamilton, J.G., Zangerl, A.R., Berenbaum, M.R. and DeLucia, E.H. 2007. Foliage of oaks grown under elevated CO2 reduces performance of Antheraea polyphemus (Lepidoptera: Saturniidae).
Environmental Entomology 36: 609-617. Lambers, H. 1993. Rising CO2, secondary plant metabolism, plant-herbivore interactions and litter decomposition. Theoretical considerations. Vegetatio
104/105: 263-271. Lincoln, D.E., Couvet, D. and Sionit, N. 1986. Responses of an insect herbivore to host plants grown in carbon dioxide enriched atmospheres. Oecologia 69: 556-560.
Lincoln, D.E., Sionit, N. and Strain, B.R. 1984. Growth and feeding response of Pseudoplusia includens (Lepidoptera: Noctuidae) to host plants grown in controlled carbon dioxide atmospheres.
Environmental Entomology 13: 1527-1530. Mauricio, R. and Rausher, M.D. 1997. Experimental manipulation of putative selective agents provides evidence for the role of natural enemies
in the evolution of plant defense. Evolution 51: 1435-1444. Rossi, A.M., Stiling, P., Moon, D.C., Cattell, M.V. and Drake, B.G. 2004. Induced defensive response of myrtle oak to foliar
insect herbivory in ambient and elevated CO2. Journal of Chemical Ecology 30: 1143-1152. Stiling, P., Moon, D.C., Hunter, M.D., Colson, J., Rossi, A.M., Hymus, G.J. and Drake, B.G. 2002.
Elevated CO2 lowers relative and absolute herbivore density across all species of a scrub-oak forest. Oecologia DOI 10.1007/s00442-002-1075-5. Stowe, K.A. 1998. Realized defense of artificially
selected lines of Brassica rapa: effects of quantitative genetic variation in foliar glucosinolate concentration. Environmental Entomology 27: 1166-1174. Williams, R.S., Lincoln, D.E.
and Norby, R.J. 2003. Development of gypsy moth larvae feeding on red maple saplings at elevated CO2 and temperature. Oecologia 137: 114-122. Wu, G., Chen, F.-J. and Ge, F. 2006. Response
of multiple generations of cotton bollworm Helicoverpa armigera Hubner, feeding on spring wheat, to elevated CO2. Journal of Applied Entomology 130: 2-9. Wu, G., Chen, F.-J., Sun, Y.-C.
and Ge, F. 2007. Response of successive three generations of cotton bollworm, Helicoverpa armigera (Hubner), fed on cotton bolls under elevated CO2. Journal of Environmental Sciences
19: 1318-1325. 7.3.3.4. Other Insects Docherty et al. (1997), in addition to studying aphids, studied two sap-feeding leafhopper species that were allowed to feed on saplings of beech
and sycamore that were grown in glasshouses maintained at atmospheric CO2 concentrations of 350 and 600 ppm. As far as they could determine, there were no significant effects of the
extra CO2 on either the feeding or performance characteristics of either leafhopper species. In a literature review of more than 30 studies published two years later, Whittaker (1999)
found that chewing insects (leaf chewers and leaf miners) showed either no change or reductions in abundance in response to atmospheric CO2 enrichment, noting, however, that population
reductions in this feeding guild were often accompanied by increased herbivory in response to CO2-induced reductions in leaf nitrogen content. In an experiment conducted on a natural
ecosystem in Wisconsin, USA—comprised predominantly of trembling aspen (Populus tremuloides Michx.)—Percy et al. (2002) studied the effects of increases in CO2 alone (to 560 ppm during
daylight hours), O3 alone (to 46.4-55.5 ppb during daylight hours), and CO2 and O3 together on the forest tent caterpillar (Malacosoma disstria), a common leaf-chewing lepidopteran found
in North American hardwood forests. By itself, elevated CO2 reduced caterpillar performance by reducing female pupal mass; while elevated O3 alone improved caterpillar performance by
increasing female pupal mass. When both gases were applied together, however, the elevated CO2 completely counteracted the enhancement of female pupal mass caused by elevated O3. Hence,
either alone or in combination with undesirable increases in the air’s O3 concentration, elevated CO2 tended to reduce the performance of the forest tent caterpillar. This finding is
particularly satisfying because, in the words of Percy et al., “historically, the forest tent catepillar has defoliated more deciduous forest than any other insect in North America,”
and because “outbreaks can reduce timber yield up to 90% in one year, and
Climate Change Reconsidered 442 increase tree vulnerability to disease and environmental stress.” In a study of yet another type of insect herbivore, Brooks and Whittaker (1999) removed
grassland monoliths from the Great Dun Fell of Cumbria, UK— which contained eggs of a destructive xylem-feeding spittlebug (Neophilaenus lineatus)—and grew them in glasshouses maintained
for two years at atmospheric CO2 concentrations of 350 and 600 ppm. During the course of their experiment, two generations of the xylem-feeding insect were produced; in each case, elevated
CO2 reduced the survival of nymphal stages by an average of 24 percent. Brooks and Whittaker suggest that this reduction in survival rate may have been caused by CO2-induced reductions
in stomatal conductance and transpirational water loss, which may have reduced xylem nutrient-water availability. Whatever the mechanism, the results of this study bode well for the
future survival of these species-poor grasslands as the air’s CO2 content continues to rise. In summing up the implications of the various phenomena described in this section, it would
appear that both CO2-induced and warming-induced changes in the physical characteristics and behavioral patterns of a diverse assemblage of insect types portend good things for the biosphere
in the years and decades to come. Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/i/insectsot
er.php. References Brooks, G.L. and Whittaker, J.B. 1999. Responses of three generations of a xylem-feeding insect, Neophilaenus lineatus (Homoptera), to elevated CO2. Global Change
Biology 5: 395-401. Docherty, M., Wade, F.A., Hurst, D.K., Whittaker, J.B. and Lea, P.J. 1997. Responses of tree sap-feeding herbivores to elevated CO2. Global Change Biology 3: 51-59.
Percy, K.E., Awmack, C.S., Lindroth, R.L., Kubiske, M.E., Kopper, B.J., Isebrands, J.G., Pregitzer, K.S., Hendrey, G.R., Dickson, R.E., Zak, D.R., Oksanen, E., Sober, J., Harrington,
R. and Karnosky, D.F. 2002. Altered performance of forest pests under atmospheres enriched by CO2 and O3. Nature 420: 403-407. Stiling, P., Moon, D.C., Hunter, M.D., Colson, J., Rossi,
A.M., Hymus, G.J. and Drake, B.G. 2002. Elevated CO2 lowers relative and absolute herbivore density across all species of a scrub-oak forest. Oecologia DOI 10.1007/s00442-002-1075-5.
Stiling, P., Rossi, A.M., Hungate, B., Dijkstra, P., Hinkle, C.R., Knot III, W.M., and Drake, B. 1999. Decreased leafminer abundance in elevated CO2: Reduced leaf quality and increased
parasitoid attack. Ecological Applications 9: 240-244. Thomas, C.D., Bodsworth, E.J., Wilson, R.J., Simmons, A.D., Davies, Z.G., Musche, M. and Conradt, L. 2001. Ecological and evolutionary
processes at expanding range margins. Nature 411: 577-581. Whittaker, J.B. 1999. Impacts and responses at population level of herbivorous insects to elevated CO2. European Journal of
Entomology 96: 149-156. 7.3.4. Shade Is the growth-enhancing effect of atmospheric CO2 enrichment reduced when light intensities are less than optimal? The question may be important
if a warmer world is also a cloudier world, as some climate models predict. In a review of the scientific literature designed to answer this question, Kerstiens (1998) analyzed the results
of 15 previously published studies of trees having differing degrees of shade tolerance, finding that elevated CO2 caused greater relative biomass increases in shade-tolerant species
than in shadeintolerant or sun-loving species. In more than half of the studies analyzed, shade-tolerant species experienced CO2-induced relative growth increases that were two to three
times greater than those of less shade-tolerant species. In an extended follow-up review analyzing 74 observations from 24 studies, Kerstiens (2001) reported that twice-ambient CO2 concentrations
increased the relative growth response of shadetolerant and shade-intolerant woody species by an average of 51 and 18 percent, respectively. Similar results were reported by Poorter
and Perez-Soba (2001), who performed a detailed meta-analysis of research results pertaining to this topic, and more recently by Kubiske et al. (2002), who measured photosynthetic acclimation
in aspen and sugar maple trees. On the other hand, a 200-ppm increase in the air’s CO2 concentration was found to enhance the photosynthetic rates of sunlit and shaded leaves of sweetgum
trees by 92 and 54 percent, respectively, at one time of year, and by 166 and 68 percent at another time (Herrick and Thomas, 1999). Likewise,
Biological Effects of Carbon Dioxide Enrichment 443 Naumburg and Ellsworth (2000) reported that a 200-ppm increase in the air’s CO2 content boosted steadystate photosynthetic rates in
leaves of four hardwood understory species by an average of 60 and 40 percent under high and low light intensities, respectively. Even though these photosynthetic responses were significantly
less in shaded leaves, they were still substantial, with mean increases ranging from 40 to 68 percent for a 60 percent increase in atmospheric CO2 concentration. Under extremely low
light intensities, the benefits arising from atmospheric CO2 enrichment may be small, but oftentimes they are very important in terms of plant carbon budgeting. In the study of Hattenschwiler
(2001), for example, seedlings of five temperate forest species subjected to an additional 200-ppm CO2 under light intensities that were only 3.4 and 1.3 percent of full sunlight exhibited
CO2-induced biomass increases that ranged from 17 to 74 percent. Similarly, in the study of Naumburg et al. (2001), a 200-ppm increase in the air’s CO2 content enhanced photosynthetic
carbon uptake in three of four hardwood understory species by more than twofold in three of the four species under light irradiances that were as low as 3 percent of full sunlight. In
a final study, in which potato plantlets inoculated with an arbuscular mycorrhizal fungus were grown at various light intensities and super CO2 enrichment of approximately 10,000 ppm,
Louche-Tessandier et al. (1999) found that the unusually high CO2 concentration produced an unusually high degree of root colonization by the beneficial mycorrhizal fungus, which typically
helps supply water and nutrients to plants. And it did so irrespective of the degree of light intensity to which the potato plantlets were exposed. So, whether light intensity is high
or low, or leaves are shaded or sunlit, when the CO2 content of the air is increased, so too are the various biological processes that lead to plant robustness. Less than optimal light
intensities do not negate the beneficial effects of atmospheric CO2 enrichment. Additional information on this topic, including reviews of newer publications as they become available,
can be found at http://www.co2science.org/subject/g/lightinteraction.php. References Hattenschwiler, S. 2001. Tree seedling growth in natural deep shade: functional traits related to
interspecific variation in response to elevated CO2. Oecologia 129: 31-42. Herrick, J.D. and Thomas, R.B. 1999. Effects of CO2 enrichment on the photosynthetic light response of sun
and shade leaves of canopy sweetgum trees (Liquidambar styraciflua) in a forest ecosystem. Tree Physiology 19: 779-786. Kerstiens, G. 1998. Shade-tolerance as a predictor of responses
to elevated CO2 in trees. Physiologia Plantarum 102: 472-480. Kerstiens, G. 2001. Meta-analysis of the interaction between shade-tolerance, light environment and growth response of woody
species to elevated CO2. Acta Oecologica 22: 61-69. Kubiske, M.E., Zak, D.R., Pregitzer, K.S. and Takeuchi, Y. 2002. Photosynthetic acclimation of overstory Populus tremuloides and understory
Acer saccharum to elevated atmospheric CO2 concentration: interactions with shade and soil nitrogen. Tree Physiology 22: 321-329. Louche-Tessandier, D., Samson, G., Hernandez-Sebastia,
C., Chagvardieff, P. and Desjardins, Y. 1999. Importance of light and CO2 on the effects of endomycorrhizal colonization on growth and photosynthesis of potato plantlets (Solanum tuberosum)
in an in vitro tripartite system. New Phytologist 142: 539-550. Naumburg, E. and Ellsworth, D.S. 2000. Photosynthetic sunfleck utilization potential of understory saplings growing under
elevated CO2 in FACE. Oecologia 122: 163-174. Naumburg, E., Ellsworth, D.S. and Katul, G.G. 2001. Modeling dynamic understory photosynthesis of contrasting species in ambient and elevated
carbon dioxide. Oecologia 126: 487-499. Poorter, H. and Perez-Soba, M. 2001. The growth response of plants to elevated CO2 under non-optimal environmental conditions. Oecologia 129:
129: 1-20. 7.3.5. Ozone Plants grown in CO2-enriched air nearly always exhibit increased photosynthetic rates and biomass production relative to plants grown at the current ambient CO2
concentration. By contrast, plants exposed to elevated ozone concentrations typically display reductions in photosynthesis and growth in comparison with plants grown at the current ambient
ozone concentration. In discussing the problem of elevated tropospheric ozone (O3) concentrations, Liu et al.
Climate Change Reconsidered 444 (2004) wrote that “ozone is considered to be one of the air pollutants most detrimental to plant growth and development in both urban and rural environments
(Lefohn, 1992; Skarby et al., 1998; Matyssek and Innes, 1999),” because it “reduces the growth and yield of numerous agronomic crops as well as fruit and forest trees (Retzlaff
et al., 1997; Fumagalli et al., 2001; Matyssek and Sandermann, 2003).” In addition, they say that ozone concentrations are “currently two to three times higher than in the early 1900s
(Galloway, 1998; Fowler et al., 1999),” and that they likely “will remain high in the future (Elvingson, 2001).” It is important to determine how major plants respond to concomitant
increases in the abundances of these two trace gases of the atmosphere, as their concentrations will likely continue to increase for many years to come. We begin with a review of the
literature with respect to various agriculture species, followed by a discussion on trees. Additional information on this topic, including reviews on the interaction of CO2 and O3 not
discussed here, can be found at http://www.co2 science.org/subject/o/subject_o.php under the heading Ozone. References Elvingson, P. 2001. For the most parts steadily down. Acid News
3: 20-21. Fowler, D., Cape, J.N., Coyle, M., Flechard, C., Kuylenstrierna, J., Hicks, K., Derwent, D., Johnson, C. and Stevenson, D. 1999. The global exposure of forests to air pollutants.
In: Sheppard, L.J. and Cape, J.N. (Eds.) Forest Growth Responses to the Pollution Climate of the 21st Century. Kluwer Academic Publisher, Dordrecht, The Netherlands, p. 5032. Fumagalli,
I., Gimeno, B.S., Velissariou, D., De Temmerman, L. and Mills, G. 2001. Evidence of ozoneinduced adverse effects on crops in the Mediterranean region. Atmospheric Environment 35: 2583-2587.
Galloway, J.N. 1998. The global nitrogen cycle: changes and consequences. Environmental Pollution 102: 15-24. Lefohn, A.S. 1992. Surface Level Ozone Exposure and Their Effects on Vegetation.
Lewis Publishers, Chelsea, UK. Liu, X.-P., Grams, T.E.E., Matyssek, R. and Rennenberg, H. 2005. Effects of elevated pCO2 and/or pO3 on C-, N-, and S-metabolites in the leaves of juvenile
beech and spruce differ between trees grown in monoculture and mixed culture. Plant Physiology and Biochemistry 43: 147-154. Liu, X., Kozovits, A.R., Grams, T.E.E., Blaschke, H., Rennenberg,
H. and Matyssek, R. 2004. Competition modifies effects of ozone/carbon dioxide concentrations on carbohydrate and biomass accumulation in juvenile Norway spruce and European beech. Tree
Physiology 24: 1045-1055. Matyssek, R. and Innes, J.L. 1999. Ozone—a risk factor for trees and forests in Europe? Water, Air and Soil Pollution 116: 199-226. Matyssek, R. and Sandermann,
H. 2003. Impact of ozone on trees: an ecophysiological perspective. Progress in Botany 64: 349-404. Retzlaff, W.A., Williams, L.E. and DeJong, T.M. 1997. Growth and yield response of
commercial bearing-age “Casselman” plum trees to various ozone partial pressures. Journal of Environmental Quality 26: 858-865. Skarby, L., Ro-Poulsen, H., Wellburn, F.A.M. and Sheppard,
L.J. 1998. Impacts of ozone on forests: a European perspective. New Phytologist 139: 109-122. 7.3.5.1. Agricultural Species Several studies have used soybean as a model plant to study
the effects of elevated CO2 and ozone on photosynthesis and growth. Reid et al. (1998), for example, grew soybeans for an entire season at different combinations of atmospheric CO2 and
ozone, reporting that elevated CO2 enhanced rates of photosynthesis in the presence or absence of ozone and that it typically ameliorated the negative effects of elevated ozone on carbon
assimilation. At the cellular level, Heagle et al. (1998a) reported that at twice the current ambient ozone concentration, soybeans simultaneously exposed to twice the current ambient
atmospheric CO2 concentration exhibited less foliar injury while maintaining significantly greater leaf chlorophyll contents than control plants exposed to elevated ozone and ambient
CO2 concentrations. By harvest time, the plants grown in the elevated ozone/elevated CO2 treatment combination had produced 53 percent more total biomass than their counterparts did
at elevated ozone and ambient CO2 concentrations (Miller et al., 1998). Finally, in analyzing seed yield, it was determined that atmospheric CO2 enrichment enhanced this parameter by
20 percent at ambient ozone, while it increased it by 74 percent at twice the ambient ozone
Biological Effects of Carbon Dioxide Enrichment 445 concentration (Heagle et al., 1998b). Thus, elevated CO2 completely ameliorated the negative effects of elevated ozone concentration
on photosynthetic rate and yield production in soybean. The ameliorating responses of elevated CO2 to ozone pollution also have been reported for various cultivars of spring and winter
wheat. In the study of Tiedemann and Firsching (2000), for example, atmospheric CO2 enrichment not only overcame the detrimental effects of elevated ozone on photosynthesis and growth,
it overcame the deleterious consequences resulting from inoculation with a biotic pathogen as well. Although infected plants displayed less absolute yield than non-infected plants at
elevated ozone concentrations, atmospheric CO2 enrichment caused the greatest relative yield increase in infected plants (57 percent vs. 38 percent). McKee et al. (2000) reported that
O3-induced reductions in leaf rubisco contents in spring wheat were reversed when plants were simultaneously exposed to twice-ambient concentrations of atmospheric CO2. In the study
of Vilhena-Cardoso and Barnes (2001), elevated ozone concentrations reduced photosynthetic rates in spring wheat grown at three different soil nitrogen levels. However, when concomitantly
exposed to twice-ambient atmospheric CO2 concentrations, elevated ozone had no effect on rates of photosynthesis, regardless of soil nitrogen. Going a step further, Pleijel et al. (2000)
observed that ozone-induced reductions in spring wheat yield were partially offset by concomitant exposure to elevated CO2 concentrations. Similar results have been reported in spring
wheat by Hudak et al. (1999) and in winter wheat by Heagle et al. (2000). Cotton plants grown at elevated ozone concentrations exhibited 25 and 48 percent reductions in leaf mass per
unit area and foliar starch concentration, respectively, relative to control plants grown in ambient air. When simultaneously exposed to twice-ambient CO2 concentrations, however, the
reductions in these parameters were only 5 and 7 percent, respectively (Booker, 2000). With respect to potato, Wolf and van Oijen (2002) used a validated potato model to predict increases
in European tuber production ranging from 1,000 to 3,000 kg of dry matter per hectare in spite of concomitant increases in ozone concentrations and air temperatures. It is clear from
these studies that elevated CO2 reduces, and nearly always completely overrides, the negative effects of ozone pollution on plant photosynthesis, growth, and yield. When explaining the
mechanisms behind such responses, most authors suggest that atmospheric CO2 enrichment tends to reduce stomatal conductance, which causes less indiscriminate uptake of ozone into internal
plant air spaces and reduces subsequent conveyance to tissues where damage often results to photosynthetic pigments and proteins, reducing plant growth and biomass production. Additional
information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/o/ozoneplantsag.php. References Booker,
F.L. 2000. Influence of carbon dioxide enrichment, ozone and nitrogen fertilization on cotton (Gossypium hirsutum L.) leaf and root composition. Plant, Cell and Environment 23: 573-583.
Heagle, A.S., Miller, J.E. and Booker, F.L. 1998a. Influence of ozone stress on soybean response to carbon dioxide enrichment: I. Foliar properties. Crop Science 38: 113-121. Heagle,
A.S., Miller, J.E. and Pursley, W.A. 1998b. Influence of ozone stress on soybean response to carbon dioxide enrichment: III. Yield and seed quality. Crop Science 38: 128-134. Heagle,
A.S., Miller, J.E. and Pursley, W.A. 2000. Growth and yield responses of winter wheat to mixtures of ozone and carbon dioxide. Crop Science 40: 1656-1664. Hudak, C., Bender, J., Weigel,
H.-J. and Miller, J. 1999. Interactive effects of elevated CO2, O3, and soil water deficit on spring wheat (Triticum aestivum L. cv. Nandu). Agronomie 19: 677-687. McKee, I.F., Mulholland,
B.J., Craigon, J., Black, C.R. and Long, S.P. 2000. Elevated concentrations of atmospheric CO2 protect against and compensate for O3 damage to photosynthetic tissues of field-grown wheat.
New Phytologist 146: 427-435. Miller, J.E., Heagle, A.S. and Pursley, W.A. 1998. Influence of ozone stress on soybean response to carbon dioxide enrichment: II. Biomass and development.
Crop Science 38: 122-128. Pleijel, H., Gelang, J., Sild, E., Danielsson, H., Younis, S., Karlsson, P.-E., Wallin, G., Skarby, L. and Sellden, G. 2000. Effects of elevated carbon dioxide,
ozone and water availability on spring wheat growth and yield. Physiologia Plantarum 108: 61-70. Reid, C.D. and Fiscus, E.L. 1998. Effects of elevated [CO2] and/or ozone on limitations
to CO2 assimilation in soybean
Climate Change Reconsidered 446 (Glycine max). Journal of Experimental Botany 18: 885-895. Tiedemann, A.V. and Firsching, K.H. 2000. Interactive effects of elevated ozone and carbon
dioxide on growth and yield of leaf rust-infected versus non-infected wheat. Environmental Pollution 108: 357-363. Vilhena-Cardoso, J. and Barnes, J. 2001. Does nitrogen supply affect
the response of wheat (Triticum aestivum cv. Hanno) to the combination of elevated CO2 and O3? Journal of Experimental Botany 52: 1901-1911. Wolf, J. and van Oijen, M. 2002. Modelling
the dependence of European potato yields on changes in climate and CO2. Agricultural and Forest Meteorology 112: 217-231. 7.3.5.2. Woody Species 7.3.5.2.1. Aspen Karnosky et al. (1999)
grew O3-sensitive and O3-tolerant aspen clones in 30-m diameter plots at the Aspen FACE site near Rhinelander, Wisconsin, USA, which were maintained at atmospheric CO2 concentrations
of 360 and 560 ppm with and without exposure to elevated O3 (1.5 times ambient ozone concentration). After one year of growth at ambient CO2, elevated O3 had caused visible injury to
leaves of both types of aspen, with the average percent damage in O3-sensitive clones being more than three times as great as that observed in O3-tolerant clones (55 percent vs. 17 percent,
respectively). In combination with elevated CO2, however, O3-induced damage to leaves of these same clones was only 38 percent and 3 percent, respectively. Thus, elevated CO2 ameliorated
much of the foliar damage induced by high O3 concentrations. King et al. (2001) studied the same plants for a period of two years, concentrating on below-ground growth, where elevated
O3 alone had no effect on fine-root biomass. When the two aspen clones were simultaneously exposed to elevated CO2 and O3, however, there was an approximate 66 percent increase in the
fine-root biomass of both of them. Also in the same experiment, Noormets et al. (2001) studied the interactive effects of O3 and CO2 on photosynthesis, finding that elevated CO2 increased
rates of photosynthesis in both clones at all leaf positions. Maximum rates of photosynthesis were increased in the O3-tolerant clone by averages of 33 and 49 percent due to elevated
CO2 alone and in combination with elevated O3, respectively, while in the O3-sensitive clone they were increased by 38 percent in both situations. Hence, CO2-induced increases in maximal
rates of net photosynthesis were typically maintained, and sometimes increased, during simultaneous exposure to elevated O3. Yet again in the same experiment, Oksanen et al. (2001) reported
that after three years of treatment, ozone exposure caused significant structural injuries to thylakoid membranes and the stromal compartment within chloroplasts, but that these injuries
were largely ameliorated by atmospheric CO2 enrichment. Likewise, leaf thickness, mesophyll tissue thickness, the amount of chloroplasts per unit cell area, and the amount of starch
in leaf chloroplasts were all decreased in the high ozone treatment, but simultaneous exposure of the ozone-stressed trees to elevated CO2 more than compensated for the ozoneinduced
reductions. After four years of growing five aspen clones with varying degrees of tolerance to ozone under the same experimental conditions, McDonald et al. (2002) developed what they
termed a “competitive stress index,” based on the heights of the four nearest neighbors of each tree, to study the influence of competition on the CO2 growth response of the various
clones as modified by ozone. In general, elevated O3 reduced aspen growth independent of competitive status, while the authors noted an “apparent convergence of competitive performance
responses in +CO2 +O3 conditions,” which they say suggests that “stand diversity may be maintained at a higher level” in such circumstances. Percy et al. (2002) utilized the same experimental
setting to assess a number of the trees’ growth characteristics, as well as the responses of one plant pathogen and two insects with different feeding strategies that typically attack
the trees. Of the plant pathogen studied, they say that “the poplar leaf rust, Melampsora medusae, is common on aspen and belongs to the most widely occurring group of foliage diseases.”
As for the two insects, they report that “the forest tent caterpillar, Malacosoma disstria, is a common leaf-chewing lepidopteran in North American hardwood forests” and that “the sapfeeding
aphid, Chaitophorus stevensis, infests aspen throughout its range.” Hence, the rust and the two insect pests the scientists studied are widespread and have significant deleterious impacts
on trembling aspen and other tree species. As but one example of this fact, the authors note that, “historically, the forest tent caterpillar has defoliated more deciduous forest
Biological Effects of Carbon Dioxide Enrichment 447 than any other insect in North America” and that “outbreaks can reduce timber yield up to 90% in one year, and increase tree vulnerability
to disease and environmental stress.” Percy et al. found that by itself, elevated O3 decreased tree height and trunk diameter, increased rust occurrence by nearly fourfold, improved
tent caterpillar performance by increasing female pupal mass by 31 percent, and had a strong negative effect on the natural enemies of aphids. The addition of the extra CO2, however,
completely ameliorated the negative effects of elevated O3 on tree height and trunk diameter, reduced the O3-induced enhancement of rust development from nearly fourfold to just over
twofold, completely ameliorated the enhancement of female tent caterpillar pupal mass caused by elevated O3, and completely ameliorated the reduction in the abundance of natural enemies
of aphids caused by elevated O3. In a final study from the Aspen FACE site, Holton et al. (2003) raised parasitized and nonparasitized forest tent caterpillars on two quaking aspen genotypes
(O3-sensitive and O3-tolerant) alone and in combination for one full growing season; they, too, found that elevated O3 improved tent caterpillar performance under ambient CO2 conditions,
but not in CO2-enriched air. In summary, it is clear that elevated ozone concentrations have a number of significant negative impacts on the well-being of North America’s most widely
distributed tree species, while elevated carbon dioxide concentrations have a number of significant positive impacts. In addition, elevated CO2 often completely eliminates the negative
impacts of elevated O3. If the tropospheric O3 concentration continues to rise as expected (Percy et al. note that “damaging O3 concentrations currently occur over 29% of the world’s
temperate and subpolar forests but are predicted to affect fully 60% by 2100”), we might hope the air’s CO2 content continues to rise as well. Additional information on this topic, including
reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/o/ozoneaspen.php. References Holton, M.K., Lindroth, R.L. and Nordheim, E.V.
2003. Foliar quality influences tree-herbivore-parasitoid interactions: effects of elevated CO2, O3, and plant genotype. Oecologia 137: 233-244. Karnosky, D.F., Mankovska, B., Percy,
K., Dickson, R.E., Podila, G.K., Sober, J., Noormets, A., Hendrey, G., Coleman, M.D., Kubiske, M., Pregitzer, K.S. and Isebrands, J.G. 1999. Effects of tropospheric O3 on trembling aspen
and interaction with CO2: results from an O3-gradient and a FACE experiment. Water, Air, and Soil Pollution 116: 311-322. King, J.S., Pregitzer, K.S., Zak, D.R., Sober, J., Isebrands,
J.G., Dickson, R.E., Hendrey, G.R. and Karnosky, D.F. 2001. Fine-root biomass and fluxes of soil carbon in young stands of paper birch and trembling aspen as affected by elevated atmospheric
CO2 and tropospheric O3. Oecologia 128: 237-250. McDonald, E.P., Kruger, E.L., Riemenschneider, Riemenschneider, D.E. and Isebrands, J.G. 2002. Competitive status influences treegrowth
responses to elevated CO2 and O3 in aggrading aspen stands. Functional Ecology 16: 792-801. Noormets, A., Sober, A., Pell, E.J., Dickson, R.E., Podila, G.K., Sober, J., Isebrands, J.G.
and Karnosky, D.F. 2001. Stomatal and non-stomatal limitation to photosynthesis in two trembling aspen (Populus tremuloides Michx.) clones exposed to elevated CO2 and O3. Plant, Cell
and Environment 24: 327-336. Oksanen, E., Sober, J. and Karnosky, D.F. 2001. Impacts of elevated CO2 and/or O3 on leaf ultrastructure of aspen (Populus tremuloides) and birch (Betula
papyrifera) in the Aspen FACE experiment. Environmental Pollution 115: 437-446. Percy, K.E., Awmack, C.S., Lindroth, R.L., Kubiske, M.E., Kopper, B.J., Isebrands, J.G., Pregitzer, K.S.,
Hendrey, G.R., Dickson, R.E., Zak, D.R., Oksanen, E., Sober, J., Harrington, R. and Karnosky, D.F. 2002. Altered performance of forest pests under atmospheres enriched by CO2 and O3.
Nature 420: 403-407. 7.3.5.2.2. Beech Liu et al. (2005) grew three-and four-year-old seedlings of European beech (Fagus sylvatica L.) for five months in well watered and fertilized soil
in containers located within walk-in phytotrons maintained at either ambient or ambient + 300 ppm CO2 (each subdivided into ambient and doubleambient O3 concentration treatments, with
maximum ozone levels restricted to <150 ppb), in both monoculture and in competition with Norway spruce, after which they examined the effects of each treatment on leaf non-structural
carbohydrate levels (soluble sugars and starch). They found that the effects of elevated O3 alone on non-structural carbohydrate levels were small when the beech
Climate Change Reconsidered 448 seedlings were grown in monoculture. When they were grown in mixed culture, however, the elevated O3 slightly enhanced leaf sugar levels, but reduced
starch levels by 50 percent. With respect to elevated CO2 alone, for the beech seedlings grown in both monoculture and mixed culture, levels of sugar and starch were significantly enhanced.
Hence, when elevated O3 and CO2 significantly affected non-structural carbohydrate levels, elevated CO2 tended to enhance them, whereas elevated O3 tended to reduce them. In addition,
the combined effects of elevated CO2 and O3 acting together were such as to produce a significant increase in leaf non-structural carbohydrates in both mixed and monoculture conditions.
As a result, the researchers concluded that “since the responses to the combined exposure were more similar to elevated pCO2 than to elevated pO3, apparently elevated pCO2 overruled
the effects of elevated pO3 on nonstructural carbohydrates.” In a slightly longer study, Grams et al. (1999) grew European beech seedlings in glasshouses maintained at average atmospheric
CO2 concentrations of either 367 or 667 ppm for a period of one year. Then, throughout the following year, in addition to being exposed to the same set of CO2 concentrations the seedlings
were exposed to either ambient or twice-ambient levels of O3. This protocol revealed that elevated O3 significantly reduced photosynthesis in beech seedlings grown at ambient CO2 concentrations
by a factor of approximately three. By contrast, in the CO2-enriched air the seedlings did not exhibit any photosynthetic reduction due to the doubled O3 concentrations. In fact, the
photosynthetic rates of the CO2-enriched seedlings actually rose by 8 percent when simultaneously fumigated with elevated O3, leading the researchers to conclude that “long-term acclimation
to elevated CO2 supply does counteract the O3-induced decline of photosynthetic light and dark reactions.” In a still longer study, Liu et al. (2004) grew three-and four-year-old old
beech seedlings for two growing seasons under the same experimental conditions as Liu et al. (2005) after the seedlings had been pre-acclimated for one year to either the ambient or
elevated CO2 treatment. At the end of the study, the plants were harvested and fresh weights and dry biomass values were determined for leaves, shoot axes, coarse roots, and fine roots,
as were carbohydrate (starch and soluble sugar) contents and concentrations for the same plant parts. This work falsified the hypothesis that “prolonged exposure to elevated CO2 does
not compensate for the adverse ozone effects on European beech,” as it revealed that all “adverse effects of ozone on carbohydrate concentrations and contents were counteracted when
trees were grown in elevated CO2.” Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/o/ozonebee
h.php. References Grams, T.E.E., Anegg, S., Haberle, K.-H., Langebartels, C. and Matyssek, R. 1999. Interactions of chronic exposure to elevated CO2 and O3 levels in the photosynthetic
light and dark reactions of European beech (Fagus sylvatica). New Phytologist 144: 95-107. Liu, X.-P., Grams, T.E.E., Matyssek, R. and Rennenberg, H. 2005. Effects of elevated pCO2 and/or
pO3 on C-, N-, and S-metabolites in the leaves of juvenile beech and spruce differ between trees grown in monoculture and mixed culture. Plant Physiology and Biochemistry 43: 147-154.
Liu, X., Kozovits, A.R., Grams, T.E.E., Blaschke, H., Rennenberg, H. and Matyssek, R. 2004. Competition modifies effects of ozone/carbon dioxide concentrations on carbohydrate and biomass
accumulation in juvenile Norway spruce and European beech. Tree Physiology 24: 1045-1055. 7.3.5.2.3. Birch At the FACE facility near Rhinelander, Wisconsin, USA, King et al. (2001) grew
a mix of paper birch and quaking aspen trees in 30-m diameter plots that were maintained at atmospheric CO2 concentrations of 360 and 560 ppm with and without exposure to elevated O3
(1.5 times the ambient O3 concentration) for a period of two years. In their study of the belowground environment of the trees, they found that the extra O3 had no effect on the growth
of fine roots over that time period, but that elevated O3 and CO2 together increased the fine-root biomass of the mixed stand by 83 percent. One year later at the same FACE facility,
Oksanen et al. (2001) observed O3-induced injuries in the thylokoid membranes of the chloroplasts of the birch trees’ leaves; the injuries were partially ameliorated in the elevated
CO2 treatment. And in a study conducted two years later, Oksanen et al.
Biological Effects of Carbon Dioxide Enrichment 449 (2003) say they “were able to visualize and locate ozone-induced H2O2 accumulation within leaf mesophyll cells, and relate oxidative
stress with structural injuries.” However, they report that “H2O2 accumulation was found only in ozone-exposed leaves and not in the presence of elevated CO2,” adding that “CO2 enrichment
appears to alleviate chloroplastic oxidative stress.” Across the Atlantic in Finland, Kull et al. (2003) constructed open-top chambers around two clones (V5952 and K1659) of silver birch
saplings that were rooted in the ground and had been growing there for the past seven years. These chambers were fumigated with air containing 360 and 720 ppm CO2 in combination with
30 and 50 ppb O3 for two growing seasons, after which it was noted that the extra O3 had significantly decreased branching in the trees’ crowns. This malady, however, was almost completely
ameliorated by a doubling of the air’s CO2 concentration. In addition, after one more year year of study, Eichelmann et al. (2004) reported that, by itself, the increase in the air’s
CO2 content increased the average net photosynthetic rates of both clones by approximately 16 percent, while the increased O3 by itself caused a 10 percent decline in the average photosynthetic
rate of clone V5952, but not of clone K1659. When both trace gases were simultaneously increased, however, the photosynthetic rate of clone V5952 once again experienced a 16 percent
increase in net photosynthesis, as if the extra O3 had had no effect when applied in the presence of the extra CO2. After working with the same trees for one additional year, Riikonen
et al. (2004) harvested them and reported finding that “the negative effects of elevated O3 were found mainly in ambient CO2, not in elevated CO2.” In fact, whereas doubling the air’s
O3 concentration decreased total biomass production by 13 percent across both clones, simultaneously doubling the air’s CO2 concentration increased total biomass production by 30 percent,
thereby more than compensating for the deleterious consequences of doubling the atmospheric ozone concentration. In commenting on this ameliorating effect of elevated CO2, the team of
Finnish scientists said it “may be associated with either increased detoxification capacity as a consequence of higher carbohydrate concentrations in leaves grown in elevated CO2, or
decreased stomatal conductance and thus decreasing O3 uptake in elevated CO2 conditions (e.g., Rao et al., 1995).” They also noted that “the ameliorating effect of elevated CO2 is in
accordance with the results of single-season open-top chamber and growth chamber studies on small saplings of various deciduous tree species (Mortensen 1995; Dickson et al., 1998; Loats
and Rebbeck, 1999) and long-term open-field and OTC studies with aspen and yellow-poplar (Percy et al., 2002; Rebbeck and Scherzer, 2002).” In another paper to come out of the Finnish
silver birch study, Peltonen et al. (2005) evaluated the impacts of doubled atmospheric CO2 and O3 concentrations on the accumulation of 27 phenolic compounds in the leaves of the trees,
finding that elevated CO2 increased the concentration of phenolic acids (+25 percent), myricetin glycosides (+18 percent), catechin derivatives (+13 percent), and soluble condensed tannins
(+19 percent). Elevated O3, on the other hand, increased the concentration of one glucoside by 22 percent, chlorogenic acid by 19 percent, and flavone aglycons by 4 percent. However,
Peltonen et al. say this latter O3-induced production of antioxidant phenolic compounds “did not seem to protect the birch leaves from detrimental O3 effects on leaf weight and area,
but may have even exacerbated them.” Last, in the combined elevated CO2 and O3 treatment, they found that “elevated CO2 did seem to protect the leaves from elevated O3 because all the
O3-derived effects on the leaf phenolics and traits were prevented by elevated CO2.” Meanwhile, back at the FACE facility near Rhinelander, Wisconsin, USA, Agrell et al. (2005) examined
the effects of ambient and elevated concentrations of atmospheric CO2 and O3 on the foliar chemistry of birch and aspen trees, plus the consequences of these effects for host plant preferences
of forest tent caterpillar larvae. In doing so, they found that “the only chemical component showing a somewhat consistent co-variation with larval preferences was condensed tannins,”
and they discovered that “the tree becoming relatively less preferred as a result of CO2 or O3 treatment was in general also the one for which average levels of condensed tannins were
most positively (or least negatively) affected by that treatment.” The mean condensed tannin concentration of birch leaves was 18 percent higher in the elevated CO2 and O3 treatment.
Consequently, as atmospheric concentrations of CO2 and O3 continue to rise, the increases in condensed tannin concentrations likely to occur in the foliage of birch trees should lead
to their leaves becoming less preferred for consumption by the forest tent caterpillar. Concurrent with the work of Agrell et al., King et al. (2005) evaluated the effect of CO2 enrichment
Climate Change Reconsidered 450 alone, O3 enrichment alone, and the net effect of both CO2 and O3 enrichment together on the growth of the Rhinelander birch trees, finding that relative
to
the ambient-air control treatment, elevated CO2 increased total biomass by 45 percent in the aspen-birch community, while elevated O3 caused a 13 percent reduction in total biomass
relative to the control. Of most interest, the combination of elevated CO2 and O3 resulted in a total biomass increase of 8.4 percent relative to the control aspen-birch community. King
et al. thus concluded that “exposure to even moderate levels of O3 significantly reduces the capacity of net primary productivity to respond to elevated CO2 in some forests.” Consequently,
they suggested it makes sense to move forward with technologies that reduce anthropogenic precursors to photochemical O3 formation, because the implementation of such a policy would
decrease an important constraint on the degree to which forest ecosystems can positively respond to the ongoing rise in the air’s CO2 concentration. Another paper to come out of the
Finnish silver birch study was that of Kostiainen et al. (2006), who studied the effects of elevated CO2 and O3 on various wood properties. Their work revealed that the elevated CO2
treatment had no effect on wood structure, but that it increased annual ring width by 21 percent, woody biomass by 23 percent, and trunk starch concentration by 7 percent. Elevated O3,
on the other hand, decreased stem vessel percentage in one of the clones by 10 percent; it had no effect on vessel percentage in the presence of elevated CO2. In discussing their results,
Kostiainen et al. note that “in the xylem of angiosperms, water movement occurs principally in vessels (Kozlowski and Pallardy, 1997),” and that “the observed decrease in vessel percentage
by elevated O3 may affect water transport,” lowering it. However, as they continue, “elevated CO2 ameliorated the O3-induced decrease in vessel percentage.” In addition, they note that
“the concentration of nonstructural carbohydrates (starch and soluble sugars) in tree tissues is considered a measure of carbon shortage or surplus for growth (Korner, 2003).” They conclude
that “starch accumulation observed under elevated CO2 in this study indicates a surplus of carbohydrates produced by enhanced photosynthesis of the same trees (Riikonen et al., 2004).”
In addition, they report that “during winter, starch reserves in the stem are gradually transformed to soluble carbohydrates involved in freezing tolerance (Bertrand et al., 1999; Piispanen
and Saranpaa, 2001), so the increase in starch concentration may improve acclimation in winter.” Rounding out the suite of Rhinelander FACE studies of paper birch is the report of Darbah
et al. (2007), who found that the total number of trees that flowered increased by 139 percent under elevated CO2 but only 40 percent under elevated O3. Likewise, with respect to the
quantity of flowers produced, they found that elevated CO2 led to a 262 percent increase, while while elevated O3 led to only a 75 percent increase. They also determined that elevated
CO2 had significant positive effects on birch catkin size, weight, and germination success rate, with elevated CO2 increasing the germination rate of birch by 110 percent, decreasing
seedling mortality by 73 percent, increasing seed weight by 17 percent, and increasing new seedling root length by 59 percent. They found just the opposite was true of elevated O3, as
it decreased the germination rate of birch by 62 percent, decreased seed weight by 25 percent, and increased new seedling root length by only 15 percent. In discussing their findings,
Darbah et al. additionally report that “the seeds produced under elevated O3 had much less stored carbohydrate, lipids, and proteins for the newly developing seedlings to depend on and,
hence, the slow growth rate.” As a result, they conclude that “seedling recruitment will be enhanced under elevated CO2 but reduced under elevated O3.” In summary, from their crowns
to their roots, roots, birch trees are generally negatively affected by rising ozone concentrations. When the air’s CO2 content is also rising, however, these negative consequences may
often be totally eliminated and replaced by positive responses. Additional information on this topic, including reviews of newer publications as they become available, can be found at
http://www.co2science.org/subject/o/ozonebirch.php. References Agrell, J., Kopper, B., McDonald, E.P. and Lindroth, R.L. 2005. CO2 and O3 effects on host plant preferences of the forest
tent caterpillar (Malacosoma disstria). Global Change Biology 11: 588-599. Bertrand, A., Robitaille, G., Nadeau, P. and Castonguay, Y. 1999. Influence of ozone on cold acclimation in
sugar maple seedlings. Tree Physiology 19: 527-534. Darbah, J.N.T., Kubiske, M.E., Nelson, N., Oksanen, E., Vaapavuori, E. and Karnosky, D.F. 2007. Impacts of
Biological Effects of Carbon Dioxide Enrichment 451 elevated atmospheric CO2 and O3 on paper birch (Betula papyrifera): Reproductive fitness. The Scientific World Journal 7(S1): 240-246.
Dickson, R.E., Coleman, M.D., Riemenschneider, D.E., Isebrands, J.G., Hogan, G.D. and Karnosky, D.F. 1998. Growth of five hybrid poplar genotypes exposed to interacting elevated [CO2]
and [O3]. Canadian Journal of Forest Research 28: 1706-1716. Eichelmann, H., Oja, V., Rasulov, B., Padu, E., Bichele, I., Pettai, H., Mols, T., Kasparova, I., Vapaavuori, E. and Laisk,
A. 2004. Photosynthetic parameters of birch (Betula pendula Roth) leaves growing in normal and in CO2-and O3-enriched atmospheres. Plant, Cell and Environment 27: 479-495. King, J.S.,
Kubiske, M.E., Pregitzer, K.S., Hendrey, G.R., McDonald, E.P., Giardina, C.P., Quinn, V.S. and Karnosky, D.F. 2005. Tropospheric O3 compromises net primary production in young stands
of trembling aspen, paper birch and sugar maple in response to elevated atmospheric CO2. New Phytologist 168: 623-636. King, J.S., Pregitzer, K.S., Zak, D.R., Sober, J., Isebrands, J.G.,
Dickson, R.E., Hendrey, G.R. and Karnosky, D.F. 2001. Fine-root biomass and fluxes of soil carbon in young stands of paper birch and trembling aspen as affected by elevated atmospheric
CO2 and tropospheric O3. Oecologia 128: 237-250. Korner, C. 2003. Carbon limitation in trees. Journal of Ecology 91: 4-17. Kostiainen, K., Jalkanen, H., Kaakinen, S., Saranpaa, P. and
Vapaavuori, E. 2006. Wood properties of two silver birch clones exposed to elevated CO2 and O3. Global Change Biology 12: 1230-1240. Kozlowski, T.T. and Pallardy, S.G. 1997. Physiology
of Woody Plants. Academic Press, San Diego, CA, USA. Kull, O., Tulva, I. and Vapaavuori, E. 2003. Influence of elevated CO2 and O3 on Betula pendula Roth crown structure. Annals of Botany
91: 559-569. Loats, K.V. and Rebbeck, J. 1999. Interactive effects of ozone and elevated carbon dioxide on the growth and physiology of black cherry, green ash, and yellow-poplar seedlings.
Environmental Pollution 106: 237-248. Mortensen, L.M. 1995. Effects of carbon dioxide concentration on biomass production and partitioning in Betula pubescens Ehrh. seedlings at different
ozone and temperature regimes. Environmental Pollution 87: 337-343. Oksanen, E., Haikio, E., Sober, J. and Karnosky, D.F. 2003. Ozone-induced H2O2 accumulation in field-grown aspen and
birch is linked to foliar ultrastructure and peroxisomal activity. New Phytologist 161: 791-799. Oksanen, E., Sober, J. and Karnosky, D.F. 2001. Impacts of elevated CO2 and/or O3 on
leaf ultrastructure of aspen (Populus tremuloides) and birch (Betula papyrifera) in the Aspen FACE experiment. Environmental Pollution 115: 437-446. Peltonen, P.A., Vapaavuori, E. and
Julkunen-Tiitto, R. 2005. Accumulation of phenolic compounds in birch leaves is changed by elevated carbon dioxide and ozone. Global Change Biology 11: 1305-1324. Percy, K.E., Awmack,
C.S., Lindroth, R.L., Kubiske, M.E., Kopper, B.J., Isebrands, J.G., Pregitzer, K.S., Hendrey, G.R., Dickson, R.E., Zak, D.R., Oksanen, E., Sober, J., Harrington, R. and Karnosky, D.F.
2002. Altered performance of forest pests enriched by CO2 and O3. Nature 420: 403-407. Piispanen, R. and Saranpaa, P. 2001. Variation of nonstructural carbohydrates in silver birch (Betula
pendula Roth) wood. Trees 15: 444-451. Rao, M.V., Hale, B.A. and Ormrod, D.P. 1995. Amelioration of ozone-induced oxidative damage in wheat plants grown under high carbon dioxide. Plant
Physiology 109: 421-432. Rebbeck, J. and Scherzer, A.J. 2002. Growth responses of yellow-poplar (Liriodendron tulipifera L.) exposed to 5 years of [O3] alone or combined with elevated
[CO2]. Plant, Cell and Environment 25: 1527-1537. Riikonen, J., Lindsberg, M.-M., Holopainen, T., Oksanen, E., Lappi, J., Peltonen, P. and Vapaavuori, E. 2004. Silver birch and climate
change: variable growth and carbon allocation responses to elevated concentrations of carbon dioxide and ozone. Tree Physiology 24: 1227-1237. 7.3.5.2.4. Yellow-Poplar Scherzel et al.
(1998) grew yellow-poplar seedlings in open-top chambers for four years at three different combinations of atmospheric O3 and CO2—(1) ambient O3 and ambient CO2, (2) doubled O3 and ambient
CO2, and (3) doubled O3 and doubled CO2— to study the interactive effects of these gases on leaflitter decomposition. This experiment revealed that the decomposition rates of yellow-poplar
leaves were similar for all three treatments for nearly five months, after which time litter produced in the elevated O3 and elevated CO2 air decomposed at a significantly slower rate,
such that even after two years of decomposition, litter from the elevated O3 and elevated CO2 treatment still contained about 12 percent more biomass than litter produced in the other
two treatments. This reduced rate of decomposition under elevated O3 and CO2 conditions will likely result in greater carbon
Climate Change Reconsidered 452 sequestration in soils supporting yellow-poplar trees over the next century or more. Loats and Rebbeck (1999) grew yellow-poplar seedlings for ten weeks
in pots they placed within growth chambers filled with ambient air, air with twice the ambient CO2 concentration, air with twice the ambient O3 concentration, and air with twice the
ambient CO2 and O3 concentrations to determine the effects of elevated CO2 and O3 on photosynthesis and growth in this deciduous tree species. In doing so, they found that doubling the
air’s CO2 concentration increased the rate of net photosynthesis by 55 percent in ambient O3 air, and that at twice the ambient level of O3 it stimulated net photosynthesis by an average
of 50 percent. Similarly, the doubled CO2 concentration significantly increased total biomass by 29 percent, while the doubled O3 concentration had little impact on growth. Last, Rebbeck
et al. (2004) grew yellow poplar seedlings for five years within open-top chambers in a field plantation at Delaware, Ohio, USA, exposing them continuously from mid-May through mid-October
of each year to either (1) charcoal-filtered air to remove ambient O3, (2) ambient O3, (3) 1.5 times ambient O3, and (4) 1.5 times ambient O3 plus 350 ppm CO2 above ambient CO2, while
they periodically measured a number of plant parameters and processes. Throughout the study, the trees were never fertilized, and they received no supplemental water beyond some given
in the first season. Averaged over the experiment’s five growing seasons, the midseason net photosynthetic rate of upper canopy foliage at saturating light intensities declined by 10
percent when the trees were grown in ambient O3-air and by 14 percent when they were grown in elevated O3-air, when compared to the trees that were grown in the charcoal-filtered air,
while seasonal net photosynthesis of foliage grown in the combination of elevated O3 and elevated CO2 was 57-80 percent higher than it was in the trees exposed to elevated O3 alone.
There was also no evidence of any photosynthetic down regulation in the trees exposed to the elevated O3 and CO2 air, with some of the highest rates being observed during the final growing
season. Consequently, Rebbeck et al. concluded that “elevated CO2 may ameliorate the negative effects of increased tropospheric O3 on yellow-poplar.” In fact, their results suggest that
a nominally doubled atmospheric CO2 concentration more than compensates for the deleterious effects of a 50 percent increase in ambient O3 levels. As the air’s CO2 content continues
to rise, earth’s yellow-poplar trees will likely display substantial increases in photosynthetic rate and biomass production, even under conditions of elevated O3 concentrations; and
the soils in which the trees grow should sequester ever greater quantities of carbon. Additional information on this topic, including reviews of newer publications as they become available,
can be found at http://www.co2science.org/subject/o/ozoneyellowpoplar.php. References Loats, K.V. and Rebbeck, J. 1999. Interactive effects of ozone and elevated carbon dioxide on the
growth and physiology of black cherry, green ash, and yellow-poplar seedlings. Environmental Pollution 106: 237-248. Rebbeck, J., Scherzer, A.J. and Loats, K.V. 2004. Foliar physiology
of yellow-poplar (Liriodendron tulipifera L.) exposed to O3 and elevated CO2 over five seasons. Trees 18: 253-263. Scherzel, A.J., Rebbeck, J. and Boerner, R.E.J. 1998. Foliar nitrogen
dynamics and decomposition of yellow-poplar and eastern white pine during four seasons of exposure to elevated ozone and carbon dioxide. Forest Ecology and Management 109: 355-366. 7.3.6.
Low Temperatures Only a handful of studies have attempted to determine what relationship, if any, exists between atmospheric CO2 enrichment and the ability of plants to withstand the
rigors of low temperatures. Loik et al. (2000) grew three Yucca species (brevifolia, schidigera, and whipplei) in pots placed within glasshouses maintained at atmospheric CO2 concentrations
of 360 and 700 700 ppm and day/night air temperatures of 40/24°C for seven months, after which some of the plants were subjected to a twoweek day/night air temperature treatment of 20/5°C.
In addition, leaves from each Yucca species were removed and placed in a freezer that was cooled at a rate of 3°C per hour until a minimum temperature of -15°C was reached. These manipulations
indicated that elevated CO2 lowered the air temperature at which 50 percent low-temperature-induced cell mortality occurred by 1.6, 1.4 and 0.8°C in brevifolia, schidigera and whipplei,
respectively. On the basis of the result obtained for Y. brevifolia, Dole et al. (2003) estimated that “the increase in freezing tolerance
Biological Effects of Carbon Dioxide Enrichment 453 caused by doubled CO2 would increase the potential habitat of this species by 14%.” By contrast, Obrist et al. (2001) observed the
opposite response. In an open-top chamber study of a temperate grass ecosystem growing on a nutrient-poor calcareous soil in northwest Switzerland, portions of which had been exposed
to atmospheric CO2 concentrations of 360 and 600 ppm for a period of six years, they determined that the average temperature at which 50 percent low-temperature-induced leaf mortality
occurred in five prominent species actually rose by an average of 0.7°C in response to the extra 240 ppm of CO2 employed in their experiment. Most relevant investigations, however, have
produced evidence of positive CO2 effects on plant low temperature tolerance. Sigurdsson (2001), for example, grew black cottonwood seedlings near Gunnarsholt, Iceland within closed-top
chambers maintained at ambient and twice-ambient atmospheric CO2 concentrations for a period of three years, finding that elevated CO2 tended to hasten the end of the growing season.
This effect was interpreted as enabling the seedlings to better avoid the severe coldinduced dieback of newly produced tissues that often occurs with the approach of winter in this region.
Likewise, Wayne et al. (1998) found that yellow birch seedlings grown at an atmospheric CO2 concentration of 800 ppm exhibited greater dormant bud survivorship at low air temperatures
than did seedlings grown at 400 ppm CO2. Schwanz and Polle (2001) investigated the effects of elevated CO2 on chilling stress in micropropagated hybrid poplar clones that were subsequently
potted and transferred to growth chambers maintained at either ambient (360 ppm) or elevated (700 ppm) CO2 for a period of three months. They determined that “photosynthesis was less
diminished and electrolyte leakage was lower in stressed leaves from poplar trees grown under elevated CO2 as compared with those from ambient CO2.” Although severe chilling did cause
pigment and and protein degradation in all stressed leaves, the damage was expressed to a lower extent in leaves from the elevated CO2 treatment. This CO2-induced chilling protection
was determined to be accompanied by a rapid induction of superoxide dismutase activity, as well as by slightly higher stabilities of other antioxidative enzymes. Another means by which
chilling-induced injury may be reduced in CO2-enriched air is suggested by the study of Sgherri et al. (1998), who reported that raising the air’s CO2 concentration from 340 to 600 ppm
increased lipid concentrations in alfalfa thylakoid membranes while simultaneously inducing a higher degree of unsaturation in the most prominent of those lipids. Under well-watered
conditions, for example, the 76 percent increase in atmospheric CO2 enhanced overall thylakoid lipid concentration by about 25 percent, while it increased the degree of unsaturation
of the two main lipids by approximately 17 percent and 24 percent. Under conditions of water stress, these responses were found to be even greater, as thylakoid lipid concentration rose
by approximately 92 percent, while the degree of unsaturation of the two main lipids rose by about 22 percent and 53 percent. Several studies conducted over the past decade explain what
these observations have to do with a plant’s susceptibility to chilling injury. Working with wild-type Arabidopsis thaliana and two mutants deficient in thylakoid lipid unsaturation,
Hugly and Somerville (1992) found that “chloroplast membrane lipid polyunsaturation contributes to the lowtemperature fitness of the organism,” and that it “is required for some aspect
of chloroplast biogenesis.” When lipid polyunsaturation was low, they observed “dramatic reductions in chloroplast size, membrane content, and organization in developing leaves.” There
was a positive correlation “between the severity of chlorosis in the two mutants at low temperatures and the degree of reduction in polyunsaturated chloroplast lipid composition.” Working
with tobacco, Kodama et al. al. (1994) demonstrated that the low-temperature-induced suppression of leaf growth and concomitant induction of chlorosis observed in wild-type plants was
much less evident in transgenic plants containing a gene that allowed for greater expression of unsaturation in the fatty acids of leaf lipids. This observation and others led them to
conclude that substantially unsaturated fatty acids “are undoubtedly an important factor contributing to cold tolerance.” In a closely related study, Moon et al. (1995) found that heightened
unsaturation of the membrane lipids of chloroplasts stabilized the photosynthetic machinery of transgenic tobacco plaints against lowtemperature photoinibition “by accelerating the recovery
of the photosystem II protein complex.” Likewise, Kodama et al. (1995), also working with transgenic tobacco plants, showed that increased fatty acid desaturation is one of the prerequisites
for normal leaf development at low, nonfreezing temperatures; and Ishizaki-Nishizawa et al. (1996) demonstrated that transgenic tobacco plants with a reduced level of saturated fatty
acids in most
Climate Change Reconsidered 454 membrane lipids “exhibited a significant increase in chilling resistance.” Many economically important crops, such as rice, maize and soybeans, are classified
as chillingsensitive and experience injury or death at temperatures between 0 and 15°C (Lyons, 1973). If atmospheric CO2 enrichment enhances their production and degree-of-unsaturation
of thylakoid lipids, as it does in alfalfa, a continuation of the ongoing rise in the air’s CO2 content could increase the abilities of these important agricultural species to withstand
periodic exposure to debilitating low temperatures. This phenomenon could provide the extra boost in food production that will be needed to sustain an increasing population in the years
and decades ahead. Earth’s natural ecosystems would also benefit from a CO2-induced increase in thylakoid lipids containing more-highly unsaturated fatty acids. Many plants of tropical
origin, for example, suffer cold damage when temperatures fall below 20°C (Graham and Patterson, 1982). With improved lipid characteristics provided by the ongoing rise in the air’s
CO2 content, such plants would be able to expand their ranges both poleward and upward in a higher-CO2 world. More research remains to be done before we can accurately assess the extent
of these potential biological benefits. In particular, we must conduct more studies of the effects of atmospheric CO2 enrichment on the properties of thylakoid lipids in a greater variety
of plants. In the same experiments, we must assess the efficacy of these lipid property changes in enhancing plant tolerance of low temperatures. Such studies should rank high on the
todo list of relevant funding agencies. Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/f/fro
thardiness.php. References Dole, K.P., Loik, M.E. and Sloan, L.C. 2003. The relative importance of climate change and the physiological effects of CO2 on freezing tolerance for the future
distribution of Yucca brevifolia. Global and Planetary Change 36: 137-146. Graham, D. and Patterson, B.D. 1982. Responses of plants to low, non-freezing temperatures: proteins, metabolism,
and acclimation. Annual Review of Plant Physiology 33: 347-372. Hugly, S. and Somerville, C. 1992. A role for membrane lipid polyunsaturation in chloroplast biogenesis at low temperature.
Plant Physiology 99: 197-202. Ishizaki-Nishizawa, O., Fujii, T., Azuma, M., Sekiguchi, K., Murata, N., Ohtani, T. and Toguri T. 1996. Lowtemperature resistance of higher plants is significantly
enhanced by a nonspecific cyanobacterial desaturase. Nature Biotechnology 14: 1003-1006. Kodama, H., Hamada, T., Horiguchi, G., Nishimura, M. and Iba, K. 1994. Genetic enhancement of
cold tolerance by expression of a gene for chloroplast w-3 fatty acid desaturase in transgenic tobacco. Plant Physiology 105: 601-605. Kodama, H., Horiguchi, G., Nishiuchi, T., Nishimura,
M. and Iba, K. 1995. Fatty acid desaturation during chilling acclimation is one of the factors involved in conferring low-temperature tolerance to young tobacco leaves. Plant Physiology
107: 1177-1185. Loik, M.E., Huxman, T.E., Hamerlynck, E.P. and Smith, S.D. 2000. Low temperature tolerance and cold acclimation for seedlings of three Mojave Desert Yucca species exposed
to elevated CO2. Journal of Arid Environments 46: 43-56. Lyons, J.M. 1973. Chilling injury in plants. Annual Review of Plant Physiology 24: 445-466. Moon, B.Y., Higashi, S.-I., Gombos,
Z. and Murata, N. 1995. Unsaturation of the membrane lipids of chloroplasts stabilizes the photosynthetic machinery against lowtemperature photoinhibition in transgenic tobacco plants.
Proceedings of the National Academy of Sciences, USA 92: 6219-6223. Obrist, D., Arnone III, J.A. and Korner, C. 2001. In situ effects of elevated atmospheric CO2 on leaf freezing resistance
and carbohydrates in a native temperate grassland. Annals of Botany 87: 839-844. Schwanz, P. and Polle, A. 2001. Growth under elevated CO2 ameliorates defenses against photo-oxidative
stress in poplar (Populus alba x tremula). Environmental and Experimental Botany 45: 43-53. Sgherri, C.L.M., Quartacci, M.F., Menconi, M., Raschi, A. and Navari-Izzo, F. 1998. Interactions
between drought and elevated CO2 on alfalfa plants. Journal of Plant Physiology 152: 118-124. Sigurdsson, B.D. 2001. Elevated [CO2] and nutrient status modified leaf phenology and growth
rhythm of young Populus trichocarpa trees in a 3-year field study. Trees 15: 403-413.
Biological Effects of Carbon Dioxide Enrichment 455 Wayne, P.M., Reekie, E.G. and Bazzaz, F.A. 1998. Elevated CO2 ameliorates birch response to high temperature and frost stress: implications
for modeling climate-induced geographic range shifts. Oecologia 114: 335-342. 7.3.7. Nitrogen Deficiency Numerous studies have investigated the effects of different soil nitrogen (N)
concentrations on plant responses to increases in the air’s CO2 content, as it has been claimed that a deficiency of soil nitrogen lessens the relative growth stimulation in plants that
is typically provided by elevated concentrations of atmospheric CO2. In this section, we evaluate the credibility of that claim for various crops, fungi, grasses and trees. The results
of these experiments indicate that some plants sometimes will not respond at all to atmospheric CO2 enrichment at low levels of soil N, while some will. Some plants respond equally well
to increases in the air’s CO2 content when growing in soils exhibiting a whole range of N concentrations. Most common, however, is the observation that plants respond ever better to
rising atmospheric CO2 concentrations as soil N concentrations rise. Interestingly, the current state of earth’s atmosphere and land surface is one of jointly increasing CO2 and N concentrations.
Hence, the outlook is good for continually increasing terrestrial vegetative productivity in the years and decades ahead, as these trends continue. Additional information on this topic,
including reviews on nitrogen not discussed here, can be found at http://www.co2science.org/subject/g/subject_g.php under the heading Growth Response to CO2 With Other Variables: Nutrients:
Nitrogen, as well as at http://www.co2science.org/subject/n/subject_n.php under the headings Nitrogen, Nitrogen Fixation and Nitrogen Use Efficiency. 7.3.7.1. Crops 7.3.7.1.1. Rice Does
a deficiency of soil nitrogen lessen the relative growth and yield stimulation of rice that is typically provided by elevated levels of atmospheric CO2? In exploring this question, Weerakoon
et al. (1999) grew seedlings of two rice cultivars for 28 days in glasshouses maintained at atmospheric CO2 concentrations of 373, 545, 723 and 895 ppm under conditions of low, medium
and high soil nitrogen content. After four weeks of treatment, photosynthesis was found to significantly increase with increasing nitrogen availability and atmospheric CO2 concentration.
Averaged across all nitrogen regimes, plants grown at 895 ppm CO2 exhibited photosynthetic rates that were 50 percent greater than those observed in plants grown at ambient CO2. Total
plant dry weight also increased with increasing atmospheric CO2. In addition, the percentage growth enhancement resulting from CO2 enrichment increased with increasing soil nitrogen;
from 21 percent at the lowest soil nitrogen concentration to 60 percent at the highest concentration. Using a different CO2 enrichment technique, Weerakoon et al. (2000) grew rice in
open-top chambers maintained at atmospheric CO2 concentrations of approximately 350 and 650 ppm during a wet and dry growing season and under a range of soil nitrogen contents. Early
in both growing seasons, plants exposed to elevated atmospheric CO2 concentrations intercepted significantly more sunlight than plants fumigated with ambient air, due to CO2-induced
increases in leaf area index. This phenomenon occurred regardless of soil nitrogen content, but disappeared shortly after canopy closure in all treatments. Later, mature canopies achieved
similar leaf area indexes at identical levels of soil nitrogen supply; but mean season-long radiation use efficiency, which is the amount of biomass produced per unit of solar radiation
intercepted, was 35 percent greater in CO2-enriched vs. ambiently grown plants and tended to increase with increasing soil nitrogen content. Utilizing a third approach to enriching the
air about a crop with elevated levels of atmospheric CO2, Kim et al. (2003) grew rice crops from the seedling stage to maturity at atmospheric CO2 concentrations of ambient and ambient
plus 200 ppm using FACE technology and three levels of applied nitrogen—low (LN, 4 g N m-2), medium (MN, 8 and 9 g N m-2) and high (HN, 15 g N m-2)—for three cropping seasons (1998-2000).
They report that “the yield response to elevated CO2 in crops supplied with MN (+14.6%) or HN (+15.2%) was about twice that of crops supplied with LN
(+7.4%),” confirming the importance of nitrogen availability to the response of rice to atmospheric CO2 enrichment previously determined by Kim et al. (2001) and Kobaysahi et al. (2001).
In light of these observations, it would appear that the maximum benefits of elevated levels of
Climate Change Reconsidered 456 atmospheric CO2 for the growth and grain production of rice cannot be realized in soils that are highly deficient in nitrogen. Increasing nitrogen concentrations
above what is considered adequate may not result in proportional gains in CO2-induced growth and yield enhancement. Additional information on this topic, including reviews of newer publications
as they become available, can be found at http://www.co2science.org/subject/n/nitrogenrice.php. References Kim, H.-Y., Lieffering, M., Kobayashi, K., Okada, M., Mitchell, M.W. and Gumpertz,
M. 2003. Effects of free-air CO2 enrichment and nitrogen supply on the yield of temperate paddy rice crops. Field Crops Research 83: 261-270. Kim, H.-Y., Lieffering, M., Miura, S., Kobayashi,
K. and Okada, M. 2001. Growth and nitrogen uptake of CO2-enriched rice under field conditions. New Phytologist 150: 223-229. Kobayashi, K., Lieffering, M. and Kim, H.-Y. 2001. Growth
and yield of paddy rice under free-air CO2 enrichment. In: Shiyomi, M. and Koizumi, H. (Eds.), Structure and Function in Agroecosystem Design and Management. CRC Press, Boca Raton, FL,
USA, pp. 371-395. Weerakoon, W.M.W., Ingram, K.T. and Moss, D.D. 2000. Atmospheric carbon dioxide and fertilizer nitrogen effects on radiation interception by rice. Plant and Soil 220:
99-106. Weerakoon, W.M., Olszyk, D.M. and Moss, D.N. 1999. Effects of nitrogen nutrition on responses of rice seedlings to carbon dioxide. Agriculture, Ecosystems and Environment 72:
1-8. 7.3.7.1.2. Wheat Smart et al. (1998) grew wheat from seed for 23 days in controlled environment chambers maintained at atmospheric CO2 concentrations of 360 and 1000 ppm and two
concentrations of soil nitrate, finding that the extra CO2 increased average plant biomass by approximately 15 percent, irrespective of soil nitrogen content. In a more realistic FACE
experiment, however, Brooks et al. (2000) grew spring wheat for two seasons at atmospheric CO2 concentrations of 370 and 570 ppm at both high and low levels of nitrogen fertility; and
they obtained twice the yield enhancement (16 percent vs. 8 percent) in the high nitrogen treatment. In an experiment with one additional variable, Vilhena-Cardoso and Barnes (2001)
grew spring wheat for two months in environmental chambers fumigated with air containing atmospheric CO2 concentrations of either 350 or 700 ppm at ambient and elevated (75 ppb) ozone
concentrations, while the plants were simultaneously subjected to either low, medium or high levels of soil nitrogen. With respect to biomass production, the elevated CO2 treatment increased
total plant dry weight by 44, 29 and 12 percent at the high, medium and low soil nitrogen levels, respectively. In addition, although elevated ozone alone reduced plant biomass, the
simultaneous application of elevated CO2 completely ameliorated its detrimental effects on biomass production, irrespective of soil nitrogen supply. Why do the plants of some studies
experience a major reduction in the relative growth stimulation provided by atmospheric CO2 enrichment under low soil nitrogen conditions, while other studies find the aerial fertilization
effect of elevated CO2 to be independent of root-zone nitrogen concentration? Based on studies of both potted and hydroponically grown plants, Farage et al. (1998) determined that low
root-zone nitrogen concentrations need not lead to photosynthetic acclimation (less than maximum potential rates of photosynthesis) in elevated CO2, as long as root-zone nitrogen supply
is adequate to meet plant nitrogen needs to maintain the enhanced relative growth rate that is made possible by atmospheric CO2 enrichment. When supply cannot meet this need, as is often
the case in soils with limited nitrogen reserves, the aerial fertilization effect of atmospheric CO2 enrichment begins to be reduced and less-thanpotential CO2-induced growth stimulation
is observed. Nevertheless, the acclimation process is the plant’s “first line of defense” to keep its productivity from falling even further than it otherwise would, as it typically
mobilizes nitrogen from “excess” rubisco and sends it to more needy plant sink tissues to allow for their continued growth and development (Theobald et al., 1998). In conclusion, although
atmospheric CO2 enrichment tends to increase the growth and yield of wheat under a wide range of soil nitrogen concentrations, including some that are very low, considerably greater
CO2-induced enhancements are possible when more soil nitrogen is available, although the response can saturate at high soil
Biological Effects of Carbon Dioxide Enrichment 457 nitrogen levels, with excess nitrogen providing little to no extra yield. Additional information on this topic, including reviews
of newer publications as they become available, can be found at http://www.co2science.org/subject/n/nitrogenwheat.php. References Brooks, T.J., Wall, G.W., Pinter Jr., P.J., Kimball,
B.A., LaMorte, R.L., Leavitt, S.W., Matthias, A.D., Adamsen, F.J., Hunsaker, D.J. and Webber, A.N. 2000. Acclimation response of spring wheat in a free-air CO2 enrichment (FACE) atmosphere
with variable soil nitrogen regimes. 3. Canopy architecture and gas exchange. Photosynthesis Research 66: 97-108. Farage, P.K., McKee, I.F. and Long, S.P. 1998. Does a low nitrogen supply
necessarily lead to acclimation of photosynthesis to elevated CO2? Plant Physiology 118: 573-580. Smart, D.R., Ritchie, K., Bloom, A.J. and Bugbee, B.B. 1998. Nitrogen balance for wheat
canopies (Triticum aestivum cv. Veery 10) grown under elevated and ambient CO2 concentrations. concentrations. Plant, Cell and Environment 21: 753-763. Theobald, J.C., Mitchell, R.A.C.,
Parry, M.A.J. and Lawlor, D.W. 1998. Estimating the excess investment in ribulose-1,5-bisphosphate carboxylase/oxygenase in leaves of spring wheat grown under elevated CO2. Plant Physiology
118: 945-955. Vilhena-Cardoso, J. and Barnes, J. 2001. Does nitrogen supply affect the response of wheat (Triticum aestivum cv. Hanno) to the combination of elevated CO2 and O3? Journal
of Experimental Botany 52: 1901-1911. 7.3.7.1.3. Other Zerihun et al. (2000) grew sunflowers for one month in pots of three different soil nitrogen concentrations that were placed within
open-top chambers maintained at atmospheric CO2 concentrations of 360 and 700 ppm. The extra CO2 of the CO2-enriched chambers reduced average rates of root nitrogen uptake by about 25
percent, which reduction, by itself, would normally tend to reduce tissue nitrogen contents and the relative growth rates of the seedlings. However, the elevated CO2 also increased photosynthetic
nitrogen-use efficiency by an average of 50 percent, which increase normally tends to increase the relative growth rates of seedlings. Of these two competing effects, the latter was
the more powerful, leading to an increase in whole plant biomass. After the one month of the study, for example, the CO2-enriched plants exhibited whole plant biomass values that were
44, 13 and 115 percent greater than those of the plants growing in ambient air at low, medium and high levels of soil nitrogen, respectively, thus demonstrating that low tissue nitrogen
contents do not necessarily preclude a growth response to atmospheric CO2 enrichment, particularly if photosynthetic nitrogen-use efficiency is enhanced, which is typically the case,
as it was in this study. Nevertheless, the greatest CO2-induced growth increase of Zerihun et al.’s study was exhibited by the plants growing in the high soil nitrogen treatment. Deng
and Woodward (1998) grew strawberries in environment-controlled glasshouses maintained at atmospheric CO2 concentrations of 390 and 560 ppm for nearly three months. In addition, the
strawberries were supplied with fertilizers containing three levels of nitrogen. The extra CO2 increased rates of net photosynthesis and total plant dry weight at all three nitrogen
levels, but the increases were not significant. Nevertheless, they provided the CO2-enriched plants with enough additional sugar and physical mass to support significantly greater numbers
of flowers and fruits than the plants grown at 390 ppm CO2. This effect consequently led to total fresh fruit weights that were 42 and 17 percent greater in the CO2-enriched plants that
received the highest and lowest levels of nitrogen fertilization, respectively, once again indicating a greater growth response at higher nitrogen levels. Newman et al. (2003) investigated
the effects of two levels of nitrogen fertilization and an approximate doubling of the air’s CO2 concentration on the growth of tall fescue, which is an important forage crop. The plants
with which they worked were initially grown from seed in greenhouse flats, but after sixteen weeks they were transplanted into 19-liter pots filled with potting media that received periodic
applications of a slow-release fertilizer. Then, over the next two years of outdoor growth, they were periodically clipped, divided and repotted to ensure they did not become root-bound;
and at the end of that time, they were placed within twenty 1.3-m-diameter open-top chambers, half of which were maintained at the ambient atmospheric CO2 concentration and half of which
were maintained at an approximately doubled CO2 concentration of 700 ppm. In addition, half of the pots in each CO2
Climate Change Reconsidered 458 treatment received 0.0673 kg N m-2 applied over a period of three consecutive days, while half of them received only one-tenth that amount, with the entire
procedure being repeated three times during the course of the 12-week experiment. Newman et al. report that the plants grown in the high-CO2 air photosynthesized 15 percent more and
produced 53 percent more dry matter (DM) under low N conditions and 61 percent more DM under high N conditions. In addition, they report that the percent organic matter (OM) was little
changed, except under elevated CO2 and high N, when %OM (as %DM) increased by 3 percent. In this study too, therefore, the greatest relative increase in productivity occurred under high,
as opposed to low, soil N availability. Demmers-Derks et al. (1998) grew sugar beets as an annual crop in controlled-environment chambers at atmospheric CO2 concentrations of 360 and
700 ppm and air temperatures of ambient and ambient plus 3°C for three consecutive years. In addition to being exposed to these CO2 and temperature combinations, the sugar beets were
supplied with solutions of low and high nitrogen content. Averaged across all three years and both temperature regimes, the extra CO2 of this study enhanced total plant biomass by 13
and 25 percent in the low and high nitrogen treatments, respectively. In addition, it increased root biomass by 12 and 26 percent for the same situations. As was the case with sunflowers,
strawberries and tall fescue, elevated CO2 elicited the largest growth responses in the sugar beets that received a high, as opposed to a low, supply of nitrogen. Also working with sugar
beets were Romanova et al. (2002), who grew them from seed for one month in controlled environment chambers maintained at atmospheric CO2 concentrations of 350 and 700 ppm, while fertilizing
them with three different levels of nitrate-nitrogen. In this study, the plants grown in CO2-enriched air exhibited rates of net photosynthesis that were approximately 50 percent greater
than those displayed by the plants grown in ambient air, regardless of soil nitrate availability. These CO2-induced increases in photosynthetic carbon uptake contributed to 60, 40 and
30 percent above-ground organ dry weight increases in plants receiving onehalf, standard, and three-fold levels of soil nitrate, respectively. Root weights, however, were less responsive
to atmospheric CO2 enrichment, displaying 10 and 30 percent increases in dry weight at one-half and standard nitrate levels, but no increase at the high soil nitrate concentration. In
this study, therefore, the role of soil nitrogen fertility was clearly opposite to that observed in the four prior studies in the case of above-ground biomass production, but was mixed
in the case of belowground biomass production. Switching to barley, Fangmeier et al. (2000) grew plants in containers placed within open-top chambers maintained at atmospheric CO2 concentrations
of either 360 or 650 ppm and either a high or low nitrogen fertilization regime. As in the case of the above-ground biomass response of the sugar beets of Romanova et al., the elevated
CO2 had the greatest relative impact on yield when the plants were grown under the less-than-optimum low-nitrogen regime, i.e., a 48 percent increase vs. 31 percent under highnitrogen
conditions. Last, we report the pertinent results of the review and analysis of Kimball et al. (2002), who summarized the findings of most FACE studies conducted on agricultural crops
since the introduction of that technology back in the late 1980s. In response to a 300-ppm increase in the air’s CO2 concentration, rates of net photosynthesis in several C3 grasses
were enhanced by an average of 46 percent under conditions of ample soil nitrogen supply and by 44 percent when nitrogen was limiting to growth. With respect to above-ground biomass
production, the differential was much larger, with the C3 grasses wheat, rice and ryegrass experiencing an average increase of 18 percent at ample nitrogen but only 4 percent at low
nitrogen; while with respect to belowground biomass production, they experienced an average increase of 70 percent at ample nitrogen and 58 percent at low nitrogen. Similarly, clover
experienced a 38 percent increase in belowground biomass production at ample soil nitrogen, and a 32 percent increase at low soil nitrogen. Finally, with respect to agricultural yield,
which is the bottom line in terms of food and fiber production, wheat and ryegrass experienced an average increase of 18 percent at ample nitrogen, while wheat experienced only a 10
percent increase at low nitrogen. In light of these several results, it can be safely concluded that although there are some significant exceptions to the rule, most agricultural crops
generally experience somewhat greater CO2-induced relative (percentage) increases in net photosynthesis and biomass production even when soil nitrogen concentrations are a limiting factor.
Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/n/nitrogenagriculture.php.
Biological Effects of Carbon Dioxide Enrichment 459 References Demmers-Derks, H., Mitchell, R.A.G., Mitchell, V.J. and Lawlor, D.W. 1998. Response of sugar beet (Beta vulgaris L.) yield
and biochemical composition to elevated CO2 and temperature at two nitrogen applications. Plant, Cell and Environment 21: 829-836. Deng, X. and Woodward, F.I. 1998. The growth and yield
responses of Fragaria ananassa to elevated CO2 and N supply. Annals of Botany 81: 67-71. Fangmeier, A., Chrost, B., Hogy, P. and Krupinska, K. 2000. CO2 enrichment enhances flag leaf
senescence in barley due to greater grain nitrogen sink capacity. Environmental and Experimental Botany 44: 151-164. Kimball, B.A., Kobayashi, K. and Bindi, M. 2002. Responses of agricultural
crops to free-air CO2 enrichment. Advances in Agronomy 77: 293-368. Newman, J.A., Abner, M.L., Dado, R.G., Gibson, D.J., Brookings, A. and Parsons, A.J. 2003. Effects of elevated CO2,
nitrogen and fungal endophyte-infection on tall fescue: growth, photosynthesis, chemical composition and digestibility. Global Change Biology 9: 425-437. Romanova, A.K., Mudrik, V.A.,
Novichkova, N.S., Demidova, R.N. and Polyakova, V.A. 2002. Physiological and biochemical characteristics of sugar beet plants grown at an increased carbon dioxide concentration and at
various nitrate doses. Russian Journal of Plant Physiology 49: 204-210. Zerihun, A., Gutschick, V.P. and BassiriRad, H. 2000. Compensatory roles of nitrogen uptake and photosynthetic
N-use efficiency in determining plant growth response to elevated CO2: Evaluation using a functional balance model. Annals of Botany 86: 723-730. 7.3.7.2. Fungi Nearly all of earth’s
plants become involved in intimate relationships with different fungal species at one point or another in their life cycles. Among other things, the fungi commonly aid plants in the
acquisition of water and important soil nutrients. In addition, fungal-plant interactions are often impacted by variations in both atmospheric CO2 and soil nitrogen concentrations. In
this subsection, we review how various aspects of fungal-plant interactions are influenced by elevated CO2 under varying soil nitrogen regimes. In a one-year study conducted by Walker
et al. (1998), ponderosa pine seedlings exposed to atmospheric CO2 concentrations of 525 and 700 ppm displayed total numbers of ectomycorrhizal fungi on their roots that were 170 and
85 percent greater, respectively, than those observed on roots of ambiently grown seedlings. In the study of Rillig et al. (1998), three grasses and two herbs fumigated with ambient
air and air containing an extra 350 ppm CO2 for four months displayed various root infection responses by arbuscular mycorrhizal fungi, which varied with soil nitrogen supply. At low
soil nitrogen contents, elevated CO2 increased the percent root infection by this type of fungi in all five annual grassland species. However, at high soil nitrogen contents, this trend
was reversed in four of the five species. Finally, in the study of Rillig and Allen (1998), several important observations were made with respect to the effects of elevated CO2 and soil
nitrogen status on fungal-plant interactions. First, after growing three-year-old shrubs at an atmospheric CO2 concentration of 750 ppm for four months, they reported insignificant 19
and 9 percent increases in percent root infected by arbuscular mycorrhizal fungi at low and high soil nitrogen concentrations, respectively. However, elevated CO2 significantly increased
the percent root infection by arbuscules, which are the main structures involved in the symbiotic exchange of carbon and nutrients between a host plant and its associated fungi, by more
than 14-fold at low soil nitrogen concentrations. In addition, the length of fungal hyphae more than doubled with atmospheric CO2 enrichment in the low soil nitrogen regime. In the high
soil nitrogen treatment, elevated CO2 increased the percent root infection by vesicles, which are organs used by arbuscular mycorrhizal fungi for carbon storage, by approximately 2.5-fold.
In conclusion, these observations suggest that elevated CO2 will indeed affect fungal-plant interactions in positive ways that often depend upon soil nitrogen status. Typically, it appears
that CO2-induced stimulations of percent root infection by various fungal components is greater under lower, rather than higher, soil nitrogen concentrations. This tendency implies that
elevated CO2 will enhance fungal-plant interactions to a greater extent when soil nutrition is less-than-optimal for plant growth, which is the common case for most of earth’s ecosystems
that are not subjected to cultural fertilization practices typical of intensive agricultural production. Additional information on this topic, including reviews of newer publications
as they become available, can be found at http://www.co2science.org/subject/n/nitrogenfungi.php.
Climate Change Reconsidered 460 References Rillig, M.C. and Allen, M.F. 1998. Arbuscular mycorrhizae of Gutierrezia sarothrae and elevated carbon dioxide: evidence for shifts in C allocation
to and within the mycobiont. Soil Biology and Biochemistry 30: 2001-2008. Rillig, M.C., Allen, M.F., Klironomous, J.N., Chiariello, N.R. and Field, C.B. 1998. Plant species-specific
changes in root-inhabiting fungi in a California annual grassland: responses to elevated CO2 and nutrients. Oecologia 113: 252-259. Walker, R.F., Johnson, D.W., Geisinger, D.R. and Ball,
J.T. 1998. Growth and ectomycorrhizal colonization of ponderosa pine seedlings supplied different levels of atmospheric CO2 and soil N and P. Forest Ecology and Management 109: 9-20.
7.3.7.3. Grasses Perennial ryegrass (Lolium perenne L.) has been used as a model species in many experiments to help elucidate grassland responses to atmospheric CO2 enrichment and soil
nitrogen availability. In the FACE study of Rogers et al. (1998), for example, plants exposed exposed to 600 ppm CO2 exhibited a 35 percent increase in their photosynthetic rates without
regard to soil nitrogen availability. However, when ryegrass was grown in plastic ventilated tunnels at twice-ambient concentrations of atmospheric CO2, the CO2-induced photosynthetic
response was about 3-fold greater in a higher, as opposed to a lower, soil nitrogen regime (Casella and Soussana, 1997). Similarly, in an open-top chamber study conducted by Davey et
al. (1999), it was reported that an atmospheric CO2 concentration of 700 ppm stimulated photosynthesis by 30 percent in this species when it was grown with moderate, but not low, soil
nitrogen availability. Thus, CO2-induced photosynthetic stimulations in perennial ryegrass can be influenced by soil nitrogen content, with greater positive responses typically occurring
under higher, as opposed to lower, soil nitrogen availability. With respect to biomass production, van Ginkel and Gorissen (1998) reported that a doubling of the atmospheric CO2 concentration
concentration increased shoot biomass of perennial ryegrass by 28 percent, regardless of soil nitrogen concentration. In the more revealing six-year FACE study of Daepp et al. (2000),
plants grown at 600 ppm CO2 and high soil nitrogen availability continually increased their dry matter production over that observed in ambienttreatment plots, from 8 percent more in
the first year to 25 percent more at the close of year six. When grown at a low soil nitrogen availability, however, CO2-enriched plants exhibited an initial 5 percent increase in dry
matter production, which dropped to a negative 11 percent in year two; but this negative trend was thereafter turned around, and it continually rose to reach a 9 percent stimulation
at the end of the study. Thus, these data demonstrate that elevated CO2 increases perennial ryegrass biomass, even under conditions of low soil nitrogen availability, especially under
conditions of long-term atmospheric CO2 enrichment. Lutze et al. (1998) reported that microcosms of the C3 grass Danthonia richardsonii grown for four years in glasshouses fumigated
with air containing 720 ppm CO2 displayed total photosynthetic carbon gains that were 15-34 percent higher than those of ambiently grown microcosms, depending on the soil nitrogen concentration.
In a clearer depiction of photosynthetic responses to soil nitrogen, Davey et al. (1999) noted that photosynthetic rates of Agrostis capillaries subjected to twice-ambient levels of
atmospheric CO2 for two years were 12 and 38 percent greater than rates measured in control plants grown at 350 ppm CO2 under high and low soil nitrogen regimes, respectively. They also
reported CO2-induced photosynthetic stimulations of 25 and 74 percent for Trifolium repens subjected to high and low soil nitrogen regimes, respectively. Thus, we see that the greatest
CO2-induced percentage increase in photosynthesis can sometimes occur under the least favorable soil nitrogen conditions. With respect to biomass production, Navas et al. (1999) reported
that 60 days’ exposure to 712 ppm CO2 increased biomass production of Danthonia richardsonii, Phalaris aquatica, Lotus pedunculatus, and Trifolium repens across a large soil nitrogen
gradient. With slightly more detail, Cotrufo and Gorissen (1997) reported average CO2-induced increases in whole-plant dry weights of Agrostis capillaries and Festuca ovina that were
20 percent greater than those of their respective controls, regardless of soil nitrogen availability. In the study of Ghannoum and Conroy (1998), three Panicum grasses grown for two
months at twice-ambient levels of atmospheric CO2 and high soil nitrogen availability displayed similar increases in total plant dry mass that were about 28 percent greater than those
of their respective ambiently grown controls. At low nitrogen, however, elevated CO2 had no significant effect on
Biological Effects of Carbon Dioxide Enrichment 461 the dry mass of two of the species, while it actually decreased that of the third species. In summary, it is clear that atmospheric
CO2 enrichment stimulates photosynthesis and biomass production in grasses and grassland species when soil nitrogen availability is high and/or moderate. Under lower soil nitrogen conditions,
it is also clear that atmospheric CO2 enrichment can have the same positive effect on these parameters, but that it can also have a reduced positive effect, no effect, or (in one case)
a negative effect. In light of the one long-term study that lasted six years, however, it is likely that— given enough time—grasslands have the ability to overcome soil nitrogen limitations
and produce positive CO2-induced growth responses. Thus, because the rising CO2 content of the air is likely to continue for a long time to come, occasional nitrogen limitations on the
aerial fertilization effect of atmospheric CO2 enrichment of grasslands will likely become less and less restrictive as time progresses. Additional information on this topic, including
reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/n/nitrogengrass.php. References Casella, E. and Soussana, J-F. 1997. Long-term
effects of CO2 enrichment and temperature increase on the carbon balance of a temperate grass sward. Journal of Experimental Botany 48: 1309-1321. Cotrufo, M.F. and Gorissen, A. 1997.
Elevated CO2 enhances below-ground C allocation in three perennial grass species at different levels of N availability. New Phytologist 137: 421-431. Daepp, M., Suter, D., Almeida, J.P.F.,
Isopp, H., Hartwig, U.A., Frehner, M., Blum, H., Nosberger, J. and Luscher, A. 2000. Yield response of Lolium perenne swards to free air CO2 enrichment increased over six years in a
high N input system on fertile soil. Global Change Biology 6: 805-816. Davey, P.A., Parsons, A.J., Atkinson, L., Wadge, K. and Long, S.P. 1999. Does photosynthetic acclimation to elevated
CO2 increase photosynthetic nitrogen-use efficiency? A study of three native UK grassland species in open-top chambers. Functional Ecology 13: 21-28. Ghannoum, O. and Conroy, J.P. 1998.
Nitrogen deficiency precludes a growth response to CO2 enrichment in C3 and C4 Panicum grasses. Australian Journal of Plant Physiology 25: 627-636. Lutze, J.L. and Gifford, R.M. 1998.
Carbon accumulation, distribution and water use of Danthonia richardsonii swards in response to CO2 and nitrogen supply over four years of growth. Global Change Biology 4: 851-861. Navas,
M.-L., Garnier, E., Austin, M.P. and Gifford, R.M. 1999. Effect of competition on the responses of grasses and legumes to elevated atmospheric CO2 along a nitrogen gradient: differences
between isolated plants, monocultures and multi-species mixtures. New Phytologist 143: 323-331. Rogers, A., Fischer, B.U., Bryant, J., Frehner, M., Blum, H., Raines, C.A. and Long, S.P.
1998. Acclimation of photosynthesis to elevated CO2 under low-nitrogen nutrition is affected by the capacity for assimilate utilization. Perennial ryegrass under free-air CO2 enrichment.
Plant Physiology 118: 683-689. Van Ginkel, J.H. and Gorissen, A. 1998. In situ decomposition of grass roots as affected by elevated atmospheric carbon dioxide. Soil Science Society of
America Journal 62: 951-958. 7.3.7.4. Trees 7.3.7.4.1. Aspen Does a deficiency of soil nitrogen lessen the relative growth stimulation of quaking aspen (Populus tremuloides Michx) that
is typically provided by elevated concentrations of atmospheric CO2? In exploring this question, Kubiske et al. (1998) grew cuttings of four quaking aspen genotypes for five months at
CO2 concentrations of 380 or 720 ppm and low or high soil nitrogen in open-top chambers in the field in Michigan, USA. They found that the elevated CO2 treatment significantly increased
net photosynthesis, regardless of soil nitrogen content, although there were no discernible increases in aboveground growth within the five-month study period. Belowground,
however, elevated CO2 significantly increased fine root production, but only in the high soil nitrogen treatment. Working at the same site, Zak et al. (2000) and Curtis et al. (2000)
grew six aspen genotypes from cuttings in open-top chambers for 2.5 growing seasons at atmospheric CO2 concentrations of 350 and 700 ppm on soils containing either adequate or inadequate
supplies of nitrogen. Curtis et al. report that at the end of this period the trees growing in the doubled-CO2 treatment exhibited rates of net photosynthesis that were 128 percent and
31 percent greater than those of the trees growing in the ambientair treatment on the high-and low-nitrogen soils,
Climate Change Reconsidered 462 respectively, while Zak et al. determined the CO2-induced biomass increases of the trees in the highand low-nitrogen soils to be 38 percent and 16 percent,
respectively. In yet another study from the Michigan site, Mikan et al. (2000) grew aspen cuttings for two years in open-top chambers receiving atmospheric CO2 concentrations of 367
and 715 ppm in soils of low and high soil nitrogen concentrations. They report finding that elevated CO2 increased the total biomass of the aspen cuttings by 50 percent and 26 percent
in the high and low soil nitrogen treatments, respectively, and that it increased coarse root biomass by 78 percent and 24 percent in the same respective treatments. Last, but again
at the same site, Wang and Curtis (2001) grew cuttings of two male and two female aspen trees for about five months in open-top chambers maintained at atmospheric CO2 concentrations
of 380 and 765 ppm on soils of high and low nitrogen content. In the male cuttings, there was a modest difference in the CO2-induced increase in total biomass (58 percent and 66 percent
in the high-and low-nitrogen soils, respectively), while in the female cuttings the difference was much greater (82 percent and 22 percent in the same respective treatments). Considering
the totality of these several observations, it would appear that the degree of soil nitrogen availability does indeed impact the aerial fertilization effect of atmospheric CO2 enrichment
on the growth of aspen trees by promoting a greater CO2-induced growth enhancement in soils of adequate, as opposed to insufficient, nitrogen content. Additional information on this
topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/n/nitrogenaspen.php. References Curtis, P.S., Vogel, C.S.,
Wang, X.Z., Pregitzer, K.S., Zak, D.R., Lussenhop, J., Kubiske, M. and Teeri, J.A. 2000. Gas exchange, leaf nitrogen, and growth efficiency of Populus tremuloides in a CO2-enriched atmosphere.
Ecological Ecological Applications 10: 3-17. Kubiske, M.E., Pregitzer, K.S., Zak, D.R. and Mikan, C.J. 1998. Growth and C allocation of Populus tremuloides genotypes in response to atmospheric
CO2 and soil N availability. New Phytologist 140: 251-260. Mikan, C.J., Zak, D.R., Kubiske, M.E. and Pregitzer, K.S. 2000. Combined effects of atmospheric CO2 and N availability on the
belowground carbon and nitrogen dynamics of aspen mesocosms. Oecologia 124: 432-445. Wang, X. and Curtis, P.S. 2001. Gender-specific responses of Populus tremuloides to atmospheric CO2
enrichment. New Phytologist 150: 675-684. Zak, D.R., Pregitzer, K.S., Curtis, P.S., Vogel, C.S., Holmes, W.E. and Lussenhop, J. 2000. Atmospheric CO2, soil-N availability, and allocation
of biomass and nitrogen by Populus tremuloides. Ecological Applications 10: 34-46. 7.3.7.4.2. Pine In a review of eleven of their previously published papers dealing with both loblolly
pine (Pinus taeda L.) and ponderosa pine (Pinus ponderosa Dougl.), Johnson et al. (1998) report that when soil nitrogen levels were so low as to be extremely deficient, or so high as
to be toxic, growth responses to atmospheric CO2 enrichment in both species were negligible. For moderate soil nitrogen deficiencies, however, a doubling of the air’s CO2 content sometimes
boosted growth by as much as 1,000 percent. In addition, atmospheric CO2 enrichment mitigated the negative growth response of ponderosa pine to extremely high soil nitrogen concentrations.
In a second paper published by some of the same scientists in the same year, Walker et al. (1998) describe how they raised ponderosa pine tree seedlings for two growing seasons in open-top
chambers having CO2 concentrations of 350, 525 and 700 ppm on soils of low, medium and high nitrogen content. They report that elevated CO2 had little effect on most growth parameters
after the first growing season, the one exception being belowground biomass, which increased with both CO2 and soil nitrogen. After two growing seasons, however, elevated CO2 significantly
increased all growth parameters, including tree height, stem diameter, shoot weight, stem volume and root volume, with the greatest responses typically occurring at the highest CO2 concentration
in the highest soil nitrogen treatment. Root volume at 700 ppm CO2 and high soil nitrogen, for example, exceeded that of all other treatments by at least 45 percent, as did shoot volume
by 42 percent. Similarly, at high CO2 and soil nitrogen, coarse root and shoot weights exceeded those at ambient CO2 and high nitrogen by 80 and 88 percent, respectively.
Biological Effects of Carbon Dioxide Enrichment 463 Walker et al. (2000) published another paper on the same trees and treatments after five years of growth. At this time, the trees
exposed to the twiceambient levels of atmospheric CO2 had heights that were 43, 64 and 25 percent greater than those of the trees exposed to ambient air and conditions of high, medium
and low levels of soil nitrogen, respectively. Similarly, the trunk diameters of the 700-ppm-trees were 24, 73 and 20 percent greater than the trunk diameters of the ambiently grown
trees exposed to high, medium and low levels of soil nitrogen. Switching to a different species, Entry et al. (1998) grew one-year-old longleaf pine seedlings for 20 months in pots of
high and low soil nitrogen content within open-top chambers maintained at atmospheric CO2 concentrations of 365 or 720 ppm, finding that the elevated CO2 caused no overall change in
whole-plant biomass at low soil nitrogen, but that at high soil nitrogen, it increased it by 42 percent. After two years of these treatments, Runion et al. (1999) also reported that
rates of net photosynthesis were about 50 percent greater in the high CO2 treatment, irrespective of soil nitrogen content … and water content too. Last, Finzi and Schlesinger (2003)
measured and analyzed the pool sizes and fluxes of inorganic and organic nitrogen (N) in the floor and top 30 cm of mineral soil of the Duke Forest at the five-year point of a long-term
FACE study, where half of the experimental plots are enriched with an extra 200 ppm of CO2. In commencing this study, they had originally hypothesized that “the increase in carbon fluxes
to the microbial community under elevated CO2 would increase the rate of N immobilization over mineralization,” leading to a decline in the significant CO2-induced stimulation of forest
net primary production that developed over the first two years of the experiment (DeLucia et al., 1999; Hamilton et al., 2002). Quite to the contrary, however, they discovered “there
was no statistically significant change in the cycling rate of N derived from soil organic matter under elevated CO2.” Neither was the rate of net N mineralization significantly altered
by elevated CO2, nor was there any statistically significant difference in the concentration or net flux of organic and inorganic N in the forest floor and top 30-cm of mineral soil
after 5 years of CO2 fumigation. Hence, at this stage of the study, they could find no support for their original hypothesis, which suggests that the growth stimulation provided by elevated
levels of atmospheric CO2 would gradually dwindle away to something rather insignificant before the stand reached its equilibrium biomass, although they continue to cling to this unsubstantiated
belief. Considering the totality of these several observations, it would appear that the degree of soil nitrogen availability impacts the effect of atmospheric CO2 enrichment on the
growth of pine trees, with greater CO2-induced growth enhancement occurring in soils of adequate, as opposed to insufficient, nitrogen content. As in the case of aspen, however, there
is evidence to suggest that at some point the response to increasing soil nitrogen saturates, and beyond that point, higher N concentrations may sometimes even reduce the forest growth
response to elevated CO2. Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/n/nitrogenpine.php.
References DeLucia, E.H., Hamilton, J.G., Naidu, S.L., Thomas, R.B., Andrews, J.A., Finzi, A., Lavine, M., Matamala, R., Mohan, J.E., Hendrey, G.R. and Schlesinger, W.H. 1999. Net primary
production of a forest ecosystem with experimental CO2 enrichment. Science 284: 1177-1179. Entry, J.A., Runion, G.B., Prior, S.A., Mitchell, R.J. and Rogers, H.H. 1998. Influence of
CO2 enrichment and nitrogen fertilization on tissue chemistry and carbon allocation in longleaf pine seedlings. Plant and Soil 200: 3-11. Finzi, A.C. and Schlesinger, W.H. 2003. Soil-nitrogen
cycling in a pine forest exposed to 5 years of elevated carbon dioxide. Ecosystems 6: 444-456. Hamilton, J.G., DeLucia, E.H., George, K., Naidu, S.L., Finzi, A.C. and Schlesinger, W.H.
2002. Forest carbon balance under elevated CO2. Oecologia 131: 250-260. Johnson, D.W., Thomas, R.B., Griffin, K.L., Tissue, D.T., Ball, J.T., Strain, B.R. and Walker, R.F. 1998. Effects
of carbon dioxide and nitrogen on growth and nitrogen uptake in ponderosa and loblolly pine. Journal of Environmental Quality 27: 414-425. Runion, G.B., Mitchell, R.J., Green, T.H.,
Prior, S.A., Rogers, H.H. and Gjerstad, D.H. 1999. Longleaf pine photosynthetic response to soil resource availability and elevated atmospheric carbon dioxide. Journal of Environmental
Quality 28: 880-887. Walker, R.F., Geisinger, D.R., Johnson, D.W. and Ball, J.T. 1998. Atmospheric CO2 enrichment and soil N fertility effects on juvenile ponderosa pine: Growth,
Climate Change Reconsidered 464 ectomycorrhizal development, and xylem water potential. Forest Ecology and Management 102: 33-44. Walker, R.F., Johnson, D.W., Geisinger, D.R. and Ball,
J.T. 2000. Growth, nutrition, and water relations of ponderosa pine in a field soil as influenced by long-term exposure to elevated atmospheric CO2. Forest Ecology and Management 137:
1-11. 7.3.7.4.3. Spruce Egli et al. (1998) rooted saplings of different genotypes of Norway spruce (Picea abies L. Karst.) directly into calcareous or acidic soils in open-top chambers
and exposed them to atmospheric CO2 concentrations of 370 or 570 ppm and low or high soil nitrogen contents. They found that elevated CO2 generally stimulated light-saturated rates of
photosynthesis under all conditions by as much as 35 percent, regardless of genotype, which consistently led to increased above-ground biomass production, also regardless of genotype,
as well as without respect to soil type or nitrogen content. Murray et al. (2000) grew Sitka spruce ((Picea sitchensis (Bong.) Carr.) seedlings for two years in pots within open-top
chambers maintained at atmospheric CO2 concentrations of 355 and 700 ppm. In the last year of the study, half of the seedlings received one-tenth of the optimal soil nitrogen supply
recommended for this species, while the other half received twice the optimal amount. Under this protocol, the extra CO2 increased the seedlings’ lightsaturated rates of net photosynthesis
by 19 percent and 33 percent in the low-and high-nitrogen treatments, respectively, while it increased their total biomass by 0 percent and 37 percent in these same treatments. Nevertheless,
Murray et al. note there was a reallocation of biomass from above-ground organs (leaves and stems) into roots in the low-nitrogen treatment; and they remark that this phenomenon “may
provide a long-term mechanism by which Sitka spruce could utilize limited resources both more efficiently and effectively,” which suggests that although low soil nitrogen precluded a
short-term CO2-induced growth response in this tree species, it is possible that the negative impact of nitrogen deficiency could be overcome in the course of much longer-term atmospheric
CO2 enrichment. In a related experiment, Liu et al. (2002) grew Sitka spruce seedlings in well-watered and fertilized pots within open-top chambers that were maintained for three years
at atmospheric CO2 concentrations of either 350 or 700 ppm, after which the seedlings were planted directly into native nutrient-deficient forest soil and maintained at the same atmospheric
CO2 concentrations for two more years in larger open-top chambers either with or without extra nitrogen being supplied to the soil. After the first three years of the study, they determined
that the CO2-enriched trees possessed 11.6 percent more total biomass than the ambient-treatment trees. At the end of the next two years, however, the CO2-enriched trees supplied with
extra nitrogen had 15.6 percent more total biomass than their similarly treated ambient-air counterparts, while the CO2-enriched trees receiving no extra nitrogen had 20.5 percent more
biomass than their ambient-treatment counterparts. In light of these several observations, it would appear that the degree of soil nitrogen availability affects the growth of spruce
trees by promoting a greater CO2-induced growth enhancement in soils of adequate, as opposed to insufficient, nitrogen content. As in the cases of aspen and pine, however, at some point
the response of spruce trees to increasing soil nitrogen saturates, and even higher nitrogen concentrations may reduce the growth response to elevated CO2 below that observed at optimal
or low soil nitrogen concentrations. Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/n/nitrog
nspruce.php. References Egli, P., Maurer, S., Gunthardt-Goerg, M.S. and Korner, C. 1998. Effects of elevated CO2 and soil quality on leaf gas exchange and aboveground growth in beech-spruce
model ecosystems. New Phytologist 140: 185-196. Liu, S.R., Barton, C., Lee, H., Jarvis, P.G. and Durrant, D. 2002. Long-term response of Sitka spruce (Picea sitchensis (Bong.) Carr.)
to CO2 enrichment and nitrogen supply. I. Growth, biomass allocation and physiology. Plant Biosystems 136: 189-198. Murray, M.B., Smith, R.I., Friend, A. and Jarvis, P.G. 2000. Effect
of elevated [CO2] and varying nutrient application rates on physiology and biomass accumulation of Sitka spruce (Picea sitchensis). Tree Physiology 20: 421-434.
Biological Effects of Carbon Dioxide Enrichment 465 7.3.7.4.4. Other Maillard et al. (2001) grew pedunculate oak seedlings for three to four months in greenhouses maintained at atmospheric
CO2 concentrations of either 350 or 700 ppm under conditions of either low or high soil nitrogen concentration. The elevated CO2 of their study stimulated belowground growth in the seedlings
growing in the nitrogen-poor soil, significantly increasing their root-to-shoot ratios. However, it increased both the below-and above-ground biomass of seedlings growing in nitrogen-rich
soil. In fact, the CO2-enriched seedlings growing in the nitrogen-rich soil produced 217 and 533 percent more stem and coarse-root biomass, respectively, than their ambientair counterparts
growing in the same fertility treatment. Overall, the doubled CO2 concentration of the air in their study enhanced total seedling biomass by approximately 30 and 140 percent under nitrogenpoor
and nitrogen-rich soil conditions, respectively. Schortemeyer et al. (1999) grew seedlings of Acacia melanoxylon (a leguminous nitrogen-fixing tree native to south-eastern Australia)
in hydroponic culture for six weeks in growth cabinets, where the air was maintained at CO2 concentrations of either 350 or 700 ppm and the seedlings were supplied with water containing
nitrogen in a number of discrete concentrations ranging from 3 to 6,400 mmol m-3. In the two lowest of these nitrogen concentration treatments, final biomass was unaffected by atmospheric
CO2 enrichment; but, as in the study of Maillard et al., it was increased by 5-to 10-fold at the highest nitrogen concentration. Temperton et al. (2003) measured total biomass production
in another N2-fixing tree—Alnus glutinosa (the common alder)—seedlings of which had been grown for three years in open-top chambers in either ambient or elevated (ambient + 350 ppm)
concentrations of atmospheric CO2 and one of two soil nitrogen regimes (full nutrient solution or no fertilizer). In their study, by contrast, they found that the trees growing under
low soil nutrient conditions exhibited essentially the same growth enhancement as that of the well-fertilized trees. Rounding out the full gamut of growth responses, Gleadow et al. (1998)
grew eucalyptus seedlings for six months in glasshouses maintained at atmospheric CO2 concentrations of either 400 or 800 ppm, fertilizing them twice daily with low or high nitrogen
solutions. They found that their doubling of the air’s CO2 concentration increased total seedling biomass by 134 percent in the low nitrogen treatment but by a smaller 98 percent in
the high nitrogen treatment. In addition, the elevated CO2 led to greater root growth in the low nitrogen treatment, as indicated by a 33 percent higher root:shoot ratio. In conclusion,
different species of trees respond differently to atmospheric CO2 enrichment under conditions of low vs. high soil nitrogen fertility. The most common response is for the growth-promoting
effects of atmospheric CO2 enrichment to be expressed to a greater degree when soil soil nitrogen fertility is optimal as opposed to less-than-optimal. Additional information on this
topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/o/ozonetreemisc.php. References Gleadow, R.M., Foley, W.J.
and Woodrow, I.E. 1998. Enhanced CO2 alters the relationship between photosynthesis and defense in cyanogenic Eucalyptus cladocalyx F. Muell. Plant, Cell and Environment 21: 12-22. Maillard,
P., Guehl, J.-M., Muller, J.-F. and Gross, P. 2001. Interactive effects of elevated CO2 concentration and nitrogen supply on partitioning of newly fixed 13C and 15N between shoot and
roots of pedunculate oak seedlings (Quercus robur L.). Tree Physiology 21: 163-172. Schortemeyer, M., Atkin, O.K., McFarlane, N. and Evans, J.R. 1999. The impact of elevated atmospheric
CO2 and nitrate supply on growth, biomass allocation, nitrogen partitioning and N2 fixation of Acacia melanoxylon. Australian Journal of Plant Physiology 26: 737-774. Temperton, V.M.,
Grayston, S.J., Jackson, G., Barton, C.V.M. , Millard, P. and Jarvis, P.G. 2003. Effects of elevated carbon dioxide concentration on growth and nitrogen fixation in Alnus glutinosa in
a long-term field experiment. Tree Physiology 23: 1051-1059. 7.3.8. High Salinity In managed agricultural ecosystems, the buildup of soil salinity from repeated irrigations can sometimes
reduce crop yields. Similarly, in natural ecosystems where exposure to brackish or salty water is commonplace, saline soils can induce growth stress in plants not normally adapted to
coping with this problem. Thus, it is important to understand how rising atmospheric CO2 concentrations may interact with soil salinity to affect plant growth.
Climate Change Reconsidered 466 In the study of Ball et al. (1997), it was found that two Australian mangrove species with differing tolerance to salinity exhibited increased rates of
net photosynthesis in response to a doubling of the atmospheric CO2 concentration, but only when exposed to salinity levels that were 25 percent, but not 75 percent, of full-strength
seawater. Mavrogianopoulos et al. (1999) reported that atmospheric CO2 concentrations of 800 and 1200 ppm stimulated photosynthesis in parnon melons by 75 and 120 percent, respectively,
regardless of soil salinity, which ranged from 0 to 50 mM NaCl. Moreover, the authors noted that atmospheric CO2 enrichment partially alleviated the negative effects of salinity on melon
yield, which increased with elevated CO2 at all salinity levels. Maggio et al. (2002) grew tomatos at 400 and 900 ppm in combination with varying degrees of soil salinity and noted that
plants grown in elevated CO2 tolerated an average root-zone salinity threshold value that was about 60 percent greater than that exhibited by plants grown at 400 ppm CO2 (51 vs. 32 mmol
dm-3 Cl). The review of Poorter and Perez-Soba (2001) found no changes in the effect of elevated CO2 on the growth responses of most plants over a wide range of soil salinities, in harmony
with the earlier findings of Idso and Idso (1994). These various studies suggest that elevated CO2 concentrations have either positive or no effects on plan growth where mild to moderate
stresses may be present due to high soil salinity levels. Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science
org/subject/s/salinity stress. php. References Ball, M.C., Cochrane, M.J. and Rawson, H.M. 1997. Growth and water use of the mangroves Rhizophora apiculata and R. stylosa in response
to salinity and humidity under ambient and elevated concentrations of atmospheric CO2. Plant, Cell and Environment 20: 1158-1166. Idso, K.E. and Idso, S.B. 1994. Plant responses to atmospheric
CO2 enrichment in the face of environmental constraints: A review of the past 10 years’ research. Agricultural and Forest Meteorology 69: 153-203. Maggio, A., Dalton, F.N. and Piccinni,
G. 2002. The effects of elevated carbon dioxide on static and dynamic indices for tomato salt tolerance. European Journal of Agronomy 16: 197-206. Mavrogianopoulos, G.N., Spanakis, J.
and Tsikalas, P. 1999. Effect of carbon dioxide enrichment and salinity on photosynthesis and yield in melon. Scientia Horticulturae 79: 51-63. Poorter, H. and Perez-Soba, M. 2001. The
growth response of plants to elevated CO2 under non-optimal environmental conditions. Oecologia 129: 1-20. 7.3.9. Elevated Temperature Will plants continue to exhibit CO2-induced growth
increases under conditions of elevated air temperature? In this section, we review the photosynthetic and growth responses of agricultural crops, grasslands and woody species to answer
this question. 7.3.9.1. Agricultural Crops The optimum growth temperature for several plants has been shown to rise substantially with increasing levels of atmospheric CO2 (McMurtrie
and Wang, 1993; McMurtrie et al., 1992; Stuhlfauth and Fock, 1990; Berry and Bjorkman, 1980). This phenomenon was predicted by Long (1991), who calculated from well-established plant
physiological principles that most C3 plants should increase their optimum growth temperature by approximately 5°C for a 300 ppm increase in the air’s CO2 content. One would thus also
expect plant photosynthetic rates to rise with concomitant increases in the air’s CO2 concentration and temperature, as has indeed been previously shown to be true by Idso and Idso (1994).
We here proceed to see if these positive CO2 and temperature interactions are still being supported in the recent scientific literature. In the study of Zhu et al. (1999), pineapples
grown at 700 ppm CO2 assimilated 15, 97 and 84 percent more total carbon than pineapples grown at the current ambient CO2 concentration in day/night air temperature regimes of 30/20
(which is optimal for pineapple growth at ambient CO2), 30/25, and 35/25 °C, respectively. Similarly, Taub et al. (2000) demonstrated that net photosynthetic rates of cucumbers grown
at twice-ambient levels of atmospheric CO2 and air temperatures of 40°C were 3.2 times greater than those displayed by control
Biological Effects of Carbon Dioxide Enrichment 467 plants grown at ambient CO2 and this same elevated air temperature. Thus, at air temperatures normally considered to be deleterious
to plant growth, rates of photosynthesis are typically considerably greater for CO2 enriched vs. ambiently grown plants. Reddy et al. (1999) grew cotton plants at air temperatures ranging
from 2°C below to 7°C above ambient air temperatures and reported that plants simultaneously exposed to 720 ppm CO2 displayed photosynthetic rates that were 137 to 190 percent greater
than those displayed by plants exposed to ambient CO2 concentrations across this temperature spectrum. Similarly, Cowling and Sage (1998) reported that a 200-ppm increase in the air’s
CO2 concentration boosted photosynthetic rates of young bean plants by 58 and 73 percent at growth temperatures of 25 and 36°C, respectively. In addition, Bunce (1998) grew wheat and
barley at 350 and 700 ppm CO2 across a wide range of temperatures and reported that elevated CO2 stimulated photosynthesis in these species by 63 and 74 percent, respectively, at an
air temperature of 10°C and by 115 and 125 percent at 30°C. Thus, the percentage increase in photosynthetic rate resulting from atmospheric CO2 enrichment often increases substantially
with increasing air temperature. Elevated CO2 often aids in the recovery of plants from high temperature-induced reductions in photosynthetic capacity, as noted by Ferris et al. (1998),
who grew soybeans for 52 days under normal air temperature and soil water conditions at atmospheric CO2 concentrations of 360 and 700 ppm, but then subjected them to an 8-day period
of high temperature and water stress. After normal air temperature and soil water conditions were restored, the CO2-enriched plants attained photosynthetic rates that were 72 percent
of their unstressed controls, while stressed plants grown at ambient CO2 attained photosynthetic rates that were only 52 percent of their respective controls. CO2-induced increases in
plant growth under high air temperatures have also been observed in a number of other agricultural species. In the previously mentioned study of Cowling and Sage (1998), for example,
the 200-ppm increase in the air’s CO2 content boosted total plant biomass for wheat and barley by a combined average of 59 and 200 percent at air temperatures of 25 and 36°C. Similarly,
Ziska (1998) reported that a doubling of the atmospheric CO2 concentration increased the total dry weight of soybeans by 36 and 42 percent at root zone temperatures of 25 and 30°C, respectively.
Likewise, Hakala (1998) noted that spring wheat grown at 700 ppm CO2 attained total biomass values that were 17 and 23 percent greater than those attained by ambiently grown plants exposed
to ambient and elevated (ambient plus 3°C) air temperatures. In addition, after inputting various observed CO2-induced growth responses of winter wheat into plant growth models, Alexandrov
and Hoogenboom (2000) predicted 12 to 49 percent increases in wheat yield in Bulgaria even if air temperatures rise by as much as 4°C. Finally, in the study of Reddy et al. (1998), it
was shown that elevated CO2 (700 ppm) increased total cotton biomass by 31 to 78 percent across an air temperature range from 20 to 40°C. Thus, the beneficial effects of elevated CO2
on agricultural crop yield is often enhanced by elevated air temperature. In some cases, however, elevated CO2 does not interact with air temperature to further increase the growth-promoting
effects of atmospheric CO2 enrichment, but simply allows the maintenance of the status quo. In the study of Demmers-Derks et al. (1998), for example, sugar beets grown at 700 ppm CO2
attained 25 percent more biomass than ambiently grown plants, regardless of air temperature, which was increased by 3°C. Similarly, in the study of Fritschi et al. (1999), elevated CO2
concentrations did not significantly interact with air temperature (4.5°C above ambient) to impact the growth of rhizoma peanut. Nonetheless, the 300-ppm increase in the air’s CO2 content
increased total biomass by 52 percent, regardless of air temperature. Finally, even if the air’s CO2 content were to cease rising or have no effect on plants, it is possible that temperature
increases alone would promote plant growth and development. This was the case in the study of Wurr et al. (2000), where elevated CO2 had essentially no effect on the yield of French
bean. However, a 4°C increase in air temperature increased yield by approximately 50 percent. In conclusion, the recent
scientific literature continues to indicate that as the air’s CO2 content rises, agricultural crops will likely exhibit enhanced rates of photosynthesis and biomass production that
will not be diminished by any global warming that might occur concurrently. In fact, if the ambient air temperature rises, the growth-promoting effects of atmospheric CO2 enrichment
will likely rise along with it. Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/org/subject/g/tempco2
g.php.
Climate Change Reconsidered 468 References Alexandrov, V.A. and Hoogenboom, G. 2000. The impact of climate variability and change on crop yield in Bulgaria. Agricultural and Forest Meteorology
104: 315-327. Berry, J. and Bjorkman, O. 1980. Photosynthetic response and adaptation to temperature in higher plants. Annual Review of Plant Physiology 31: 491-543. Bunce, J.A. 1998.
The temperature dependence of the stimulation of photosynthesis by elevated carbon dioxide in wheat and barley. Journal of Experimental Botany 49: 1555-1561. Cowling, S.A. and Sage,
R.F. 1998. Interactive effects of low atmospheric CO2 and elevated temperature on growth, photosynthesis and respiration in Phaseolus vulgaris. Plant, Cell and Environment 21: 427-435.
Demmers-Derks, H., Mitchell, R.A.G., Mitchell, V.J. and Lawlor, D.W. 1998. Response of sugar beet (Beta vulgaris L.) yield and biochemical composition to elevated CO2 and temperature
at two nitrogen applications. Plant, Cell and Environment 21: 829-836. Ferris, R., Wheeler, T.R., Hadley, P. and Ellis, R.H. 1998. Recovery of photosynthesis after environmental stress
in soybean grown under elevated CO2. Crop Science 38: 948-955. Fritschi, F.B., Boote, K.J., Sollenberger, L.E., Allen, Jr. L.H. and Sinclair, T.R. 1999. Carbon dioxide and temperature
effects on forage establishment: photosynthesis and biomass production. Global Change Biology 5: 441-453. Hakala, K. 1998. Growth and yield potential of spring wheat in a simulated changed
climate with increased CO2 and higher temperature. European Journal of Agronomy 9: 41-52. Idso, K.E. and Idso, S.B. 1994. Plant responses to atmospheric CO2 enrichment in the face of
environmental constraints: A review of the past 10 years’ research. Agricultural and Forest Meteorology 69: 153-203. Long, S.P. 1991. Modification of the response of photosynthetic productivity
to rising temperature by atmospheric CO2 concentrations: Has its importance been underestimated? Plant, Cell and Environment 14: 729-739. McMurtrie, R.E. and Wang, Y.-P. 1993. Mathematical
models of the photosynthetic response of tree stands to rising CO2 concentrations and temperatures. Plant, Cell and Environment 16: 1-13. McMurtrie, R.E., Comins, H.N., Kirschbaum, M.U.F.
and Wang, Y.-P. 1992. Modifying existing forest growth models to take account of effects of elevated CO2. Australian Journal of Botany 40: 657-677. Reddy, K.K., Davidonis, G.H., Johnson,
A.S. and Vinyard, B.T. 1999. Temperature regime and carbon dioxide enrichment alter cotton boll development and fiber properties. Agronomy Journal 91: 851-858. Reddy, K.R., Robana, R.R.,
Hodges, H.F., Liu, X.J. and McKinion, J.M. 1998. Interactions of CO2 enrichment and temperature on cotton growth and leaf characteristics. Environmental and Experimental Botany 39: 117-129.
Stuhlfauth, T. and Fock, H.P. 1990. Effect of whole season CO2 enrichment on the cultivation of a medicinal plant, Digitalis lanata. Journal of Agronomy and Crop Science 164: 168-173.
Taub, D.R., Seeman, J.R. and Coleman, J.S. 2000. Growth in elevated CO2 protects photosynthesis against hightemperature damage. Plant, Cell and Environment 23: 649-656. Wurr, D.C.E.,
Edmondson, R.N. and Fellows, J.R. 2000. Climate change: a response surface study of the effects of CO2 and temperature on the growth of French beans. Journal of Agricultural Science
135: 379-387. Zhu, J., Goldstein, G. and Bartholomew, D.P. 1999. Gas exchange and carbon isotope composition of Ananas comosus in response to elevated CO2 and temperature. Plant, Cell
and Environment 22: 999-1007. Ziska, L.H. 1998. The influence of root zone temperature on photosynthetic acclimation to elevated carbon dioxide concentrations. Annals of Botany 81: 717-721.
7.3.9.2. Grassland Species In the study of Lilley et al. (2001), swards of Trifolium subterraneum were grown at 380 and 690 ppm CO2 in combination with simultaneous exposure to ambient
and elevated (ambient plus 3.4°C) air temperature. After one year of treatment, they reported that elevated CO2 increased foliage growth by 19 percent at ambient air temperature. At
elevated air temperature, however, plants grown at ambient CO2 exhibited a 28 percent reduction in foliage growth, while CO2-enriched plants still displayed a growth enhancement of 8
percent. Similarly, Morgan et al. (2001) reported that twice-ambient levels of atmospheric CO2 increased above-ground biomass in native shortgrass steppe ecosystems by an average of
38 percent, in spite of an average air temperature increase of 2.6°C. Likewise, when bahiagrass was grown across a temperature gradient of 4.5°C, Fritschi et al. (1999) reported that
a 275 ppm increase in the air’s CO2 content boosted photosynthesis and aboveground biomass by 22 and 17 percent, respectively,
Biological Effects of Carbon Dioxide Enrichment 469 independent of air temperature. Thus, at elevated air temperature, CO2-induced increases in rates of photosynthesis and biomass production
are typically equal to or greater than what they are at ambient air temperature. Other studies report similar results. Greer et al. (2000), for example, grew five pasture species at
18 and 28°C and reported that plants concomitantly exposed to 700 ppm CO2 displayed average photosynthetic rates that were 36 and 70 percent greater, respectively, than average rates
exhibited by control plants subjected to ambient CO2 concentrations. Moreover, the average CO2-induced biomass increase for these five species rose dramatically with increasing air temperature:
from only 8 percent at 18°C to 95 percent at 28°C. Thus, the beneficial effects of elevated CO2 on grassland productivity is often significantly enhanced by elevated air temperature.
Finally, temperature increases alone can promote grass growth and development. Norton et al. (1999) found elevated CO2 had essentially no effect on the growth of the perennial grass
Agrostis curtisii after two years of fumigation; however, a 3°C increase in air temperature increased the growth of this species considerably. In conclusion, grassland plants will likely
exhibit enhanced rates of photosynthesis and biomass production as the air’s CO2 content rises that will not be diminished by any global warming that might occur concurrently. If the
ambient air temperature rises, the growth-promoting effects of atmospheric CO2 enrichment will likely rise along with it. Additional information on this topic, including reviews of newer
publications as they become available, can be found at http://www.co2science.org/subject/g/tempco2grass.php. References Fritschi, F.B., Boote, K.J., Sollenberger, L.E., Allen, Jr. L.H.
and Sinclair, T.R. 1999. Carbon dioxide and temperature effects on forage establishment: photosynthesis and biomass production. Global Change Biology 5: 441-453. Greer, D.H., Laing,
W.A., Campbell, B.D. and Halligan, E.A. 2000. The effect of perturbations in temperature and photon flux density on the growth and photosynthetic responses of five pasture species. Australian
Journal of Plant Physiology 27: 301-310. Idso, K.E. and Idso, S.B. 1994. Plant responses to atmospheric CO2 enrichment in the face of environmental constraints: A review of the past
10 years’ research. Agricultural and Forest Meteorology 69: 153-203. Lilley, J.M., Bolger, T.P. and Gifford, R.M. 2001. Productivity of Trifolium subterraneum and Phalaris aquatica under
warmer, higher CO2 conditions. New Phytologist 150: 371-383. Morgan, J.A., LeCain, D.R., Mosier, A.R. and Milchunas, D.G. 2001. Elevated CO2 enhances water relations and productivity
and affects gas exchange in C3 and C4 grasses of the Colorado shortgrass steppe. Global Change Biology 7: 451-466. Norton, L.R., Firbank, L.G., Gray, A.J. and Watkinson, A.R. 1999. Responses
to elevated temperature and CO2 in the perennial grass Agrostis curtisii in relation to population origin. Functional Ecology 13: 29-37. 7.3.9.3. Trees In the study of Kellomaki and
Wang (2001), birch seedlings were grown at atmospheric CO2 concentrations of 350 and 700 ppm in combination with ambient and elevated (ambient plus 3°C) air temperatures. After five
months of treatment, the authors reported that photosynthetic rates of CO2-enriched seedlings were 21 and 28 percent greater than those displayed by their ambiently grown counterparts
at ambient and elevated air temperatures, respectfully. In another study, Carter et al. (2000) observed that a 300 ppm increase in the air’s CO2 content allowed leaves of sugar maple
seedlings to remain green and non-chlorotic when exposed to air temperatures 3°C above ambient air temperature. On the other hand, seedlings fumigated with ambient air exhibited severe
foliar chlorosis when exposed to the same elevated air temperatures. These results thus indicate that at elevated air temperatures, rates of photosynthesis are greater and foliar health
is typically better in birch and sugar maples trees in CO2-enriched as opposed to ambient air. Other studies report similar results. Sheu et al. (1999) grew a sub-tropical tree at day/night
temperatures of 25/20°C (ambient) and 30/25°C (elevated) for six months and reported that seedlings exposed to 720 ppm CO2 displayed photosynthetic rates that were 20 and 40 percent
higher, respectively, than that of their ambiently grown controls. In addition, the CO2-induced increases in total dry weight for this species were 14 and 49 percent,
Climate Change Reconsidered 470 respectively, at ambient and elevated air temperatures. Likewise, Maherali et al. (2000) observed that a 5°C increase in ambient air temperature increased
the CO2-induced biomass enhancement resulting from a 750 ppm CO2 enrichment of ponderosa pine seedlings from 42 to 62 percent. Wayne et al. (1998) reported that a 5°C increase in the
optimal growth temperature of yellow birch seedlings fumigated with an extra 400 ppm CO2 increased the CO2-induced increase in biomass from 60 to 227 percent. The beneficial effects
of elevated CO2 on tree species photosynthesis and growth can also be assessed during natural seasonal temperature changes, as documented by Hymus et al. (1999) for loblolly pine and
Roden et al. (1999) for snow gum seedlings. In some cases, however, there appear to be little interactive effects between elevated CO2 and temperature on photosynthesis and growth in
tree species. When Tjoelker et al. (1998a), for example, grew seedlings of quaking aspen, paper birch, tamarack, black spruce and jack pine at atmospheric CO2 concentrations of 580 ppm,
they reported average increases in photosynthetic rates of 28 percent, regardless of temperature, which varied from 18 to 30°C. After analyzing the CO2-induced increases in dry mass
for these seedlings, Tjoelker et al. (1998b) further reported that dry mass values were about 50 and 20 percent greater for the deciduous and coniferous species, respectively, regardless
of air temperature. Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/g/tempco2trees.php.
References Carter, G.A., Bahadur, R. and Norby, R.J. 2000. Effects of elevated atmospheric CO2 and temperature on leaf optical properties in Acer saccharum. Environmental and Experimental
Botany 43: 267-273. Hymus, G.J., Ellsworth, D.S., Baker, N.R. and Long, S.P. 1999. Does free-air carbon dioxide enrichment affect photochemical energy use by evergreen trees in different
seasons? A chlorophyll fluorescence study of mature loblolly pine. Plant Physiology 120: 1183-1191. Idso, K.E. and Idso, S.B. 1994. Plant responses to atmospheric CO2 enrichment in the
face of environmental constraints: A review of the past 10 years’ research. Agricultural and Forest Meteorology 69: 153-203. Kellomaki, S. and Wang, K.-Y. 2001. Growth and resource use
of birch seedlings under elevated carbon dioxide and temperature. Annals of Botany 87: 669-682. Maherali, H. and DeLucia, E.H. 2000. Interactive effects of elevated CO2 and temperature
on water transport in ponderosa pine. American Journal of Botany 87: 243-249. Roden, J.S., Egerton, J.J.G. and Ball, M.C. 1999. Effect of elevated [CO2] on photosynthesis and growth
of snow gum (Eucalyptus pauciflora) seedlings during winter and spring. Australian Journal of Plant Physiology 26: 37-46. Sheu, B.-H. and Lin, C.-K. 1999. Photosynthetic response of
seedlings of the sub-tropical tree Schima superba with exposure to elevated carbon dioxide and temperature. temperature. Environmental and Experimental Botany 41: 57-65. Tjoelker, M.G.,
Oleksyn, J. and Reich, P.B. 1998a. Seedlings of five boreal tree species differ in acclimation of net photosynthesis to elevated CO2 and temperature. Tree Physiology 18: 715-726. Tjoelker,
M.G., Oleksyn, J. and Reich, P.B. 1998b. Temperature and ontogeny mediate growth response to elevated CO2 in seedlings of five boreal tree species. New Phytologist 140: 197-210. Wayne,
P.M., Reekie, E.G. and Bazzaz, F.A. 1998. Elevated CO2 ameliorates birch response to high temperature and frost stress: implications for modeling climate-induced geographic range shifts.
Oecologia 114: 335-342. 7.3.9.4. Other In a mechanistic model study of Mediterranean shrub vegetation, Osborne et al. (2000) reported that increased warming and reduced precipitation
would likely decrease net primary production. However, when the same model was run at twice the ambient atmospheric CO2 concentration, it predicted a 25 percent increase in vegetative
productivity, in in spite of the increased warming and reduced precipitation. Although we tend to not review studies based on mechanistic models, it is also interesting to note that
Bunce (2000) demonstrated that field-grown Taraxacum officinale plants exposed to 525 ppm CO2 and low air temperatures (between 15 and 25°C) displayed photosynthetic rates that were
10 to 30 percent greater than what was predicted by state-ofthe-art biochemical models of photosynthesis for this range of temperatures. Thus, at both high and low air temperatures,
elevated CO2 appears to be capable of significantly increasing the photosynthetic prowess of some plants.
Biological Effects of Carbon Dioxide Enrichment 471 In the real world, Stirling et al. (1998) grew five fast-growing native species at various atmospheric CO2 concentrations and air
temperatures, finding that twice-ambient levels of atmospheric CO2 increased photosynthetic rates by 18-36 percent for all species regardless of air temperature, which was up to 3°C
higher than ambient air temperature. In addition, atmospheric CO2 enrichment increased average plant biomass by 25 percent, also regardless of air temperature. Likewise, in a study of
vascular plants from Antarctica, Xiong et al. (2000) reported that a 13°C rise in air temperature increased plant biomass by 2-to 3-fold. We can only imagine what the added benefit of
atmospheric CO2 enrichment would do for these species. Hamerlynck et al. (2000) demonstrated that the desert perennial shrub Larrea tridentata maintained more favorable midday leaf water
potentials during a nine-day high-temperature treatment when fumigated with 700 ppm CO2, as compared to to 350 ppm. Additional information on this topic, including reviews of newer publications
as they become available, can be found at http://www.co2science.org/subject/g/tempco2growthres.php. References Bunce, J.A. 2000. Acclimation to temperature of the response of photosynthesis
to increased carbon dioxide concentration in Taraxacum officinale. Photosynthesis Research 64: 89-94. Hamerlynck, E.P., Huxman, T.E., Loik, M.E. and Smith, S.D. 2000. Effects of extreme
high temperature, drought and elevated CO2 on photosynthesis of the Mojave Desert evergreen shrub, Larrea tridentata. Plant Ecology 148: 183-193. Idso, K.E. and Idso, S.B. 1994. Plant
responses to atmospheric CO2 enrichment in the face of environmental constraints: A review of the past 10 years’ research. Agricultural and Forest Meteorology 69: 153-203. Osborne, C.P.,
Mitchell, P.L., Sheehy, J.E. and Woodward, F.I. 2000. Modeling the recent historical impacts of atmospheric CO2 and climate change on Mediterranean vegetation. Global Change Biology
6: 445-458. Stirling, C.M., Heddell-Cowie, M., Jones, M.L., Ashenden, T.W. and Sparks, T.H. 1998. Effects of elevated CO2 and temperature on growth and allometry of five native fastgrowing
annual species. New Phytologist 140: 343-354. Xiong, F.S., Meuller, E.C. and Day, T.A. 2000. Photosynthetic and respiratory acclimation and growth response of Antarctic vascular plants
to contrasting temperature regimes. American Journal of Botany 87: 700-710. 7.3.10. UV-B Radiation Zhao et al. (2004) report that “as a result of stratospheric ozone depletion, UV-B
radiation (280-320 nm) levels are still high at the Earth’s surface and are projected to increase in the near future (Madronich et al., 1998; McKenzie et al., 2003).” In reference to
this potential development, they note that “increased levels of UV-B radiation are known to affect plant growth, development and physiological processes (Dai et al., 1992; Nogués et
al., 1999),” stating that high UV-B levels often result in “inhibition of photosynthesis, degradation of protein and DNA, and increased oxidative stress (Jordan et al., 1992; Stapleton,
1992).” In light of the above observations, it is important to clarify how the ongoing rise in the air’s CO2 content might affect the deleterious effects of UV-B radiation on earth’s
vegetation. To investigate this question, Zhao et al. grew well watered and fertilized cotton plants in sunlit controlled environment chambers maintained at atmospheric CO2 concentrations
of 360 or 720 ppm from emergence until three weeks past first-flower stage under three levels of UV-B radiation (0, 8 and 16 kJ m-2 d-1). On five dates between 21 and 62 days after emergence,
they measured a number of plant physiological processes and parameters. Over the course of the experiment, the mean net photosynthetic rate of the upper-canopy leaves in the CO2-enriched
chambers was increased—relative to that in the ambient-air chambers—by 38.3 percent in the low UV-B treatment (from 30.3 to 41.9 m m-2 s-1), 41.1 percent in the medium UV-B treatment
(from 28.7 to 40.5 m m-2 s-1), and 51.5 percent in the high UV-B treatment (from 17.1 to 25.9 m m-2 s-1). In the medium UV-B treatment, the growth stimulation from the elevated CO2 was
sufficient to raise net photosynthesis rates 33.7 percent above the rates experienced in the ambient air and no UV-B treatment (from 30.3 to 40.5 m m-2 s-1); but in the high UV-B treatment
the radiation damage was so great that even with the help of the 51.5 percent increase in net photosynthesis provided by the doubled-CO2 air, the mean net photosynthesis rate of the
cotton leaves was 14.5 percent less than that experienced in the
Climate Change Reconsidered 472 ambient air and no UV-B treatment (dropping from 30.3 to 25.9 m m-2 s-1). It should be noted that the medium UV-B treatment of this study was chosen to
represent the intensity of UV-B radiation presently received on a clear summer day in the major cotton production region of Mississippi, USA, under current stratospheric ozone conditions,
while the high UV-B treatment was chosen to represent what might be expected there following a 30 percent depletion of the ozone layer, which has been predicted to double the region’s
reception of UV-B radiation from 8 to 16 kJ m-2 d-1. Consequently, a doubling of the current CO2 concentration and the current UV-B radiation level would reduce the net photosynthetic
rate of cotton leaves by just under 10 percent (from 28.7 to 25.9 m m-2 s-1), whereas in the absence of a doubling of the air’s CO2 content, a doubling of the UV-B radiation level would
reduce cotton net photosynthesis by just over 40 percent (from 28.7 to 17.1 m m-2 s-1). Viewed in this light, it can be seen that a doubling the current atmospheric CO2 concentration
would compensate for over three-fourths of the loss of cotton photosynthetic capacity caused by a doubling of the current UV-B radiation intensity. It may do better than that, for in
the study of Zhao et al. (2003), it was reported that both Adamse and Britz (1992) and Rozema et al. (1997) found that doubled CO2 totally compensated for the negative effects of equally
high UV-B radiation. In another study (Qaderi and Reid, 2005), well watered and fertilized canola (Brassica napus L.) plants were grown from seed to maturity in pots within controlled
environment chambers maintained at either 370 or 740 ppm CO2 with and without a daily dose of UV-B radiation in the amount of 4.2 kJ m-2, while a number of plant parameters were measured
at various times throughout the growing season. With respect to the bottom-line result of final seed yield, this parameter was determined to be 0.98 g/plant in the control treatment
(ambient CO2, CO2, with UV-B). Doubling the CO2 concentration increased yield by 25.5 percent to 1.23 g/plant. Alternatively, removing the UV-B radiation flux increased yield by 91.8
percent to 1.88 g/plant. Doing both (doubling the CO2 concentration while simultaneously removing the UV-B flux) increased final seed yield most of all, by 175.5 percent to 2.7 g/plant.
Viewed from a different perspective, doubling the air’s CO2 concentration in the presence of the UV-B radiation flux enhanced final seed yield by 25.5 percent, while doubling CO2 in
the absence of the UV-B radiation flux increased seed yield by 43.6 percent. In concluding their paper, the authors note that “previous studies have shown that elevated CO2 increases
biomass and seed yield, whereas UV-B decreases them (Sullivan, 1997; Teramura et al., 1990).” Finding much the same thing in their study, they thus reckoned that “elevated CO2 may have
a positive effect on plants by mitigating the detrimental effects caused by UV-B radiation.” Two years later in a similar study of the same plant, Qaderi et al. (2007) grew well watered
and fertilized canola plants from the 30-day-old stage until 25 days after anthesis in 1-L pots within controlled environment chambers exposed to either 4.2 kJ m-2 d-1 of UV-B radiation
or no such radiation in air of either 370 or 740 ppm CO2, in order to determine the effects of these two parameters on the photosynthetic rates and water use efficiency of the maturing
husks or siliquas that surround the plants’ seeds. Results indicated that for the plants exposed to 4.2 kJ m-2 d-1 of UV-B radiation, the experimental doubling of the air’s CO2 concentration
led to a 29 percent increase in siliqua net photosynthesis, an 18 percent decrease in siliqua transpiration, and a 58 percent increase in siliqua water use efficiency; while for the
plants exposed to no UV-B radiation, siliqua net photosynthesis was increased by a larger 38 percent, transpiration was decreased by a larger 22 percent and water use efficiency was
increased by a larger 87 percent in the CO2-enriched air. In another noteworthy study, Deckmyn et al. (2001) grew white clover plants for four months in four small greenhouses, two of
which allowed 88 percent of the incoming UV-B radiation to pass through their roofs and walls and two of which allowed 82 percent to pass through, while one of the two greenhouses in
each of the UV-B treatments was maintained at ambient CO2 (371 ppm) and the other at elevated CO2 (521 ppm). At the mid-season point of their study, they found that the 40 percent increase
in atmospheric CO2 concentration stimulated the production of flowers in the low UV-B treatment by 22 percent and in the slightly higher UV-B treatment by 43 percent; while at the end
of the season, the extra CO2 was determined to have provided no stimulation of biomass production in the low UV-B treatment, but it significantly stimulated biomass production by 16
percent in the high UV-B treatment. The results of this study indicate that the positive effects of atmospheric CO2 enrichment on on flower and biomass production in white clover are
greater at more realistic or natural values of UV-B radiation than those found in many greenhouses. As a result,
Biological Effects of Carbon Dioxide Enrichment 473 Deckmyn et al. say their results “clearly indicate the importance of using UV-B transmittant greenhouses or open-top chambers when
conducting CO2 studies,” for if this is not done, their work suggests that the results obtained could significantly underestimate the magnitude of the benefits that are being continuously
accrued by earth’s vegetation as a result of the ongoing rise in the air’s CO2 content. In 2007, Koti et al. (2007) used Soil-Plant-Atmosphere-Research (SPAR) chambers at Mississippi
State University (USA) to investigate the effects of doubled atmospheric CO2 concentration (720 vs. 360 ppm) on the growth and development of six well watered and fertilized soybean
(Glycine max L.) genotypes grown from seed in pots filled with fine sand and exposed to the dual stresses of high day/night temperatures (38/30°C vs. 30/22°C) and high UV-B radiation
levels (10 vs. 0 kJ/m2/day). Results led this group of authors to report that “elevated CO2 partially compensated [for] the damaging effects on vegetative growth and physiology caused
by negative stressors such as high temperatures and enhanced UV-B radiation levels in soybean,” specifically noting, in this regard, CO2’s positive influence on the physiological parameters
of plant height, leaf area, total biomass, net photosynthesis, total chlorophyll content, phenolic content and wax content, as well as relative plant injury. In a study that did not
include UV-B radiation as an experimental parameter, Estiarte et al. (1999) grew spring wheat in FACE plots in Arizona, USA, at atmospheric CO2 concentrations of 370 and 550 ppm and
two levels of soil moisture (50 and 100 percent of potential evapotranspiration). They found that leaves of plants grown in elevated CO2 had 14 percent higher total flavonoid concentrations
than those of plants grown in ambient air, and that soil water content did not affect the relationship. An important aspect of this finding is that one of the functions of flavonoids
in plant leaves leaves is to protect them against UV-B radiation. More studies of this nature should be conducted to see how general this beneficial response may be throughout the plant
world. In a study of UV-B and CO2 effects on a natural ecosystem, which was conducted at the Abisko Scientific Research Station in Swedish Lapland, Johnson et al. (2002) studied plots
of subarctic heath composed of open canopies of downy birch and dense dwarf-shrub layers containing scattered herbs and grasses. For a period of five years, they exposed the plots to
factorial combinations of UV-B radiation— ambient and that expected to result from a 15 percent stratospheric ozone depletion—and atmospheric CO2 concentration—ambient (around 365 ppm)
and enriched (around 600 ppm)—after which they determined the amounts of microbial carbon (Cmic) and nitrogen (Nmic) in the soils of the plots. When the plots were exposed to the enhanced
UV-B radiation, the amount of Cmic in the soil was reduced to only 37 percent of what it was at the ambient UV-B B level when the air’s CO2 content was maintained at the ambient concentration.
When the UV-B increase was accompanied by the CO2 increase, however, not only was there not a decrease in Cmic, there was an actual increase of 37 percent. The amount of Nmic in the
soil experienced a 69 percent increase when the air’s CO2 content was maintained at the ambient concentration; and when the UV-B increase was accompanied by the CO2 increase, Nmic rose
even more, by 138 percent. These findings, in the words of Johnson et al., “may have far-reaching implications ... because the productivity of many semi-natural ecosystems is limited
by N (Ellenberg, 1988).” The 138 percent increase in soil microbial N observed in this study to accompany a 15 percent reduction in stratospheric ozone and a 64 percent increase in atmospheric
CO2 concentration (experienced in going from 365 ppm to 600 ppm) should significantly enhance the input of plant litter to the soils of these ecosystems, which phenomenon
represents the first half of of the carbon sequestration process, i.e., the carbon input stage. With respect to the second stage of keeping as much of that carbon as possible in the
soil, Johnson et al. note that “the capacity for subarctic semi-natural heaths to act as major sinks for fossil fuel-derived carbon dioxide is [also] likely to be critically dependent
on the supply of N,” as is indeed indicated to be the case in the literature review of Berg and Matzner (1997), who report that with more nitrogen in the soil, the long-term storage
of carbon is significantly enhanced, as more litter is chemically transformed into humic substances when nitrogen is more readily available, and these more recalcitrant carbon compounds
can be successfully stored in the soil for many millennia. In light of these several findings, we conclude that the ongoing rise in the air’s CO2 content is a powerful antidote for the
deleterious biological impacts that might possibly be caused by an increase in the flux of UV-B radiation at the surface of the earth due to any further depletion of the planet’s stratospheric
ozone layer.
Climate Change Reconsidered 474 Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/u/uvbradiatio
.php. References Adamse, P. and Britz, S.J. 1992. Amelioration of UV-B damage under high irradiance. I. Role of photosynthesis. Photochemistry and Photobiology 56: 645-650. Berg, B.
and Matzner, E. 1997. Effect of N deposition on decomposition of plant litter and soil organic matter in forest ecosystems. Environmental Reviews 5: 1-25. Dai, Q., Coronal, V.P., Vergara,
B.S., Barnes, P.W. and Quintos, A.T. 1992. Ultraviolet-B radiation effects on growth and physiology of four rice cultivars. Crop Science 32: 1269-1274. Deckmyn, G., Caeyenberghs, E.
and Ceulemans, R. 2001. Reduced UV-B in greenhouses decreases white clover response to enhanced CO2. Environmental and Experimental Botany 46: 109-117. Ellenberg, H. 1988. Vegetation
Ecology of Central Europe. Cambridge University Press, Cambridge, UK. Estiarte, M., Penuelas, J., Kimball, B.A., Hendrix, D.L., Pinter Jr., P.J., Wall, G.W., LaMorte, R.L. and Hunsaker,
D.J. 1999. Free-air CO2 enrichment of wheat: leaf flavonoid concentration throughout the growth cycle. Physiologia Plantarum 105: 423-433. Johnson, D., Campbell, C.D., Lee, J.A., Callaghan,
T.V. and Gwynn-Jones, D. 2002. Arctic microorganisms respond more to elevated UV-B radiation than CO2. Nature 416: 82-83. Jordan, B.R., Chow, W.S. and Anderson, J.M. 1992. Changes in
mRNA levels and polypeptide subunits of ribulose 1,5-bisphosphate carboxylase in response to supplementary ultraviolet-B radiation. Plant, Cell and Environment 15: 91-98. Koti, S., Reddy,
K.R., Kakani, V.G., Zhao, D. and Gao, W. 2007. Effects of carbon dioxide, temperature and ultraviolet-B radiation and their interactions on soybean (Glycine max L.) growth and development.
Environmental and Experimental Botany 60: 1-10. Madronich, S., McKenzie, R.L., Bjorn, L.O. and Caldwell, M.M. 1998. Changes in biologically active ultraviolet radiation reaching the
Earth’s surface. Journal of Photochemistry and Photobiology B 46: 5-19. McKenzie, R.L., Bjorn, L.O., Bais, A. and Ilyasd, M. 2003. Changes in biologically active ultraviolet radiation
reaching the earth’s surface. Photochemical and Photobiological Sciences 2: 5-15. Nogués, S., Allen, D.J., Morison, J.I.L. and Baker, N.R. 1999. Characterization of stomatal closure
caused by ultraviolet-B radiation. Plant Physiology 121: 489-496. Qaderi, M.M. and Reid, D.M. 2005. Growth and physiological responses of canola (Brassica napus) to UVB and CO2 under
controlled environment conditions. Physiologia Plantarum 125: 247-259. Qaderi, M.M., Reid, D.M. and Yeung, E.C. 2007. Morphological and physiological responses of canola (Brassica napus)
siliquas and seeds to UVB and CO2 under controlled environment conditions. Environmental and Experimental Botany 60: 428-437. Rozema, J., Lenssen, G.M., Staaij, J.W.M., Tosserams, M.,
Visser, A.J. and Brockman, R.A. 1997. Effects of UV-B radiation on terrestrial plants and ecosystems: interaction with CO2 enrichment. Plant Ecology 128: 182-191. Stapleton, A.E. 1992.
Ultraviolet radiation and plants: Burning questions. The Plant Cell 105: 881-889. Sullivan, J.H. 1997. Effects of increasing UV-B radiation and atmospheric CO2 on photosynthesis and
growth: implications for terrestrial ecosystems. Plant Ecology 128: 195-206. Teramura, A.H., Sullivan, J.H. and Ziska, L.H. 1990. Interaction of elevated UV-B radiation and CO2 on productivity
and photosynthetic characteristics in wheat, rice, and soybean. Plant Physiology 94: 470-475. Zhao, D., Reddy, K.R., Kakani, V.G., Mohammed, A.R., Read, J.J. and Gao, W. 2004. Leaf and
canopy photosynthetic characteristics of cotton (Gossypiuym hirsutum) under elevated CO2 concentration and UV-B radiation. Journal of Plant Physiology 161: 581-590. Zhao, D., Reddy,
K.R., Kakani, V.G., Read, J.J. and Sullivan, J.H. 2003. Growth and physiological responses of cotton (Gossypium hirsutum L.) to elevated carbon dioxide and ultraviolet-B radiation under
controlled environmental conditions. Plant, Cell and Environment 26: 771-782. 7.3.11. Water Stress As the CO2 content of the air continues to rise, nearly all of earth’s plants will
exhibit increases in photosynthesis and biomass production. However, some experts claim that water stress will negate these benefits. In reviewing the scientific literature of the ten-year
period 1983-1994, however, Idso and Idso (1994) found that water stress typically will not
Biological Effects of Carbon Dioxide Enrichment 475 negate the CO2-induced stimulation of plant productivity. They found that the CO2-induced percentage increase in plant biomass production
was often greater under water-stressed conditions than it was when plants were well-watered. We here review some more recent scientific literature in this area for agricultural, grassland
and woody plant species. 7.3.11.1. Agricultural Species During times of water stress, atmospheric CO2 enrichment often stimulates plants to develop largerthan-usual and more robust root
systems to probe greater volumes of soil for scarce and much-needed moisture. Wechsung et al. (1999), for example, observed a 70 percent increase in lateral root dry weights of water-stressed
wheat grown at 550 ppm CO2, while De Luis et al. (1999) reported a 269 percent increase in root-to-shoot ratio of waterstressed alfalfa growing at 700 ppm CO2. Thus, elevated CO2 elicits
stronger-than-usual positive root responses in agricultural species under conditions of water stress. Elevated levels of atmospheric CO2 also tend to reduce the openness of stomatal
pores on leaves, thus decreasing plant stomatal conductance. This phenomenon, in turn, reduces the amount of water lost to the atmosphere by transpiration and, consequently, lowers overall
plant water use. Serraj et al. (1999) report that water-stressed soybeans grown at 700 ppm CO2 reduced their total seasonal water loss by 10 percent relative to that of water-stressed
control plants grown at 360 ppm CO2. In addition, Conley et al. (2001) noted that a 200-ppm increase in the air’s CO2 concentration reduced cumulative evapotranspiration in water-stressed
sorghum by approximately 4 percent. Atmospheric CO2 enrichment thus increases plant water acquisition, by stimulating root growth, while it reduces plant water loss, by constricting
stomatal apertures; and these dual effects typically enhance plant water-use efficiency, even under conditions of less-than-optimal soil water content. But these phenomena have other
implications as well. CO2-induced increases in root development together with CO2-induced reductions in stomatal conductance often contribute to the maintenance of a more favorable plant
water status during times of drought. Sgherri et al. (1998) reported that leaf water potential, which is a good indicator of overall plant water status, was 30 percent higher (less negative
and therefore more favorable) in water-stressed alfalfa grown at an atmospheric CO2 concentration of 600 ppm CO2 versus 340 ppm CO2. Wall (2001) reports that leaf water potentials were
similar in CO2-enriched water-stressed plants and ambiently grown well-watered control plants, which implies a complete CO2-induced amelioration of water stress in the CO2-enriched plants.
Similarly, Lin and Wang (2002) demonstrated that elevated CO2 caused a several-day delay in the onset of the water stress-induced production of the highly reactive oxygenated compound
H2O2 in spring wheat. If atmospheric CO2 enrichment thus allows plants to maintain maintain a better water status during times of water stress, it is only logical to expect that such
plants should exhibit greater rates of photosynthesis than ambiently grown plants. And so they do. With the onset of water stress in Brassica juncea, for example, photosynthetic rates
dropped by 40 percent in plants growing in ambient air, while plants growing in air containing 600 ppm CO2 only experienced a 30 percent reduction in net photosynthesis (Rabha and Uprety,
1998). Ferris et al. (1998) reported that after imposing water-stress conditions on soybeans and allowing them to recover following complete rewetting of the soil, plants grown in air
containing 700 ppm CO2 reached pre-stressed rates of photosynthesis after six days, while plants grown in ambient air never recovered to pre-stressed rates. Reasoning analogously, it
is also to be expected that plant biomass production would be enhanced by elevated CO2 concentrations under drought conditions. In exploring this idea, Ferris et al. (1999) reported
that water-stressed soybeans grown at 700 ppm CO2 attained seed yields that were 24 percent greater than those of similarly water-stressed plants grown at ambient CO2 concentrations,
while Hudak et al. (1999) reported that water-stress had no effect on yield in CO2-enriched spring wheat. In some cases, the CO2-induced percentage biomass increase is actually greater
for water-stressed plants than it is for well-watered plants. Li et al. (2000), for example, reported that a 180-ppm increase in the air’s CO2 content increased lower stem grain weights
in water-stressed and well-watered spring wheat by 24 and 14 percent, respectively. Similarly, spring wheat grown in air containing an additional 280 ppm CO2 exhibited 57 and 40 percent
increases in grain yield under water-stressed and well-watered conditions, respectively (Schutz and Fangmeier, 2001). Likewise, Ottman et al. (2001) noted that elevated CO2 increased
plant biomass in water
Climate Change Reconsidered 476 stressed sorghum by 15 percent, while no biomass increase occurred in well-watered sorghum. In summary, the conclusions of Idso and Idso (1994) are well
supported by the recent peer-reviewed scientific literature, which indicates that the ongoing rise in the air’s CO2 content will likely lead to substantial increases in plant photosynthetic
rates and biomass production, even in the face of stressful conditions imposed by less-than-optimum soil moisture conditions. Additional information on this topic, including reviews
of newer publications as they become available, can be found at http://www.co2science.org/subject/g/growthwaterag.php. References Conley, M.M., Kimball, B.A., Brooks, T.J., Pinter Jr.,
P.J., Hunsaker, D.J., Wall, G.W., Adams, N.R., LaMorte, R.L., Matthias, A.D., Thompson, T.L., Leavitt, S.W., Ottman, M.J., Cousins, A.B. and Triggs, J.M. 2001. CO2 enrichment increases
water-use efficiency in sorghum. New Phytologist 151: 407-412. De Luis, J., Irigoyen, J.J. and Sanchez-Diaz, M. 1999. Elevated CO2 enhances plant growth in droughted N2-fixing alfalfa
without improving water stress. Physiologia Plantarum 107: 84-89. Ferris, R., Wheeler, T.R., Hadley, P. and Ellis, R.H. 1998. Recovery of photosynthesis after environmental stress in
soybean grown under elevated CO2. Crop Science 38: 948-955. Ferris, R., Wheeler, T.R., Ellis, R.H. and Hadley, P. 1999. Seed yield after environmental stress in soybean grown under elevated
CO2. Crop Science 39: 710-718. Hudak, C., Bender, J., Weigel, H.-J. and Miller, J. 1999. Interactive effects of elevated CO2, O3, and soil water deficit on spring wheat (Triticum aestivum
L. cv. Nandu). Agronomie 19: 677-687. Idso, K.E. and Idso, S.B. 1994. Plant responses to atmospheric CO2 enrichment in the face of environmental constraints: A review of the past 10
years’ research. Agricultural and Forest Meteorology 69: 153-203. Li, A.-G., Hou, Y.-S., Wall, G.W., Trent, A., Kimball, B.A. and Pinter Jr., P.J. 2000. Free-air CO2 enrichment and drought
stress effects on grain filling rate and duration in spring wheat. Crop Science 40: 1263-1270. Lin, J.-S and Wang, G.-X. 2002. Doubled CO2 could improve the drought tolerance better
in sensitive cultivars than in tolerant cultivars in spring wheat. Plant Science 163: 627-637. Ottman, M.J., Kimball, B.A., Pinter Jr., P.J., Wall, G.W., Vanderlip, R.L., Leavitt, S.W.,
LaMorte, R.L., Matthias, A.D. and Brooks, T.J. 2001. Elevated CO2 increases sorghum biomass under drought conditions. New Phytologist 150: 261-273. Rabha, B.K. and Uprety, D.C. 1998.
Effects of elevated CO2 and moisture stress on Brassica juncea. Photosynthetica 35: 597-602. Serraj, R., Allen, L.H., Jr., Sinclair, T.R. 1999. Soybean leaf growth and gas exchange response
to drought under carbon dioxide enrichment. Global Change Biology 5: 283-291. Sgherri, C.L.M., Quartacci, M.F., Menconi, M., Raschi, A. and Navari-Izzo, F. 1998. Interactions between
drought and elevated CO2 on alfalfa plants. Journal of Plant Physiology 152: 118-124. Schutz, M. and Fangmeier, A. 2001. Growth and yield responses of spring wheat (Triticum aestivum
L. cv. Minaret) to elevated CO2 and water limitation. Environmental Pollution 114: 187-194. Wall, G.W. 2001. Elevated atmospheric CO2 alleviates drought stress in wheat. Agriculture,
Ecosystems and Environment 87: 261-271. Wechsung, G., Wechsung, F., Wall, G.W., Adamsen, F.J., Kimball, B.A., Pinter Jr., P.J., LaMorte, R.L., Garcia, R.L. and Kartschall, T. 1999. The
effects of free-air CO2 enrichment and soil water availability on spatial and seasonal patterns of wheat root growth. Global Change Biology 5: 519-529. 7.3.11.2. Grassland Species In
the study of Leymarie et al. (1999), twice-ambient levels of atmospheric CO2 caused significant reductions in the stomatal conductance of waterstressed Arabidopsis thaliana. Similarly,
Volk et al. (2000) reported that calcareous grassland species exposed to elevated CO2 concentrations (600 ppm) consistently exhibited reduced stomatal conductance, regardless of soil
moisture availability. availability. Thus, atmospheric CO2 enrichment clearly reduces stomatal conductance and plant transpiration and soil water depletion in grassland ecosystems. In
the case of four grassland species comprising a pasture characteristic of New Zealand, Clark et al. (1999) found that leaf water potential, which is a good indicator of plant water status,
was consistently higher (less negative and, therefore, less stressful) under elevated atmospheric CO2 concentrations. Similarly, leaf water potentials of the water-stressed
Biological Effects of Carbon Dioxide Enrichment 477 C4 grass Panicum coloratum grown at 1000 ppm CO2 were always higher than those of their water-stressed counterparts growing in ambient
air (Seneweera et al., 2001). Indeed, Seneweera et al. (1998) reported that leaf water potentials observed in CO2-enriched waterstressed plants were an amazing three-and-a-half times
greater than those observed in control plants grown at 350 ppm during drought conditions (Seneweera et al., 1998). If atmospheric CO2 enrichment thus allows plants to maintain improved
water status during times of water stress, it is only logical to expect that such plants will exhibit greater photosynthetic rates than similar plants growing in ambient air. In a severe
test of this concept, Ward et al. (1999) found that extreme water stress caused 93 and 85 percent reductions in the photosynthetic rates of two CO2-enriched grassland species; yet their
rates of carbon fixation were still greater than those observed under ambient CO2 conditions. In view of the fact that elevated CO2 enhances photosynthetic rates during times of water
stress, one would expect that plant biomass production would also be enhanced by elevated CO2 concentrations under drought conditions. And so it is. On the American prairie, for example,
Owensby et al. (1999) reported that tallgrass ecosystems exposed to twiceambient concentrations of atmospheric CO2 for eight years only exhibited significant increases in aboveand below-ground
biomass during years of less-thanaverage rainfall. Also, in the study of Derner et al. (2001), the authors reported that a 150-ppm increase in the CO2 content of the air increased shoot
biomass in two C4 grasses by 57 percent, regardless of soil water content. Seneweera et al. (2001) reported that a 640-ppm increase in the air’s CO2 content increased shoot dry mass
in a C4 grass by 44 and 70 percent under well-watered and water-stressed conditions, respectively. Likewise, Volk et al. (2000) grew calcareous grassland assemblages at 360 and 600 ppm
CO2 and documented 18 and 40 percent CO2-induced increases in whole-community biomass under wellwatered and water-stressed conditions, respectively. In summary, the peer-reviewed scientific
literature indicates that the ongoing rise in the air’s CO2 content will likely lead to substantial increases in plant photosynthetic rates and biomass production for grassland species
even in the face of stressful environmental conditions imposed by less-thanoptimum soil moisture contents. Additional information on this topic, including reviews of newer publications
as they become available, can be found at http://www.co2science.org/subject/g/growthwatergrass.php. References Clark, H., Newton, P.C.D. and Barker, D.J. 1999. Physiological and morphological
responses to elevated CO2 and a soil moisture deficit of temperate pasture species growing in an established plant community. Journal of Experimental Botany 50: 233-242. Derner, J.D.,
Polley, H.W., Johnson, H.B. and Tischler, C.R. 2001. Root system response of C4 grass seedlings to CO2 and soil water. Plant and Soil 231: 97-104. Leymarie, J., Lasceve, G. and Vavasseur,
A. 1999. Elevated CO2 enhances stomatal responses to osmotic stress and abscisic acid in Arabidopsis thaliana. Plant, Cell and Environment 22: 301-308. Owensby, C.E., Ham, J.M., Knapp,
A.K. and Auen, L.M. 1999. Biomass production and species composition change in a tallgrass prairie ecosystem after long-term exposure to elevated atmospheric CO2. Global Change Biology
5: 497-506. Seneweera, S.P., Ghannoum, O. and Conroy, J. 1998. High vapor pressure deficit and low soil water availability enhance shoot growth responses of a C4 grass (Panicum coloratum
cv. Bambatsi) to CO2 enrichment. Australian Journal of Plant Physiology 25: 287-292. Seneweera, S., Ghannoum, O. and Conroy, J.P. 2001. Root and shoot factors contribute to the effect
of drought on photosynthesis and growth of the C4 grass Panicum coloratum at elevated CO2 partial pressures. Australian Journal of Plant Physiology 28: 451-460. Volk, M., Niklaus, P.P.A.
and Korner, C. 2000. Soil moisture effects determine CO2 responses of grassland species. Oecologia 125: 380-388. Ward, J.K., Tissue, D.T., Thomas, R.B. and Strain, B.R. 1999. Comparative
responses of model C3 and C4 plants to drought in low and elevated CO2. Global Change Biology 5: 857-867. 7.3.11.3. Woody Species During times of water stress, atmospheric CO2 enrichment
often stimulates the development of larger-than-usual and more robust root systems in woody perennial species, which allows them to probe greater volumes of soil for scarce and much-needed
moisture. Tomlinson and Anderson (1998), for
Climate Change Reconsidered 478 example, report that greater root development in water-stressed red oak seedlings grown at 700 ppm CO2 helped them effectively deal with the reduced availability
of moisture. These trees eventually produced just as much biomass as well-watered controls exposed to ambient air containing 400 ppm CO2. In addition, Polley et al. (1999) note that
waterstressed honey mesquite trees subjected to an atmospheric CO2 concentration of 700 ppm produced 37 percent more root biomass than water-stressed control seedlings growing at 370
ppm. Elevated levels of atmospheric CO2 also tend to reduce the area of open stomatal pore space on leaf surfaces, thus reducing plant stomatal conductance. This phenomenon, in turn,
reduces the amount of water lost to the atmosphere via transpiration. Tognetti et al. (1998), for example, determined that stomatal conductances of mature oak trees growing near natural
CO2 springs in central Italy were significantly lower than those of similar trees growing further away from the springs during periods of severe summer drought. CO2-induced increases
in root development together with CO2-induced reductions in stomatal conductance often contribute to the maintenance of a more favorable plant water status during times of drought. In
the case of three Mediterranean shrubs, Tognetti et al. (2002) found that leaf water potential, which is a good indicator of plant water status, was consistently higher (less negative
and, hence, less stressful) under twice-ambient CO2 concentrations. Similarly, leaf water potentials of water-stressed mesquite seedlings grown at 700 ppm CO2 were 40 percent higher
than those of their water-stressed counterparts growing in ambient air (Polley et al., 1999), which is comparable to values of -5.9 and -3.4 MPa observed in water-stressed evergreen
shrubs (Larrea tridentata) exposed to 360 and 700 ppm CO2, respectively (Hamerlynck et al., 2000). Palanisamy (1999) observed water-stressed Eucalyptus seedlings grown at 800 ppm CO2
display greater net net photosynthetic rates than their ambiently grown and water-stressed counterparts. Runion et al. (1999) observed the CO2-induced photosynthetic stimulation of water-stressed
pine seedlings grown at 730 ppm CO2 to be nearly 50 percent greater than that of similar water-stressed pine seedlings grown at 365 ppm CO2. Similarly, Centritto et al. (1999a) found
that water-stressed cherry trees grown at 700 ppm CO2 displayed net photosynthetic rates that were 44 percent greater than those of water-stressed trees grown at 350 ppm CO2. And Anderson
and Tomlinson (1998) found that a 300-ppm increase in the air’s CO2 concentration boosted photosynthetic rates in well-watered and water-stressed red oak seedlings by 34 and 69 percent,
respectively, demonstrating that the CO2-induced percentage enhancement in net photosynthesis in this species was essentially twice as great in water-stressed seedlings as in well-watered
ones. Sometimes, plants suffer drastically when subjected to extreme water stress. However, the addition of CO2 to the atmosphere often gives them an edge over ambiently growing plants.
Tuba et al. (1998), for example, reported that leaves of a waterstressed woody shrub exposed to an atmospheric CO2 concentration of 700 ppm continued to maintain positive rates of net
carbon fixation for a period that lasted three times longer than that observed for leaves of equally water-stressed control plants growing in ambient air. Similarly, Fernandez et al.
(1998) discovered that herb and tree species growing near natural CO2 vents in Venezuela continued to maintain positive rates of net photosynthesis during that location’s dry season,
while the same species growing some distance away from the CO2 source displayed net losses of carbon during this stressful time. Likewise, Fernandez et al. (1999) noted that after four
weeks of drought, the deciduous Venezuelan shrub Ipomoea carnea continued to exhibit positive carbon gains under elevated CO2 conditions, whereas ambiently growing plants displayed net
carbon losses. Polley et al. (2002) reported that seedlings of five woody species grown at twice-ambient CO2 concentrations survived 11 days longer (on average) than control seedlings
when subjected to maximum drought conditions. Thus, in some cases of water stress, enriching the air with CO2 can mean the difference between life or death. Arp et al. (1998) reported
that six perennial plants common to the Netherlands increased their biomass under CO2-enriched conditions even when suffering from lack of water. In other cases, the CO2-induced percentage
biomass increase is sometimes even greater for water-stressed plants than it is for wellwatered plants. Catovsky and Bazzaz (1999), for example, reported that the CO2-induced biomass
increase for paper birch was 27 percent and 130 percent for well-watered and water-stressed seedlings, respectively. Similarly, Schulte et al. (1998) noted that the CO2-induced biomass
increase of oak seedlings was greater under water-limiting conditions than under well-watered conditions (128 percent vs. 92 percent), as did Centritto et al. (1999b) for basal
Biological Effects of Carbon Dioxide Enrichment 479 trunk area in cherry seedlings (69 percent vs. 22 percent). Finally, Knapp et al. (2001) developed tree-ring index chronologies from
western juniper stands in Oregon, USA, finding that the trees recovered better from the effects of drought in the 1990’s, when the air’s CO2 concentration was around 340 ppm, than they
did from 1900-1930, when the atmospheric CO2 concentration was around 300 ppm. In a loosely related study, Osborne et al. (2002) looked at the warming and reduced precipitation experienced
in Mediterranean shrublands over the last century and concluded that primary productivity should have been negatively impacted in those areas. However, when the concurrent increase in
atmospheric CO2 concentration was factored into their mechanistic model, a 25 percent increase in primary productivity was projected. In summary, the peer-reviewed scientific literature
indicates that the ongoing rise in the air’s CO2 content will likely lead to substantial increases in photosynthetic rates and biomass production in earth’s woody species in the years
and decades ahead, even in the face of stressful conditions imposed by less-than-optimal availability of soil moisture. Additional information on this topic, including reviews of newer
publications as they become available, can be found at http://www.co2science.org/subject/g/growthwaterwood.php. References Anderson, P.D. and Tomlinson, P.T. 1998. Ontogeny affects response
of northern red oak seedlings to elevated CO2 and water stress. I. Carbon assimilation and biomass production. New Phytologist 140: 477-491. Arp, W.J., Van Mierlo, J.E.M., Berendse,
F. and Snijders, W. 1998. Interactions between elevated CO2 concentration, nitrogen and water: effects on growth and water use of six perennial plant species. Plant, Cell and Environment
21: 1-11. Catovsky, S. and Bazzaz, F.A. 1999. Elevated CO2 influences the responses of two birch species to soil moisture: implications for forest community structure. Global Change
Biology 5: 507-518. Centritto, M., Magnani, F., Lee, H.S.J. and Jarvis, P.G. 1999a. Interactive effects of elevated [CO2] and drought on cherry (Prunus avium) seedlings. II. Photosynthetic
capacity and water relations. New Phytologist 141: 141-153. Centritto, M., Lee, H.S.J. and Jarvis, P.G. 1999b. Interactive effects of elevated [CO2] and drought on cherry (Prunus avium)
seedlings. I. Growth, whole-plant water use efficiency and water loss. New Phytologist 141: 129-140. Fernandez, M.D., Pieters, A., Azuke, M., Rengifo, E., Tezara, W., Woodward, F.I.
and Herrera, A. 1999. Photosynthesis in plants of four tropical species growing under elevated CO2. Photosynthetica 37: 587-599. Fernandez, M.D., Pieters, A., Donoso, C., Tezara, W.,
Azuke, M., Herrera, C., Rengifo, E. and Herrera, A. 1998. Effects of a natural source of very high CO2 concentration on the leaf gas exchange, xylem water potential and stomatal characteristics
of plants of Spatiphylum cannifolium and Bauhinia multinervia. New Phytologist 138: 689-697. Hamerlynck, E.P., Huxman, T.E., Loik, M.E. and Smith, S.D. 2000. Effects of extreme high
temperature, drought and elevated CO2 on photosynthesis of the Mojave Desert evergreen shrub, Larrea tridentata. Plant Ecology 148: 183-193. Knapp, P.A., Soule, P.T. and Grissino-Mayer,
H.D. 2001. Post-drought growth responses of western juniper (Juniperus occidentalis var. occidentalis) in central Oregon. Geophysical Research Letters 28: 2657-2660. Osborne, C.P., Mitchell,
P.L.,
Sheehy, J.E. and Woodward, F.I. 2000. Modellng the recent historical impacts of atmospheric CO2 and climate change on Mediterranean vegetation. Global Change Biology 6: 445-458. Palanisamy,
K. 1999. Interactions of elevated CO2 concentration and drought stress on photosynthesis in Eucalyptus cladocalyx F. Muell. Photosynthetica 36: 635-638. Polley, H.W., Tischler, C.R.,
Johnson, H.B. and Derner, J.D. 2002. Growth rate and survivorship of drought: CO2 effects on the presumed tradeoff in seedlings of five woody legumes. Tree Physiology 22: 383-391. Polley,
H.W., Tischler, C.R., Johnson, H.B. and Pennington, R.E. 1999. Growth, water relations, and survival of drought-exposed seedlings from six maternal families of honey mesquite (Prosopis
glandulosa): responses to CO2 enrichment. Tree Physiology 19: 359-366. Runion, G.B., Mitchell, R.J., Green, T.H., Prior, S.A., Rogers, H.H. and Gjerstad, D.H. 1999. Longleaf pine photosynthetic
response to soil resource availability and elevated atmospheric carbon dioxide. Journal of Environmental Quality 28: 880-887. Schulte, M., Herschbach, C. and Rennenberg, H. 1998. Interactive
effects of elevated atmospheric CO2, mycorrhization and drought on long-distance transport of
Climate Change Reconsidered 480 reduced sulphur in young pedunculate oak trees (Quercus robur L.). Plant, Cell and Environment 21: 917-926. Tognetti, R., Longobucco, A., Miglietta, F.
and Raschi, A. 1998. Transpiration and stomatal behaviour of Quercus ilex plants during the summer in a Mediterranean carbon dioxide spring. Plant, Cell and Environment 21: 613-622.
Tognetti, R., Raschi, A. and Jones M.B. 2002. Seasonal changes in tissue elasticity and water transport efficiency in three co-occurring Mediterranean shrubs under natural long-term
CO2 enrichment. Functional Plant Biology 29: 1097-1106. Tomlinson, P.T. and Anderson, P.D. 1998. Ontogeny affects response of northern red oak seedlings to elevated CO2 and water stress.
II. Recent photosynthate distribution and growth. New Phytologist 140: 493-504. Tuba, Z., Csintalan, Z., Szente, K., Nagy, Z. and Grace, J. 1998. Carbon gains by desiccation-tolerant
plants at elevated CO2. Functional Ecology 12: 39-44. 7.4. Acclimation Plants grown in elevated CO2 environments often exhibit some degree of photosynthetic acclimation or down regulation,
which is typically characterized by long-term rates of photosynthesis that are somewhat lower than what would be expected on the basis of measurements made during short-term exposure
to CO2-enriched air. These downward adjustments result from modest long-term decreases in the activities and/or amounts of the primary plant carboxylating enzyme rubisco. Acclimation
is said to be present when the photosynthetic rates of long-term CO2-enriched plants are found to be lower than those of long-term non-CO2-enriched plants when the normally CO2-enriched
plants are measured during brief exposures to ambient CO2 concentrations. In this section, we review research that has been published on acclimation in agricultural, desert, grassland
and woody species. Additional information on this topic, including reviews on acclimation not discussed here, can be found at http://www.co2science.org/subject/a/subject _a.php under
the heading Acclimation. 7.4.1. Agricultural Species Several studies have examined the effects of elevated CO2 on acclimation in agricultural crops. Ziska (1998), for example, reported
that soybeans grown at an atmospheric CO2 concentration of 720 ppm initially exhibited photosynthetic rates that were 50 percent greater than those observed in control plants grown at
360 ppm. However, after the onset of photosynthetic acclimation, CO2-enriched plants displayed subsequent photosynthetic rates that were only 30 percent greater than their ambiently
grown counterparts. In another study, Theobald et al. (1998) grew spring wheat at twice-ambient atmospheric CO2 concentrations and determined that elevated CO2 reduced the amount of
rubisco required to sustain enhanced rates of photosynthesis, which led to a significant increase in plant photosynthetic nitrogenuse efficiency. CO2-induced increases in photosynthetic
nitrogen-use efficiency have also been reported in spring wheat by Osborne et al. (1998). In an interesting study incorporating both hydroponically and pot-grown wheat plants, Farage
et al. (1998) demonstrated that low nitrogen fertilization does not lead to photosynthetic acclimation in elevated CO2 environments, as long as the nitrogen supply keeps pace with the
relative growth rate of the plants. Indeed, when spring wheat was grown at an atmospheric CO2 concentration of 550 ppm in a freeair CO2 enrichment (FACE) experiment with optimal soil
nutrition and unlimited rooting volume, Garcia et al. (1998) could find no evidence of photosynthetic acclimation. CO2-induced photosynthetic acclimation often results from insufficient
plant sink strength, which can lead to carbohydrate accumulation in source leaves and the triggering of photosynthetic end product feedback inhibition, which reduces net photosynthetic
rates. Indeed, Gesch et al. (1998) reported that rice plants—which have relatively limited potential for developing additional carbon sinks—grown at an atmospheric CO2 concentration
of 700 ppm exhibited increased leaf carbohydrate contents, which likely reduced rbcS mRNA levels and rubisco protein content. Similarly, Sims et al. (1998) reported that photosynthetic
acclimation was induced in CO2-enriched soybean plants from the significant accumulation of nonstructural carbohydrates in their leaves. However, in growing several different Brassica
species at 1,000 ppm CO2, Reekie et al. (1998) demonstrated that CO2-induced acclimation was avoided in species having well-developed carbon sinks (broccoli and cauliflower) and only
appeared in those lacking significant sink strength (rape and mustard). Thus, acclimation does not appear to be a direct consequence of atmospheric CO2 enrichment
Biological Effects of Carbon Dioxide Enrichment 481 but rather an indirect effect of low sink strength, which results in leaf carbohydrate accumulation that can trigger acclimation.
In some cases, plants can effectively increase their sink strength, and thus reduce the magnitude of CO2-induced acclimation, by forming symbiotic relationships with certain species
of soil fungi. Under such conditions, photosynthetic down regulation is not triggered as rapidly, or as frequently, by end product feedback inhibition, as excess carbohydrates are mobilized
out of source leaves and sent belowground to symbiotic fungi. Indeed, Louche-Tessandier et al. (1999) report that photosynthetic acclimation in CO2-enriched potatoes was less apparent
when plants were simultaneously colonized by a mycorrhizal fungus. Thus, CO2-induced acclimation appears to be closely related to the source:sink balance that exists within plants, being
triggered when sink strength falls below, and source strength rises above, critical thresholds in in a speciesdependent manner. Acclimation is generally regarded as a process that reduces
the amount of rubisco and/or other photosynthetic proteins, which effectively increases the amount of nitrogen available for enhancing sink development or stimulating other nutrient-limited
processes. In the study of Watling et al. (2000), for example, the authors reported a 50 percent CO2-induced reduction in the concentration of PEPcarboxylase, the primary carboxylating
enzyme in C4 plants, within sorghum leaves. Similarly, Maroco et al. (1999) documented CO2-induced decreases in both PEP-carboxylase and rubisco in leaves of the C4 crop maize. In some
cases, however, acclimation to elevated CO2 is manifested by an “up-regulation” of certain enzymes. When Gesch et al. (2002) took rice plants from ambient air and placed them in air
containing 700 ppm, for example, they noticed a significant increase in the activity of sucrose-phosphate synthase (SPS), which is a key enzyme involved in the production of sucrose.
Similarly, Hussain et al. (1999) reported that rice plants grown at an atmospheric CO2 concentration of 660 ppm displayed 20 percent more SPS activity during the growing season than
did ambiently grown rice plants. Such increases in the activity of this enzyme could allow CO2-enriched plants to avoid the onset of photosynthetic acclimation by synthesizing and subsequently
exporting sucrose from source leaves into sink tissues before they accumulate and trigger end product feedback inhibition. In an interesting experiment, Gesch et al. (2000) took ambiently
growing rice plants and placed them in an atmospheric CO2 concentration of 175 ppm, which reduced photosynthetic rates by 45 percent. However, after five days exposure to this sub-ambient
CO2 concentration, the plants manifested an upregulation of rubisco, which stimulated photosynthetic rates by 35 percent. Thus, plant acclimation responses can involve both an increase
or decrease in specific enzymes, depending on the atmospheric CO2 concentration. In summary, many peer-reviewed studies suggest that as the CO2 content of the air slowly but steadily
rises, agricultural species may not necessarily exhibit photosynthetic acclimation, even under conditions of low soil nitrogen. If a plant can maintain a balance between its sources
and sinks for carbohydrates at the whole-plant level, acclimation should not be necessary. Because earth’s atmospheric CO2 content is rising by an average of only 1.5 ppm per year, most
plants should be able to either (1) adjust their relative growth rates by the small amount that would be needed to prevent low nitrogen-induced acclimation from ever occurring, or (2)
expand their root systems by the small amount that would be needed to supply the extra nitrogen required to take full advantage of the CO2-induced increase in leaf carbohydrate production.
In the event a plant cannot initially balance its sources and sinks for carbohydrates at the whole-plant level, CO2-induced acclimation represents a beneficial secondary mechanism for
achieving achieving that balance through redistributing limiting resources away from the plant’s photosynthetic machinery to strengthen sink development or enhance other nutrient-limiting
processes. Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/a/acclimationag.php.
References Farage, P.K., McKee, I.F. and Long, S.P. 1998. Does a low nitrogen supply necessarily lead to acclimation of photosynthesis to elevated CO2? Plant Physiology 118: 573-580.
Garcia, R.L., Long, S.P., Wall, G.W., Osborne, C.P., Kimball, B.A., Nie, G.Y., Pinter Jr., P.J., LaMorte, R.L. and Wechsung, F. 1998. Photosynthesis and conductance of spring-wheat leaves:
field response to continuous freeair atmospheric CO2 enrichment. Plant, Cell and Environment 21: 659-669.
Climate Change Reconsidered 482 Gesch, R.W., Boote, K.J., Vu, J.C.V., Allen Jr., L.H. and Bowes, G. 1998. Changes in growth CO2 result in rapid adjustments of ribulose-1,5-bisphosphate
carboxylase/oxygenase small subunit gene expression in expanding and mature leaves of rice. Plant Physiology 118: 521-529. Gesch, R.W., Vu, J.C.V., Boote, K.J., Allen Jr., L.H. and Bowes,
G. 2000. Subambient growth CO2 leads to increased Rubisco small subunit gene expression in developing rice leaves. Journal of Plant Physiology 157: 235-238. Gesch, R.W., Vu, J.C.V.,
Boote, K.J., Allen Jr., L.H. and Bowes, G. 2002. Sucrose-phosphate synthase activity in mature rice leaves following changes in growth CO2 is unrelated to sucrose pool size. New Phytologist
154: 77-84. Hussain, M.W., Allen, L.H., Jr. and Bowes, G. 1999. Upregulation of sucrose phosphate synthase in rice grown under elevated CO2 and temperature. Photosynthesis Research 60:
199-208. Louche-Tessandier, D., Samson, G., Hernandez-Sebastia, C., Chagvardieff, P. and Desjardins, Y. 1999. Importance of light and CO2 on the effects of endomycorrhizal colonization
on growth and photosynthesis of potato plantlets (Solanum tuberosum) in an in vitro tripartite system. New Phytologist 142: 539-550. Maroco, J.P., Edwards, G.E. and Ku, M.S.B. 1999.
Photosynthetic acclimation of maize to growth under elevated levels of carbon dioxide. Planta 210: 115-125. Osborne, C.P., LaRoche, J., Garcia, R.L., Kimball, B.A., Wall, G.W., Pinter,
P.J., Jr., LaMorte, R.L., Hendrey, G.R. and Long, S.P. 1998. Does leaf position within a canopy affect acclimation of photosynthesis to elevated CO2? Plant Physiology 117: 1037-1045.
Reekie, E.G., MacDougall, G., Wong, I. and Hicklenton, P.R. 1998. Effect of sink size on growth response to elevated atmospheric CO2 within the genus Brassica. Canadian Journal of Botany
76: 829-835. Sims, D.A., Luo, Y. and Seeman, J.R. 1998. Comparison of photosynthetic acclimation to elevated CO2 and limited nitrogen supply in soybean. Plant, Cell and Environment 21:
945-952. 952. Theobald, J.C., Mitchell, R.A.C., Parry, M.A.J. and Lawlor, D.W. 1998. Estimating the excess investment in ribulose-1,5-bisphosphate carboxylase/oxygenase in leaves of
spring wheat grown under elevated CO2. Plant Physiology 118: 945-955. Watling, J.R., Press, M.C. and Quick, W.P. 2000. Elevated CO2 induces biochemical and ultrastructural changes in
leaves of the C4 cereal sorghum. Plant Physiology 123: 1143-1152. Ziska, L.H. 1998. The influence of root zone temperature on photosynthetic acclimation to elevated carbon dioxide concentrations.
Annals of Botany 81: 717-721. 7.4.2. Chaparral and Desert Species Roberts et al. (1998) conducted a FACE experiment in southern California, USA, exposing Adenostoma fassciculatum shrubs
to atmospheric CO2 concentrations of 360 and 550 ppm while they studied the nature of gas-exchange in this chaparral species. After six months of CO2 fumigation, photosynthetic acclimation
occurred. However, because of reductions in stomatal conductance and transpirational water loss, the CO2-enriched shrubs exhibited leaf water potentials that were less negative (and,
hence, less stressful) than those of control plants. This CO2-induced water conservation phenomenon should enable this woody perennial to better withstand the periods of drought that
commonly occur in this southern California region, while the photosynthetic down regulation it exhibits should allow it to more equitably distribute the limiting resources it possesses
among different essential plant physiological processes. Huxman and Smith (2001) measured seasonal gas exchange during an unusually wet El Niño year in an annual grass (Bromus madritensis
ssp. rubens) and a perennial forb (Eriogonum inflatum) growing within FACE plots established in the Mojave Desert, USA, which they maintained at atmospheric CO2 concentrations of 350
and 550 ppm. The elevated CO2 consistently increased net photosynthetic rates in the annual grass without inducing photosynthetic acclimation. In fact, even as seasonal photosynthetic
rates declined post-flowering, the reduction was much less in the CO2-enriched plants. However, elevated CO2 had no consistent effect on stomatal conductance in this species. By contrast,
Eriogonum plants growing at 550 ppm CO2 exhibited significant photosynthetic acclimation, especially late in the season, which led to similar rates of net photosynthesis in these plants
in both CO2 treatments. But in this species, elevated CO2 reduced stomatal conductance over most of the growing season. Although the two desert plants exhibited different stomatal and
photosynthetic responses to elevated CO2, both experienced significant CO2-induced increases in water use efficiency and biomass production, thus highlighting the existence of different,
but equally effective, species-specific
Biological Effects of Carbon Dioxide Enrichment 483 mechanisms for responding positively to atmospheric CO2 enrichment in a desert environment. In another study conducted at the Mojave
Desert FACE site, Hamerlynck et al. (2002) determined that plants of the deciduous shrub Lycium andersonii grown in elevated CO2 displayed photosynthetic acclimation, as maximum rubisco
activity in the plants growing in the CO2-enriched air was 19 percent lower than in the plants growing in ambient air. Also, the elevated CO2 did not significantly impact rates of photosynthesis.
Leaf stomatal conductance, on the other hand, was consistently about 27 percent lower in the plants grown in the CO2-enriched air; and during the last month of the spring growing season,
the plants in the elevated CO2 plots displayed leaf water potentials that were less negative than those exhibited by the control plants growing in ambient air. Hence, as the CO2 content
of the air increases, Lycium andersonii will likely respond by exhibiting significantly enhanced water use efficiency, which should greatly increase its ability to cope with the highly
variable precipitation and temperature regimes of the Mojave Desert. The acclimation observed within the shrub’s photosynthetic apparatus should allow it to reallocate more resources
to producing and sustaining greater amounts of biomass. Thus, it is likely that future increases in the air’s CO2 content will favor a “greening” of the American Mojave Desert. In summary,
the few studies of the acclimation phenomenon that have been conducted on chaparral and desert plants indicate that although it can sometimes be complete, other physiological changes,
such as the reductions in stomatal conductance that typically produce large increases in water use efficiency, often more than compensate for the sometimes small to negligible increases
in photosynthesis. Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/a/acclimationdesert.php.
References Hamerlynck, E.P., Huxman, T.E., Charlet, T.N. and Smith, S.D. 2002. Effects of elevated CO2 (FACE) on the functional ecology of the drought-deciduous Mojave Desert shrub,
Lycium andersonii. Environmental and Experimental Botany 48: 93-106. Huxman, T.E. and Smith, S.D. 2001. Photosynthesis in an invasive grass and native forb at elevated CO2 during an
El Niño year in the Mojave Desert. Oecologia 128: 193-201. Roberts, S.W., Oechel, W.C., Bryant, P.J., Hastings, S.J., Major, J. and Nosov, V. 1998. A field fumigation system for elevated
carbon dioxide exposure in chaparral shrubs. Functional Ecology 12: 708-719. 7.4.3. Grassland Species In nearly every reported case of photosynthetic acclimation in CO2-enriched plants,
rates of photosynthesis displayed by grassland species grown and measured at elevated CO2 concentrations are typically greater than those exhibited by control plants grown and measured
at ambient CO2 concentrations (Davey et al., 1999; Bryant et al., 1998). As mentioned in prior sections, CO2-induced photosynthetic acclimation often results from insufficient plant
sink strength, which can lead to carbohydrate accumulation in source leaves and the triggering of photosynthetic end-product feedback inhibition, which reduces rubisco activity and rates
of net photosynthesis (Roumet et al., 2000). As one example of this phenomenon, Rogers et al. (1998) reported that perennial ryegrass grown at an atmospheric CO2 concentration of 600
ppm and low soil nitrogen exhibited leaf carbohydrate contents and rubisco activities that were 100 percent greater and 25 percent less, respectively, than those observed in control
plants grown at 360 ppm CO2, prior to a cutting event. Following the cutting, which effectively reduced the source:sink ratio of the plants, leaf carbohydrate contents in CO2-enriched
plants decreased and rubisco activities increased, completely ameliorating the photosynthetic acclimation in this species. However, at high soil nitrogen, photosynthetic acclimation
to elevated CO2 did not occur. Thus, photosynthetic acclimation appears to result from the inability of plants to develop adequate sinks at low soil nitrogen, and is not necessarily
induced directly by atmospheric CO2 enrichment. In some cases, plants can effectively increase their sink strength and thus reduce the magnitude of CO2-induced acclimation by forming
symbiotic relationships with certain species of soil fungi. Under such conditions, photosynthetic down regulation is not triggered as rapidly, or as frequently, by endproduct feedback
inhibition, as excess carbohydrates are mobilized out of source leaves and sent
Climate Change Reconsidered 484 belowground to symbiotic fungi. Staddon et al. (1999) reported that photosynthetic acclimation was not induced in CO2-enriched Plantago lanceolata plants
that were inoculated with a mycorrhizal fungus, while it was induced in control plants that were not inoculated with the fungus. Thus, CO2-induced acclimation appears to be closely related
to the source:sink balance that exists within plants, and is triggered when sink strength falls below, and source strength rises above, certain critical thresholds in a species-dependent
manner. As the CO2 content of the air slowly but steadily rises, these peer-reviewed studies suggest that grassland species may not exhibit photosynthetic acclimation if they can maintain
a balance between their sources and sinks for carbohydrates at the whole-plant level. But in the event this balancing act is not initially possible, acclimation represents a beneficial
secondary mechanism for achieving that balance by redistributing limiting resources away from the plant’s photosynthetic machinery to strengthen its sink development and/or nutrientgathering
activities. Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/a/acclimationgrass.php.
References Bryant, J., Taylor, G. and Frehner, M. 1998. Photosynthetic acclimation to elevated CO2 is modified by source:sink balance in three component species of chalk grassland swards
grown in a free air carbon dioxide enrichment (FACE) experiment. Plant, Cell and Environment 21: 159-168. Davey, P.A., Parsons, A.J., Atkinson, L., Wadge, K. and Long, S.P. 1999. Does
photosynthetic acclimation to elevated CO2 increase photosynthetic nitrogen-use efficiency? A study of three native UK grassland species in open-top chambers. Functional Ecology 13:
21-28. Rogers, A., Fischer, B.U., Bryant, J., Frehner, M., Blum, H., Raines, C.A. and Long, S.P. 1998. Acclimation of photosynthesis to elevated CO2 under low-nitrogen nutrition is affected
by the capacity for assimilate utilization. Perennial ryegrass under free-air CO2 enrichment. Plant Physiology 118: 683-689. Roumet, C., Garnier, E., Suzor, H., Salager, J.-L. and Roy,
J. 2000. Short and long-term responses of whole-plant gas exchange to elevated CO2 in four herbaceous species. Environmental and Experimental Botany 43: 155-169. Staddon, P.L., Fitter,
A.H. and Robinson, D. 1999. Effects of mycorrhizal colonization and elevated atmospheric carbon dioxide on carbon fixation and below-ground carbon partitioning in Plantago lanceolata.
Journal of Experimental Botany 50: 853-860. 7.4.4. Tree Species Trees grown for long periods of time in elevated CO2 environments often, but not always (Marek et al., 2001; Stylinski
et al., 2000; Bartak et al., 1999; Schortemeyer et al., 1999), exhibit some degree of photosynthetic acclimation or down regulation, which is typically characterized by modestly reduced
rates of photosynthesis (compared to what might be expected on the basis of short-term term exposure to CO2-enriched air) that result from a long-term decrease in the activity and/or
amount of the primary plant carboxylating enzyme rubisco (Kubiske et al., 2002; Egli et al., 1998). This acclimation response in plants accustomed to growing in CO2-enriched air is characterized
by short-term reductions in their rates of net photosynthesis when measured during short-term exposure to ambient air relative to net photosynthesis rates of comparable plants that have
always been grown in ambient air. Jach and Ceulemans (2000) grew one-year-old Scots pine seedlings for two additional years at twiceambient atmospheric CO2 concentrations and reported
that the elevated CO2 increased the trees’ mean rate of net photosynthesis by 64 percent. However, when measured during a brief return to ambient CO2 concentrations, the normally CO2-enriched
seedlings exhibited an approximate 21 percent reduction in average net photosynthesis rate relative to that of seedlings that had always been exposed to ambient air. Similarly, Spunda
et al. (1998) noted that a 350-ppm increase in the air’s CO2 concentration boosted rates of net photosynthesis in fifteen-year-old Norway spruce trees by 78 percent; but when net photosynthesis
in the normally CO2-enriched trees was measured at a temporary atmospheric CO2 concentration of 350 ppm, an 18 percent reduction was observed relative to what was observed in comparable
trees that had always been grown in ambient air. After reviewing the results of 15 different atmospheric CO2 enrichment studies of European forest species growing in field environments
maintained at twice-ambient CO2 concentrations, Medlyn et al. (1999) found that the mean photosynthetic acclimation effect in the CO2-enriched
Biological Effects of Carbon Dioxide Enrichment 485 trees was characterized by an average reduction of 19 percent in their rates of net photosynthesis when measured at temporary ambient
CO2 concentrations relative to the mean rate of net photosynthesis exhibited by those trees that had always been exposed to ambient air. Nonetheless, in nearly every reported case of
CO2-induced photosynthetic acclimation in trees, the photosynthetic rates of trees growing and measured in CO2-enriched air have still been much greater than those exhibited by trees
growing and measured in ambient air. CO2-induced photosynthetic acclimation, when it occurs, often results from insufficient plant sink strength, which can lead to carbohydrate accumulation
in source leaves and the triggering of photosynthetic end-product feedback inhibition, which results in reduced rates of net photosynthesis. Pan et al. (1998), for example, reported
that apple seedlings grown at an atmospheric CO2 concentration of 1600 ppm had foliar starch concentrations 17-fold greater than those observed in leaves of seedlings grown at 360 ppm
CO2, suggesting that this phenomenon likely triggered the reductions in leaf net photosynthesis rates they observed. Similarly, Rey and Jarvis (1998) reported that the accumulation of
starch within leaves of CO2-enriched silver birch seedlings (100 percent above ambient) may have induced photosynthetic acclimation in that species. Also, in the study of Wiemken and
Ineichen (2000), a 300-ppm increase in the air’s CO2 concentration induced seasonal acclimation in young spruce trees, where late-summer, fall and winter rates of net photosynthesis
declined in conjunction with 40 to 50 percent increases in foliar glucose levels. In some cases, trees can effectively increase their sink strength, and thus reduce the magnitude of
CO2-induced photosynthetic acclimation, by forming symbiotic relationships with certain species of soil fungi. Under such conditions, photosynthetic down regulation is not triggered
as rapidly, or as frequently, by end-product feedback inhibition, as excess carbohydrates are mobilized out of the trees’ source leaves and sent belowground to support the growth of
symbiotic fungi. Jifon et al. (2002), for example, reported that the degree of CO2-induced photosynthetic acclimation in sour orange tree seedlings was significantly reduced by the presence
of mycorrhizal fungi, which served as sinks for excess carbohydrates synthesized by the CO2-enriched seedlings. During acclimation to elevated CO2, the amounts and activities of rubisco
and/or other photosynthetic proteins are often reduced, which effectively increases the amount of nitrogen available for enhancing sink development or stimulating other nutrient-limited
processes. As an example of this phenomenon operating in trees, in the study of Blaschke et al. (2001), the authors reported that a doubling of the atmospheric CO2 concentration reduced
foliar rubisco concentrations by 15 and 30 percent in two mature oak species growing near CO2-emitting springs. Similarly, a 200-ppm increase in the air’s CO2 content reduced foliar
rubisco concentrations in young aspen and birch seedlings by 39 percent (Takeuchi et al., 2001) and 24 percent (Tjoelker et al., 1998), respectively. Also, in two Pinus radiata studies,
seedlings fumigated with air containing 650 ppm CO2 displayed 30 to 40 percent reductions in
rubisco concentration and rubisco activity relative to measurements made on seedlings grown in ambient air (Griffin et al., 2000; Turnbull et al., 1998). Other studies of CO2-enriched
Norway spruce and Scots pines have documented CO2-induced reductions in foliar chlorophyll contents of 17 percent (Spunda et al., 1998) and 26 percent (Gielen et al., 2000), respectively.
And in the study of Gleadow et al. (1998), elevated CO2 led to acclimation in eucalyptus seedlings, which mobilized nitrogen away from rubisco and into prunasin, a sugar-based defense
compound that deters herbivory. In another interesting experiment, Polle et al. (2001) germinated acorns from oak trees exposed to ambient and CO2-enriched air and subsequently grew
them at ambient and twice-ambient atmospheric CO2 concentrations. They discovered that seedlings derived from acorns produced on CO2-enriched trees exhibited less-pronounced photosynthetic
acclimation to elevated CO2 than did seedlings derived from acorns produced on ambiently grown trees, suggesting the possibility of generational adaptation to higher atmospheric CO2
concentrations over even longer periods of time. In summary, these many peer-reviewed scientific studies suggest that as the air’s CO2 content slowly but steadily rises, trees may be
able to avoid photosynthetic acclimation if they maintain a proper balance between carbohydrate sources and sinks at the whole-tree level, which they may well be able to do in response
to the current rate of rise of the air’s CO2 concentration (a mere 1.5 ppm per year). If a tree cannot initially balance its sources and sinks of carbohydrates, however, acclimation
is an important and effective means of achieving that balance through redistributing essential resources away from the tree’s
Climate Change Reconsidered 486 photosynthetic machinery in an effort to strengthen sink development, enhance various nutrient-limited processes, and increase nutrient acquisition by,
for example, stimulating the development of roots and their symbiotic fungal partners. And if those adjustments are not entirely successful, it is still the case that the acclimation
process is hardly ever 100 percent complete (in the studies reviewed here it was on the order of 20 percent), so that tree growth is almost always significantly enhanced in CO2-enriched
air in nearly every real-world setting. Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/a/acc
imationtree.php. References Bartak, M., Raschi, A. and Tognetti, R. 1999. Photosynthetic characteristics of sun and shade leaves in the canopy of Arbutus unedo L. trees exposed to in
situ long-term elevated CO2. Photosynthetica 37: 1-16. Blaschke, L., Schulte, M., Raschi, A., Slee, N., Rennenberg, H. and Polle, A. 2001. Photosynthesis, soluble and structural carbon
compounds in two Mediterranean oak species (Quercus pubescens and Q. ilex) after lifetime growth at naturally elevated CO2 concentrations. Plant Biology 3: 288-297. Egli, P., Maurer,
S., Gunthardt-Goerg, M.S. and Korner, C. 1998. Effects of elevated CO2 and soil quality on leaf gas exchange and aboveground growth in beech-spruce model ecosystems. New Phytologist
140: 185-196. Gielen, B., Jach, M.E. and Ceulemans, R. 2000. Effects of season, needle age and elevated atmospheric CO2 on chlorophyll fluorescence parameters and needle nitrogen concentration
in (Pinus sylvestris L.). Photosynthetica 38: 13-21. Gleadow, R.M., Foley, W.J. and Woodrow, I.E. 1998. Enhanced CO2 alters the relationship between photosynthesis and defense in cyanogenic
Eucalyptus cladocalyx F. Muell. Plant, Cell and Environment 21: 12-22. Griffin, K.L., Tissue, D.T., Turnbull, M.H. and Whitehead, D. 2000. The onset of photosynthetic acclimation to
elevated CO2 partial pressure in field-grown Pinus radiata D. Don. after 4 years. Plant, Cell and Environment 23: 1089-1098. Jach, M.E. and Ceulemans, R. 2000. Effects of season, needle
age and elevated atmospheric CO2 on photosynthesis in Scots pine (Pinus sylvestris L.). Tree Physiology 20: 145-157. Jifon, J.L., Graham, J.H., Drouillard, D.L. and Syvertsen, J.P. 2002.
Growth depression of mycorrhizal Citrus seedlings grown at high phosphorus supply is mitigated by elevated CO2. New Phytologist 153: 133-142. Kubiske, M.E., Zak, D.R., Pregitzer, K.S.
and Takeuchi, Y. 2002. Photosynthetic acclimation of overstory Populus tremuloides and understory Acer saccharum to elevated atmospheric CO2 concentration: interactions with shade and
soil nitrogen. Tree Physiology 22: 321-329. Marek, M.V., Sprtova, M., De Angelis, P. and Scarascia-Mugnozza, G. 2001. Spatial distribution of photosynthetic response to long-term influence
of elevated CO2 in a Mediterranean macchia mini-ecosystem. Plant Science 160: 1125-1136. Medlyn, B.E., Badeck. F.-W., De Pury, D.G.G., Barton, C.V.M., Broadmeadow, M., Ceulemans, R.,
De Angelis, P., Forstreuter, M., Jach, M.E., Kellomaki, S., Laitat, E., Marek, M., Philippot, S., Rey, A., Strassemeyer, J., Laitinen, K., Liozon, R., Portier, B., Roberntz, P., Wang,
K. and Jarvis, P.G. 1999. Effects of elevated [CO2] on photosynthesis in European forest species: a meta-analysis of model parameters. Plant, Cell and Environment 22: 1475-1495. Pan,
Q., Wang, Z. and Quebedeaux, B. 1998. Responses of the apple plant to CO2 enrichment: changes in photosynthesis, sorbitol, other soluble sugars, and starch. Australian Journal of Plant
Physiology 25: 293-297. Polle, A., McKee, I. and Blaschke, L. 2001. Altered physiological and growth responses to elevated [CO2] in offspring from holm oak (Quercus ilex L.) mother trees
with lifetime exposure to naturally elevated [CO2]. Plant, Cell and Environment 24: 1075-1083. Rey, A. and Jarvis, P.G. 1998. Long-Term photosynthetic acclimation to increased atmospheric
CO2 concentration in young birch (Betula pendula) trees. Tree Physiology 18: 441-450. Schortemeyer, M., Atkin, O.K., McFarlane, N. and Evans, J.R. 1999. The impact of elevated atmospheric
CO2 and nitrate supply on growth, biomass allocation, nitrogen partitioning and N2 fixation of Acacia melanoxylon. Australian Journal of Plant Physiology 26: 737-774. Spunda, V., Kalina,
J., Cajanek, M., Pavlickova, H. and Marek, M.V. 1998. Long-term exposure of Norway spruce to elevated CO2 concentration induces changes in photosystem II mimicking an adaptation to increased
irradiance. Journal of Plant Physiology 152: 413-419. Stylinski, C.D., Oechel, W.C., Gamon, J.A., Tissue, D.T., Miglietta, F. and Raschi, A. 2000. Effects of lifelong [CO2] enrichment
on carboxylation and light utilization of Quercus pubescens Willd. examined with gas exchange,
Biological Effects of Carbon Dioxide Enrichment 487 biochemistry and optical techniques. Plant, Cell and Environment 23: 1353-1362. Takeuchi, Y., Kubiske, M.E., Isebrands, J.G., Pregitzer,
K.S., Hendrey, G. and Karnosky, D.F. 2001. Photosynthesis, light and nitrogen relationships in a young deciduous forest canopy under open-air CO2 enrichment. Plant, Cell and Environment
24: 1257-1268. Tjoelker, M.G., Oleksyn, J. and Reich, P.B. 1998. Seedlings of five boreal tree species differ in acclimation of net photosynthesis to elevated CO2 and temperature. Tree
Physiology 18: 715-726. Turnbull, M.H., Tissue, D.T., Griffin, K.L., Rogers, G.N.D. and Whitehead, D. 1998. Photosynthetic acclimation to long-term exposure to elevated CO2 concentration
in Pinus radiata D. Don. is related to age of needles. Plant, Cell and Environment 21: 1019-1028. Wiemken, V. and Ineichen, K. 2000. Seasonal fluctuations of the levels of soluble carbohydrates
in spruce needles exposed to elevated CO2 and nitrogen fertilization and glucose as a potential mediator of acclimation to elevated CO2. Journal of Plant Physiology 156: 746-750. 7.5.
Competition Do higher levels of atmospheric CO2 favor some plants over others? Could this result in ecological changes that could be judged “bad” because of their effects on wildlife
or plants that are beneficial to mankind? This section seeks to answer these questions by surveying research on the effects of CO2 enhancement on C3 versus C4 plants, and weeds versus
crops. Additional information on this topic, including reviews on competition not discussed here, can be found at http://www.co2science.org/subject/c/subject _c.php under the heading
Competition. 7.5.1. C3 vs C4 Plants C3 plants typically respond better to atmospheric CO2 enrichment than do C4 plants in terms of increasing their rates of photosynthesis and biomass
production. Hence, it has periodically been suggested that in a world of rising atmospheric CO2 concentration, C3 plants may out-compete C4 plants and displace them, thereby decreasing
the biodiversity of certain ecosystems. However, the story is much more complex that what is suggested by this simple scenario. Wilson et al. (1998) grew 36 species of perennial grass
common to tallgrass prairie ecosystems with and without arbuscular mycorrhizal fungi, finding that the dry matter production of the C3 species that were colonized by the fungi was the
same as that of the non-inoculated C3 species, but that the fungalcolonized C4 species produced, on average, 85 percent more dry matter than the non-inoculated C4 species. This finding
is of pertinence to the relative responsiveness of C3 and C4 plants to atmospheric CO2 enrichment; for elevated levels of atmospheric CO2 tend to enhance the mycorrhizal colonization
of plant roots, which is known to make soil minerals and water more available for plant growth. Hence, this CO2-induced fungal-mediated growth advantage, which from this study appears
to be more readily available to C4 plants, could well counter the inherently greater biomass response of of C3 plants relative to that of C4 plants, leveling the playing field relative
to their competition for space in any given ecosystem. Another advantage that may come to C4 plants as a consequence of the ongoing rise in the air’s CO2 content was elucidated by BassiriRad
et al. (1998), who found that elevated CO2 enhanced the ability of the perennial C4 grass Bouteloua eriopoda to increase its uptake of NO3-and PO43-considerably more than the perennial
C3 shrubs Larrea tridentata and Prosopis glandulosa. In an eight-year study of the effects of twice-ambient atmospheric CO2 concentrations on a pristine tallgrass prairie in Kansas,
Owensby et al. (1999) found that the elevated CO2 did not affect the basal coverage of its C4 species or their relative contribution to the composition of the ecosystem. The antitranspirant
effect of atmospheric CO2 enrichment (Pospisilova and Catsky, 1999) is often more strongly expressed in C4 plants than in C3 plants and typically allows C4 plants to better cope with
water stress. In a study of the C3 dicot Abutilon theophrasti and the C4 dicot Amaranthus retroflexus, for example, Ward et al. (1999) found that Amaranthus retroflexus exhibited a greater
relative recovery from drought than did the C3 species, which suggests, in their words, that “the C4 species would continue to be more competitive than the C3 species in regions receiving
more frequent and severe droughts,” which basically characterizes regions where C4 plants currently exist. Two years later, Morgan et al. (2001) published the results of an open-top
chamber study of a native shortgrass steppe ecosystem in Colorado, USA, where
Climate Change Reconsidered 488 they had exposed the enclosed ecosystems to atmospheric CO2 concentrations of 360 and 720 ppm for two six-month growing seasons. In spite of an average
air temperature increase of 2.6°C, which was caused by the presence of the open-top chambers, the elevated CO2 increased above-ground biomass production by an average of 38 percent in
both years of the study; and when 50 percent of the standing green plant biomass was defoliated to simulate grazing halfway through the growing season, atmospheric CO2 enrichment still
increased aboveground biomass by 36 percent. It was also found that the communities enriched with CO2 tended to have greater amounts of moisture in their soils than communities exposed
to ambient air; and this phenomenon likely contributed to the less negative and, therefore, less stressful plant water potentials that were measured in the CO2-enriched plants. Last,
the elevated CO2 did not preferentially stimulate the growth of C3 species over that of C4 species in in these communities. Elevated CO2 did not significantly affect the percentage composition
of C3 and C4 species in these grasslands; they maintained their original level of vegetative biodiversity. This would also appear to be the conclusion of the study of Wand et al. (1999),
who in a massive review of the scientific literature published between 1980 and 1997 analyzed nearly 120 individual responses of C3 and C4 grasses to elevated CO2. On average, they found
photosynthetic enhancements of 33 and 25 percent, respectively, for C3 and C4 plants, along with biomass enhancements of 44 and 33 percent, respectively, for a doubling of the air’s
CO2 concentration. These larger-than-expected growth responses in the C4 species led them to conclude that “it may be premature to predict that C4 grass species will lose their competitive
advantage over C3 grass species in elevated CO2.” Further support for this conclusion comes from the study of Campbell et al. (2000), who reviewed research work done between 1994 and
1999 by a worldwide network of 83 scientists associated with the Global Change and Terrestrial Ecosystems (GCTE) Pastures and Rangelands Core Research Project 1, which resulted in the
publication of over 165 peer-reviewed scientific journal articles. After analyzing this body of research, they concluded that the “growth of C4 species is about as responsive to CO2
concentration as [is that of] C3 species when water supply restricts growth, as is usual in grasslands containing C4 species.” The work of this group of scientists also provides no evidence
for the suggestion that C3 plants may out-compete C4 plants and thereby replace them in a high-CO2 world of the future. Additional information on this topic, including reviews of newer
publications as they become available, can be found at http://www.co2science.org/subject/b/biodivc3vsc4.php. References BassiriRad, H., Reynolds, J.F., Virginia, R.A. and Brunelle, M.H.
1998. Growth and root NO3-and PO43-uptake capacity of three desert species in response to atmospheric CO2 enrichment. Australian Journal of Plant Physiology 24: 353-358. Campbell, B.D.,
Stafford Smith, D.M., Ash, A.J., Fuhrer, J., Gifford, R.M., Hiernaux, P., Howden, S.M., Jones, M.B., Ludwig, J.A., Manderscheid, R., Morgan, J.A., Newton, P.C.D., Nosberger, J., Owensby,
C.E., Soussana, J.F., Tuba, Z. and ZuoZhong, C. 2000. A synthesis of recent global change research on pasture and rangeland production: reduced uncertainties and their management implications.
Agriculture, Ecosystems and Environment 82: 39-55. Morgan, J.A., Lecain, D.R., Mosier, A.R. and Milchunas, D.G. 2001. Elevated CO2 enhances water relations and productivity and affects
gas exchange in C3 and C4 grasses of the Colorado shortgrass steppe. Global Change Biology 7: 451-466. Owensby, C.E., Ham, J.M., Knapp, A.K. and Auen, L.M. 1999. Biomass production and
species composition change in a tallgrass prairie ecosystem after long-term exposure to elevated atmospheric CO2. Global Change Biology 5: 497-506. Pospisilova, J. and Catsky, J. 1999.
Development of water stress under increased atmospheric CO2 concentration. Biologia Plantarum 42: 1-24. Wand, S.J.E., Midgley, G.F., Jones, M.H. and Curtis, P.S. 1999. Responses of wild
C4 and C3 grass (Poaceae) species to elevated atmospheric CO2 concentration: a meta-analytic test of current theories and perceptions. Global Change Biology 5: 723-741. Ward, J.K., Tissue,
D.T., Thomas, R.B. and Strain, B.R. 1999. Comparative responses of model C3 and C4 plants to drought in low and elevated CO2. Global Change Biology 5: 857-867. Wilson, G.W.T. and Hartnett,
D.C. 1998. Interspecific variation in plant responses to mycorrhizal colonization in tallgrass prairie. American Journal of Botany 85: 1732-1738.
Biological Effects of Carbon Dioxide Enrichment 489 7.5.2. N-Fixers vs. Non-N-Fixers Will nitrogen-fixing (N-fixing) plants benefit more from atmospheric CO2 enrichment than non-N-fixers
and thus obtain a competitive advantage over them that could lead to some non-N-fixers being excluded from certain plant communities, thereby decreasing the biodiversity of those ecosystems?
In a two-year glasshouse study of simulated lowfertility ecosystems composed of grassland species common to Switzerland, Stocklin and Korner (1999) found that atmospheric CO2 enrichment
gave nitrogen-fixing legumes an initial competitive advantage over non-N-fixers. However, it would be expected that, over time, a portion of the extra nitrogen fixed by these legumes
would become available to neighboring non-N-fixing species, which would then be able to use it to their own advantage, thereby preserving the species richness of the ecosystem over the
long haul. Indeed, in a four-year study of an established (non-simulated) high grassland ecosystem located in the Swiss Alps, Arnone (1999) found no difference between the minimal to
non-existent growth responses of N-fixing and non-N-fixing species to elevated levels of atmospheric CO2. In a study of mixed plantings of the grass Lolium perenne and the legume Medicago
sativa, Matthies and Egli (1999) found that elevated CO2 did not influence the competition between the two plants, either directly or indirectly via its effects upon the root hemiparasite
Rhinanthus alectorolophus. In a study of mixed plantings of two grasses and two legumes, Navas et al. (1999) observed that plant responses to atmospheric CO2 enrichment are more dependent
upon neighboring plant density than they are upon neighboring plant identity. In the few studies of this question that have been conducted to date, therefore, it would appear that there
is little evidence to suggest that N-fixing legumes will out-compete non-N-fixing plants. Additional information on this topic, including reviews of newer publications as they become
available, can be found at http://www.co2science.org/subject/b/biodivnfixers.php. References Arnone III, J.A. 1999. Symbiotic N2 fixation in a high Alpine grassland: effects of four
growing seasons of elevated CO2. Functional Ecology 13: 383-387. Matthies, D. and Egli, P. 1999. Response of a root hemiparasite to elevated CO2 depends on host type and soil nutrients.
Oecologia 120: 156-161. Navas, M.-L., Garnier, E., Austin, M.P. and Gifford, R.M. 1999. Effect of competition on the responses of grasses and legumes to elevated atmospheric CO2 along
a nitrogen gradient: differences between isolated plants, monocultures and multi-species mixtures. New Phytologist 143: 323-331. Stocklin, J. and Korner, C. 1999. Interactive effects
of elevated CO2, P availability and legume presence on calcareous grassland: results of a glasshouse experiment. Functional Ecology 13: 200-209. 7.5.3. Weeds vs. Non-Weeds Elevated CO2
typically stimulates the growth of nearly all plant species in monoculture, including those deemed undesirable by humans, i.e., weeds. Consequently, it is important to determine how
future increases in the air’s CO2 content may influence relationships between weeds and non-weeds when they grow competitively in mixed-species stands. Dukes (2002) grew model serpentine
grasslands common to California, USA, in competition with the invasive forb Centaurea solstitialis at atmospheric CO2 concentrations of 350 and 700 ppm for one year, determining that
elevated CO2 increased the biomass proportion of this weedy species in the community by a mere 1.2 percent, while total community biomass increased by 28 percent. Similarly, Gavazzi
et al. (2000) grew loblolly pine seedlings for four months in competition with both C3 and C4 weeds at atmospheric CO2 concentrations of 260 and 660 ppm, reporting that elevated CO2
increased pine biomass by 22 percent while eliciting no response from either type of weed. In a study of pasture ecosystems near Montreal, Canada, Taylor and Potvin (1997) found that
elevated CO2 concentrations did not influence the number of native species returning after their removal (to simulate disturbance), even in the face of the introduced presence of the
C3 weed Chenopodium album, which normally competes quite effectively with several slower-growing crops in ambient air. In fact, atmospheric CO2 enrichment did not impact the growth of
this weed in any measurable way. Ziska et al. (1999) also studied the C3 weed C. album, along with the C4 weed Amaranthus retroflexus, in glasshouses maintained at atmospheric CO2 concentrations
of 360 and 720 ppm. They
Climate Change Reconsidered 490 determined that elevated CO2 significantly increased the photosynthetic rate and total dry weight of the C3 weed, but that it had no effect on the C4
weed. Also, they found that the growth response of the C3 weed to a doubling of the air’s CO2 content was approximately 51 percent, which is about the same as the average 52 percent
growth response tabulated by Idso (1992), and that obtained by Poorter (1993) for rapidly growing wild C3 species (54 percent), which finding suggests there is no enhanced dominance
of the C3 weed over other C3 plants in a CO2-enriched environment. Wayne et al. (1999) studied another agricultural weed, field mustard (Brassica kaber), which was sown in pots at six
densities, placed in atmospheric CO2 concentrations of 350 and 700 ppm, and sequentially harvested during the growing season. Early in stand development, elevated CO2 increased above-ground
weed biomass in a density-dependent manner, with the greatest stimulation of 141 percent occurring at the lowest density (corresponding to 20 plants per square meter) and the smallest
stimulation of 59 percent occurring at the highest density (corresponding to 652 plants per square meter). However, as stands matured, the density-dependence of the CO2-induced growth
response disappeared, and CO2-enriched plants exhibited an average aboveground biomass that was 34 percent greater than that of ambiently grown plants across a broad range of plant densities.
This final growth stimulation was similar to that of most other herbaceous plants exposed to atmospheric CO2 enrichment (30 to 50 percent biomass increases for a doubling of the air’s
CO2 content), evidence once again that atmospheric CO2 enrichment confers no undue advantage upon weeds at the expense of other plants. In a study of a weed that affects both plants
and animals, Caporn et al. (1999) examined bracken (Pteridium aquilinum), which poses a serious weed problem and potential threat to human health in the United Kingdom and other regions,
growing specimens for 19 months in controlled environment chambers maintained at atmospheric CO2 concentrations of 370 and 570 ppm and normal or high levels of soil fertility. They found
that the high CO2 treatment consistently increased rates of net photosynthesis by 30 to 70 percent, depending on soil fertility and time of year. However, elevated CO2 did not increase
total plant dry mass or the dry mass of any plant organ, including rhizomes, roots and fronds. In fact, the only significant effect of elevated CO2 on bracken growth was observed in
the normal nutrient regime, where elevated CO2 reduced mean frond area. Finally, in a study involving two parasitic species (Striga hermonthica and Striga asiatica), Watling and Press
(1997) reported that total parasitic biomass per host plant at an atmospheric CO2 concentration of 700 ppm was 65 percent less than it was in ambient air. And in a related study, Dale
and Press (1999) observed that the presence of a parasitic plant (Orobanche minor) reduced its host’s biomass by 47 percent in ambient air of 360 ppm CO2, but by only 20 percent in air
of 550 ppm CO2. These several studies suggest that, contrary to what is claimed by the IPCC, the ongoing rise in the air’s CO2 content will not favor the growth of weedy species over
that of crops and native plants. In fact, it may provide non-weeds greater protection against weed-induced decreases in their productivity and growth. Future increases in the air’s CO2
content may actually increase the competitiveness of non-weeds over weeds. Additional information on this topic, including reviews of newer publications as they become available, can
be found at http://www.co2science.org/subject/b/weedsvsnonw.php. References Caporn, S.J.M., Brooks, A.L., Press, M.C. and Lee, J.A. 1999. Effects of long-term exposure to elevated CO2
and increased nutrient supply on bracken (Pteridium aquilinum). Functional Ecology 13: 107-115. Dale, H. and Press, M.C. 1999. Elevated atmospheric CO2 influences the interaction between
the parasitic angiosperm Orobanche minor and its host Trifolium repens. New Phytologist 140: 65-73. Dukes, J.S. 2002. Comparison of the effect of elevated CO2 on an invasive species
(Centaurea solstitialis) in monoculture and community settings. Plant Ecology 160: 225-234. Gavazzi, M., Seiler, J., Aust, W. and Zedaker, S. 2000. The influence of elevated carbon dioxide
and water availability on herbaceous weed development and growth of transplanted loblolly pine (Pinus taeda). Environmental and Experimental Botany 44: 185-194. Idso, K.E. 1992. Plant
Responses to Rising Levels of Carbon Dioxide: A Compilation and Analysis of the Results of a Decade of International Research into the Direct Biological Effects of Atmospheric CO2 Enrichment.
Climatological Publications Scientific Paper #23, Office of Climatology, Arizona State University, Tempe, AZ.
Biological Effects of Carbon Dioxide Enrichment 491 Poorter, H. 1993. Interspecific variation in the growth response of plants to an elevated and ambient CO2 concentration. Vegetatio
104/105: 77-97. Taylor, K. and Potvin, C. 1997. Understanding the longterm effect of CO2 enrichment on a pasture: the importance of disturbance. Canadian Journal of Botany 75: 1621-1627.
Watling, J.R. and Press, M.C. 1997. How is the relationship between the C4 cereal Sorghum bicolor and the C3 root hemi-parasites Striga hermonthica and Striga asiatica affected by elevated
CO2? Plant, Cell and Environment 20: 1292-1300. Wayne, P.M., Carnelli, A.L., Connolly, J. and Bazzaz, F.A. 1999. The density dependence of plant responses to elevated CO2. Journal of
Ecology 87: 183-192. Ziska, L.H., Teasdale, J.R. and Bunce, J.A. 1999. Future atmospheric carbon dioxide may increase tolerance to glyphosate. Weed Science 47: 608-615. 7.6. Respiration
Nearly all of earth’s plants respond favorably to increases in the air’s CO2 concentration by exhibiting enhanced rates of net photosynthesis and biomass production during the light
part of each day. In many cases, observed increases in these parameters (especially biomass production) are believed to be due, in part, to CO2-induced reductions in carbon losses via
respiration during the day and especially at night (called “dark respiration”). In this summary, we examine what has been learned about this subject from experiments conducted on various
herbaceous and woody plants. Additional information on this topic, including reviews on respiration not discussed here, can be found at http://www.co2science.org/subject/r/subject _r.php
under the heading Respiration. 7.6.1. Herbaceous Plants 7.6.1.1. Crops Baker et al. (2000) grew rice in Soil-Plant-Atmosphere Research (SPAR) units at atmospheric CO2 concentrations
of 350 and 700 ppm during daylight hours. Under these conditions, rates of dark respiration decreased in both CO2 treatments with short-term increases in the air’s CO2 concentration
at night. However, when dark respiration rates were measured at the CO2 growth concentrations of the plants, they were not significantly different from each other. Cousins et al. (2001)
grew sorghum at atmospheric CO2 concentrations of 370 and 570 ppm within a free-air CO2 enrichment (FACE) facility near Phoenix, Arizona, USA. Within six days of planting, the photosynthetic
rates of the second leaves of the CO2-enriched plants were 37 percent greater than those of the second leaves of the ambiently grown plants. However, this CO2-induced photosynthetic
enhancement slowly declined with time, stabilizing at approximately 15 percent between 23 and 60 days after planting. In addition, when measuring photosynthetic rates at a reduced oxygen
concentration of 2 percent, they observed 16 and 9 percent increases in photosynthesis for the ambient and CO2-enriched plants, respectively.
These observations suggest that the extra 200 ppm of CO2 was reducing photorespiratory carbon losses, although this phenomenon did not account for all of the CO2-induced stimulation
of photosynthesis. Das et al. (2002) grew tropical nitrogen-fixing mungbean plants in open-top chambers maintained at atmospheric CO2 concentrations of either 350 or 600 ppm for two
growing seasons, with the extra CO2 being provided between either days 0 and 20 or days 21 and 40 after germination. This work revealed that the elevated CO2 decreased rates of respiration
by 54-62 percent, with the greatest declines occurring during the first 20 days after germination. Wang et al. (2004) grew well-watered and fertilized South American tobacco plants from
seed in 8.4-liter pots (one plant per pot) filled with sand and housed in controlled-environment growth chambers maintained at atmospheric CO2 concentrations of either 365 or 730 ppm
for a total of nine weeks. Over this period they found that the ratio of net photosynthesis per unit leaf area (A) to dark respiration per unit leaf area (Rd) “changed dramatically.”
Whereas A/Rd was the same in both treatments at the beginning of the measurement period, a month later it had doubled in the CO2-enriched environment but had risen by only 58 percent
in the ambient treatment. Speaking of this finding, the three researchers say that “if the dynamic relationship between A and Rd observed in N. sylvestris is applicable to other species,
it will have important implications for carbon cycling in terrestrial ecosystems, since plants will assimilate CO2 more efficiently as they mature.”
Climate Change Reconsidered 492 Bunce (2005) grew soybeans in the field in opentop chambers maintained at atmospheric CO2 concentrations of ambient and ambient +350 ppm at the Beltsville
Agricultural Research Center in Maryland (USA), where net carbon dioxide exchange rate measurements were performed on a total of 16 days between 18 July and 11 September of 2000 and
2003, during the flowering to early pod-filling stages of the growing season. Averaged over the course of the study, he found that daytime net photosynthesis per unit leaf area was 48
percent greater in the plants growing in the CO2-enriched air, while nighttime respiration per unit leaf area was unaffected by elevated CO2. However, because the extra 350 ppm of CO2
increased leaf dry mass per unit area by an average of 23 percent, respiration per unit of mass was significantly lower for the leaves of the soybeans growing in the CO2-enriched air.
Wang and Curtis (2002) conducted a metaanalysis of the results of 45 area-based dark respiration (Rda) and 44 mass-based dark respiration (Rdm) assessments of the effects of a doubling
of the air’s CO2 concentration on 33 species of plants derived from 37 scientific studies. This work revealed that the mean leaf Rda of the suite of herbaceous plants studied was significantly
higher (+29 percent, P < 0.01) at elevated CO2 than at ambient CO2. However, when the herbaceous plants were separated into groups that had experienced durations of CO2 enrichment that
were either shorter or longer than 60 days, it was found that the short-term studies exhibited a mean Rda increase of 51 percent (P < 0.05), while the long-term studies exhibited no
effect. Hence, for conditions of continuous atmospheric CO2 enrichment, herbaceous plants would likely experience no change in leaf Rda. In addition, the two researchers found that plants
exposed to elevated CO2 for < 100 days “showed significantly less of a reduction in leaf Rdm due to CO2 enrichment (-12%) than did plants exposed for longer periods (-35%, P < 0.01).”
Hence, for long-term conditions of continuous atmospheric CO2 enrichment, herbaceous crops would likely experience an approximate 35 percent decrease in leaf Rdm. Bunce (2004) grew six
different 16-plant batches of soybeans within a single controlled-environment chamber, one to a pot filled with 1.8 liters of vermiculite that was flushed daily with a complete nutrient
solution. In three experiments conducted at day/night atmospheric CO2 concentrations of 370/390 ppm, air temperatures were either 20, 25 or 30°C, while in three other experiments conducted
at an air temperature of 25°C, atmospheric CO2 concentrations were either 40, 370 or 1400 ppm. At the end of the normal 16 hours of light on the 17th day after planting, half of the
plants were harvested and used for the measurement of a number of physical parameters, while measurements of the plant physiological processes of respiration, translocation and nitrate
reduction were made on the other half of the plants over the following 8-hour dark period. Plotting translocation and nitrate reduction as functions of respiration, Bunce found that
“a given change in the rate of respiration was accompanied by the same change in the rate of translocation or nitrate reduction, regardless of whether the altered respiration was caused
by a change in temperature or by a change in atmospheric CO2 concentration.” As a result, and irrespective of whatever mechanisms may have been involved in eliciting the responses observed,
Bunce concluded that “the parallel responses of translocation and nitrate reduction for both the temperature and CO2 treatments make it unlikely that the response of respiration to one
variable [CO2] was an artifact while the response to the other [temperature] was real.” Hence, there is reason to believe that the oft-observed decreases in dark respiration experienced
by plants exposed to elevated levels of atmospheric CO2, as per the review and analysis studies of Drake et al. (1999) and Wang and Curtis (2002), are indeed real and not the result
of measurement system defects. In light of these several findings, it can be concluded that the balance of evidence suggests that the growth of herbaceous crops is generally enhanced
by CO2-induced decreases in respiration during the dark period. Additional information on this topic, including reviews of newer publications as they become available, can be found at
http://www.co2science.org/subject/r/respirationcrops.php. References Baker, J.T., Allen, L.H., Jr., Boote, K.J. and Pickering, N.B. 2000. Direct effects of atmospheric carbon dioxide
concentration on whole canopy dark respiration of rice. Global Change Biology 6: 275-286. Bunce, J.A. 2004. A comparison of the effects of carbon dioxide concentration and temperature
on respiration, translocation and nitrate reduction in darkened soybean leaves. Annals of Botany 93: 665-669.
Biological Effects of Carbon Dioxide Enrichment 493 Bunce, J.A. 2005. Response of respiration of soybean leaves grown at ambient and elevated carbon dioxide concentrations to day-to-day
variation in light and temperature under field conditions. Annals of Botany 95: 1059-1066. Cousins, A.B., Adam, N.R., Wall, G.W., Kimball, B.A., Pinter Jr., P.J., Leavitt, S.W., LaMorte,
R.L., Matthias, A.D., Ottman, M.J., Thompson, T.L. and Webber, A.N. 2001. Reduced photorespiration and increased energy-use efficiency in young CO2-enriched sorghum leaves. New Phytologist
150: 275-284. Das, M., Zaidi, P.H., Pal, M. and Sengupta, U.K. 2002. Stage sensitivity of mungbean (Vigna radiata L. Wilczek) to an elevated level of carbon dioxide. Journal of Agronomy
and Crop Science 188: 219-224. Drake, B.G., Azcon-Bieto, J., Berry, J., Bunce, J., Dijkstra, P., Farrar, J., Gifford, R.M., Gonzalez-Meler, M.A., Koch, G., Lambers, H., Siedow, J. and
Wullschleger, S. 1999. Does elevated atmospheric CO2 inhibit mitochondrial respiration in green plants? Plant, Cell and Environment 22: 649-657. Wang, X., Anderson, O.R. and Griffin,
K.L. 2004. Chloroplast numbers, mitochondrion numbers and carbon assimilation physiology of Nicotiana sylvestris as affected by CO2 concentration. Environmental and Experimental Botany
51: 21-31. Wang, X. and Curtis, P. 2002. A meta-analytical test of elevated CO2 effects on plant respiration. Plant Ecology 161: 251-261. 7.6.1.2. Other Herbaceous Plants In this section
we review the results of studies of noncrop herbaceous plants to determine if atmospheric CO2 enrichment tends to increase or decrease (or leave unaltered) their respiration rates. Rabha
and Uprety (1998) grew India mustard plants for an entire season in open-top chambers with either ambient or enriched (600 ppm) atmospheric CO2 concentrations and adequate or inadequate
soil moisture levels. Their work revealed that the elevated CO2 concentration reduced leaf dark respiration rates by about 25 percent in both soil moisture treatments, which suggests
that a greater proportion of the increased carbohydrate pool in the CO2-enriched plants remained within them to facilitate increases in growth and development. Ziska and Bunce (1999)
grew four C4 plants in controlled environment chambers maintained at either full-day (24-hour) atmospheric CO2 concentrations of 350 and 700 ppm or a nocturnal-only CO2 concentration
of 700 ppm (with 350 ppm CO2 during the day) for about three weeks. In this particular study, 24-hour CO2 enrichment caused a significant increase in the photosynthesis (+13 percent)
and total dry mass (+21 percent) of only one of the four C4 species (Amaranthus retroflexus). However, there was no significant effect of nocturnal-only CO2 enrichment on this species,
indicating that the observed increase in biomass, resulting from 24-hour atmospheric CO2 enrichment, was not facilitated by greater carbon conservation stemming from a CO2-induced reduction
in dark respiration. In an experiment that produced essentially the same result, Grunzweig and Korner Korner (2001) constructed model grasslands representative of the Negev of Israel
and placed them in growth chambers maintained at atmospheric CO2 concentrations of 280, 440 and 600 ppm for five months. This study also revealed that atmospheric CO2 enrichment had
no effect on nighttime respiratory carbon losses. Moving to the other end of the moisture spectrum, Van der Heijden et al. (2000) grew peat moss hydroponically within controlled environment
chambers maintained at atmospheric CO2 concentrations of 350 and 700 ppm for up to six months, while simultaneously subjecting the peat moss to three different levels of nitrogen deposition.
In all cases, they found that the elevated CO2 reduced rates of dark respiration consistently throughout the study by 40 to 60 percent. In a final multi-species study, Gonzalez-Meler
et al. (2004) reviewed the scientific literature pertaining to the effects of atmospheric CO2 enrichment on plant respiration from the cellular level to the level of entire ecosystems.
They report finding that “contrary to what was previously thought, specific respiration rates are generally not reduced when plants are grown at elevated CO2.” Nevertheless, they note
that “whole ecosystem studies show that canopy respiration does not increase proportionally to increases in biomass in response to elevated CO2,” which suggests that respiration per
unit biomass is likely somewhat reduced by atmospheric CO2 enrichment. However, they also find that “a larger proportion of respiration takes place in the root system [when plants are
grown in CO2-enriched air],” which once again obfuscates the issue. The three researchers say “fundamental information is still lacking on how respiration and the processes supported
by it are physiologically controlled, thereby preventing sound interpretations of what seem to be species-specific responses of
Climate Change Reconsidered 494 respiration to elevated CO2.” They conclude that “the role of plant respiration in augmenting the sink capacity of terrestrial ecosystems is still uncertain.”
Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/r/respirationherbaceous.php.
References Gonzalez-Meler, M.A., Taneva, L. and Trueman, R.J. 2004. Plant respiration and elevated atmospheric CO2 concentration: Cellular responses and global significance. Annals of
Botany 94: 647-656. Grunzweig, J.M. and Korner, C. 2001. Growth, water and nitrogen relations in grassland model ecosystems of the semi-arid Negev of Israel exposed to elevated CO2.
Oecologia 128: 251-262. Rabha, B.K. and Uprety, D.C. 1998. Effects of elevated CO2 and moisture stress on Brassica juncea. Photosynthetica 35: 597-602. Van der Heijden, E., Verbeek,
S.K. and Kuiper, P.J.C. 2000. Elevated atmospheric CO2 and increased nitrogen deposition: effects on on C and N metabolism and growth of the peat moss Sphagnum recurvum P. Beauv. Var.
mucronatum (Russ.) Warnst. Global Change Biology 6: 201-212. Ziska, L.H. and Bunce, J.A. 1999. Effect of elevated carbon dioxide concentration at night on the growth and gas exchange
of selected C4 species. Australian Journal of Plant Physiology 26: 71-77. 7.6.2. Woody Plants 7.6.2.1. Coniferous Trees Jach and Ceulemans (2000) grew three-year old Scots pine seedlings
out-of-doors and rooted in the ground in open-top chambers maintained at atmospheric CO2 concentrations of either 350 or 750 ppm for two years. To make the experiment more representative
of the natural world, they applied no nutrients or irrigation water to the soils in which the trees grew for the duration of the study. After two years of growth under these conditions,
dark respiration on a needle mass basis in the CO2-enriched seedlings was 27 percent and 33 percent lower in current-year and one-year-old needles, respectively, with the greater reduction
in the older needles being thought to arise from the greater duration of elevated CO2 exposure experienced by those needles. Hamilton et al. (2001) studied the short-and long-term respiratory
responses of loblolly pines in a free-air CO2-enrichment (FACE) study that was established in 1996 on 13-year-old trees in a North Carolina (USA) plantation, where the CO2-enriched trees
were exposed to an extra 200 ppm of CO2. This modest increase in the atmosphere’s CO2 concentration produced no significant short-term suppression of dark respiration rates in the trees’
needles. Neither did long-term exposure to elevated CO2 alter maintenance respiration, which is the amount of CO2 respired to maintain existing plant tissues. However, growth respiration,
which is the amount of CO2 respired when constructing new tissues, was reduced by 21 percent. McDowell et al. (1999) grew five-month-old seedlings of western hemlock in root boxes subjected
to various root-space CO2 concentrations (ranging from 90 to 7000 ppm) for periods of several hours to determine the effects of soil CO2 concentration on growth, maintenance and total
root respiration. In doing so, they found that although elevated CO2 had no effect on growth respiration, it significantly impacted maintenance and total respiration. At a soil CO2 concentration
of 1585 ppm, for example, total and maintenance respiration rates of roots were 55 percent and 60 percent lower, respectively, than they were at 395 ppm. The impact of elevated CO2 on
maintenance respiration was so strong that it exhibited an exponential decline of about 37 percent for every doubling of soil CO2 concentration. The implications of this observation
are especially important because maintenance respiration comprised 85 percent of total root respiration in this study. The results of these experiments suggest that both above and below
the soil surface, coniferous trees may exhibit reductions in total respiration in a high-CO2 world of the future. Three studies of three species, however, is insufficent evidence evidence
to reach a firm conclusion. Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/r/respirationconi
ers.php. References Hamilton, J.G., Thomas, R.B. and DeLucia, E.H. 2001. Direct and indirect effects of elevated CO2 on leaf
Biological Effects of Carbon Dioxide Enrichment 495 respiration in a forest ecosystem. Plant, Cell and Environment 24: 975-982. Jach, M.E. and Ceulemans, R. 2000. Short-versus longterm
effects of elevated CO2 on night-time respiration of needles of Scots pine (Pinus sylvestris L.). Photosynthetica 38: 57-67. McDowell, N.G., Marshall, J.D., Qi, J. and Mattson, K. 1999.
Direct inhibition of maintenance respiration in western hemlock roots exposed to ambient soil carbon dioxide concentrations. Tree Physiology 19: 599-605. 7.6.2.2. Deciduous Trees Wang
and Curtis (2001) grew cuttings of two male and two female trembling aspen trees for about five months on soils containing low and high nitrogen contents in open-top chambers maintained
at atmospheric CO2 concentrations of 380 and 765 ppm, finding that gender had little effect on dark respiration rates, but that elevated CO2 increased them, by 6 percent and 32 percent
in the low and high soil nitrogen treatments, respectively. On the other hand, Karnosky et al. (1999) grew both O3-sensitive and O3-tolerant aspen clones for one full year in free-air
CO2-enrichment (FACE) plots maintained at atmospheric CO2 concentrations of 360 and 560 ppm, finding that the extra CO2 decreased dark respiration rates by 24 percent. Gielen et al.
(2003) measured stem respiration rates of white, black and robusta poplar trees in a high-density forest plantation in the third year of a FACE experiment in which the CO2 concentration
of the air surrounding the trees was increased to a value of approximately 550 ppm. This study revealed, in their words, that “stem respiration rates were not affected by the FACE treatment,”
and that “FACE did not influence the relationships between respiration rate and both stem temperature and relative growth rate.” In addition, they say they could find “no effect of the
FACE treatment on Rm [maintenance respiration, which is related to the sustaining of existing cells] and Rg [growth respiration, which is related to the synthesis of new tissues].” Hamilton
et al. (2001) studied respiratory responses of sweetgum trees growing in the understory of a loblolly pine plantation (but occasionally reaching the top of the canopy) to an extra 200
ppm of CO2 in a FACE study conducted in North Carolina, USA. As a result of their measurement program, they determined that the modest increase in atmospheric CO2 concentration did not
appear to alter maintenance respiration to any significant degree, but that it reduced dark respiration by an average of 10 percent and growth respiration of leaves at the top of the
canopy by nearly 40 percent. In reviewing the results of these several deciduous tree studies, we see cases of both increases and decreases in respiration rates in response to atmospheric
CO2 enrichment, as well as cases of no change in respiration. More data are needed before any general conclusions may safely be drawn. Additional information on this topic, including
reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/r/respirationdeciduous.php. References Gielen, B., Scarascia-Mugnozza, G. and
Ceulemans, R. 2003. Stem respiration of Populus species in the third year of free-air CO2 enrichment. Physiologia Plantarum 117: 500-507. Hamilton, J.G., Thomas, R.B. and DeLucia, E.H.
2001. Direct and indirect effects of elevated CO2 on leaf respiration in a forest ecosystem. Plant, Cell and Environment 24: 975-982. Karnosky, D.F., Mankovska, B., Percy, K., Dickson,
R.E., Podila, G.K., Sober, J., Noormets, A., Hendrey, G., Coleman, M.D., Kubiske, M., Pregitzer, K.S. and Isebrands, J.G. 1999. Effects of tropospheric O3 on trembling aspen and interaction
with CO2: results from an O3-gradient and a FACE experiment. Water, Air, and Soil Pollution 116: 311-322. Wang, X. and Curtis, P.S. 2001. Gender-specific responses of Populus tremuloides
to atmospheric CO2 enrichment. New Phytologist 150: 675-684. 7.6.2.3. Multiple Tree Studies Amthor (2000) measured dark respiration rates of intact leaves of nine different tree species
growing naturally in an American deciduous forest. Within a specially designed leaf chamber, the CO2 concentration surrounding individual leaves was stabilized at 400 ppm for 15 minutes,
whereupon their respiration rates were measured for 30 minutes, after which the CO2 concentration in the leaf chamber was raised to 800 ppm for 15 minutes and respiration data were again
recorded for the same leaves. This protocol revealed that elevated CO2 had little effect on leaf dark respiration rates. The extra 400 ppm of
Climate Change Reconsidered 496 CO2 within the measurement cuvette decreased the median respiration rate by only 1.5 percent across the nine tree species. This observation led Amthor
to state that the “rising atmospheric CO2 concentration has only a small direct effect on tree leaf respiration in deciduous forests;” and he calculated that it can be “more than eliminated
by a 0.22°C temperature increase.” Upon this premise, he concluded that “future direct effects of increasing CO2 in combination with warming could stimulate tree leaf respiration in
their sum,” and that this consequence “would translate into only slight, if any, effects on the carbon balance of temperate deciduous forests in a future atmosphere containing as much
as [800 ppm] CO2.” Amthor’s conclusion is debatable, for it is based upon the extrapolation of the short-term respiratory responses of individual leaves, exposed to elevated CO2 for
only an hour or two, to that of entire trees, many of which will experience rising CO2 levels for a century or more during their lifetimes. Trees are longlived organisms that should
not be expected to reveal the nature of their long-term responses to elevated atmospheric CO2 concentrations on as short a time scale as 15 minutes. Indeed, their respiratory responses
may change significantly with the passage of time as they acclimate and optimize their physiology and growth patterns to the gradually rising CO2 content of earth’s atmosphere, as evidenced
by the findings of the following two studies. Wang and Curtis (2002) conducted a metaanalysis of the results of 45 area-based dark respiration (Rda) and 44 mass-based dark respiration
(Rdm) assessments of the effects of an approximate doubling of the air’s CO2 concentration on 33 species of plants (both herbaceous and woody) derived from 37 scientific publications.
This effort revealed that the mean leaf Rda of the woody plants they analyzed was unaffected by elevated CO2. However, there was an effect on mean leaf Rdm, and it was determined to
be time-dependent. The woody plants exposed to elevated CO2 for < 100 days, in the reviewing scientists’ words, “showed significantly less of a reduction in leaf Rdm due to CO2 enrichment
(-12%) than did plants exposed for longer periods (-35%, P < 0.01).” Hence, for conditions of continuous long-term atmospheric CO2 enrichment, the results of Wang and Curtis’ analysis
suggest woody plants may experience an approximate 35 percent decrease in leaf Rdm. Drake et al. (1999) also conducted a comprehensive analysis of the peer-reviewed scientific literature
to determine the effects of elevated atmospheric CO2 concentrations on plant respiration rates. They found that atmospheric CO2 enrichment typically decreased respiration rates in mature
foliage, stems, and roots of CO2-enriched plants relative to rates measured in plants grown in ambient air; and when normalized on a biomass basis, they determined that a doubling of
the atmosphere’s CO2 concentration would likely reduce plant respiration rates by an average of 18 percent. To determine the potential effects of this phenomenon on annual global carbon
cycling, which the twelve researchers say “will enhance the quantity of carbon stored by forests,” they input a 15 percent CO2-induced respiration reduction into a carbon sequestration
model, finding that an additional 6 to 7 Gt of carbon would remain sequestered within the terrestrial biosphere each year, thus substantially strengthening the terrestrial carbon sink.
Davey et al. (2004) reached a different conclusion. “Averaged across many previous investigations, doubling the CO2 concentration has frequently been reported to cause an instantaneous
reduction of leaf dark respiration measured as CO2 efflux.” However, as they continue, “no known mechanism accounts for this effect, and four recent studies [Amthor (2000); Anthor et
al. (2001); Jahnke (2001); Jahnke and Krewitt (2002)] have shown that the measurement of respiratory CO2 efflux is prone to experimental artifacts that could account for the reported
response.” Using a technique that avoids the potential artifacts of prior attempts to resolve the issue, Davey et al. employed a high-resolution dual channel oxygen analyzer in an open
gas exchange system to measure the respiratory O2 uptake of nine different species of plants in response to a short-term increase in atmospheric CO2 concentration, as well as the response
of seven species to long-term elevation of the air’s CO2 content in four different field experiments. In doing so, they found that “over six hundred separate measurements of respiration
failed to reveal any decrease in respiratory O2 uptake with an instantaneous increase in CO2.” Neither could they detect any response to a five-fold increase in the air’s CO2 concentration
nor to the total removal of CO2 from the air. They also note that “this lack of response of respiration to elevated CO2 was independent of treatment method, developmental stage, beginning
or end of night, and the CO2 concentration at which the plants had been grown.” In the long-term field studies, however, there was a respiratory response;
Biological Effects of Carbon Dioxide Enrichment 497 but it was small (7 percent on a leaf mass basis), and it was positive, not negative. In light of these contradicgtory results, the
most reasonable conclusion is that atmospheric CO2 enrichment may either increase or decrease woodyplant respiration, but not to any great degree, and that in the mean, the net result
for the conglomerate of earth’s trees would likely be something of little impact. Additional information on this topic, including reviews of newer publications as they become available,
can be found at http://www.co2science.org/subject/r/respirationtrees.php. References Amthor, J.S. 2000. Direct effect of elevated CO2 on nocturnal in situ leaf respiration in nine temperate
deciduous tree species is small. Tree Physiology 20: 139-144. Amthor, J.S., Koch, G.W., Willms, J.R. and Layzell, D.B. 2001. Leaf O2 uptake in the dark is independent of coincident CO2
partial pressure. Journal of Experimental Botany 52: 2235-2238. Davey, P.A., Hunt, S., Hymus, Hymus, G.J., DeLucia, E.H., Drake, B.G., Karnosky, D.F. and Long, S.P. 2004. Respiratory
oxygen uptake is not decreased by an instantaneous elevation of [CO2], but is increased with long-term growth in the field at elevated [CO2]. Plant Physiology 134: 520-527. Drake, B.G.,
Azcon-Bieto, J., Berry, J., Bunce, J., Dijkstra, P., Farrar, J., Gifford, R.M., Gonzalez-Meler, M.A., Koch, G., Lambers, H., Siedow, J. and Wullschleger, S. 1999. Does elevated atmospheric
CO2 inhibit mitochondrial respiration in green plants? Plant, Cell and Environment 22: 649-657. Jahnke, S. 2001. Atmospheric CO2 concentration does not directly affect leaf respiration
in bean or poplar. Plant, Cell and Environment 24: 1139-1151. Jahnke, S. and Krewitt, M. 2002. Atmospheric CO2 concentration may directly affect leaf respiration measurement in tobacco,
but not respiration itself. Plant, Cell and Environment 25: 641-651. Wang, X. and Curtis, P. 2002. A meta-analytical test of elevated CO2 effects on plant respiration. Plant Ecology
161: 251-261. 7.7. Carbon Sequestration As the CO2 content of the air continues to rise, nearly all of earth’s plants respond by increasing their photosynthetic rates and producing more
biomass. This results in more carbon being captured and stored, or sequestered, in plant fibers and soil, which counterbalances some of the CO2 emissions produced by mankind’s use of
fossil fuels. In this section we begin with a research review of what is known about forest and forest-species responses to atmospheric CO2 enrichment and subsequent carbon sequestration.
Then we survey research on how temperatures affect sequestration and whether CO2 enhancement offsets the rate at which carbon is re-released from soil (decomposition). 7.7.1. CO2 Enhancement
and Carbon Sequestration 7.7.1.1 Forests The planting and preservation of forests has long been acknowledged to be an effective and environmentally friendly (indeed, enhancing) means
for slowing climate-model-predicted CO2-induced global warming. This prescription for moderating
potential climate change is based on two well-established and very straightforward facts: (1) the carbon trees use to construct their tissues comes from the air, and (2) its extraction
from the atmosphere slows the rate of rise of the air’s CO2 content. In an open-top chamber experiment conducted in Switzerland, Nitschelm et al. (1997) reported that a 71 percent increase
in the atmospheric CO2 concentration above white clover monocultures led to a 50 percent increase in soil organic carbon content. Related studies on wheat and soybean agroecosystems
(Islam et al., 1999) provided similar results, as did a free-air CO2 enrichment (FACE) experiment on cotton, which documented a 10 percent increase in soil organic carbon content in
plots receiving 550 ppm CO2 relative to those receiving 370 ppm (Leavitt et al., 1994). These phenomena will allow long-lived perennial species characteristic of forest ecosystems to
sequester large amounts of carbon within their wood for extended periods of time (Chambers et al., 1998). In reviewing studies that have been conducted on individual trees, it is clear
that elevated levels of atmospheric CO2 increase photosynthesis and growth
Climate Change Reconsidered 498 in both broad-leaved and coniferous species. When broad-leaved trembling aspen (Populus tremuloides) were exposed to twice-ambient levels of atmospheric
CO2 for 2.5 years, for example, Pregitzer et al. (2000) reported 17 and 65 percent increases in fine root biomass at low and high levels of soil nitrogen, respectively; while Zak et
al. (2000) observed 16 and 38 percent CO2-induced increases in total tree biomass when subjected to the same respective levels of soil nitrogen. Similar results have been reported for
coniferous trees. When branches of Sitka spruce (Picea sitchensis) were fumigated with air of 700 ppm CO2 for four years, rates of net photosynthesis in current and second-year needles
were 100 and 43 percent higher, respectively, than photosynthetic rates of needles exposed to ambient air (Barton and Jarvis, 1999). In addition, ponderosa pine (Pinus ponderosa) grown
at 700 ppm CO2 for close to 2.5 years exhibited rates of net photosynthesis in current-year needles that were 49 percent greater than those of needles exposed to air containing 350 ppm
CO2 (Houpis et al., 1999). Elevated CO2 also enhances carbon sequestration by reducing carbon losses arising from plant respiration. Karnosky et al. (1999) reported that aspen seedlings
grown for one year at 560 ppm CO2 displayed dark respiration rates that were 24 percent lower than rates exhibited by trembling aspen grown at 360 ppm CO2. Also, elevated CO2 has been
shown to decrease maintenance respiration, which it did by 60 percent in western hemlock seedlings exposed to an atmospheric CO2 concentration of nearly 1600 ppm (McDowell et al., 1999).
In a thorough review of these topics, Drake et al. (1999) concluded that, on average, a doubling of the atmospheric CO2 concentration reduces plant respiration rates by approximately
17 percent. This finding contrasts strikingly with the much smaller effects reported by Amthor (2000), who found an average reduction in dark respiration of only 1.5 percent for nine
deciduous tree species exposed to 800 ppm CO2. The period of CO2 exposure in his much shorter experiments, however, was a mere 15 minutes. If the air’s CO2 content doubles, plants will
likely sequester something on the order of 17 percent more carbon than ambiently grown plants, solely as a consequence of CO2-induced reductions in respiration. And it is good to remember
that this stored carbon is in addition to that sequestered as a result of CO2-induced increases in plant photosynthetic rates. Based upon several different types of empirical data, a
number of researchers have concluded that current rates of carbon sequestration are robust and that future rates will increase with increasing atmospheric CO2 concentrations. In the
study of Fan et al. (1998) based on atmospheric measurements, for example, the broad-leaved forested region of North America between 15 and 51°N latitude was calculated to possess a
current carbon sink that can annually remove all the CO2 emitted into the air from fossil fuel combustion in both Canada and the United States. On another large scale, Phillips et al.
(1998) used data derived from tree basal area to show that average forest biomass in the tropics has increased substantially over the past 40 years and that growth in the Neotropics
alone (south and central South America, the Mexican lowlands, the Carribean islands, and southern Florida) can account for 40 percent of the missing carbon of the entire globe. And in
looking to the future, White et al. (2000) have calculated that coniferous and mixed forests north of 50°N latitude will likely expand their northern and southern boundaries by about
50 percent due to the combined effects of increasing atmospheric CO2, rising temperature, and nitrogen deposition. The latter of these factors, nitrogen deposition, is an important variable.
As indicated in the study of White et al., it can play an interactive role with increasing atmospheric CO2 to increase plant growth and carbon sequestration. However, the magnitude of
that role is still being debated. Nadelhoffer et al. (1999), for example, concluded that nitrogen deposition from human activities is “unlikely to be a major contributor” to the large
CO2 sink that exists in northern temperate forests. Houghton et al. (1998), however, feel that nitrogen deposition holds equal weight with CO2 fertilization in the production of terrestrial
carbon sinks; and Lloyd (1999) demonstrated that when CO2 and nitrogen increase together, modeled forest productivity is greater than that predicted by the sum of the individual contributions
of these two variables. Thus, anthropogenic nitrogen deposition can have anywhere from small to large positive effects on carbon sequestration, as well as everything in between. In conclusion,
as the air’s CO2 content continues to rise, the ability of earth’s forests to sequester carbon should also rise. With more CO2 in the atmosphere, trees will likely exhibit greater rates
of photosynthesis and reduced rates of respiration. Together, these observations suggest that biologically fixed fixed carbon will experience greater residency times
Biological Effects of Carbon Dioxide Enrichment 499 within plant tissues. And if this carbon is directed into wood production, which increases substantially with atmospheric CO2 enrichment,
some of it can be kept out of circulation for a very long time, possibly even a millennium or more. Additional information on this topic, including reviews of newer publications as they
become available, can be found at http://www.co2science.org/subject/c/carbonforests.php. References Amthor, J.S. 2000. Direct effect of elevated CO2 on nocturnal in situ leaf respiration
in nine temperate deciduous tree species is small. Tree Physiology 20: 139-144. Barton, C.V.M. and Jarvis, P.G. 1999. Growth response of branches of Picea sitchensis to four years exposure
to elevated atmospheric carbon dioxide concentration. New Phytologist 144: 233-243. Chambers, J.Q., Higuchi, N. and Schimel, J.P. 1998. Ancient trees in Amazonia. Nature 391: 135-136.
De Angelis, P., Chigwerewe, K.S. and Mugnozza, G.E.S. 2000. Litter quality and decomposition in a CO2-enriched Mediterranean forest ecosystem. Plant and Soil 224: 31-41. Drake, B.G.,
Azcon-Bieto, J., Berry, J., Bunce, J., Dijkstra, P., Farrar, J., Gifford, R.M., Gonzalez-Meler, M.A., Koch, G., Lambers, H., Siedow, J. and Wullschleger, S. 1999. Does elevated atmospheric
CO2 inhibit mitochondrial respiration in green plants? Plant, Cell and Environment 22: 649-657. Fan, S., Gloor, M., Mahlman, J., Pacala, S., Sarmiento, J., Takahashi, T. and Tans, P.
1998. A large terrestrial carbon sink in North America implied by atmospheric and oceanic carbon dioxide data and models. Science 282: 442-446. Hirschel, G., Korner, C. and Arnone III,
J.A. 1997. Will rising atmospheric CO2 affect leaf litter quality and in situ decomposition rates in native plant communities? Oecologia 110: 387-392. Houghton, R.A., Davidson, E.A.
and Woodwell, G.M. 1998. Missing sinks, feedbacks, and understanding the role of terrestrial ecosystems in the global carbon balance. Global Biogeochemical Cycles 12: 25-34. Houpis,
J.L.J., Anderson, P.D., Pushnik, J.C. and Anschel, D.J. 1999. Among-provenance variability of gas exchange and growth in response to long-term elevated CO2 exposure. Water, Air, and
Soil Pollution 116: 403-412. Islam, K.R., Mulchi, C.L. and Ali, A.A. 1999. Tropospheric carbon dioxide or ozone enrichments and moisture effects on soil organic carbon quality. Journal
of Environmental Quality 28: 1629-1636. Karnosky, D.F., Mankovska, B., Percy, K., Dickson, R.E., Podila, G.K., Sober, J., Noormets, A., Hendrey, G., Coleman, M.D., Kubiske, M., Pregitzer,
K.S. and Isebrands, J.G. 1999. Effects of tropospheric O3 on trembling aspen and interaction with CO2: results from an O3-gradient and a FACE experiment. Water, Air, and Soil Pollution
116: 311-322. King, J.S., Pregitzer, K.S. and Zak, D.R. 1999. Clonal variation in above-and below-ground responses of Populus tremuloides Michaux: Influence of soil warming and nutrient
availability. Plant and Soil 217: 119-130. Leavitt, S.W., Paul, E.A., Kimball, B.A., Hendrey, G.R., Mauney, J.R., Rauschkolb, R., Rogers, H., Nagy, J., Pinter Jr., P.J. and Johnson,
H.B. 1994. Carbon isotope dynamics of free-air CO2-enriched cotton and soils. Agricultural and Forest Meteorology 70: 87-101. Lloyd, J. 1999. The CO2 dependence of photosynthesis, plant
growth responses to elevated CO2 concentrations and their interaction with soil nutrient status, II. Temperate and boreal forest productivity and the combined effects of increasing CO2
concentrations and increased nitrogen deposition at a global scale. Functional Ecology 13: 439-459. McDowell, N.G., Marshall, J.D., Qi, J. and Mattson, K. 1999. Direct inhibition of
maintenance respiration in western hemlock roots exposed to ambient soil carbon dioxide concentrations. Tree Physiology 19: 599-605. Nadelhoffer, K.J., Emmett, B.A., Gundersen, P., Kjonaas,
O.J., Koopmans, C.J., Schleppi, P., Tietema, A. and Wright, R.F. 1999. Nitrogen deposition makes a minor contribution to carbon sequestration in temperate forests. Nature 398: 145-148.
Nitschelm, J.J., Luscher, A., Hartwig, U.A. and van Kessel, C. 1997. Using stable isotopes to determine soil carbon input differences under ambient and elevated atmospheric CO2 conditions.
Global Change Biology 3: 411-416. Phillips, O.L., Malhi, Y., Higuchi, N., Laurance, W.F., Nunez, P.V., Vasquez, R.M., Laurance, S.G., Ferreira, L.V., Stern, M., Brown, S. and Grace,
J. 1998. Changes in the carbon balance of tropical forests: Evidence from longterm plots. Science 282: 439-442. Pregitzer, K.S., Zak, D.R., Maziaasz, J., DeForest, J., Curtis, P.S. and
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and Boerner, R.E.J. 1998. Foliar nitrogen dynamics and decomposition of yellow-poplar and eastern white pine during four seasons of exposure to
Climate Change Reconsidered 500 elevated ozone and carbon dioxide. Forest Ecology and Management 109: 355-366. White, A., Cannell, M.G.R. and Friend, A.D. 2000. The high-latitude terrestrial
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W.E. and Lussenhop, J. 2000. Atmospheric CO2, soil-N availability, and allocation of biomass and nitrogen by Populus tremuloides. Ecological Applications 10: 34-46. 7.7.1.2. Old Forests
In addition to enhancing the growth and production of young forests, available research indicates that rising atmospheric CO2 concentrations will also increase the productivity and growth
of older forests. For most of the past century it was believed that old-growth forests, such as those of Amazonia, should be close to dynamic equilibrium. Just the opposite, however,
has been repeatedly observed over the past two decades. In one of the first studies to illuminate this reality, Phillips and Gentry (1994) analyzed the turnover rates—which are close
correlates of net productivity (Weaver and Murphy, 1990)—of 40 tropical forests from all around the world. They found that the growth rates of these already highly productive forests
had been rising ever higher since at least 1960, and that they had experienced an apparent acceleration in growth rate sometime after 1980. Commenting on these findings, Pimm and Sugden
(1994) reported that the consistency and simultaneity of the forest growth trends that Phillips and Gentry had documented on several continents led them to conclude that “enhanced productivity
induced by increased CO2 is the most plausible candidate for the cause of the increased turnover.” A few years later, Phillips et al. (1998) analyzed forest growth rate data for the
period 1958 to 1996 for several hundred plots of mature tropical trees scattered around the world, finding that tropical forest biomass, as a whole, increased substantially over the
period of record. In fact, the increase in the Neotropics was equivalent to approximately 40 percent of the missing terrestrial carbon sink of the entire globe. Consequently, they concluded
that tropical forests “may be helping to buffer the rate of increase in atmospheric CO2, thereby reducing the impacts of global climate change.” And, again, they identified the aerial
fertilization effect of the ongoing rise in the air’s CO2 content as one of the primary factors likely to be responsible for this phenomenon. More recently, Laurance et al. (2004a) reported
accelerated growth in the 1990s relative to the 1980s for the large majority (87 percent) of tree genera in 18 one-hectare plots spanning an area of about 300 km2 in central Amazonia,
while Laurance et al. (2004b) observed similarly accelerated tree community dynamics in the 1990s relative to the 1980s. In addition, Baker et al. (2004) reported there has been a net
increase in biomass in old-growth Amazonian forests in recent decades at a rate of 1.22 ± 0.42 Mg ha-1 yr-1, slightly greater than that originally estimated by Phillips et al. And once
again, it was suggested, in the words of Laurance et al. (2005), that these “pervasive changes in central Amazonian tree communities were most likely caused by global-or regional-scale
drivers, such as increasing atmospheric CO2 concentrations (Laurance et al., 2004a,b).” Expanding upon this theme, Laurance et al. (2005) say they “interpreted these changes as being
consistent with an ecological ‘signature’ expected from increasing forest productivity (cf., Phillips and Gentry, 1994; Lewis et al. 2004a,b; Phillips et al., 2004).” They note, however,
that they have been challenged in this conclusion by Nelson (2005), and they thus go on to consider his arguments in some detail, methodically dismantling them one by one. Evidence of
increasing dynamism and productivity of intact tropical forests continued, with Lewis et al., (2004a) reporting that among 50 oldgrowth plots scattered across tropical South America,
“stem recruitment, stem mortality, and biomass growth, and loss, all increased significantly.” In summarizing these and other findings, Lewis (2006) reports that over the past two decades,
“these forests have shown concerted changes in their ecology, becoming, on average, faster growing—more productive—and more dynamic, and showing a net increase in above-ground biomass,”
all of which rates of increase are greater than the previously documented increases in the rates of these phenomena. What is more, Lewis says that “preliminary analyses also suggest
the African and Australian forests are showing structural changes similar to South American forests.” So why should we care about growth trends of old forests? People who seek to address
the issue
Biological Effects of Carbon Dioxide Enrichment 501 solely on the basis of forced reductions in anthropogenic CO2 emissions claim that carbon sequestration by forests is viable only
when forests are young and growing vigorously. (Pearce, 1999) As forests age, as claimed by the IPCC, they gradually lose their carbon-sequestering prowess, such that forests more than
one hundred years old become essentially useless for removing CO2 from the air, as they claim such ancient and decrepit stands yearly lose as much CO2 via respiration as they take in
via photosynthesis. Although demonstrably erroneous, with repeated telling the twisted tale actually begins to sound reasonable. After all, doesn’t the metabolism of every living thing
slow down as it gets older? We grudgingly admit that it does—even with trees—but some trees live a remarkably long time. In Panama (Condit et al., 1995), Brazil (Chambers et al., 1998;
Laurance et al., 2004; Chambers et al., 2001), and many parts of the southwestern United States (Graybill and Idso, 1993), for example, individuals of a number of different species have
been shown to live for nearly one and a half millennia. At a hundred years of age, these super-slurpers of CO2 are mere youngsters. And in their really old age, their appetite for the
vital gas, though diminished, is not lost. In fact, Chambers et al. (1998) indicate that the longlived trees of Brazil continue to experience protracted slow growth even at 1,400 years
of age. And protracted slow growth (evident in yearly increasing trunk diameters) of very old and large trees can absorb a huge amount of CO2 out of the air each year, especially when,
as noted by Chanbers et al. (1998) with respect to the Brazilian forests in the central Amazon, about 50 percent of their above-ground biomass is contained in less than the largest 10
percent of their trees. Consequently, since the life span of these massive long-lived trees is considerably greater than the projected life span of the entire “Age of Fossil Fuels,”
their cultivation and preservation represents an essentially permanent, though only partial, solution to the perceived problem of the anthropogenic global warming. Another important
fact about forests and their ability to sequester carbon over long periods of time is that the forest itself is the unit of primary importance when it comes to determining the amount
of carbon that can be sequestered on a unit area of land. Cary et al. (2001) note most models of forest carbon sequestration wrongly assume that “agerelated growth trends of individual
trees and evenaged, monospecific stands can be extended to natural forests.” When they compared the predictions of such models against real-world data they gathered from northern Rocky
Mountain subalpine forests that ranged in age from 67 to 458 years, for example, they found that above-ground net primary productivity in 200-year-old natural stands was almost twice
as great as that of modeled stands, and that the difference between the two increased linearly throughout the entire sampled sampled age range. So what’s the explanation for the huge
discrepancy? Cary et al. suggest that long-term recruitment and the periodic appearance of additional late-successional species (increasing biodiversity) may have significant effects
on stand productivity, infusing the primary unit of concern, i.e., the everevolving forest super-organism, with greater vitality than would have been projected on the basis of characteristics
possessed by the unit earlier in its life. They also note that by not including effects of size-or age-dependent decreases in stem and branch respiration per unit of sapwood volume in
models of forest growth, respiration in older stands can be overestimated by a factor of two to five. How serious are these model shortcomings? For the real-world forests studied by
Cary et al., they produce predictions of carbon sequestration that are only a little over half as large as what is observed in nature for 200-year-old forests, while for 400-yearold
forests they produce results that are only about a third as large as what is characteristic of the real world. And as the forests grow older still, the difference between reality and
model projections grows with them. Another study relevant to the ability of forests to act as long-term carbon sinks was conducted by Luo et al. (2003), who analyzed data obtained from
the Duke Forest FACE experiment, in which three 30-meter-diameter plots within a 13-year-old forest (composed primarily of loblolly pines with sweetgum and yellow poplar trees as sub-dominants,
together with numerous other trees, shrubs, and vines that occupy still smaller niches) began to be enriched with an extra 200 ppm of CO2 in August 1996, while three similar plots were
maintained at the ambient atmospheric CO2 concentration. A number of papers describing different facets of this still-ongoing longterm study have been published; and as recounted by
Luo et al., they have revealed the existence of a CO2-induced “sustained photosynthetic stimulation at leaf and canopy levels [Myers et et al., 1999; Ellsworth, 2000; Luo et al., 2001;
Lai et al., 2002], which resulted in sustained stimulation of wood biomass
Climate Change Reconsidered 502 increment [Hamilton et al., 2002] and a larger carbon accumulation in the forest floor at elevated CO2 than at ambient CO2 [Schlesinger and Lichter, 2001].”
Based upon these findings and what they imply about rates of carbon removal from the atmosphere and its different residence times in plant, litter, and soil carbon pools, Luo et al.
developed a model for studying the sustainability of forest carbon sequestration. Applying this model to a situation where the atmospheric CO2 concentration gradually rises from a value
of 378 ppm in 2000 to a value of 710 ppm in 2100, they calculated that the carbon sequestration rate of the Duke Forest would rise from an initial value of 69 g m-2 yr-1 to a final value
of 201 g m-2 yr-1, which is a far cry from the scenario promulgated by those who claim earth’s forests will have released much of the carbon they had previously absorbed as early as
the year 2050 (Pearce, 1999). Another study that supports the long-term viability of carbon sequestration by forests was conducted by Paw U et al. (2004), who also note that old-growth
forests have generally been considered to “represent carbon sources or are neutral (Odum, 1963, 1965),” stating that “it is generally assumed that forests reach maximum productivity
at an intermediate age and productivity declines in mature and old-growth stands (Franklin, 1988), presumably as dead woody debris and other respiratory demands increase.” More particularly,
they report that a number of articles have suggested that “old-growth conifer forests are at equilibrium with respect to net ecosystem productivity or net ecosystem exchange (DeBell
and Franklin, 1987; Franklin and DeBell, 1988; Schulze et al., 1999), as an age-class end point of ecosystem development.” To see if these claims had any merit, Paw U et al. used an
eddy covariance technique to estimate the CO2 exchange rate of the oldest forest ecosystem (500 years old) in the AmeriFlux network of carbon-flux measurement stations—the Wind River
old-growth forest in southwestern Washington, USA, which is composed mainly of Douglas fir and western hemlock—over a period of 16 months, from May 1998 to August 1999. Throughout this
period, the 14 scientists report “there were no monthly averages with net release of CO2,” and that the cumulative net ecosystem exchange showed “remarkable sequestration of carbon,
comparable to many younger forests.” They concluded that “in contrast to frequently stated opinions, old-growth forests can be significant carbon sinks,” noting that “the old-growth
forests of the Pacific Northwest can contribute to optimizing carbon sequestration strategies while continuing to provide ecosystem services essential to supporting biodiversity.” Binkley
et al. (2004) revisited an aging aspen forest in the Tesuque watershed of northern New Mexico, USA—which between 1971 and 1976 (when it was between 90 and 96 years old) was thought to
have had a negative net ecosystem production rate of -2.0 Mg ha-1 yr-1—and measured the basal diameters of all trees trees in the central 0.01 ha of each of 27 plots arrayed across the
watershed, after which they used the same regression equations employed in the earlier study to calculate live tree biomass as of 2003. “Contrary to expectation,” as they describe it,
Binkley et al. report that “live tree mass in 2003 [186 Mg ha-1] was significantly greater than in 1976 [149 Mg ha-1] (P = 0.02), refuting the hypothesis that live tree mass declined.”
In fact, they found that the annual net increment of live tree mass was about 1.37 Mg ha-1 yr-1 from age 96 to age 123 years, which is only 12 percent less than the mean annual increment
of live tree mass experienced over the forest’s initial 96 years of existence (149 Mg ha-1 /96 yr = 1.55 Mg ha-1 yr-1). Consequently, in response to the question they posed when embarking
on their study—“Do old forests gain or lose carbon?”—Binkley et al. concluded that “old aspen forests continue to accrue live stem mass well into their second century, despite declining
current annual increments,” which, we might add, are not all that much smaller than those the forests exhibited in their younger years. Similar results have been obtained by Hollinger
et al. (1994) for a 300-year-old Nothofagus site in New Zealand, by Law et al. (2001) for a 250-year-old ponderosa pine site in the northwestern United States, by Falk et al. (2002)
for a 450-year-old Douglas fir/western hemlock site in the same general area, and by Knohl et al. (2003) for a 250-year-old deciduous forest in Germany. In commenting on these findings,
the latter investigators say they found “unexpectedly high carbon uptake rates during 2 years for an unmanaged ‘advanced’ beech forest, which is in contrast to the widely spread hypothesis
that ‘advanced’ forests are insignificant as carbon sinks.” For the forest they studied, as they describe it, “assimilation is clearly not balanced by respiration, although this site
shows typical characteristics of an ‘advanced’ forest at a comparatively late stage of development.” These observations about forests are remarkably similar to recent findings regarding
humans; i.e., that nongenetic interventions, even late in life, can put one
Biological Effects of Carbon Dioxide Enrichment 503 on a healthier trajectory that extends productive lifespan. So what is the global “intervention” that has put the planet’s trees on
the healthier trajectory of being able to sequester significant amounts of carbon in their old age, when past theory (which was obviously based on past observations) decreed they should
be in a state of no-net-growth or even negative growth? The answer is probably CO2 enhancement. For any tree of age 250 years or more, the greater portion of its life (at least two-thirds
of it) has been spent in an atmosphere of reduced CO2 content. Up until 1920, for example, the air’s CO2 concentration had never been above 300 ppm throughout the lives of such trees,
whereas it is currently 375 ppm or 25 percent higher. And for older trees, even greater portions of their lives have been spent in air of even lower CO2 concentration. Hence, the “intervention”
that has given new life to old trees would appear to be the aerial fertilization effect produced by the CO2 that resulted from the Industrial Revolution and is being maintained by its
ever-expanding aftermath (Idso, 1995). Based on these many observations, as well as the results of the study of Greenep et al. (2003), which strongly suggest, in their words, that “the
capacity for enhanced photosynthesis in trees growing in elevated CO2 is unlikely to be lost in subsequent generations,” it would appear that earth’s forests will remain strong sinks
for atmospheric carbon well into the distant future. A wealth of scientific data confirms the reality of the ever-increasing productivity of earth’s older forests, especially those of
Amazonia, concomitant with the rise in the air’s CO2 content. An even greater wealth of laboratory and field data demonstrates that rising forest productivity is what one would expect
to observe in response to the stimulus provided
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Biological Effects of Carbon Dioxide Enrichment 505 Phillips, O.L. and Gentry, A.H. 1994. Increasing turnover through time in tropical forests. Science 263: 954-958. Phillips, O.L.,
Malhi, Y., Higuchi, N., Laurance, W.F., Nunez, P.V., Vasquez, R.M., Laurance, S.G., Ferreira, L.V., Stern, M., Brown, S. and Grace, J. 1998. Changes in the carbon balance of tropical
forests: Evidence from longterm plots. Science 282: 439-442. Pimm, S.L. and Sugden, A.M. 1994. Tropical diversity and global change. Science 263: 933-934. Schlesinger, W.H. and Lichter,
J. 2001. Limited carbon storage in soil and litter of experimental forest plots under increased atmospheric CO2. Nature 411: 466-469. Schulze, E.-D., Lloyd, J., Kelliher, F.M., Wirth,
C., Rebmann, C., Luhker, B., Mund, M., Knohl, A., Milyuokova, I.M. and Schulze, W. 1999. Productivity of forests in the Eurosiberian boreal region and their potential to act as a carbon
sink: a synthesis. Global Change Biology 5: 703-722. Weaver, P.L. and Murphy, P.G. 1990. Forest structure and productivity in Puerto Rico’s Luquillo Mountains. Biotropica 22: 69-82.
7.7.2. Decomposition What is the fate of the extra carbon that is stored within plant tissues as a consequence of atmospheric CO2 enrichment? Is it rapidly returned to the atmosphere
following tissue senescence and decomposition? Or is it locked away for long periods of time? Experiments and real-world observations reveal that atmospheric CO2 enrichment typically
reduces, or has no effect upon, decomposition rates of senesced plant material. 7.7.2.1. Processes and Properties Atmospheric CO2 enrichment stimulates photosynthesis and growth in nearly
all plants, typically producing more non-structural carbohydrates, which can be used to manufacture more carbon-based secondary compounds (CBSCs) or phenolics. This observation is important
because phenolics tend to inhibit the decomposition of the organic matter in which they are found (Freeman et al., 2001). If elevated levels of atmospheric CO2 lead to the production
of more of these decay-resistant substances, one would expect the ongoing rise in the air’s CO2 content to lead to the enhanced sequestration of plant-litter-derived carbon in the world’s
soils, producing a negative feedback phenomenon that would tend to slow the rate of rise of the air’s CO2 content and thereby moderate CO2-induced global warming. For a long time, research
on this matter was rather muddled. Many studies reported the expected increases in CBSC concentrations with experimentally created increases in the air’s CO2 content. Others, however,
could find no significant plant phenolic content changes; a few even detected CO2-induced decreases in CBSC concentrations. Penuelas et al. (1997) finally brought order to the issue
when they identified the key role played by soil nitrogen. In analyzing the results of several different studies, Penuelas et al. noticed that when soil nitrogen supply was less than
adequate, some of the CBSC responses to a doubling of the air’s CO2 content were negative, i.e., a portion of the studies indicated that plant CBSC concentrations declined as the air’s
CO2 content rose. When soil nutrient supply was more than adequate, however, the responses were almost all positive, with plant CBSC concentrations rising in response to a doubling of
the air’s CO2 concentration. In addition, when the CO2 content of the air was tripled, all CBSC responses, under both low and high soil nitrogen conditions, were positive. The solution
to the puzzle was thus fairly simple. With a tripling of the air’s CO2 content, nearly all plants exhibited increases in CBSC production; but with only a doubling of the atmospheric
CO2 concentration, adequate nitrogen is needed to ensure a positive CBSC response. What makes these observations exciting is that atmospheric CO2 enrichment, in addition to enhancing
plant growth, typically stimulates nitrogen fixation in both woody (Olesniewicz and Thomas, 1999) and non-woody (Niklaus et al., 1998; Dakora and Drake, 2000) legumes. As the air’s CO2
content continues to rise, earth’s nitrogen-fixing plants should become ever more proficient in this important enterprise. In addition, some of the extra nitrogen thus introduced into
earth’s ecosystems will likely be shared with non-nitrogen-fixing plants. Also, since the microorganisms responsible for nitrogen fixation are found in nearly all natural ecosystems
(Gifford, 1992), and since atmospheric CO2 enrichment can directly stimulate the nitrogen-fixing activities of these microbes (Lowe and Evans, 1962), it can be appreciated that the ongoing
rise in the air’s CO2 content will likely provide more nitrogen for the
Climate Change Reconsidered 506 production of more CBSCs in all of earth’s plants. And with ever-increasing concentrations of decayresistant materials being found throughout plant tissues,
the plant-derived organic matter that is incorporated into soils should remain there for ever longer periods of time. On the other hand, in a meta-analysis of the effects of atmospheric
CO2 enrichment on leaf-litter chemistry and decomposition rate that was based on a total of 67 experimental observations, Norby et al. (2001) found that elevated atmospheric CO2 concentrations—mostly
between 600 and 700 ppm— reduced leaf-litter nitrogen concentration by about 7 percent. But in experiments where plants were grown under as close to natural conditions as possible, such
as in open-top chambers, free-air CO2 enrichment (FACE) plots, or in the proximity of CO2-emitting springs, there were no significant effects of elevated CO2 on leaf-litter nitrogen
content. In addition, based on a total of 46 experimental observations, Norby et al. determined that elevated atmospheric CO2 concentrations increased leaf-litter lignin concentrations
by an average of 6.5 percent. However, these increases in lignin content occurred in woody but not in herbaceous species. And again, the lignin concentrations of leaf litter were not
affected by elevated CO2 when plants were grown in open-top chambers, FACE plots, or in the proximity of CO2-emitting springs. In an analysis of a total of 101 observations, Norby et
al. found elevated CO2 had no consistent effect on leaf-litter decomposition rate in any type of experimental setting. As the air’s CO2 content continues to rise, it will likely have
little to no impact on leaf-litter chemistry and rates of leaf-litter decomposition. Since there will be more leaf litter produced in a high-CO2 world of the future, however, that fact
alone will ensure that more carbon is sequestered in the world’s soils for longer periods of time. Additional information on this topic, including reviews of newer publications as they
become available, can be found at http://www.co2science.org/subject/d/decompprocesses.php. References Agren, G.I. and Bosatta, E. 2002. Reconciling differences in predictions of temperature
response of soil organic matter. Soil Biology & Biochemistry 34: 129-132. Dakora, F.D. and Drake, B.G. 2000. Elevated CO2 stimulates associative N2 fixation in a C3 plant of the Chesapeake
Bay wetland. Plant, Cell and Environment 23: 943-953. Fitter, A.H., Self, G.K., Brown, T.K., Bogie, D.S., Graves, J.D., Benham, D. and Ineson, P. 1999. Root production and turnover in
an upland grassland subjected to artificial soil warming respond to radiation flux and nutrients, not temperature. Oecologia 120: 575-581. Freeman, C., Ostle, N. and Kang, H. 2001. An
enzymic ‘latch’ on a global carbon store. Nature 409: 149. Giardina, C.P. and Ryan, M.G. 2000. Evidence that decomposition rates of organic carbon in mineral soil do not vary with temperature.
Nature 404: 858-861. Gifford, R.M. 1992. Interaction of carbon dioxide with growth-limiting environmental factors in vegetative productivity: Implications for the global carbon cycle.
Advances in Bioclimatology 1: 24-58. Grisi, B., Grace, C., Brookes, P.C., Benedetti, A. and Dell’abate, M.T. 1998. Temperature effects on organic matter and microbial biomass dynamics
in temperate and tropical soils. Soil Biology & Biochemistry 30: 1309-1315. Johnson, L.C., Shaver, G.R., Cades, D.H., Rastetter, E., Nadelhoffer, K., Giblin, A., Laundre, J. and Stanley,
A. 2000. Plant carbon-nutrient interactions control CO2 exchange in Alaskan wet sedge tundra ecosystems. Ecology 81: 453-469. Liski, J., Ilvesniemi, H., Makela, A. and Westman, C.J.
1999. CO2 emissions from soil in response to climatic warming are overestimated—The decomposition of old soil organic matter is tolerant of temperature. Ambio 28: 171-174. Lowe, R.H.
and Evans, H.J. 1962. Carbon dioxide requirement for growth of legume nodule bacteria. Soil Science 94: 351-356. Niklaus, P.A., Leadley, P.W., Stocklin, J. and Korner, C. 1998. Nutrient
relations in calcareous grassland under elevated CO2. Oecologia 116: 67-75. Norby, R.J., Cotrufo, M.F., Ineson, P., O’Neill, E.G. and Canadell, J.G. 2001. Elevated CO2, litter chemistry,
and decomposition: a synthesis. Oecologia 127: 153-165. Olesniewicz, K.S. and Thomas, R.B. 1999. Effects of mycorrhizal colonization on biomass production and nitrogen fixation of black
locust (Robinia pseudoacacia) seedlings grown under elevated atmospheric carbon dioxide. New Phytologist 142: 133-140. Peñuelas, J., Estiarte, M. and Llusia, J. 1997. Carbon-based secondary
compounds at elevated CO2. Photosynthetica 33: 313-316.
Biological Effects of Carbon Dioxide Enrichment 507 Thornley, J.H.M. and Cannell, M.G.R. 2001. Soil carbon storage response to temperature: an hypothesis. Annals of Botany 87: 591-598.
7.7.2.2. Agricultural Crops In the study by Booker et al. (2000), leaves from defoliated cotton plants grown at an atmospheric CO2 concentration of 720 ppm displayed significantly greater
amounts of starch and soluble sugars and significantly lower concentrations of nitrogen than the leaves of plants grown in ambient air. These changes in the quality of the leaf litter
produced under high CO2 likely affected its subsequent decomposition rate, which was 10 to 14 percent slower than that observed for leaf litter collected from plants grown in air of
normal CO2 concentration. Likewise, when crop residues from soybean and sorghum plants that were raised in twice-ambient CO2 environments were mixed with soils to study their decomposition
rates, Torbert et al. (1998) noted they lost significantly less carbon – up to 40 percent less less – than similarly treated crop residues from ambiently grown crops. In contrast to
the aforementioned studies, neither Van Vuuren et al. (2000), for spring wheat, nor Henning et al. (1996), for soybean and sorghum, found any significant differences in the decomposition
rates of the residues of crops grown under conditions of high or normal atmospheric CO2 concentration. As the air’s CO2 content continues to rise, therefore, and agricultural crops grow
more robustly and return greater amounts of litter to the soil, it is likely that greater amounts of carbon will be sequestered in the soil in which they grew, as crop residue decomposition
rates are significantly decreased or remain unchanged. Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.or
/subject/d/decompositionagri.php. References Booker, F.L., Shafer, S.R., Wei, C.-M. and Horton, S.J. 2000. Carbon dioxide enrichment and nitrogen fertilization effects on cotton (Gossypium
hirsutum L.) plant residue chemistry and decomposition. Plant and Soil 220: 89-98. Henning, F.P., Wood, C.W., Rogers, H.H., Runion, G.B. and Prior, S.A. 1996. Composition and decomposition
of soybean and sorghum tissues grown under elevated atmospheric carbon dioxide. Journal of Environmental Quality 25: 822-827. Torbert, H.A., Prior, S.A., Rogers, H.H. and Runion, G.B.
1998. Crop residue decomposition as affected by growth under elevated atmospheric CO2. Soil Science 163: 412-419. Van Vuuren, M.M.I., Robinson, D., Scrimgeour, C.M., Raven, J.A. and
Fitter, A.H. 2000. Decomposition of 13Clabelled wheat root systems following growth at different CO2 concentrations. Soil Biology & Biochemistry 32: 403-413. 7.7.2.3. Grassland Species
In the study of Nitschelm et al. (1997), white clover exposed to an atmospheric CO2 concentration of 600 ppm for one growing season channeled 50 percent more newly fixed carbon compounds
into the soil than similar plants exposed to ambient air. In addition, the clover’s roots decomposed at a rate that was 24 percent slower than that observed for roots of control plants,
as has also been reported for white clover by David et al. (2001). These observations suggest that soil carbon sequestration under white clover ecosystems will be greatly enhanced as
the air’s CO2 content continues to rise, as was also shown for moderately fertile sandstone grasslands (Hu et al., 2001). Similar results have been observed with miniecosystems comprised
entirely of perennial ryegrass. Van Ginkel et al. (1996), for example, demonstrated that exposing this species to an atmospheric CO2 concentration of 700 ppm for two months caused a
92 percent increase in root growth and 19 percent and 14 percent decreases in root decomposition rates one and two years, respectively, after incubating ground roots within soils. This
work was later followed up by Van Ginkel and Gorissen (1998), who showed a 13 percent reduction in the decomposition rates of CO2-enriched perennial ryegrass roots in both disturbed
and undisturbed root profiles. This and other work led the authors to calculate that CO2-induced reductions in the decomposition of perennial ryegrass litter, which enhances soil carbon
sequestration, could well be large enough to remove over half of the anthropogenic CO2 emissions that may be released in the next century (Van Ginkel et al., 1999). In some cases, atmospheric
CO2 enrichment has little or no significant effect on litter quality and subsequent rates of litter decomposition, as was the case in the study of Hirschel et al. (1997) for lowland
Climate Change Reconsidered 508 calcareous and high alpine grassland species. Similar non-effects of elevated CO2 on litter decomposition have also been reported in a California grassland
(Dukes and Field, 2000). In light of these experimental findings, it would appear that as the air’s CO2 concentration increases, litter decomposition rates of grassland species will
likely decline, increasing the amount of carbon sequestered in grassland soils. Since this phenomenon is augmented by the aerial fertilization effect of atmospheric CO2 enrichment, which
leads to the production of greater amounts of litter, there is thus a double reason for expecting more carbon to be removed from the atmosphere by earth’s grasslands in the future. Additional
information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/d/decompositiongrass.php. References David,
J.-F., Malet, N., Couteaux, M.-M. and Roy, J. 2001. Feeding rates of the woodlouse Armadillidium vulgare on herb litters produced at two levels of atmospheric CO2. Oecologia 127: 343-349.
Dukes, J.S. and Field, C.B. 2000. Diverse mechanisms for CO2 effects on grassland litter decomposition. Global Change Biology 6: 145-154. Hirschel, G., Korner, C. and Arnone III, J.A.
1997. Will rising atmospheric CO2 affect leaf litter quality and in situ decomposition rates in native plant communities? Oecologia 110: 387-392. Hu, S., Chapin III, F.S., Firestone,
M.K., Field, C.B. and Chiariello, N.R. 2001. Nitrogen limitation of microbial decomposition in a grassland under elevated CO2. Nature 409: 188-191. Nitschelm, J.J., Luscher, A., Hartwig,
U.A. and van Kessel, C. 1997. Using stable isotopes to determine soil carbon input differences under ambient and elevated atmospheric CO2 conditions. Global Change Biology 3: 411-416.
Van Ginkel, J.H. and Gorissen, A. 1998. In situ decomposition of grass roots as affected by elevated atmospheric carbon dioxide. Soil Science Society of America Journal 62: 951-958.
Van Ginkel, J.H., Gorissen, A. and Polci, D. 2000. Elevated atmospheric carbon dioxide concentration: effects of increased carbon input in a Lolium perenne soil on microorganisms and
decomposition. Soil Biology & Biochemistry 32: 449-456. Van Ginkel, J.H., Whitmore, A.P. and Gorissen, A. 1999. Lolium perenne grasslands may function as a sink for atmospheric carbon
dioxide. Journal of Environmental Quality 28: 1580-1584. Van Ginkel, J.H., Gorissen, A. and van Veen, J.A. 1996. Long-term decomposition of grass roots as affected by elevated atmospheric
carbon dioxide. Journal of Environmental Quality 25: 1122-1128. 7.7.2.4. Woody Plants The sequestering of carbon in the soils upon which woody plants grow has the potential to provide
a powerful brake on the rate of rise of the air’s CO2 content if the plant litter that is incorporated into those soils does not decompose more rapidly in a CO2-enriched atmosphere than
it does in current ambient air. It is important to determine if this latter constraint is true or false. In this section we review this question with respect to litter produced by conifers
and deciduous trees. Scherzel et al. (1998) exposed seedlings of two eastern white pine genotypes to elevated concentrations of atmospheric CO2 and O3 in opentop chambers for four full
growing seasons, finding no changes in the decomposition rates of the litter of either genotype to the concentration increases of either of these two gases. Likewise, Kainulainen et
al. (2003) could find no evidence that the litter of 22-year-old Scots pine trees that had been exposed to elevated concentrations of CO2 and O3 for three full years decomposed any faster
or slower than litter produced in ambient air. In addition, Finzi and Schlesinger (2002) found that the decomposition rate of litter from 13-year-old loblolly pine trees was unaffected
by elevated CO2 concentrations maintained for a period of two full years in a FACE study. In light of these observations, plus the fact that Saxe et al. (1998) have determined that a
doubling of the air’s CO2 content leads to more than a doubling of the biomass production of coniferous species, it logically follows that the ongoing rise in the atmosphere’s CO2 concentration
is increasing carbon sequestration rates in the soils upon which conifers grow and producing a significant negative feedback phenomenon that slows the rate of rise of the air’s CO2 content.
What about deciduous trees? Scherzel et al. (1998) exposed seedlings of yellow poplar trees to elevated concentrations of atmospheric CO2 and O3 in
Biological Effects of Carbon Dioxide Enrichment 509 open-top chambers for four full growing seasons, finding that rates of litter decomposition were similar for all treatments for the
first five months of the study. Thereafter, however, litter produced in the elevated O3 and CO2 treatment decomposed at a significantly slower rate, such that after two years had passed,
the litter from the elevated O3 and CO2 treatment contained approximately 12 percent more biomass than the litter from any other treatment. Cotrufo et al. (1998) grew two-year-old ash
and sycamore seedlings for one growing season in closedtop chambers maintained at atmospheric CO2 concentrations of 350 and 600 ppm. The high-CO2 air increased lignin contents in the
litter produced from both tree species, which likely contributed to the decreased litter decomposition rates observed in the CO2-enriched chambers. After one year of incubation, for
example, litter bags from the CO2-enriched trees of both species had about 30 percent more dry mass remaining in them than litter bags from the ambient trees. In addition, woodlouse
arthropods consumed 16 percent less biomass when fed litter generated from seedlings grown at 600 ppm CO2 than when fed litter generated from seedlings grown in ambient air. De Angelis
et al. (2000) constructed large opentop chambers around 30-year-old mixed stands of naturally growing Mediterranean forest species (dominated by Quercus ilex, Phillyrea augustifolia,
and Pistacia lentiscus) near the coast of central Italy. Half of the chambers were exposed to ambient air of 350 ppm CO2, while half were exposed to air of 710 ppm CO2; and after three
years, the lignin and carbon concentrations of the leaf litter of all three species were increased by 18 and 4 percent, respectively, while their nitrogen concentrations were reduced
by 13 percent. These changes resulted in a 20 percent CO2-induced increase in the carbon-to-nitrogen ratio of the leaf litter, which parameter is commonly used to predict decomposition
rates, where larger ratios are
generally associated with less rapid decomposition than smaller ratios. This case was no exception, with 4 percent less decomposition occurring in the leaf litter gathered from beneath
the CO2-enriched trees than in the litter collected from beneath the trees growing in ambient air. Cotrufo and Ineson (2000) grew beech seedlings for five years in open-top chambers
fumigated with air containing either 350 or 700 ppm CO2. Subsequently, woody twigs from each CO2 treatment were collected and incubated in native forest soils for 42 months, after which
they determined there was no significant effect of the differential CO2 exposure during growth on subsequent woody twig decomposition, although the mean decomposition rate of the CO2-enriched
twigs was 5 percent less than that of the ambient-treatment twigs. Conway et al. (2000) grew two-year-old ash tree seedlings in solardomes maintained at atmospheric CO2 concentrations
of 350 and 600 ppm, after which naturally senesced leaves were collected, inoculated with various fungal species, and incubated for 42 days. They found the elevated CO2 significantly
reduced the amount of nitrogen in the senesced leaves, thus giving the CO2-enriched leaf litter a higher carbon-to-nitrogen ratio than the litter collected from the seedlings growing
in ambient air. This change likely contributed to the observed reductions in the amount of fungal colonization present on the senesced leaves from the CO2-enriched treatment, which would
be expected to result in reduced rates of leaf decomposition. King et al. (2001) grew aspen seedlings for five months in open-top chambers receiving atmospheric CO2 concentrations of
350 and 700 ppm. At the end of this period, naturally senesced leaf litter was collected, analyzed, and allowed to decompose under ambient conditions for 111 days. Although the elevated
CO2 slightly lowered leaf litter nitrogen content, it had no effect on litter sugar, starch, or tannin concentrations. With little to no CO2-induced effects on leaf litter quality, there
there was no CO2-induced effect on litter decomposition. Dilustro et al. (2001) erected open-top chambers around portions of a regenerating oak-palmetto scrub ecosystem in Florida, USA
and maintained them at CO2 concentrations of either 350 or 700 ppm, after which they incubated ambient-and elevated-CO2-produced fine roots for 2.2 years in the chamber soils, which
were nutrient-poor and often water-stressed. They found the elevated CO2 did not significantly affect the decomposition rates of the fine roots originating from either the ambient or
CO2-enriched environments. Of these seven studies of deciduous tree species, five are suggestive of slight reductions in litter decomposition rates under CO2-enriched growth conditions,
while two show no effect. With deciduous trees exhibiting large growth enhancements in response to atmospheric CO2 enrichment, we can expect to see large increases in the amounts of
carbon they sequester in the soils on which they grow as the air’s CO2 content continues to rise. And this phenomenon should slow the rate of rise of the
Climate Change Reconsidered 510 atmosphere’s CO2 concentration and thereby reduce the impetus for CO2-induced global warming. To summarize, scientific theory and empirical research show
that the ongoing rise in the air’s CO2 content will not materially alter the rate of decomposition of the world’s soil organic matter. This means the rate at which carbon is sequestered
in the world’s soils should continue to increase, a joint function of the rate at which the productivity of earth’s plants is increased by the aerial fertilization effect of the rising
atmospheric CO2 concentration and the rate of expansion of the planet’s vegetation into drier regions of the globe that is made possible by the concomitant CO2-induced increase in vegetative
water use efficiency. Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/d/decompconifers.php
and http://www.co2 science.org/subject/d/decompdeciduous.php. References Conway, D.R., Frankland, J.C., Saunders, V.A. and Wilson, D.R. 2000. Effects of elevated atmospheric CO2 on fungal
competition and decomposition of Fraxinus excelsior litter in laboratory microcosms. Mycology Research 104: 187-197. Cotrufo, M.F. and Ineson, P. 2000. Does elevated atmospheric CO2
concentration affect wood decomposition? Plant and Soil 224: 51-57. Cotrufo, M.F., Briones, M.J.I. and Ineson, P. 1998. Elevated CO2 affects field decomposition rate and palatability
of tree leaf litter: importance of changes in substrate quality. Soil Biology and Biochemistry 30: 1565-1571. De Angelis, P., Chigwerewe, K.S. and Mugnozza, G.E.S. 2000. Litter quality
and decomposition in a CO2-enriched Mediterranean forest ecosystem. Plant and Soil 224: 31-41. Dilustro, J.J., Day, F.P. and Drake, B.G. 2001. Effects of elevated atmospheric CO2 on
root decomposition in a scrub oak ecosystem. Global Change Biology 7: 581-589. Finzi, A.C. and Schlesinger, W.H. 2002. Species control variation in litter decomposition in a pine forest
exposed to elevated CO2. Global Change Biology 8: 1217-1229. Kainulainen, P., Holopainen, T. and Holopainen, J.K. 2003. Decomposition of secondary compounds from needle litter of Scots
pine grown under elevated CO2 and O3. Global Change Biology 9: 295-304. King, J.S., Pregitzer, K.S., Zak, D.R., Kubiske, M.E., Ashby, J.A. and Holmes, W.E. 2001. Chemistry and decomposition
of litter from Populus tremuloides Michaux grown at elevated atmospheric CO2 and varying N availability. Global Change Biology 7: 65-74. Saxe, H., Ellsworth, D.S. and Heath, J. 1998.
Tree and forest functioning in an enriched CO2 atmosphere. New Phytologist 139: 395-436. Scherzel, A.J., Rebbeck, J. and Boerner, R.E.J. 1998. Foliar nitrogen dynamics and decomposition
of yellow-poplar and eastern white pine during four seasons of exposure to elevated ozone and carbon dioxide. Forest Ecology and Management 109: 355-366. 7.7.3. Temperature and Carbon
Sequestration 7.7.3.1. General It must noted, as stated by Agren and Bosatta (2002), that that “global warming has long been assumed to lead to an increase in soil respiration and, hence,
decreasing soil carbon stores.” Indeed, this dictum was accepted as gospel for many years, for a number of laboratory experiments seemed to suggest that nature would not allow more carbon
to be sequestered in the soils of a warming world. As one nonlaboratory experiment after another has recently demonstrated, however, such is not the case, and theory has been forced
to change to accommodate reality. The old-school view of things began to unravel in 1999 when two studies presented evidence refuting the long-standing orthodoxy. Abandoning the laboratory
for the world of nature, Fitter et al. (1999) heated natural grass ecosystems by 3°C and found that the temperature increase had “no direct effect on the soil carbon store.” That same
year, Liski et al. (1999) showed that carbon storage in the soils of both high-and low-productivity boreal forests in Finland actually increased with rising temperatures along a natural
temperature gradient. The following year, Johnson et al. (2000) warmed natural Arctic tundra ecosystems by nearly 6°C for eight full years and found no significant effect on ecosystem
respiration. Likewise, Giardina and Ryan (2000) analyzed organic carbon decomposition data derived from the forest soils of 82 different sites on five continents, reporting that “despite
a 20°C gradient in mean annual temperature, soil carbon mass loss ... was insensitive to temperature.”
Biological Effects of Carbon Dioxide Enrichment 511 Thornley and Cannell (2001) ventured forth with what they called “a hypothesis” concerning the matter. Specifically, they proposed
the idea that warming may increase the rate of certain physicochemical processes that transfer organic carbon from less-stable to more-stable soil organic matter pools, thereby enabling
the better-protected organic matter to avoid, or more strongly resist, decomposition. Then, they developed a dynamic soil model in which they demonstrated that if their thinking were
correct, long-term soil carbon storage would appear to be insensitive to a rise in temperature, even if the respiration rates of all soil carbon pools rose in response to warming, as
they indeed do. Agren and Bosatta’s 2002 paper is an independent parallel development of much the same concept, although they describe the core idea in somewhat different terms, and
they upgrade the concept from what Thornley and Cannell call a “hypothesis” to what they refer to as the ““continuousquality theory.” Quality, in this context, refers to the degradability
of soil organic matter; and continuous quality suggests there is a wide-ranging continuous spectrum of soil organic carbon “mini-pools” that possess differing degrees of resistance to
decomposition. The continuous quality theory states that soils from naturally higher temperature regimes will contain relatively more organic matter in carbon pools that are more resistant
to degradation and are consequently characterized by lower rates of decomposition, which has been observed experimentally to be the case by Grisi et al. (1998). In addition, it states
that this shift in the distribution of soil organic matter qualities—i.e., the highertemperature-induced creation of more of the moredifficult-to-decompose organic matter—will counteract
the decomposition-promoting influence of the higher temperatures, so that the overall decomposition rate of the totality of organic matter in a higher-temperature soil is either unaffected
or reduced. Rising CO2 levels tend to maintain (Henning et al., 1996) or decrease (Torbert et al., 1998; Nitschelm et al., 1997) CO2 fluxes from agricultural soils. Consequently, these
phenomena tend to increase the carbon contents of most soils in CO2-enriched atmospheres. However, it is sometimes suggested that rising air temperatures, which can accelerate the breakdown
of soil organic matter and increase biological respiration rates, could negate this CO2-induced enhancement of carbon sequestration, possibly leading to an even greater release of carbon
back to the atmosphere. Casella and Soussana (1997) grew perennial ryegrass in controlled environments receiving ambient and elevated (700 ppm) atmospheric CO2 concentrations, two levels
of soil nitrogen, and ambient and elevated (+3°C) air temperatures for a period of two years, finding that “a relatively large part of the additional photosynthetic carbon is stored
below-ground during the two first growing seasons after exposure to elevated CO2, thereby increasing significantly the below-ground carbon pool.” At the low and high levels of soil nitrogen
supply, for example, the elevated CO2 increased soil carbon storage by 32 and 96 percent, respectively, “with no significant increased temperature effect.” The authors thus concluded
that in spite of predicted increases in temperature, “this stimulation of the below-ground carbon sequestration in temperate grassland soils could exert a negative feed-back on the current
rise of the atmospheric CO2 concentration.” Much the same conclusion was reached by Van Ginkel et al. (1999). After reviewing prior experimental work that established the growth and
decomposition responses of perennial ryegrass to both atmospheric CO2 enrichment and increased temperature, these researchers concluded that, at both low and high soil nitrogen contents,
CO2-induced increases in plant growth and CO2-induced decreases in plant decomposition rate are more than sufficient to counteract any enhanced soil respiration rate that might be caused
by an increase in air temperature. In addition, after reconstructing carbon storage in the terrestrial vegetation of Northern Eurasia as far back as 125,000 years ago, Velichko et al.
(1999) determined that plants in this part of the world were more productive and efficient in sequestering carbon at higher temperatures than they were at lower temperatures. Similarly,
Allen et al., (1999) used sediment cores from a lake in southern Italy and from the Mediterranean Sea to conclude that, over the past 102,000 years, warmer climates have been better
for vegetative productivity and carbon sequestration than have cooler climates. In conclusion, research conducted to date strongly suggests that the CO2-induced enhancement of vegetative
carbon sequestration will not be reduced by any future rise in air temperature, regardless of its cause. The rest of this section looks more closely at research regarding forests and
peatlands. Additional information on this topic, including reviews of newer publications as they become
Climate Change Reconsidered 512 available, can be found at http://www.co2science.org/subject/c/carbonco2xtemp.php. References Allen, J.R.M., Brandt, U., Brauer, A., Hubberten, H.-W.,
Huntley, B., Keller, J., Kraml, M., Mackensen, A., Mingram, J., Negendank, J.F.W., Nowaczyk, N.R., Oberhansli, H., Watts, W.A., Wulf, S. and Zolitschka, B. 1999. Rapid environmental
changes in southern Europe during the last glacial period. Nature 400: 740-743. Casella, E. and Soussana, J-F. 1997. Long-term effects of CO2 enrichment and temperature increase on the
carbon balance of a temperate grass sward. Journal of Experimental Botany 48: 1309-1321. Fan, S., Gloor, M., Mahlman, J., Pacala, S., Sarmiento, J., Takahashi, T. and Tans, P. 1998.
A large terrestrial carbon sink in North America implied by atmospheric and oceanic carbon dioxide data and models. Science 282: 442-446. Henning, F.P., Wood, C.W., Rogers, H.H., Runion,
G.B. and Prior, S.A. 1996. Composition and decomposition of soybean and sorghum tissues grown under elevated atmospheric carbon dioxide. Journal of Environmental Quality 25: 822-827.
Nitschelm, J.J., Luscher, A., Hartwig, U.A. and van Kessel, C. 1997. Using stable isotopes to determine soil carbon input differences under ambient and elevated atmospheric CO2 conditions.
Global Change Biology 3: 411-416. Torbert, H.A., Prior, S.A., Rogers, H.H. and Runion, G.B. 1998. Crop residue decomposition as affected by growth under elevated atmospheric CO2. Soil
Science 163: 412-419. Van Ginkel, J.H., Whitmore, A.P. and Gorissen, A. 1999. Lolium perenne grasslands may function as a sink for atmospheric carbon dioxide. Journal of Environmental
Quality 28: 1580-1584. Velichko, A.A., Zelikson, E.M. and Borisova, O.K. 1999. Vegetation, phytomass and carbon storage in Northern Eurasia during the last glacial-interglacial cycle
and the Holocene. Chemical Geology 159: 191-204. 7.7.3.2. Forests Liski et al. (1999) studied soil carbon storage across a temperature gradient in a modern-day Finnish boreal forest,
determining that carbon sequestration in the soil of this forest increased with temperature. In deciduous forests of the eastern United States, White et al. (1999) also determined that
persistent increases in growing season length (due to rising air temperatures) may lead to long-term increases in carbon storage, which tend to counterbalance the effects of increasing
air temperature on respiration rates. A data-driven analysis by Fan et al. (1998) suggests that the carbon-sequestering abilities of North America’s forests between 15 and 51°N latitude
are so robust that they can yearly remove from the atmosphere all of the CO2 annually released to it by fossil fuel consumption in both the United States and Canada (and this calculation
was done during a time touted as having the warmest temperatures on record). Moreover, Phillips et al. (1998) have shown that carbon sequestration in tropical forests has increased substantially
over the past 42 years, in spite of any temperature increases that may have occurred during that time. Similarly, King et al. (1999) showed that aspen seedlings increased their photosynthetic
rates and biomass production as temperatures rose from 10 to 29°C, putting to rest the idea that high-temperatureinduced increases in respiration rates would cause net losses in carbon
fixation. White et al. (2000) showed that rising temperatures increased the growing season by about 15 days for 12 sites in deciduous forests located within the United States, causing
a 1.6 percent increase in net ecosystem productivity per day. Thus, rather than exerting a negative influence on forest carbon sequestration, if air temperatures rise in the future they
will likely have a positive effect on carbon storage in forests and their associated soils. References King, J.S., Pregitzer, K.S. and Zak, D.R. 1999. Clonal variation in above-and below-ground
responses of Populus tremuloides Michaux: Influence of soil warming and nutrient availability. Plant and Soil 217: 119-130. Liski, J., Ilvesniemi, H., Makela, A. and Westman, C.J. 1999.
CO2 emissions from soil in response to climatic warming are overestimated -The decomposition of old soil organic matter is tolerant of temperature. Ambio 28: 171-174. Phillips, O.L.,
Malhi, Y., Higuchi, N., Laurance, W.F., Nunez, P.V., Vasquez, R.M., Laurance, S.G., Ferreira, L.V., Stern, M., Brown, S. and Grace, J. 1998. Changes in the carbon balance of tropical
forests: Evidence from longterm plots. Science 282: 439-442.
Biological Effects of Carbon Dioxide Enrichment 513 White, A., Cannell, M.G.R. and Friend, A.D. 2000. The high-latitude terrestrial carbon sink: a model analysis. Global Change Biology
6: 227-245. White, M.A., Running, S.W. and Thornton, P.E. 1999. The impact of growing-season length variability on carbon assimilation and evapotranspiration over 88 years in the eastern
US deciduous forest. International Journal of Biometeorology 42: 139-145. 7.7.3.3. Peatlands Putative CO2-induced global warming has long been predicted to turn boreal and tundra biomes
into major sources of carbon emissions. Until just a few short years ago it was nearly universally believed that rising air temperatures would lead to the thawing of extensive areas
of permafrost and the subsequent decomposition of their vast stores of organic matter, which, it was thought, would release much of the peatlands’ tightly held carbon, enabling it to
make its way back to the atmosphere as CO2. Improved soil drainage and increased aridity were also envisioned to help the process along, possibly freeing enough carbon at a sufficiently
rapid rate to rival the amount released to the atmosphere as CO2 by all anthropogenic sources combined. The end result was claimed to be a tremendous positive feedback to the ongoing
rise in the air’s CO2 content, which was envisioned to produce a greatly amplified atmospheric greenhouse effect that would lead to catastrophic global warming. This scenario was always
too bad to be true. Why? Because it did not begin to deal with the incredible complexity of the issue, several important neglected aspects of which have been briefly described by Weintraub
and Schimel (2005). One of the first cracks in the seemingly sound hypothesis was revealed by the study of Oechel et al. (2000), wherein long-term measurements of net ecosystem CO2 exchange
rates in wet-sedge and moist-tussock tundra communities of the Alaskan Arctic indicated that these ecosystems were gradually changing from carbon sources to carbon sinks. The transition
occurred between 1992 and 1996, at the apex of a regional warming trend that culminated with the highest summer temperature and surface water deficit of the previous four decades. How
did this dramatic and unexpected biological transformation happen? The answer of the scientists who documented the phenomenon was “a previously undemonstrated capacity for ecosystems
to metabolically adjust to long-term changes in climate.” Just as people can change their behavior in response to environmental stimuli, so can plants. And this ecological acclimation
process is only one of several newly recognized phenomena that have caused scientists to radically revise the way they think about global change in Arctic regions. Camill et al. (2001)
investigated (1) changes in peat accumulation across a regional gradient of mean annual temperature in Manitoba, Canada, (2) net above-ground primary production and decomposition for
major functional plant groups of the region, and (3) soil cores from several frozen and thawed bog sites that were used to determine long-term changes in organic matter accumulation
following the thawing of boreal peatlands. In direct contradiction of earlier thinking on the subject, but in confirmation of the more recent findings of Camill (1999a,b), the researchers
discovered that above-ground biomass and decomposition “were more strongly controlled by local succession than regional climate.” In other words, they determined that over a period of
several years, natural changes in plant community composition generally “have stronger effects on carbon sequestration than do simple increases in temperature and aridity.” Their core-derived
assessments of peat accumulation over the past two centuries demonstrated that rates of biological carbon sequestration can almost double following the melting of permafrost, in harmony
with the findings of Robinson and Moore (2000) and Turetsky et al. (2000), who found rates of organic matter accumulation in other recently thawed peatlands to have risen by 60-72 percent.
Griffis and Rouse (2001) drew upon the findings of a number of experiments conducted over the past quarter-century at a subarctic sedge fen near Churchill, Manitoba, Canada, in order
to develop an empirical model of net ecosystem CO2 exchange there. The most fundamental finding of this endeavor was that “carbon acquisition is greatest during wet and warm conditions,”
such as is generally predicted for the world as a whole by today’s most advanced climate models. However, since regional climate change predictions are not very dependable, the two scientists
investigated the consequences of a 4°C increase in temperature accompanied by both a 30 percent increase and decrease in precipitation; and “in all cases,” as they put it, “the equilibrium
response showed substantial increases in carbon acquisition.” One of the reasons behind this finding, as explained by Griffis and Rouse, is that “arctic ecosystems
Climate Change Reconsidered 514 photosynthesize below their temperature optimum over the majority of the growing season,” so that increasing temperatures enhance plant growth rates considerably
more than they increase plant decay rates. In summing up their findings, Griffis and Rouse say “warm surface temperatures combined with wet soil conditions in the early growing season
increase above-ground biomass and carbon acquisition throughout the summer season.” Indeed, they note that “wet spring conditions can lead to greater CO2 acquisition through much of
the growing period even when drier conditions persist.” They thus conclude that if climate change plays out as described by current climate models—i.e., if the world becomes warmer and
wetter—“northern wetlands should therefore become larger sinks for atmospheric CO2.” In a somewhat different type of study, Mauquoy et al. (2002) analyzed three cores obtained from a
raised peat bog in the UK (Walton Moss) and a single core obtained from a similar bog in Denmark (Lille Vildmose) for macro-and micro-fossils (pollen), bulk density, loss on ignition,
carbon/nitrogen ratios, and humification, while they were 14C dated by accelerator mass spectrometry. Among a variety of other things, it was determined, in their words, that “the lowest
carbon accumulation values for the Walton Moss monoliths between ca. cal AD 1300 and 1800 and between ca. cal AD 1490 and 1580 for Lille Vildmose occurred during the course of Little
Ice Age deteriorations,” which finding they describe as being much the same as the observation “made by Oldfield et al. (1997) for a Swedish ‘aapa’ mire between ca. cal AD 1400 and 1800.”
They also report that carbon accumulation before this, in the Medieval Warm Period, was higher, as was also the case following the Little Ice Age, as the earth transitioned to the Modern
Warm Period. Consequently, whereas the IPCC claims that warming will hasten the release of carbon from ancient peat bogs, these real-world data demonstrate that just the opposite is
more likely. In a somewhat similar study, but one that concentrated more on the role of nitrogen than of temperature, Turunen et al. (2004) derived recent (0-150 years) and long-term
(2,000-10,000 years) apparent carbon accumulation rates for several ombrotrophic peatlands in eastern Canada with the help of 210Pb-and 14C-dating of soil-core materials. This work revealed
that the average long-term apparent rate of C accumulation at 15 sites was 19 ± 8 g C m-2 yr-1, which is comparable to long-term rates observed in Finnish bogs by Tolonen and Turunen
(1996) and Turunen et al. (2002). Recent C accumulation rates at 23 sites, on the other hand, were much higher, averaging 73 ± 17 g C m-2 yr-1, which results, in their words, are also
“similar to results from Finland (Tolonen and Turunen, 1996; Pitkanen et al., 1999) and for boreal Sphagnum dominated peat deposits in North America (Tolonen et al., 1988; Wieder et
al., 1994; Turetsky et al., 2000).” Noting that recent rates of C accumulation are “strikingly higher” than than long-term rates, Turunen et al. suggested that increased N deposition
“leads to larger rates of C and N accumulation in the bogs, as has been found in European forests (Kauppi et al., 1992; Berg and Matzner, 1997), and could account for some of the missing
C sink in the global C budget.” Returning to the role of temperature, Payette et al. (2004) quantified the main patterns of change in a subarctic peatland on the eastern coast of Canada’s
Hudson Bay, which were caused by permafrost decay between 1957 and 2003, based on detailed surveys conducted in 1973, 1983, 1993 and 2003. This work revealed there was continuous permafrost
thawing throughout the period of observation, such that “about 18 percent of the initial frozen peatland surface was melted in 1957,” while thereafter “accelerated thawing occurred with
only 38 percent, 28 percent and 13 percent of the original frozen surface still remaining in 1983, 1993 and 2003, respectively.” This process, in their words, was one of “terrestrialization”
via the establishment of fen/bog vegetation, which nearly always results in either no net loss of carbon or actual carbon sequestration. As a result, Payette et al. concluded that “contrary
to current expectations, the melting of permafrost caused by recent climate change does not [our italics] transform the peatland to a carbon-source ecosystem.” Instead, they say that
“rapid terrestrialization exacerbates carbon-sink conditions and tends to balance the local carbon budget.” In a study of experimental warming of Icelandic plant communities designed
to see if the warming of high-latitude tundra ecosystems would result in significant losses of species and reduced biodiversity, Jonsdottir et al. (2005) conducted a field experiment
to learn how vegetation might respond to moderate warming at the low end of what is predicted by most climate models for a doubling of the air’s CO2 content. Specifically, they studied
the effects of 3-5 years of modest surface warming (1°-2°C) on two widespread but contrasting tundra plant communities, one of which was a nutrient-deficient and speciespoor moss heath
and the other of which was a
Biological Effects of Carbon Dioxide Enrichment 515 species-rich dwarf shrub heath. At the conclusion of the study, no changes in community structure were detected in the moss heath.
In the dwarf shrub heath, on the other hand, the number of deciduous and evergreen dwarf shrubs increased more than 50 percent, bryophytes decreased by 18 percent, and canopy height
increased by 100 percent, but with the researchers reporting they “detected no changes in species richness or other diversity measures in either community and the abundance of lichens
did not change.” Although Jonsdottir et al.’s study was a relatively short-term experiment as far as ecosystem studies go, its results indicate a rise in temperature need not have a
negative effect on the species diversity of high-latitude tundra ecosystems and may have a positive influence on plant growth. In a study that included an entirely new element of complexity,
Cole et al. (2002) constructed 48 small microcosms from soil and litter they collected near the summit of Great Dun Fell, Cumbria, England. Subsequent to “defaunating” this material
by reducing its temperature to -80°C for 24 hours, they thawed and inoculated it with native
soil microbes, after which half of the microcosms were incubated in the dark at 12°C and half at 18°C for two weeks, in order to establish near-identical communities of the soils’ natural
complement of microflora in each microcosm. The former of these temperatures was chosen to represent mean August soil temperature at a depth of 10 cm at the site of soil collection,
while the latter was picked to be “close to model predictions for soil warming that might result from a doubling of CO2 in blanket peat environments.” Next, 10 seedlings of Festuca ovina,
an indigenous grass of blanket peat, were planted in each of the microcosms, while 100 enchytraeid worms were added to each of half of the mini-ecosystems, producing four experimental
treatments: ambient temperature, ambient temperature plus enchytraeid worms, elevated temperature, and elevated temperature plus enchytraeid worms. Then, the 48 microcosms—sufficient
to destructively harvest three replicates of each treatment four different times throughout the course of the 64-day experiment— were arranged in a fully randomized design and maintained
at either 12° or 18°C with alternating 12-hour light and dark periods, while being given distilled water every two days to maintain their original weights. So what did the researchers
learn? First, they found that elevated temperature reduced the ability of the enchytraeid worms to enhance the loss of carbon from the microcosms. At the normal ambient temperature,
the presence of the worms enhanced dissolved organic carbon (DOC) loss by 16 percent, while at the elevated temperature expected for a doubling of the air’s CO2 content they had no effect
on DOC. In addition, Cole et al. note that “warming may cause drying at the soil surface, forcing enchytraeids to burrow to deeper subsurface horizons;” and since the worms are known
to have little influence on soil carbon dynamics below a depth of about 4 cm (Cole et al., 2000), the researchers concluded that this additional consequence of warming would further
reduce the ability of enchytraeids to enhance carbon loss from blanket peatlands. In summing up their findings, Cole et al. concluded that “the soil biotic response to warming in this
study was negative,” in that it resulted in a reduced loss of carbon to the atmosphere. But what about the effects of elevated CO2 itself on the loss of DOC from soils? Freeman et al.
(2004) note that riverine transport of DOC has increased markedly in many places throughout the world over the past few decades (Schindler et al., 1997; Freeman et al., 2001; Worrall
et al., 2003); they suggest this phenomenon may be related to the historical increase in the air’s CO2 content. The researchers’ first piece of evidence for this conclusion came from
a three-year study of monoliths (11-cm diameter x 20-cm deep cores) taken from three Welsh peatlands—a bog that received nutrients solely from rainfall, a fen that gained more nutrients
from surrounding soils and groundwater, and a riparian peatland that gained even more nutrients from nutrient-laden water transported from other terrestrial ecosystems via drainage streams—which
they exposed to either ambient air or air enriched with an extra 235 ppm of CO2 within a solardome facility. This study revealed that the DOC released by monoliths from the three peatlands
was significantly enhanced—by 14 percent in the bog, 49 percent in the fen, and 61 percent in the riparian peatland—by the additional CO2 to which they were exposed, which is the order
of response one would expect from what we know about the stimulation of net primary productivity due to atmospheric CO2 enrichment, i.e., it is low in the face of low soil nutrients,
intermediate when soil nutrient concentrations are intermediate, and high when soil nutrients are present in abundance. Consequently, Freeman et al. concluded that the DOC increases
they observed “were induced by increased primary production and DOC exudation from plants,”
Climate Change Reconsidered 516 which conclusion logically follows from their findings. Nevertheless, and to further test their hypothesis, they followed the translocation of labeled
13C through the plant-soil systems of the different peat monoliths for about two weeks after exposing them to ~99 percent-pure 13CO2 for a period of five hours. This exercise revealed
that (1) the plants in the ambient-air and CO2-enriched treatments assimilated 22.9 and 35.8 mg of 13C from the air, respectively, (2) the amount of DOC that was recovered from the leachate
of the CO2-enriched monoliths was 0.6 percent of that assimilated, or 0.215 mg (35.8 mg x 0.006 = 0.215 mg), and (3) the proportion of DOC in the soil solution of the CO2-enriched monoliths
that was derived from recently assimilated CO2 (the 13C labeled CO2) was 10 times higher than that of the control. This latter observation suggests that the amount of DOC recovered from
the leachate of the ambientair monoliths was only about a tenth as much as that recovered recovered from the leachate of the CO2-enriched monoliths, which puts the former amount at about
0.022 mg. Hence, what really counts, i.e., the net sequestration of 13C experienced by the peat monoliths over the two-week period (which equals the amount that went into them minus
the amount that went out), comes to 22.9 mg minus 0.022 mg = 22.878 mg for the ambient-air monoliths and 35.8 mg minus 0.215 mg = 35.585 mg for the CO2-enriched monoliths. In the end,
therefore, even though the CO2-enriched monoliths lost 10 times more 13C via root exudation than did the ambient-air monoliths, they still sequestered about 55 percent more 13C overall,
primarily in living-plant tissues. In light of this impressive array of pertinent findings, it would appear that continued increases in the air’s CO2 concentration and temperature would
not result in losses of carbon from earth’s peatlands. Quite to the contrary, these environmental changes— if they persist—would likely work together to enhance carbon capture by these
particular ecosystems. Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/c/carbonpeat.php.
References Berg, B. and Matzner, E. 1997. Effect of N deposition on decomposition of plant litter and soil organic matter in forest systems. Environmental Reviews 5: 1-25. Camill, P.
1999a. Patterns of boreal permafrost peatland vegetation across environmental gradients sensitive to climate warming. Canadian Journal of Botany 77: 721-733. Camill, P. 1999b. Peat accumulation
and succession following permafrost thaw in the boreal peatlands of Manitoba, Canada. Ecoscience 6: 592-602. Camill, P., Lynch, J.A., Clark, J.S., Adams, J.B. and Jordan, B. 2001. Changes
in biomass, aboveground net primary production, and peat accumulation following permafrost thaw in the boreal peatlands of Manitoba, Canada. Ecosystems 4: 461-478. Cole, L., Bardgett,
R.D. and Ineson, P. 2000. Enchytraeid worms (Oligochaeta) enhance mineralization of carbon in organic upland soils. European Journal of Soil Science 51: 185-192. Cole, L., Bardgett,
R.D., Ineson, P. and Hobbs, P.J. 2002. Enchytraeid worm (Oligochaeta) influences on microbial community structure, nutrient dynamics and plant growth in blanket peat subjected to warming.
Soil Biology & Biochemistry 34: 83-92. Freeman, C., Evans, C.D., Monteith, D.T., Reynolds, B. and Fenner, N. 2002. Export of organic carbon from peat soils. Nature 412: 785. Freeman,
C., Fenner, N., Ostle, N.J., Kang, H., Dowrick, D.J., Reynolds, B., Lock, M.A., Sleep, D., Hughes, S. and Hudson, J. 2004. Export of dissolved organic carbon from peatlands under elevated
carbon dioxide levels. Nature 430: 195-198. Griffis, T.J. and Rouse, W.R. 2001. Modelling the interannual variability of net ecosystem CO2 exchange at a subarctic sedge fen. Global Change
Biology 7: 511-530. Jonsdottir, I.S., Magnusson, B., Gudmundsson, J., Elmarsdottir, A. and Hjartarson, H. 2005. Variable sensitivity of plant communities in Iceland to experimental warming.
Global Change Biology 11: 553-563. Kauppi, P.E., Mielikainen, K. and Kuusela, K. 1992. Biomass and carbon budget of European forests. Science 256: 70-74. Mauquoy, D., Engelkes, T., Groot,
M.H.M., Markesteijn, F., Oudejans, M.G., van der Plicht, J. and van Geel, B. 2002. High-resolution records of late-Holocene climate change and carbon accumulation in two north-west European
ombrotrophic peat bogs. Palaeogeography, Palaeoclimatology, Palaeoecology 186: 275-310.
Biological Effects of Carbon Dioxide Enrichment 517 Oechel, W.C., Vourlitis, G.L., Hastings, S.J., Zulueta, R.C., Hinzman, L. and Kane, D. 2000. Acclimation of ecosystem CO2 exchange
in the Alaskan Arctic in response to decadal climate warming. Nature 406: 978-981. Payette, S., Delwaide, A., Caccianiga, M. and Beauchemin, M. 2004. Accelerated thawing of subarctic
peatland permafrost over the last 50 years. Geophysical Research Letters 31: 10.1029/2004GL020358. Pitkanen, A., Turunen, J. and Tolonen, K. 1999. The role of fire in the carbon dynamics
of a mire, Eastern Finland. The Holocene 9: 453-462. Robinson, S.D. and Moore, T.R. 2000. The influence of permafrost and fire upon carbon accumulation in high boreal peatlands, Northwest
Territories, Canada. Arctic, Antarctic and Alpine Research 32: 155-166. Schindler, D.W., Curtis, P.J., Bayley, S.E., Parker, B.R., Beaty, K.G. and Stainton, M.P. 1997. Climate-induced
changes in the dissolved organic carbon budgets of boreal lakes. Biogeochemistry 36: 9-28. Tolonen, K., Davis, R.B. and Widoff, L. 1988. Peat accumulation rates in selected Maine peat
deposits. Maine Geological Survey, Department of Conservation Bulletin 33: 1-99. Tolonen, K. and Turunen, J. 1996. Accumulation rates of carbon in mires in Finland and implications for
climate change. The Holocene 6: 171-178. Turetsky, M.R., Wieder, R.K., Williams, C.J, and Vitt, D.H. 2000. Organic matter accumulation, peat chemistry, and permafrost melting in peatlands
of boreal Alberta. Ecoscience 7: 379-392. Turunen, J., Roulet, N.T., Moore, T.R. and Richard, P.J.H. 2004. Nitrogen deposition and increased carbon accumulation in ombrotrophic peatlands
in eastern Canada. Global Biogeochemical Cycles 18: 10.1029/2003 GB002154. Turunen, J., Tomppo, E., Tolonen, K. and Reinikainen, A. 2002. Estimating carbon accumulation rates of undrained
mires in Finland: Application to boreal and subarctic regions. The Holocene 12: 69-80. Weintraub, M.N. and Schimel, J.P. 2005. Nitrogen cycling and the spread of shrubs control changes
in the carbon balance of Arctic tundra ecosystems. BioScience 55: 408-415. Wieder, R.K., Novak, M., Schell, W.R. and Rhodes, T. 1994. Rates of peat accumulation over the past 200 years
in five Sphagnum-dominated peatlands in the United States. Journal of Paleolimnology 12: 35-47. Worrall, F., Burt, T. and Shedden, R. 2003. Long term records of riverine dissolved organic
matter. Biogeochemistry 64: 165-178. 7.8. Other Benefits Other benefits to plants of CO2 enhancement documented in this section include superior nitrogenuse efficiency, increased nutrient
acquisition, greater resistance to pathogens and parasitic plants, greater root development, greater seed and tannin production, and improved performance of transgenic plants. In addition
to these benefits to plants, CO2 enrichment benefits all life on earth by reducing plant emissions of isoprene, a chemical responsible for the production of tropospheric ozone. 7.8.1.
Nitrogen-Use Efficiency Long-term exposure to elevated atmospheric CO2 concentrations often, but not always, elicits photosynthetic acclimation or down regulation in plants, which is
typically accompanied by reduced amounts of rubisco and/or other photosynthetic proteins that are typically present in excess amounts in plants grown in ambient air. As a consequence,
foliar nitrogen concentrations often decrease with atmospheric CO2 enrichment, as nitrogen is mobilized out of leaves and into other areas of the plant to increase its availability for
enhancing sink development or stimulating other nutrient-limited processes. In reviewing the literature in this area, one quickly notices that in spite of the fact that photosynthetic
acclimation has occurred, CO2-enriched plants nearly always display rates of photosynthesis that are greater than those of control plants exposed to ambient air. Consequently, photosynthetic
nitrogen-use efficiency, i.e., the amount of carbon converted into sugars during the photosynthetic process per unit of leaf nitrogen, often increases dramatically in CO2-enriched plants.
In the study of Davey et al. (1999), for example, CO2-induced reductions in foliar nitrogen contents and concomitant increases in photosynthetic rates led to photosynthetic nitrogen-use
efficiencies in the CO2-enriched (to 700 ppm CO2) grass Agrostis capillaris that were 27 and 62 percent greater than those observed in control plants grown at 360 ppm CO2 under moderate
and low soil nutrient conditions,
Climate Change Reconsidered 518 respectively. Similarly, elevated CO2 enhanced photosynthetic nitrogen-use efficiencies in Trifolium repens by 66 and 190 percent under moderate and low
soil nutrient conditions, respectively, and in Lolium perenne by 50 percent, regardless of soil nutrient status. Other researchers have found comparable CO2-induced enhancements of photosynthetic
nitrogen-use efficiency in wheat (Osborne et al., 1998) and in Leucadendron species (Midgley et al., 1999). In some cases, researchers report nitrogen-use efficiency in terms of the
amount of biomass produced per unit of plant nitrogen. Niklaus et al. (1998), for example, reported that intact swards of CO2-enriched calcareous grasslands grown at 600 ppm CO2 attained
total biomass values that were 25 percent greater than those of control swards exposed to ambient air while extracting the same amount of nitrogen from the soil as ambiently grown swards.
Similar results have been reported for strawberry by Deng and Woodward (1998), who noted that the growth nitrogen-use efficiencies of plants grown at 560 ppm CO2 were 23 and 17 percent
greater than those of ambiently grown plants simultaneously subjected to high and low soil nitrogen availability, respectively. In conclusion, the scientific literature indicates that
as the air’s CO2 content continues to rise, earth’s plants will likely respond by reducing the amount of nitrogen invested in rubisco and other photosynthetic proteins, while still maintaining
enhanced rates of photosynthesis, which consequently should increase their photosynthetic nitrogen-use efficiencies. As overall plant nitrogen-use efficiency increases, it is likely
plants will grow ever better on soils containing less-than-optimal levels of nitrogen, a point addressed in more detail in Section 7.3.7 of this report. Additional information on this
topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/n/nitrogenefficiency.php. References Davey, P.A., Parsons,
A.J., Atkinson, L., Wadge, K. and Long, S.P. 1999. Does photosynthetic acclimation to elevated CO2 increase photosynthetic nitrogen-use efficiency? A study of three native UK grassland
species in open-top chambers. Functional Ecology 13: 21-28. Deng, X. and Woodward, F.I. 1998. The growth and yield responses of Fragaria ananassa to elevated CO2 and N supply. Annals
of Botany 81: 67-71. Midgley, G.F., Wand, S.J.E. and Pammenter, N.W. 1999. Nutrient and genotypic effects on CO2-responsiveness: photosynthetic regulation in Leucadendron species of
a nutrient-poor environment. Journal of Experimental Botany 50: 533-542. Niklaus, P.A., Leadley, P.W., Stocklin, J. and Korner, C. 1998. Nutrient relations in calcareous grassland under
elevated CO2. Oecologia 116: 67-75. Osborne, C.P., LaRoche, J., Garcia, R.L., Kimball, B.A., Wall, G.W., Pinter, P.J., Jr., LaMorte, R.L., Hendrey, G.R. and Long, S.P. 1998. Does leaf
position within a canopy affect acclimation of photosynthesis to elevated CO2? Plant Physiology 117: 117: 1037-1045. 7.8.2. Nutrient Acquisition Most species of plants respond to increases
in the air’s CO2 content by displaying enhanced rates of photosynthesis and biomass production. Oftentimes, the resulting growth stimulation is preferentially expressed belowground,
thereby causing significant increases in fine-root numbers and surface area. This phenomenon tends to increase total nutrient uptake under CO2-enriched conditions, which further stimulates
plant growth and development. In this summary, we review how the acquisition of plant nutrients—primarily nitrate and phosphate—is affected by atmospheric CO2 enrichment. The effects
of elevated CO2 on nitrogen fixation are addressed elsewhere (Sections 7.3.7 and 7.8.1) and more research on the effects of CO2 enhancement on roots appears in Section 7.8.5. Smart et
al. (1998) noted there were no differences on a per-unit-biomass basis in the total amounts of nitrogen within CO2-enriched and ambiently grown wheat seedlings after three weeks of exposure
to atmospheric CO2 concentrations of 360 and 1,000 ppm. Nevertheless, the CO2-enriched seedlings exhibited greater rates of soil nitrate extraction than did the ambiently grown plants.
Similarly, BassiriRad et al. (1998) reported that a doubling of the atmospheric CO2 concentration doubled the uptake rate of nitrate in the C4 grass Bouteloua eriopoda. However, they
also reported that elevated CO2 had no effect on the rate of nitrate uptake in Prosopis, and that it decreased the rate of nitrate uptake by 55 percent in Larrea. Nonetheless, atmospheric
CO2 enrichment increased total biomass in these two species by 55 and 69 percent, respectively. Thus, although the uptake rate of this nutrient was depressed under elevated CO2 conditions
Biological Effects of Carbon Dioxide Enrichment 519 in the latter species, the much larger CO2-enriched plants likely still extracted more total nitrate from the soil than did the ambiently
grown plants of the experiment. Nasholm et al. (1998) determined that trees, grasses and shrubs can all absorb significant amounts of organic nitrogen from soils. Thus, plants do not
have to wait for the mineralization of organic nitrogen before they extract the nitrogen they need from soils to support their growth and development. Hence, the forms of nitrogen removed
from soils by plants (nitrate vs. ammonium) and their abilities to remove different forms may not be as important as was once thought. With respect to the uptake of phosphate, Staddon
et al. (1999) reported that Plantago lanceolata and Trifolium repens plants grown at 650 ppm CO2 for 2.5 months exhibited total plant phosphorus contents that were much greater than
those displayed by plants grown at 400 ppm CO2, due to the fact that atmospheric CO2 enrichment enrichment significantly enhanced plant biomass. Similarly, Rouhier and Read (1998) reported
that enriching the air around Plantago lanceolata plants with an extra 190 ppm of CO2 for a period of three months led to increased uptake of phosphorus and greater tissue phosphorus
concentrations than were observed in plants growing in ambient air. Greater uptake of phosphorus also can occur due to CO2-induced increases in root absorptive surface area or enhancements
in specific enzyme activities. In addressing the first of these phenomena, BassiriRad et al. (1998) reported that a doubling of the atmospheric CO2 concentration significantly increased
the belowground biomass of Bouteloua eriopoda and doubled its uptake rate of phosphate. However, elevated CO2 had no effect on uptake rates of phosphate in Larrea and Prosopis. Because
the CO2-enriched plants grew so much bigger, they still removed more phosphate from the soil on a per-plant basis. With respect to the second phenomenon, phosphatase—the primary enzyme
responsible for the conversion of organic phosphate into usable inorganic forms—had its activity increased by 30 to 40 percent in wheat seedlings growing at twice-ambient CO2 concentrations
(Barrett et al., 1998). In summary, as the CO2 content of the air increases, experimental data to date suggest that much of earth’s vegetation will likely extract enhanced amounts of
mineral nutrients from the soils in which they are rooted. Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2scienc
.org/subject/n/nutrientacquis.php. References Barrett, D.J., Richardson, A.E. and Gifford, R.M. 1998. Elevated atmospheric CO2 concentrations increase wheat root phosphatase activity
when growth is limited by phosphorus. Australian Journal of Plant Physiology 25: 87-93. BassiriRad, H., Reynolds, J.F., Virginia, R.A. and Brunelle, M.H. 1998. Growth and root NO3-and
PO43-uptake capacity of three desert species in response to atmospheric CO2 enrichment. Australian Journal of Plant Physiology 24: 353-358. Nasholm, T., Ekblad, A., Nordin, A., Giesler,
R., Hogberg, M. and Hogberg, P. 1998. Boreal forest plants take up organic nitrogen. Nature 392: 914-916. Rouhier, H. and Read, D.J. 1998. The role of mycorrhiza in determining the response
of Plantago lanceolata to CO2 enrichment. New Phytologist 139: 367-373. Smart, D.R., Ritchie, K., Bloom, A.J. and Bugbee, B.B. 1998. Nitrogen balance for wheat canopies (Triticum aestivum
cv. Veery 10) grown under elevated and ambient CO2 concentrations. Plant, Cell and Environment 21: 753-763. Staddon, P.L., Fitter, A.H. and Graves, J.D. 1999. Effect of elevated atmospheric
CO2 on mycorrhizal colonization, external mycorrhizal hyphal production and phosphorus inflow in Plantago lanceolata and Trifolium repens in association with the arbuscular mycorrhizal
fungus Glomus mosseae. Global Change Biology 5: 347-358. 7.8.3. Pathogens As the air’s CO2 content continues to rise, it is natural to wonder—and important to determine—determine—how
this phenomenon may impact plant-pathogen interactions. One thing we know about the subject is that atmospheric CO2 enrichment nearly always enhances photosynthesis, which commonly leads
to increased plant production of carbon-based secondary compounds, including lignin and various phenolics, both of which substances tend to increase plant resistance to pathogen attack.
Enlarging upon this topic, Chakraborty and Datta (2003) report that “changes in plant physiology,
Climate Change Reconsidered 520 anatomy and morphology that have been implicated in increased resistance or [that] can potentially enhance host resistance at elevated CO2 include: increased
net photosynthesis allowing mobilization of resources into host resistance (Hibberd et al., 1996a.); reduced stomatal density and conductance (Hibberd et al., 1996b); greater accumulation
of carbohydrates in leaves; more waxes, extra layers of epidermal cells and increased fibre content (Owensby, 1994); production of papillae and accumulation of silicon at penetration
sites (Hibberd et al., 1996a); greater number of mesophyll cells (Bowes, 1993); and increased biosynthesis of phenolics (Hartley et al., 2000), among others.” Chakraborty and Datta found
another way atmospheric CO2 enrichment may tip the scales in favor of plants in a study of the aggressiveness of the fungal anthracnose pathogen Colletotrichum gloeosporioides. They
inoculated two isolates of the pathogen onto two cultivars of the tropical pasture legume Stylosanthes scabra (Fitzroy, which is susceptible to the fungal pathogen, and Seca, which is
more resistant) over the course of 25 sequential infection cycles at ambient (350 ppm) and elevated (700 ppm) atmospheric CO2 concentrations in controlled environment chambers. This
protocol revealed that “at twice-ambient CO2 the overall level of aggressiveness of the two [pathogen] isolates was significantly reduced on both [host] cultivars.” In addition, they
say that “as shown previously (Chakraborty et al., 2000), the susceptible Fitzroy develops a level of resistance to anthracnose at elevated CO2, but resistance in Seca remains largely
unchanged.” Simultaneously, however, pathogen fecundity was found to increase at twice-ambient CO2. Of this finding, they report that their results “concur with the handful of studies
that have demonstrated increased pathogen fecundity at elevated CO2 (Hibberd et al., 1996a; Klironomos et al., 1997; Chakraborty et al., 2000).” How this happened in the situation they
investigated, investigated, as they describe it, is that the overall increase in fecundity at high CO2 “is a reflection of the altered canopy environment,” wherein “the 30 percent larger
S. scabra plants at high CO2 (Chakraborty et al., 2000) makes the canopy microclimate more conducive to anthracnose development.” In view of the opposing changes induced in pathogen
behavior by elevated levels of atmospheric CO2 in this specific study—reduced aggressiveness but increased fecundity—it is difficult to know the outcome of atmospheric CO2 enrichment
for the pathogen-host relationship. More research, especially under realistic field conditions, will be needed to clarify the situation; and, of course, different results are likely
to be observed for different pathogen-host associations. Results also could differ under different climatic conditions. Nevertheless, the large number of ways in which elevated CO2 has
been demonstrated to increase plant resistance to pathogen attack suggests that plants may well gain the advantage over pathogens as the air’s CO2 content continues to climb in the years
ahead. McElrone et al. (2005) “assessed how elevated CO2 affects a foliar fungal pathogen, Phyllosticta minima, of Acer rubrum [red maple] growing in the understory at the Duke Forest
free-air CO2 enrichment experiment in Durham, North Carolina, USA … in the 6th, 7th, and 8th years of the CO2 exposure.” Surveys conducted in those years, in their words, “revealed that
elevated CO2 [to 200 ppm above ambient] significantly reduced disease incidence, with 22%, 27% and 8% fewer saplings and 14%, 4%, and 5% fewer leaves infected per plant in the three
consecutive years, respectively.” In addition, they report that the elevated CO2 “also significantly reduced disease severity in infected plants in all years (e.g. mean lesion area reduced
35%, 50%, and 10% in 2002, 2003, and 2004, respectively).” With respect to identifying the underlying mechanism or mechanisms that produced these beneficent consequences, thinking it
could have been a direct deleterious effect of elevated CO2 on the fungal pathogen, McElrone et al. performed some side experiments in controlled environment chambers. However, they
found that the elevated CO2 benefited the fungal pathogen as well as the red maple saplings, observing that “exponential growth rates of P. minima were 17% greater under elevated CO2.”
And they obtained similar results when they repeated the in vitro growth analysis two additional times in different growth chambers. Taking another tack when “scanning electron micrographs
verified that conidia germ tubes of P. minima infect A. rubrum leaves by entering through the stomata,” the researchers turned their attention to the pathogen’s mode of entry into the
saplings’ foliage. In this investigation they found that both stomatal size and density were unaffected by atmospheric CO2 enrichment, but that “stomatal conductance was reduced by 21-36%
under elevated CO2, providing smaller openings for infecting germ tubes.” They concluded that reduced disease severity under under elevated CO2 was likely due to altered leaf
Biological Effects of Carbon Dioxide Enrichment 521 chemistry, as elevated CO2 increased total leaf phenolic concentrations by 15 percent and tannin concentrations by 14 percent. Because
the phenomena they found to be important in reducing the amount and severity of fungal pathogen infection (leaf spot disease) of red maple have been demonstrated to be operative in most
other plants as well, McElrone et al. say these CO2-enhanced leaf defensive mechanisms “may be prevalent in many plant pathosystems where the pathogen targets the stomata.” They state
their results “provide concrete evidence for a potentially generalizable mechanism to predict disease outcomes in other pathosystems under future climatic conditions.” Malmstrom and
Field (1997) grew individual oat plants for two months in pots within phytocells maintained at CO2 concentrations of 350 and 700 ppm, while a third of each CO2 treatment’s plants were
infected with the barley yellow dwarf virus (BYDV), which plagues more than 150 plant species worldwide, including all major cereal crops. They found that the elevated CO2 stimulated
net
photosynthesis rates in all plants, but with the greatest increase occurring in diseased individuals (48 percent vs. 34 percent). In addition, atmospheric CO2 enrichment decreased stomatal
conductance by 34 percent in healthy plants, but by 50 percent in infected ones, thus reducing transpirational water losses more in infected plants. Together, these two phenomena contributed
to a CO2-induced doubling of the instantaneous water-use efficiency of healthy control plants, but to a much larger 2.7-fold increase in diseased plants. Thus, although BYDV infection
did indeed reduce overall plant biomass production, the growth response to elevated CO2 was greatest in the diseased plants. After 60 days of CO2 enrichment, for example, total plant
biomass increased by 36 percent in infected plants, while it increased by only 12 percent in healthy plants. In addition, while elevated CO2 had little effect on root growth in healthy
plants, it increased root biomass in infected plants by up to 60 percent. In their concluding remarks, therefore, Malmstrom and Field say that CO2 enrichment “may reduce losses of infected
plants to drought” and “may enable diseased plants to compete better with healthy neighbors.” Tiedemann and Firsching (2000) grew spring wheat from germination to maturity in controlled
environment chambers maintained at either ambient (377 ppm) or enriched (612 ppm) atmospheric CO2 concentrations and either ambient (20 ppb) or enriched (61 ppb) atmospheric ozone (O3)
concentrations, while half of the plants in each of the four resulting treatments were inoculated with a leaf rust-causing pathogen. These procedures revealed that the percent of leaf
area infected by rust in inoculated plants was largely unaffected by atmospheric CO2 enrichment but strongly reduced by elevated O3. With respect to photosynthesis, elevated CO2 increased
rates in inoculated plants by 20 and 42 percent at ambient and elevated O3 concentrations, respectively. Although inoculated plants produced lower yields than non-inoculated plants,
atmospheric CO2 enrichment still stimulated yield in infected plants, increasing it by fully 57 percent at high O3. Consequently, the beneficial effects of elevated CO2 on wheat photosynthesis
and yield continued to be expressed in the presence of both O3 and pathogenic stresses. In another joint CO2/O3 study, Percy et al. (2002) grew the most widely distributed North American
tree species—trembling aspen—in twelve 30-m-diameter free-air CO2 enrichment (FACE) rings near Rhinelander, Wisconsin, USA in air maintained at ambient CO2 and O3 concentrations, ambient
O3 and elevated CO2 (560 ppm during daylight hours), ambient CO2 and elevated O3 (46.4-55.5 ppb during daylight hours), and elevated CO2 and O3 over the period of each growing season
from 1998 through 2001. Throughout this experiment they assessed a number of the young trees’ growth characteristics, as well as their responses to poplar leaf rust (Melampsora medusae),
which they say “is common on aspen and belongs to the most widely occurring group of foliage diseases.” Their work revealed that elevated CO2 alone did not alter rust occurrence, but
that elevated O3 alone increased it by nearly fourfold. When applied together, however, elevated CO2 reduced the enhancement of rust development caused by elevated O3 from nearly fourfold
to just over twofold. Jwa and Walling (2001) grew tomato plants in hydroponic culture for eight weeks in controlled environment chambers maintained at atmospheric CO2 concentrations
of 350 and 700 ppm. At week five of their study, half of the plants growing in each CO2 concentration were infected with the fungal pathogen Phytophthora parasitica, which attacks plant
roots and induces water stress that decreases plant growth and yield. This infection procedure reduced total plant biomass by nearly 30 percent at both atmospheric CO2 concentrations.
However, the elevated CO2 treatment increased the total biomass of healthy and
Climate Change Reconsidered 522 infected plants by the same percentage, so that infected tomato plants grown at 700 ppm CO2 exhibited biomass values similar to those of healthy tomato
plants grown at 350 ppm CO2. Consequently, atmospheric CO2 enrichment completely counterbalanced the negative effects of Phytophthora parasitica infection on tomato productivity. Pangga
et al. (2004) grew well-watered and fertilized pencilflower (cultivar Fitzroy) seedlings— an important legume crop susceptible to anthracnose disease caused by Colletotrichum gloeosporioides—
within a controlled environment facility maintained at atmospheric CO2 concentrations of either 350 or 700 ppm, where they inoculated six-, nine-and twelveweek-old plants with conidia
of C. gloeosporioides. Then, ten days after inoculation, they counted the anthracnose lesions on the plants and classified them as either resistant or susceptible. In doing so, they
found that “the mean number of susceptible, resistant, and total lesions per leaf averaged over the three plant ages was significantly (P<0.05) greater at 350 ppm than at 700 ppm CO2,
reflecting the development of a level of resistance in susceptible cv. Fitzroy at high CO2.” With respect to plants inoculated at twelve weeks of age, they say that those grown “at 350
ppm had 60 and 75 percent more susceptible and resistant lesions per leaf, respectively, than those [grown] at 700 ppm CO2.” The Australian scientists say their work “clearly shows that
at 350 ppm overall susceptibility of the canopy increases with increasing age because more young leaves are produced on secondary and tertiary branches of the more advanced plants.”
However, “at 700 ppm CO2, infection efficiency did not increase with increasing plant age despite the presence of many more young leaves in the enlarged canopy,” which finding, in their
words, “points to reduced pathogen efficiency or an induced partial resistance to anthracnose in Fitzroy at 700 ppm CO2.” Finally, according to Plessl et al. (2007), “potato late blight
caused by the oomycete Phytophthora infestans (Mont.) de Bary is the most devastating disease of potato worldwide,” adding that “infection occurs through leaves and tubers followed by
a rapid spread of the pathogen finally causing destructive necrosis.” In an effort to ascertain the effects of atmospheric CO2 enrichment on this pathogen, Plessl et al. grew individual
well watered and fertilized plants of the potato cultivar Indira in 3.5-liter pots filled with a 1:2 mixture of soil and “Fruhstorfer TErde” in controlled-environment chambers maintained
at atmospheric CO2 concentrations of either 400 or 700 ppm. Four weeks after the start of the experiment, the first three fully developed pinnate leaves were cut from the plants and
inoculated with zoospores of P. infestans in Petri dishes containing water-agar, after which their symptoms were evaluated daily via comparison with control leaves that were similarly
treated but unexposed to the pathogen. Results of the German researchers analysis revealed that the 300 ppm increase in CO2 “dramatically reduced symptom development,” including extent
of necrosis (down by 44 percent four days after inoculation and 65 percent five days after inoculation), area of sporulation (down by 100 percent four days after inoculation and 61 percent
five days after inoculation), and sporulation intensity (down by 73 percent four days after inoculation and 17 percent five days after inoculation). Plessl et al. conclude that their
results “clearly demonstrated that the potato cultivar Indira, which under normal conditions shows a high susceptibility to P. infestans, develops resistance against this pathogen after
exposure to 700 ppm CO2,” noting that “this finding agrees with results from Ywa et al. (1995), who reported an increased tolerance of tomato plants to Phytophthora root rot when grown
at elevated CO2.” These similar observations bode well for both potato and tomato cultivation in a CO2-enriched world of the future. In conclusion, the balance of evidence obtained to
date demonstrates an enhanced ability of plants to withstand pathogen attacks in CO2-enriched as opposed to ambient-CO2 air. As the atmosphere’s CO2 concentration continues to rise in
the years to come, earth’s vegetation should fare ever better in its battle against myriad debilitating plant diseases. Additional information on this topic, including reviews of newer
publications as they become available, can be found at http://www.co2science.org/subject/p/pathogens.php. References Bowes, G. 1993. Facing the inevitable: Plants and increasing atmospheric
CO2. Annual Review of Plant Physiology and Plant Molecular Biology 44: 309-332. Chakraborty, S. and Datta, S. 2003. How will plant pathogens adapt to host plant resistance at elevated
CO2 under a changing climate? New Phytologist 159: 733-742. Chakraborty, S., Pangga, I.B., Lupton, J., Hart, L., Room, P.M. and Yates, D. 2000. Production and dispersal of
Biological Effects of Carbon Dioxide Enrichment 523 Colletotrichum gloeosporioides spores on Stylosanthes scabra under elevated CO2. Environmental Pollution 108: 381-387. Hartley, S.E.,
Jones, C.G. and Couper, G.C. 2000. Biosynthesis of plant phenolic compounds in elevated atmospheric CO2. Global Change Biology 6: 497-506. Hibberd, J.M., Whitbread, R. and Farrar, J.F.
1996a. Effect of elevated concentrations of CO2 on infection of barley by Erysiphe graminis. Physiological and Molecular Plant Pathology 48: 37-53. Hibberd, J.M., Whitbread, R. and Farrar,
J.F. 1996b. Effect of 700 µmol per mol CO2 and infection of powdery mildew on the growth and partitioning of barley. New Phytologist 134: 309-345. Jwa, N.-S. and Walling, L.L. 2001.
Influence of elevated CO2 concentration on disease development in tomato. New Phytologist 149: 509-518. Klironomos, J.N., Rillig, M.C., Allen, M.F., Zak, D.R., Kubiske, M. and Pregitzer,
K.S. 1997. Soil fungalarthropod responses to Populus tremuloides grown under enriched atmospheric CO2 under field conditions. Global Change Biology 3: 473-478. Malmstrom, C.M. and Field,
C.B. 1997. Virus-induced differences in the response of oat plants to elevated carbon dioxide. Plant, Cell and Environment 20: 178-188. McElrone, A.J., Reid, C.D., Hoye, K.A., Hart,
E. and Jackson, R.B. 2005. Elevated CO2 reduces disease incidence and severity of a red maple fungal pathogen via changes in host physiology and leaf chemistry. Global Change Biology
11: 1828-1836. Owensby, C.E. 1994. Climate change and grasslands: ecosystem-level responses to elevated carbon dioxide. Proceedings of the XVII International Grassland Congress. Palmerston
North, New Zealand: New Zealand Grassland Association, pp. 1119-1124. Pangga, I.B., Chakraborty, S. and Yates, D. 2004. Canopy size and induced resistance in Stylosanthes scabra determine
anthracnose severity at high CO2. Phytopathology 94: 221-227. Percy, K.E., Awmack, C.S., Lindroth, R.L., Kubiske, M.E., Kopper, B.J., Isebrands, J.G., Pregitzer, K.S., Hendrey, G.R.,
Dickson, R.E., Zak, D.R., Oksanen, E., Sober, J., Harrington, R. and Karnosky, D.F. 2002. Altered performance of forest pests under atmospheres enriched by CO2 and O3. Nature 420: 403-407.
Plessl, M., Elstner, E.F., Rennenberg, H., Habermeyer, J. and Heiser, I. 2007. Influence of elevated CO2 and ozone concentrations on late blight resistance and growth of potato plants.
Environmental and Experimental Botany 60: 447-457. Tiedemann, A.V. and Firsching, K.H. 2000. Interactive effects of elevated ozone and carbon dioxide on growth and yield of leaf rust-infected
versus non-infected wheat. Environmental Pollution 108: 357-363. Ywa, N.S., Walling, L. and McCool, P.M. 1995. Influence of elevated CO2 on disease development and induction of PR proteins
in tomato roots by Phytophthora parasitica. Plant Physiololgy 85 (Supplement): 1139. 7.8.4. Parasitic Plants Parasitic plants obtain energy, water and nutrients from their host plants
and cause widespread reductions in harvestable crop yields around the globe. Hence, it is important to understand how rising atmospheric CO2 levels may impact the growth of parasitic
plants and the relationships between them and their hosts. Matthies and Egli (1999) grew Rhinanthus alectorolophus (a widely distributed parasitic plant of Central Europe) for a period
of two months on the grass Lolium perenne and the legume Medicago sativa in pots placed within open-top chambers maintained at atmospheric CO2 concentrations of 375 and 590 ppm, half
of which pots were fertilized to produce an optimal soil nutrient regime and half of which were unfertilized. At low nutrient supply, they found that atmospheric CO2 enrichment decreased
mean parasite biomass by an average of 16 percent, while at high nutrient supply it increased parasite biomass by an average of 123 percent. Nevertheless, the extra 215 ppm of CO2 increased
host plant biomass in both situations: by 29 percent under high soil nutrition and by 18 percent under low soil nutrition. Dale and Press (1999) infected white clover (Trifolium repens)
plants with Orobanche minor (a parasitic weed that primarily infects leguminous crops in the United Kingdom and the Middle East) and exposed them to atmospheric CO2 concentrations of
either 360 or 550 ppm for 75 days in controlledenvironment growth cabinets. The elevated CO2 in this study had no effect on the total biomass of parasite per host plant, nor did it impact
the number of parasites per host plant or the time to parasitic attachment to host roots. On the other hand, whereas infected host plants growing in ambient air produced 47 percent less
biomass than uninfected plants growing in ambient air, infected plants growing at 550 ppm CO2 exhibited final dry weights that were only 20 percent less than those displayed by
Climate Change Reconsidered 524 uninfected plants growing in the CO2-enriched air, indicative of a significant CO2-induced partial alleviation of parasite-induced biomass reductions
in the white clover host plants. Watling and Press (1997) infected several C4 sorghum plants with Striga hermonthica and Striga asiatica (parasitic C3 weeds of the semi-arid tropics
that infest many grain crops) and grew them, along with uninfected control plants, for approximately two months in controlled-environment cabinets maintained at atmospheric CO2 concentrations
of 350 and 700 ppm. In the absence of parasite infection, the extra 350 ppm of CO2 increased sorghum biomass by approximately 36 percent. When infected with S. hermonthica, however,
the sorghum plants grown at ambient and elevated CO2 concentrations only produced 32 and 43 percent of the biomass displayed by their respective uninfected controls. Infection with S.
asiatica was somewhat less stressful and led to host biomass production that was about half that of uninfected controls in both ambient and CO2-enriched air. The end result was that
the doubling of the air’s CO2 content employed in this study increased sorghum biomass by 79 percent and 35 percent in the C4 sorghum plants infected with S. hermonthica and S. asiatica,
respectively. Hwangbo et al. (2003) grew Kentucky Bluegrass (Poa pratensis L.) with and without infection by the C3 chlorophyllous parasitic angiosperm Rhinanthus minor L. (a facultative
hemiparasite found in natural and semi-natural grasslands throughout Europe) for eight weeks in open-top chambers maintained at ambient and elevated (650 ppm) CO2 concentrations. At
the end of the study, the parasite’s biomass (when growing on its host) was 47 percent greater in the CO2-enriched chambers, while its host exhibited only a 10 percent CO2-induced increase
in biomass in the parasite’s absence but a nearly doubled 19 percent increase when infected by it. Watling and Press (2000) grew upland rice (Oryza sativa L.) in pots in controlled-environment
chambers maintained at 350 and 700 ppm CO2 in either the presence or absence of the root parasite S. hermonthica for a period of 80 days after sowing, after which time the plants were
harvested and weighed. In ambient air, the presence of the parasite reduced the biomass of the rice to only 35 percent of what it was in the absence of the parasite; whereas in air enriched
with CO2 the presence of the parasite reduced the biomass of infected plants to but 73 percent of what it was in the absence of the parasite. In summary, these several observations suggest
that the rising CO2 content of the air can have wide and variable effects on parasitic plants, ranging from negative to positive growth responses, depending upon soil nutrition and host
plant specificity. With respect to the infected host plants, elevated CO2 generally tends to reduce the negative effects of parasitic infection, so that infected host plants continue
to exhibit positive growth responses to elevated CO2. It is likely that whatever the scenario with regard to parasitic infection, host plants will fare better under higher atmospheric
CO2 conditions than they do currently. Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/p/para
ites.php. References Dale, H. and Press, M.C. 1999. Elevated atmospheric CO2 influences the interaction between the parasitic angiosperm Orobanche minor and its host Trifolium repens.
New Phytologist 140: 65-73. Hwangbo, J.-K., Seel, W.E. and Woodin, S.J. 2003. Shortterm exposure to elevated atmospheric CO2 benefits the growth of a facultative annual root hemiparasite,
Rhinanthus minor (L.), more than that of its host, Poa pratensis (L.). Journal of Experimental Botany 54: 1951-1955. Matthies, D. and Egli, P. 1999. Response of a root hemiparasite to
elevated CO2 depends on host type and soil nutrients. Oecologia 120: 156-161. Watling, J.R. and Press, M.C. 1997. How is the relationship between the C4 cereal Sorghum bicolor and the
C3 root hemi-parasites Striga hermonthica and Striga asiatica affected by elevated CO2? Plant, Cell and Environment 20: 1292-1300. Watling, J.R. and Press, M.C. 2000. Infection with
the parasitic angiosperm Striga hermonthica influences the response of the C3 cereal Oryza sativa to elevated CO2. Global Change Biology 6: 919-930. 7.8.5. Roots In reviewing the scientific
literature pertaining to atmospheric CO2 enrichment effects on belowground plant growth and development, Weihong et al. (2000) briefly summarize what is known about this subject. They
report that atmospheric CO2 enrichment
Biological Effects of Carbon Dioxide Enrichment 525 typically enhances the growth rates of roots, especially those of fine roots, and that CO2-induced increases in root production eventually
lead to increased carbon inputs to soils, due to enhanced root turnover and exudation of various organic carbon compounds, which can potentially lead to greater soil carbon sequestration.
In addition, they note that increased soil carbon inputs stimulate the growth and activities of soil microorganisms that utilize plantderived carbon as their primary energy source; and
they report that subsequently enhanced activities of fungal and bacterial plant symbionts often lead to increased plant nutrient acquisition. In a much more narrowly focused study, Crookshanks
et al. (1998) sprouted seeds of the small and fast-growing Arabidopsis thaliana plant on agar medium in Petri dishes and grew the resulting immature plants in controlled environment
chambers maintained at atmospheric CO2 concentrations of either 355 or 700 ppm. Visual assessments of root growth were made after emergence of the roots from the seeds, while microscopic
investigations of root cell properties were also conducted. The scientists learned that the CO2-enriched plants directed a greater proportion of their newly produced biomass into root,
as opposed to shoot, growth. In addition, the young plants produced longer primary roots and more and longer lateral roots. These effects were found to be related to the CO2-induced
stimulation of mitotic activity, accelerated cortical cell expansion, and increased cell wall plasticity. Gouk et al. (1999) grew an orchid plantlet, Mokara Yellow, in plastic bags flushed
with 350 and 10,000 ppm CO2 for three months to study the effects of elevated CO2 on this epiphytic CAM species. They determined that the super-elevated CO2 of their experiment enhanced
the total dry weight of the orchid plantlets by more than two-fold, while increasing the growth of existing roots and stimulating the induction of new roots from internodes located on
the orchid stems. Total chlorophyll content was also increased by elevated CO2—by 64 percent in young leaves and by 118 percent in young roots. This phenomenon permitted greater light
harvesting during photosynthesis and likely led to the tissue starch contents of the CO2-enriched plantlets rising nearly 20-fold higher than those of the control-plantlets. In spite
of this large CO2-induced accumulation of starch, however, no damage or disruption of chloroplasts was evident in the leaves and roots of the CO2-enriched plants. A final question that
has periodically intrigued researchers is whether plants take up carbon through their roots in addition to through their leaves. Although a definitive answer eludes us, various aspects
of the issue have been described by Idso (1989), who we quote as follows. “Although several investigators have claimed that plants should receive little direct benefit from dissolved
CO2 (Stolwijk et al., 1957; Skok et al., 1962; Splittstoesser, 1966), a number of experiments have produced significant increases in root growth (Erickson, 1946; Leonard and Pinckard,
1946; Geisler, 1963; Yorgalevitch and Janes, 1988), as well as yield itself (Kursanov et al., 1951; Grinfeld, 1954; Nakayama and Bucks, 1980; Baron and Gorski, 1986), with CO2-enriched
irrigation water. Early on, Misra (1951) suggested that this beneficent effect may be related to CO2-induced changes in soil nutrient availability; and this hypothesis may well be correct.
Arteca et al. (1979), for example, have observed K, Ca and Mg to be better absorbed by potato roots when the concentration of CO2 in the soil solution is increased; while Mauney and
Hendrix (1988) found Zn and Mn to be better absorbed by cotton under such conditions, and Yurgalevitch and Janes (1988) found an enhancement of the absorption of Rb by tomato roots.
In all cases, large increases in either total plant growth or yield accompanied the enhanced uptake of nutrients. Consequently, as it has been suggested that CO2 concentration plays
a major role in in determining the porosity, plasticity and charge of cell membranes (Jackson and Coleman, 1959; Mitz, 1979), which could thereby alter ion uptake and organic acid production
(Yorgalevitch and Janes, 1988), it is possible that some such suite of mechanisms may well be responsible for the plant productivity increases often observed to result from enhanced
concentrations of CO2 in the soil solution.” In the next two sections we survey the scientific literature on root responses to atmospheric CO2 enrichment for crops and then trees. Additional
information on this topic, including reviews on roots not discussed here, can be found at http://www.co2science.org/subject/r/subject_r.php under the heading Roots. References Arteca,
R.N., Pooviah, B.W. and Smith, O.E. 1979. Changes in carbon fixation, tuberization, and growth induced by CO2 applications to the root zones of potato plants. Science 205: 1279-1280.
Climate Change Reconsidered 526 Baron, J.J. and Gorski, S.F. 1986. Response of eggplant to a root environment enriched with CO2. HortScience 21: 495-498. Crookshanks, M., Taylor, G.
and Dolan, L. 1998. A model system to study the effects of elevated CO2 on the developmental physiology of roots: the use of Arabidopsis thaliana. Journal of Experimental Botany 49:
593-597. Erickson, L.C. 1946. Growth of tomato roots as influenced by oxygen in the nutrient solution. American Journal of Botany 33: 551-556. Geisler, G. 1963. Morphogenetic influence
of (CO2 + HCO3-) on roots. Plant Physiology 38: 77-80. Gouk, S.S., He, J. and Hew, C.S. 1999. Changes in photosynthetic capability and carbohydrate production in an epiphytic CAM orchid
plantlet exposed to superelevated CO2. Environmental and Experimental Botany 41: 219-230. Grinfeld, E.G. 1954. On the nutrition of plants with carbon dioxide through the roots. Dokl.
Akad. Nauk SSSR 94: 919-922. Idso, S.B. 1989. Carbon Dioxide and Global Change: Earth in Transition. IBR IBR Press, Tempe, AZ. Jackson, W.A. and Coleman, N.T. 1959. Fixation of carbon
dioxide by plant roots through phosphoenolpyruvate carboxylase. Plant and Soil 11: 1-16. Kursanov, A.L., Kuzin, A.M. and Mamul, Y.V. 1951. On the possibility for assimilation by plants
of carbonates taken in with the soil solution. Dokl. Akad. Nauk SSSR 79: 685-687. Leonard, O.A. and Pinckard, J.A. 1946. Effect of various oxygen and carbon dioxide concentrations on
cotton root development. Plant Physiology 21: 18-36. Mauney, J.R. and Hendrix, D.L. 1988. Responses of glasshouse grown cotton to irrigation with carbon dioxidesaturated water. Crop
Science 28: 835-838. Misra, R.K. 1951. Further studies on the carbon dioxide factor in the air and soil layers near the ground. Indian Journal of Meteorology and Geophysics 2: 284-292.
Mitz, M.A. 1979. CO2 biodynamics: A new concept of cellular control. Journal of Theoretical Biology 80: 537-551. Nakayama, F.S. and Bucks, D.A. 1980. Using subsurface trickle system
for carbon dioxide enrichment. In Jensen, M.H. and Oebker, N.F. (Eds.), Proceedings of the 15th Agricultural Plastics Congress, National Agricultural Plastics Association, Manchester,
MO, pp. 13-18. Skok, J., Chorney, W. and Broecker, W.S. 1962. Uptake of CO2 by roots of Xanthium plants. Botanical Gazette 124: 118-120. Splittstoesser, W.E. 1966. Dark CO2 fixation
and its role in the growth of plant tissue. Plant Physiology 41: 755-759. Stolwijk, J.A.J. and Thimann, K.V. 1957. On the uptake of carbon dioxide and bicarbonate by roots and its influence
on growth. Plant Physiology 32: 513-520. Weihong, L., Fusuo, Z. and Kezhi, B. 2000. Responses of plant rhizosphere to atmospheric CO2 enrichment. Chinese Science Bulletin 45: 97-101.
Yorgalevitch, C.M. and Janes, W.H. 1988. Carbon dioxide enrichment of the root zone of tomato seedlings. Journal of Horticultural Science 63: 265-270. 7.8.5.1. Crops Hodge and Millard
(1998) grew narrowleaf plantain (Plantago lanceolata) seedlings for a period of six weeks in controlled environment growth rooms maintained at atmospheric CO2 concentrations of either
400 or 800 ppm. By the end of this period, the plants in the 800-ppm air exhibited increases in shoot and root dry matter production that were 159 percent and 180 percent greater, respectively,
than the corresponding dry matter increases experienced by the plants growing in 400-ppm air, while the amount of plant carbon recovered from the potting medium (sand) was 3.2 times
greater in the elevated-CO2 treatment. Thus, these investigators found that the belowground growth stimulation provided by atmospheric CO2 enrichment was greater than that experienced
above-ground. Wechsung et al. (1999) grew spring wheat (Triticum aestivum) in rows in a FACE study employing atmospheric CO2 concentrations of 370 and 550 ppm and irrigation treatments
that periodically replaced either 50 percent or 100 percent of prior potential evapotranspiration in an effort to determine the effects of elevated CO2 and water stress on root growth.
They found that elevated CO2 increased in-row root dry weight by an average of 22 percent during the growing season under both the wet and dry irrigation regimes. In addition, during
the vegetative growth phase, atmospheric CO2 enrichment increased inter-row root dry weight by 70 percent, indicating that plants grown in elevated CO2 developed greater lateral root
systems than plants grown at ambient CO2. During the reproductive growth phase, elevated CO2 stimulated the branching
Biological Effects of Carbon Dioxide Enrichment 527 of lateral roots into inter-row areas, but only when water was limiting to growth. In addition, the CO2-enriched plants tended to
display greater root dry weights at a given depth than did ambiently grown plants. In a comprehensive review of all prior FACE experiments conducted on agricultural crops, Kimball et
al. (2002) determined that for a 300-ppm increase in atmospheric CO2 concentration, the root biomass of wheat, ryegrass and rice experienced an average increase of 70 percent at ample
water and nitrogen, 58 percent at low nitrogen and 34 percent at low water, while clover experienced a 38 percent increase at ample water
and nitrogen, plus a 32 percent increase at low nitrogen. Outdoing all of the other crops was cotton, which exhibited a 96 percent increase in root biomass at ample water and nitrogen.
Zhao et al. (2000) germinated pea (Pisum sativum) seeds and exposed the young plants to various atmospheric CO2 concentrations in controlled environment chambers to determine if elevated
CO2 impacts root border cells, which are major contributors of root exudates in this and most other agronomic plants. They found that elevated CO2 increased the production of root border
cells in pea seedlings. In going from ambient air to air enriched to 3,000 and 6,000 ppm CO2, border-cell numbers increased by over 50 percent and 100 percent, respectively. Hence, as
the CO2 content of the air continues to rise, peas (and possibly many other crop plants) will likely produce greater numbers of root border cells, which should increase the amounts of
root exudations occurring in their rhizospheres, which further suggests that associated soil microbial and fungal activities will be stimulated as a result of the increases in plant-derived
carbon inputs that these organisms require to meet their energy needs. Van Ginkel et al. (1996) grew perennial ryegrass (Lolium perenne) plants from seed in two growth chambers for 71
days under continuous 14CO2-labeling of the atmosphere at at CO2 concentrations of 350 and 700 ppm at two different soil nitrogen levels. At the conclusion of this part of the experiment,
the plants were harvested and their roots dried, pulverized and mixed with soil in a number of one-liter pots that were placed within two wind tunnels in an open field, one of which
had ambient air of 361 ppm CO2 flowing through it, and one of which had air of 706 ppm CO2 flowing through it. Several of the containers were then seeded with more Lolium perenne, others
were similarly seeded the following year, and still others were kept bare for two years. Then, at the ends of the first and second years, the different degrees of decomposition of the
original plant roots were assessed. It was determined, first, that shoot and root growth were enhanced by 13 and 92 percent, respectively, by the extra CO2 in the initial 71-day portion
of the experiment, once again demonstrating the significant benefits that are often conferred upon plant roots by atmospheric CO2 enrichment. Secondly, it was found that the decomposition
of the high-CO2-grown roots in the high-CO2 wind tunnel was 19 percent lower than that of the low-CO2-grown roots in the low-CO2 wind tunnel at the end of the first year, and that it
was 14 percent lower at the end of the second year in the low-nitrogen-grown plants but equivalent in the high-nitrogen-grown plants. It was also determined that the presence of living
roots reduced the decomposition rate of dead roots below the dead-root-only decomposition rate observed in the bare soil treatment. Based on these findings, van Ginkel et al. conclude
that “the combination of higher root yields at elevated CO2 combined with a decrease in root decomposition will lead to a longer residence time of C in the soil and probably to a higher
C storage.” In conclusion, as the CO2 content of the air continues to rise, many crops will likely develop larger and more extensively branching root systems that may help them to better
cope with periods of reduced soil moisture availability. This chain of events should make the soil environment even more favorable for plant growth and development in a high-CO2 world
of the future. Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/r/rootscrops.php.
References Hodge, A. and Millard, P. 1998. Effect of elevated CO2 on carbon partitioning and exudate release from Plantago lanceolata seedlings. Physiologia Plantarum 103: 280-286. Kimball,
B.A., Kobayashi, K. and Bindi, M. 2002. Responses of agricultural crops to free-air CO2 enrichment. Advances in Agronomy 77: 293-368. Van Ginkel, J.H., Gorissen, A. and van Veen, J.A.
1996. Long-term decomposition of grass roots as affected by elevated atmospheric carbon dioxide. Journal of Environmental Quality 25: 1122-1128.
Climate Change Reconsidered 528 Wechsung, G., Wechsung, F., Wall, G.W., Adamsen, F.J., Kimball, B.A., Pinter Jr., P.J., LaMorte, R.L., Garcia, R.L. and Kartschall, T. 1999. The effects
of free-air CO2 enrichment and soil water availability on spatial and seasonal patterns of wheat root growth. Global Change Biology 5: 519-529. Zhao, X., Misaghi, I.J. and Hawes, M.C.
2000. Stimulation of border cell production in response to increased carbon dioxide levels. Plant Physiology 122: 181-188. 7.8.5.2. Trees Janssens et al. (1998) grew three-year-old Scots
pine seedlings for a period of six months in open-top chambers maintained at ambient and 700 ppm atmospheric CO2, finding that the extra CO2 increased total root length by 122 percent
and total root dry mass by 135 percent. In a similar study that employed close to the same degree of enhancement of the air’s CO2 content, Pritchard et al. (2001a) grew idealized ecosystems
representative of regenerating longleaf pine forests of the southeastern USA for a period of of 18 months in large soil bins located within open-top chambers. The above-ground parts
of these seedlings experienced a growth enhancement of 20 percent. The root biomass of the trees, however, was increased by more than three times as much (62 percent). Working with FACE
technology, Pritchard et al. (2001b) studied 14-year-old loblolly pine trees after a year of exposure to an extra 200 ppm of CO2, finding that total standing root length and root numbers
were 16 and 34 percent greater, respectively, in the CO2-enriched plots than in the ambient-air plots. In addition, the elevated CO2 increased the diameter of living and dead roots by
8 and 6 percent, respectively, while annual root production was found to be 26 percent greater in the CO2-enriched plots. For the degree of CO2 enrichment employed in the prior two studies,
this latter enhancement corresponds to a root biomass increase of approximately 45 percent. In an open-top chamber study of a model ecosystem composed of a mixture of spruce and beech
seedlings, Wiemken et al. (2001) investigated the effects of a 200 ppm increase in the air’s CO2 concentration that prevailed for a period of four years. On nutrient-poor soils, the
extra CO2 led to a 30 percent increase in fine-root biomass, while on nutrient-rich soils it led to a 75 percent increase. These numbers correspond to increases of about 52 percent and
130 percent, respectively, for atmospheric CO2 enhancements on the order of those employed by Janssens et al. (1998) and Pritchard et al. (2001a). Another interesting aspect of the Wiemken
et al. study was their finding that the extra CO2 increased the amount of symbiotic fungal biomass associated with the trees’ fine roots by 31 percent on nutrientpoor soils and by 100
percent on nutrient-rich soils, which for the degree of atmospheric CO2 enrichment used in the studies of Janssens et al. (1998) and Pritchard et al. (2001a) translate into increases
of about 52 percent and 175 percent, respectively. Berntsen and Bazzaz (1998) removed intact chunks of of soil from the Hardwood-White Pine-Hemlock forest region of New England and placed
them in plastic containers within controlled environment glasshouses maintained at either 375 or 700 ppm CO2 for a period of two years in order to study the effects of elevated CO2 on
the regeneration of plants from seeds and rhizomes present in the soil. At the conclusion of the study, total mesocosm plant biomass (more than 95 percent of which was supplied by yellow
and white birch tree seedlings) was found to be 31 percent higher in the elevated CO2 treatment than in ambient air, with a mean enhancement of 23 percent above-ground and 62 percent
belowground. The extra CO2 also increased the mycorrhizal colonization of root tips by 45 percent in white birch and 71 percent in yellow birch; and the CO2-enriched yellow birch seedlings
exhibited 322 percent greater root length and 305 percent more root surface area than did the yellow birch seedlings growing in ambient air. Kubiske et al. (1998) grew cuttings of four
quaking aspen genotypes in open-top chambers for five months at atmospheric CO2 concentrations of either 380 or 720 ppm and low or high soil nitrogen concentrations. They found, surprisingly,
that the cuttings grown in elevated CO2 displayed no discernible increases in above-ground growth. However, the extra CO2 significantly increased fineroot length and root turnover rates
at high soil nitrogen by increasing fine-root production, which would logically be expected to produce benefits (not the least of which would be a larger belowground water-and nutrient-gathering
system) that would eventually lead to enhanced above-ground growth as well. Expanding on this study, Pregitzer et al. (2000) grew six quaking aspen genotypes for 2.5 growing seasons
in open-top chambers maintained at atmospheric CO2 concentrations of 350 and 700 ppm with both adequate and inadequate supplies of soil
Biological Effects of Carbon Dioxide Enrichment 529 nitrogen. This work demonstrated that the trees exposed to elevated CO2 developed thicker and longer roots than the trees growing
in ambient air, and that the fine-root biomass of the CO2-enriched trees was enhanced by 17 percent in the nitrogen-poor soils and by 65 percent in the nitrogen-rich soils. Yet another
study of quaking aspen conducted by King et al. (2001) demonstrated that trees exposed to an atmospheric CO2 concentration 560 ppm in a FACE experiment produced 133 percent more fineroot
biomass than trees grown in ambient air of 360 ppm, which roughly equates to 233 percent more fineroot biomass for the degree of CO2 enrichment employed in the prior study of Pregitzer
et al. And when simultaneously exposed to air of 1.5 times the normal ozone concentration, the degree of fine-root biomass stimulation produced by the extra CO2 was still as great as
66 percent, or roughly 115 percent when extrapolated to the greater CO2 enrichment employed by Pregitzer et al. In a final quaking aspen study, King et al. (1999) grew four clones at
two different temperature regimes (separated by 5°C) and two levels of soil nitrogen (N) availability (high and low) for 98 days, while measuring photosynthesis, growth, biomass allocation,
and root production and mortality. They found that the higher of the two temperature regimes increased rates of photosynthesis by 65 percent and rates of whole-plant growth by 37 percent,
while it simultaneously enhanced root production and turnover. It was thus their conclusion that “trembling aspen has the potential for substantially greater growth and root turnover
under conditions of warmer soil at sites of both high and low N-availability” and that “an immediate consequence of this will be greater inputs of C and nutrients to forest soils.” In
light of these several findings pertaining to quaking aspen trees, it is evident that increases in atmospheric CO2 concentration, air temperature and soil nitrogen content all enhance
their belowground growth, which positively impacts their above-ground growth. Turning our attention to other deciduous trees, Gleadow et al. (1998) grew eucalyptus seedlings for six
months in glasshouses maintained at atmospheric CO2 concentrations of either 400 or 800 ppm, fertilizing them twice daily with low or high nitrogen solutions. The elevated CO2 of their
experiment increased total plant biomass by 98 and 134 percent relative to plants grown at ambient CO2 in the high and low nitrogen treatments, respectively. In addition, in the low
nitrogen treatment, elevated CO2 stimulated greater root growth, as indicated by a 33 percent higher root:shoot ratio. In a more complex study, Day et al. (1996) studied the effects
of elevated CO2 on fine-root production in open-top chambers erected over a regenerating oak-palmetto scrub ecosystem in Florida, USA, determining that a 350-ppm increase in the atmosphere’s
CO2 concentration increased fineroot length densities by 63 percent while enhancing the distribution of fine roots at both the soil surface (0-12 cm) and at a depth of 50-60 cm. These
findings suggest that the ongoing rise in the atmosphere’s CO2 concentration will likely increase the distribution of fine roots near the soil surface, where the greatest concentrations
of nutrients are located, and at a depth that coincides with the upper level of the site’s water table, both of which phenomena should increase the trees’ ability to acquire the nutrients
and water they will need to support CO2-enhanced biomass production in the years ahead. In another study that employed CO2, temperature and nitrogen as treatments, Uselman et al. (2000)
grew seedlings of the nitrogen-fixing black locust tree for 100 days in controlled environments maintained at atmospheric CO2 concentrations of 350 and 700 ppm and air temperatures of
26°C (ambient) and 30°C, with either some or no additional nitrogen fertilization, finding that the extra CO2 increased total seedling biomass by 14 percent, the elevated temperature
increased it by 55 percent, and nitrogen fertilization increased it by 157 percent. With respect to root exudation, a similar pattern was seen. Plants grown in elevated CO2 exuded 20
percent more organic carbon compounds than plants grown in ambient air, while elevated temperature and fertilization increased root exudation by 71 and 55 percent, respectively. Hence,
as the air’s CO2 content continues to rise, black locust trees will likely exhibit enhanced rates of biomass production and exudation of dissolved organic compounds from their roots.
Moreover, if air temperature also rises, even by as much as 4°C, its positive effect on biomass production and root exudation will likely be even greater than that resulting from the
increasing atmospheric CO2 concentration. The same would appear to hold true for anthropogenic nitrogen deposition, reinforcing what was learned about the impacts of these three environmental
factors on the growth of quaking aspen trees. In light of these several experimental findings, it can confidently be concluded that the ongoing rise in the air’s CO2 content, together
with possible
Climate Change Reconsidered 530 concurrent increases in air temperature and nitrogen deposition, will likely help earth’s woody plants increase their root mass and surface area to become
ever more robust and productive. Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/r/rootsconif
rs.php and http://www.co2 science.org/subject/r/rootsdeciduous.php. References Berntson, G.M. and Bazzaz, F.A. 1998. Regenerating temperate forest mesocosms in elevated CO2: belowground
growth and nitrogen cycling. Oecologia 113: 115-125. Day, F.P., Weber, E.P., Hinkle, C.R. and Drake, B.G. 1996. Effects of elevated atmospheric CO2 on fine root length and distribution
in an oak-palmetto scrub ecosystem in central Florida. Global Change Biology 2: 143-148. Gleadow, R.M., Foley, W.J. and Woodrow, I.E. 1998. Enhanced CO2 alters the relationship between
photosynthesis and defense in cyanogenic Eucalyptus cladocalyx F. Muell. Plant, Plant, Cell and Environment : 12-22. Janssens, I.A., Crookshanks, M., Taylor, G. and Ceulemans, R. 1998.
Elevated atmospheric CO2 increases fine root production, respiration, rhizosphere respiration and soil CO2 efflux in Scots pine seedlings. Global Change Biology 4: 871-878. King, J.S.,
Pregitzer, K.S. and Zak, D.R. 1999. Clonal variation in above-and below-ground growth responses of Populus tremuloides Michaux: Influence of soil warming and nutrient availability. Plant
and Soil 217: 119-130. King, J.S., Pregitzer, K.S., Zak, D.R., Sober, J., Isebrands, J.G., Dickson, R.E., Hendrey, G.R. and Karnosky, D.F. 2001. Fine-root biomass and fluxes of soil
carbon in young stands of paper birch and trembling aspen as affected by elevated atmospheric CO2 and tropospheric O3. Oecologia 128: 237-250. Kubiske, M.E., Pregitzer, K.S., Zak, D.R.
and Mikan, C.J. 1998. Growth and C allocation of Populus tremuloides genotypes in response to atmospheric CO2 and soil N availability. New Phytologist 140: 251-260. Pregitzer, K.S.,
Zak, D.R., Maziaasz, J., DeForest, J., Curtis, P.S. and Lussenhop, J. 2000. Interactive effects of atmospheric CO2 and soil-N availability on fine roots of Populus tremuloides. Ecological
Applications 10: 18-33. Pritchard, S.G., Davis, M.A., Mitchell, R.J., Prior, A.S., Boykin, D.L., Rogers, H.H. and Runion, G.B. 2001a. Root dynamics in an artificially constructed regenerating
longleaf pine ecosystem are affected by atmospheric CO2 enrichment. Environmental and Experimental Botany 46: 35-69. Pritchard, S.G., Rogers, H.H., Davis, M.A., Van Santen, E., Prior,
S.A. and Schlesinger, W.H. 2001b. The influence of elevated atmospheric CO2 on fine root dynamics in an intact temperate forest. Global Change Biology 7: 829-837. Uselman, S.M., Qualls,
R.G. and Thomas, R.B. 2000. Effects of increased atmospheric CO2, temperature, and soil N availability on root exudation of dissolved organic carbon by a N-fixing tree (Robinia pseudoacacia
L.). Plant and Soil 222: 191-202. Wiemken, V., Ineichen, K. and Boller, T. 2001. Development of ectomycorrhizas in model beech-spruce ecosystems on siliceous and calcareous soil: a 4-year
experiment with atmospheric CO2 enrichment and nitrogen fertilization. Plant and Soil 234: 99-108. 7.8.6. Seeds Elevated CO2 levels are known to have effects on seeds that are different
from their effects on total biomass, roots, and other dimensions examined so far. In this section we survey the scientific literature on this topic regarding crops, grasslands, and trees.
Additional information on this topic, including reviews on seeds not discussed here, can be found at http://www.co2science.org/subject/s/subject_s.php under the heading Seeds. 7.8.6.1.
Crops In a greenhouse study of the various components of seed biomass production, Palta and Ludwig (2000) grew narrow-leafed lupin in pots filled with soil within Mylar-film tunnels
maintained at either 355 or 700 ppm CO2. They found that the extra CO2 increased (1) the final number of pods and (2) the number of pods that filled large seeds, while it (3) reduced
to zero the number of pods that had small seeds, (4) reduced the number of pods with unfilled seeds from 16 to 1 pod per plant, and increased (5) pod set and (6) dry matter accumulation
on the developing branches. These several CO2-induced improvements to key physiological processes resulted in 47 to 56 percent increases in dry matter per plant, which led to increases
of 44 to 66 percent in seed yield per plant. Sanhewe et al. (1996) grew winter wheat in polyethylene tunnels maintained at atmospheric CO2 concentrations of 380 and 680 ppm from the
time of
Biological Effects of Carbon Dioxide Enrichment 531 seed germination to the time of plant maturity, while maintaining a temperature gradient of approximately 4°C in each tunnel. In addition
to the elevated CO2 increasing seed yield per unit area, they found it also increased seed weight, but not seed survival or germination. Increasing air temperature, on the other hand,
increased seed longevity across the entire range of temperatures investigated (14 to 19°C). Thomas et al. (2003) grew soybean plants to maturity in sunlit controlled-environment chambers
under sinusoidally varying day/night-max/min temperatures of 28/18, 32/22, 36/26, 40/30 and 44/34°C and two levels of atmospheric CO2 concentration (350 and 700 ppm). They determined,
in their words, that the effect of temperature on seed composition and gene expression was “pronounced,” but that “there was no effect of CO2.” In this regard, however, they note that
“Heagle et al. (1998) observed a positive significant effect of CO2 enrichment on soybean seed oil and oleic acid concentration,” the latter of which parameters Thomas et al. found to
increase with rising temperature all the way from 28/18 to 44/34°C. In another soybean study, Ziska et al. (2001) grew one modern and eight ancestral genotypes in glasshouses maintained
at atmospheric CO2 concentrations of 400 and 710 ppm, finding that the extra CO2 increased photosynthetic rates by an average of 75 percent. This enhancement in photosynthetic sugar
production led to increases in seed yield that averaged 40 percent for all cultivars, except for one ancestral variety that exhibited an 80 percent increase in seed yield. Hence, if
plant breeders were to utilize the highly CO2-responsive ancestral cultivar identified in this study in their breeding programs, it is possible that soybean seed yields could be made
to rise even faster and higher in the days and years ahead. Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2scien
e.org/subject/s/seedscrops.php. References Heagle, A.S., Miller, J.E. and Pursley, W.A. 1998. Influence of ozone stress on soybean response to carbon dioxide enrichment: III. Yield and
seed quality. Crop Science 38: 128-134. Palta, J.A. and Ludwig, C. 2000. Elevated CO2 during pod filling increased seed yield but not harvest index in indeterminate narrow-leafed lupin.
Australian Journal of Agricultural Research 51: 279-286. Sanhewe, A.J., Ellis, R.H., Hong, T.D., Wheeler, T.R., Batts, G.R., Hadley, P. and Morison, J.I.L. 1996. The effect of temperature
and CO2 on seed quality development in wheat (Triticum aestivum L.). Journal of Experimental Botany 47: 631-637. Thomas, J.M.G., Boote, K.J., Allen Jr., L.H., Gallo-Meagher, M. and Davis,
J.M. 2003. Elevated temperature and carbon dioxide effects on soybean seed composition and transcript abundance. Crop Science 43: 1548-1557. Ziska, L.H., Bunce, J.A. and Caulfield, F.A.
2001. Rising atmospheric carbon dioxide and seed yields of soybean genotypes. Crop Science 41: 385-391. 7.8.6.2. Grasslands Steinger et al. (2000) collected seeds from Bromus erectus
plants that had been grown at atmospheric CO2 concentrations of 360 and 650 ppm and germinated some of both groups of seeds under those same two sets of conditions. They found the elevated
CO2 treatment increased individual seed mass by about 9 percent and increased seed carbon-to-nitrogen ratio by almost 10 percent. However, they also learned that these changes in seed
properties had little impact on subsequent seedling growth. In fact, when the seeds produced by ambient or CO2-enriched plants were germinated and grown in ambient air, there was no
significant size difference between the two groups of resultant seedlings after a period of 19 days. Likewise, when the seeds produced from ambient or CO2-enriched plants were germinated
and grown in the high CO2 treatment, there was also no significant difference between the sizes of the seedlings derived from the two groups of seeds. However, the CO2-enriched seedlings
produced from both groups of seeds were almost 20 percent larger than the seedlings produced from both groups of seeds grown in ambient air, demonstrating that the direct effects of
elevated atmospheric CO2 concentration on seedling growth and development were more important than the differences in seed characteristics produced by the elevated atmospheric CO2 concentration
in which their parent plants grew. In another study conducted about the same time, Edwards et al. (2001) utilized a FACE experiment where daytime atmospheric CO2 concentrations above
a sheep-grazed pasture in New Zealand were increased by 115 ppm to study the effects of elevated CO2 on seed production, seedling recruitment and
Climate Change Reconsidered 532 species compositional changes. In the two years of their study, the extra daytime CO2 increased seed production and dispersal in seven of the eight most
abundant species, including the grasses Anthoxanthum odoratum, Lolium perenne and Poa pratensis, the legumes Trifolium repens and T. subterranean, and the herbs Hypochaeris radicata
and Leontodon saxatilis. In some of these plants, elevated CO2 increased the number of seeds per reproductive structure, while all of them exhibited CO2-induced increases in the number
of reproductive structures per unit of ground area. In addition, they determined that the CO2-induced increases in seed production contributed in a major way to the increase in the numbers
of species found within the CO2-enriched plots. In a five-year study of a nutrient-poor calcareous grassland in Switzerland, Thurig et al. (2003) used screen-aided CO2 control (SACC)
technology (Leadley et al., 1997) to enrich the air over half of their experimental plots with an extra 300 ppm of CO2, finding that “the effect of elevated CO2 on the number of flowering
shoots (+24%) and seeds (+29%) at the community level was similar to above ground biomass response.” In terms of species functional groups, there was a 42 percent increase in the mean
seed number of graminoids and a 33 percent increase in the mean seed number of forbs, but no change in legume seed numbers. In most species, mean seed weight also tended to be greater
in plants grown in CO2-enriched air (+12 percent); and Thurig et al. say it is known from many studies that heavier seeds result in seedlings that “are more robust than seedlings from
lighter seeds (Baskin and Baskin, 1998).” Wang and Griffin (2003) grew dioecious white cockle plants from seed to maturity in sand-filled pots maintained at optimum moisture and fertility
conditions in environmentally controlled growth chambers in which the air was continuously maintained at CO2 concentrations of either 365 or 730 ppm. In response to this doubling of
the air’s CO2 content, the vegetative mass of both male and female plants rose by approximately 39 percent. Reproductive mass, on the other hand, rose by 82 percent in male plants and
by 97 percent in females. In the female plants, this feat was accomplished, in part, by increases of 36 percent and 44 percent in the number and mass of seeds per plant, and by a 15
percent increase in the mass of individual seeds, in harmony with the findings of Jablonski et al. (2002), which they derived from a meta-analysis of the results of 159 CO2 enrichment
experiments conducted on 79 species of agricultural and wild plants. Because dioecious plants comprise nearly half of all angiosperm families, we may expect to see a greater proportion
of plant biomass allocated to reproduction in a high-CO2 world of the future, which result should bode well for the biodiversity of earth’s many ecosystems. Additional information on
this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/s/seedsgrasslands.php. References Baskin, C.C. and Baskin,
J.M. 1998. Seeds: Ecology, Biogeography, and Evolution of Dormancy and Germination. Academic Press, San Diego, CA. Edwards, G.R., Clark, H. and Newton, P.C.D. 2001. The effects of elevated
CO2 on seed production and seedling recruitment in a sheep-grazed pasture. Oecologia 127: 383-394. Jablonski, L.M., Wang, X. and Curtis, P.S. 2002. Plant reproduction under elevated
CO2 conditions: a metaanalysis of reports on 79 crop and wild species. New Phytologist 156: 9-26. Leadley, P.W., Niklaus, P.A., Stocker, R. et al. 1997. Screen-aided CO2 control (SACC):
a middle ground between FACE and open-top chambers. Acta Oecologica 18: 39-49. Steinger, T., Gall, R. and Schmid, B. 2000. Maternal and direct effects of elevated CO2 on seed provisioning,
germination and seedling growth in Bromus erectus. Oecologia 123: 475-480. Thurig, B., Korner, C. and Stocklin, J. 2003. Seed production and seed quality in a calcareous grassland in
elevated CO2. Global Change Biology 9: 873-884. Wang, X. and Griffin, K.L. 2003. Sex-specific physiological and growth responses to elevated atmospheric CO2 in Silene latifolia Poiret.
Global Change Biology 9: 612-618. 7.8.6.3. Trees How does enriching the air with carbon dioxide impact the reproductive capacity of trees? LaDeau and Clark (2001) determined the reproductive
response of loblolly pine trees to atmospheric CO2 enrichment at Duke Forest in the Piedmont region of
Biological Effects of Carbon Dioxide Enrichment 533 North Carolina, USA, where in August of 1996 three 30-m-diameter FACE rings began to enrich the air around the 13-year-old trees they
encircled to 200 ppm above the atmosphere’s normal background concentration, while three other FACE rings served as control plots. Because the
trees were not mature at the start of the experiment they did not produce any cones until a few rare ones appeared in 1998. By the fall of 1999, however, the two scientists found that,
compared to the trees growing in ambient air, the CO2-enriched trees were twice as likely to be reproductively mature, and they produced three times more cones per tree. Similarly, the
trees growing in the CO2-enriched air produced 2.4 times more cones in the fall of 2000; and from August 1999 through July 2000, they collected three times as many seeds in the CO2-fertilized
FACE rings as in the control rings. Also working on this aspect of the Duke Forest FACE study were Hussain et al. (2001), who report that (1) seeds collected from the CO2-enriched trees
were 91 percent heavier than those collected from the trees growing in ambient air, (2) the CO2-enriched seeds had a lipid content that was 265 percent greater than that of the seeds
produced on the ambienttreatment trees, (3) the germination success for seeds developed under atmospheric CO2 enrichment was more than three times greater than that observed for control
seeds developed at ambient CO2, regardless of germination CO2 concentration, (4) seeds from the CO2-enriched trees germinated approximately five days earlier than their ambiently produced
counterparts, again regardless of germination CO2 concentration, and (5) seedlings developing from seeds collected from CO2-enriched trees displayed significantly greater root lengths
and needle numbers than seedlings developing from trees exposed to ambient air, also regardless of growth CO2 concentration. The propensity for elevated levels of atmospheric CO2 to
hasten the production of more plentiful seeds seeds on the trees of this valuable timber species bodes well for naturally regenerating loblolly pine stands of the southeastern United
States, where LaDeau and Clark report the trees “are profoundly seed-limited for at least 25 years.” As the air’s CO2 content continues to climb, they conclude that “this period of seed
limitation may be reduced.” In addition, the observations of Hussain et al. suggest that loblolly pine trees in a CO2-enriched world of the future will likely display significant increases
in their photosynthetic rates. Enhanced carbohydrate supplies resulting from this phenomenon will likely be used to increase seed weight and lipid content. Such seeds should consequently
exhibit significant increases in germination success, and their enhanced lipid supplies will likely lead to greater root lengths and needle numbers in developing seedlings. Consequently,
when CO2-enriched loblolly pine seedlings become photosynthetically active, they will likely produce biomass at greater rates than those those exhibited by seedlings growing under current
CO2 concentrations. Five years later, LaDeau and Clark (2006a) conducted a follow-up study that revealed “carbon dioxide enrichment affected mean cone production both through early maturation
and increased fecundity,” so that “trees in the elevated CO2 plots produced twice as many cones between 1998 and 2004 as trees in the ambient plots.” They also determined that the trees
grown in elevated CO2 “made the transition to reproductive maturation at smaller [trunk] diameters,” and that they “not only reached reproductive maturation at smaller diameters, but
also at younger ages.” By 2004, for example, they say that “roughly 50% of ambient trees and 75% of fumigated trees [had] produced cones.” In addition, they observed that “22% of the
trees in high CO2 produced between 40 and 100 cones during the study, compared with only 9% of ambient trees.” “In this 8-year study,” in the words of the two researchers, “we find that
previous short-term responses indeed persist.” In addition, they note that “P. taeda trees that produce large seed crops early in their life span tend to continue to be prolific producers
(Schutlz, 1997),” and they conclude that this fact, together with their findings, suggests that “individual responses seen in this young forest may be sustained over their life span.”
In a concurrent report, LaDeau and Clark (2006b) analyzed the seed and pollen responses of the loblolly pines to atmospheric CO2 enrichment, finding that the “trees grown in high-CO2
plots first began producing pollen while younger and at smaller sizes relative to ambient-grown trees,” and that cone pollen and airborne pollen grain abundances were significantly greater
in the CO2-enriched stands. More specifically, they found that “by spring 2005, 63% of all trees growing in high CO2 had produced both pollen and seeds vs. only 36% of trees in the ambient
plots.” The researchers say precocious pollen production “could enhance the production of viable seeds by increasing the percentage percentage of fertilized ovules,” and that “more pollen
disseminated from multiple-source trees may also increase rates of gene flow among stands, and could further reduce rates of self-pollination,
Climate Change Reconsidered 534 indirectly enhancing the production of viable seeds.” They also say “pine pollen is not a dangerous allergen for the public at large.” Another major study
of the reproductive responses of trees to elevated levels of atmospheric CO2 was conducted at the Kennedy Space Center, Florida, USA, where in 1996 three species of scruboak (Quercus
myrtifolia, Q. chapmanii, and Q. geminata) were enclosed within sixteen open-top chambers, half of which were maintained at 379 ppm CO2 and half at 704 ppm. Five years later—in August,
September and October of 2001—Stiling et al. (2004) counted the numbers of acorns on randomly selected twigs of each species, while in November of that year they counted the numbers
of fallen acorns of each species within equal-size quadrates of ground area, additionally evaluating mean acorn weight, acorn germination rate, and degree of acorn infestation by weevils.
They found acorn germination rate and degree of predation by weevils were unaffected by elevated CO2, while acorn size was enhanced by a small amount: 3.6 percent for Q. myrtifolia,
7.0 percent for Q. chapmanii, and 7.7 percent for Q. geminata. Acorn number responses, on the other hand, were enormous, but for only two of the three species, as Q. geminata did not
register any CO2-induced increase in reproductive output, in harmony with its unresponsive overall growth rate. For Q. myrtifolia, however, Stiling et al. report “there were four times
as many acorns per 100 twigs in elevated CO2 as in ambient CO2 and for Q. chapmanii the increase was over threefold.” On the ground, the enhancement was greater still, with the researchers
reporting that “the number of Q. myrtifolia acorns per meter squared in elevated CO2 was over seven times greater than in ambient CO2 and for Q. chapmanii, the increase was nearly sixfold.”
Stiling et al. say these results lead them to believe “there will be large increases in seedling production in scrub-oak forests in an atmosphere of elevated CO2,” noting that “this
is important because many forest systems are ‘recruitment-limited’ (Ribbens et al., 1994; Hubbell et al., 1999),” which conclusion echoes that of LaDeau and Clark with respect to loblolly
pines. A third major study of CO2 effects on seed production in trees has been conducted at the FACE facility near Rhinelander, Wisconsin (USA), where young paper birch (Betula papyrifera
Marsh.) seedlings were planted in 1997 and have been growing since 1998 in open-top chambers maintained at atmospheric CO2 concentrations of either 360 or 560 ppm, as well as at atmospheric
ozone (O3) concentrations of either ambient or 1.5 times ambient. There, Darbah et al. (2007) collected many types of data pertaining to flowering, seed production, seed germination
and new seedling growth and development over the 2004-2006 growing seasons; and as they describe it, “elevated CO2 had significant positive effect[s] on birch catkin size, weight, and
germination success rate.” More specifically, they note that “elevated CO2 increased germination rate of birch by 110%, compared to ambient CO2 concentrations, decreased seedling mortality
by 73%, increased seed weight by 17% [and] increased [new seedling] root length by 59%.” In conclusion, research on a variety of tree species finds that CO2 enhancement increases the
production of viable seeds and seedlings, meaning these species should flourish as CO2 levels conntinue to rise. Additional information on this topic, including reviews of newer publications
as they become available, can be found at http://www.co2science.org/subject/s/seedstrees.php. References Darbah, J.N.T., Kubiske, M.E., Nelson, N., Oksanen, E., Vaapavuori, E. and Karnosky,
D.F. 2007. Impacts of elevated atmospheric CO2 and O3 on paper Birch (Betula papyrifera): Reproductive fitness. The Scientific World JOURNAL 7(S1): 240-246. Hubbell, S.P., Foster, R.B.,
O’Brien, S.T., Harms, K.E., Condit, R., Wechsler, B., Wright, S.J. and Loo de Lao, S. 1999. Light-gap disturbances, recruitment limitation, and tree diversity in a neotropical forest.
Science 283: 554-557. Hussain, M., Kubiske, M.E. and Connor, K.F. 2001. Germination of CO2-enriched Pinus taeda L. seeds and subsequent seedling growth responses to CO2 enrichment. Functional
Ecology 15: 344-350. LaDeau, S.L. and Clark, J.S. 2001. Rising CO2 levels and the fecundity of forest trees. Science 292: 95-98. LaDeau, S.L. and Clark, J.S. 2006a. Elevated CO2 and
tree fecundity: the role of tree size, interannual variability, and population heterogeneity. Global Change Biology 12: 822-833. LaDeau, S.L. and Clark, J.S. 2006b. Pollen production
by Pinus taeda growing in elevated atmospheric CO2. Functional Ecology 20: 541-547. Ribbens, E., Silander, J.A. and Pacala, S.W. 1994. Seedling recruitment in forests: calibrating models
to predict
Biological Effects of Carbon Dioxide Enrichment 535 patterns of tree seedling dispersion. Ecology 75: 1794-1806. Schutlz, R.P. 1997. Loblolly Pine—The Ecology and Culture of Loblolly
Pine (Pinus taeda L.). USDA Forest Service Agricultural Handbook 713. USDA Forest Service, Washington, DC, USA. Stiling, P., Moon, D., Hymus, G. and Drake, B. 2004. Differential effects
of elevated CO2 on acorn density, weight, germination, and predation among three oak species in a scrub-oak forest. Global Change Biology 10: 228-232. 7.8.7. Tannins Condensed tannins
are naturally occurring secondary carbon compounds produced in the leaves of a number of different plants that often act to deter herbivorous insects. How do condensed tannin concentrations
in the leaves and roots of trees respond to atmospheric CO2 enrichment? Additional information on this topic, including reviews on tannins not discussed here, can be found at http://www.co2science.or
/subject/t/subject_t.php under the heading Tannins. 7.8.7.1. Aspen Trees King et al. (2001) grew aspen seedlings for five months in open-top chambers maintained at atmospheric CO2 concentrations
of either 350 or 700 ppm. At the end of this period, naturally senesced leaf litter was collected and analyzed; and it was found that the elevated CO2 of this particular study had no
effect on leaf litter tannin concentration. A substantially different result was obtained in an earlier study of aspen leaves that was conducted by McDonald et al. (1999), who grew aspen
seedlings in controlled environment greenhouses that were maintained at either ambient (387 ppm) or elevated (696 ppm) CO2 concentrations under conditions of either low or high light
availability (half and full sunlight, respectively) for 31 days after the mean date of bud break. In this case it was determined that under low light conditions, the CO2-enriched seedlings
exhibited an increase of approximately 15 percent in leaf condensed tannin concentration, while under high light conditions the CO2-induced increase in leaf condensed tannin concentration
was 175 percent. In a much more complex study than either of the two preceding ones, Agrell et al. (2005) examined the effects of ambient and elevated concentrations of atmospheric CO2
(360 ppm and 560 ppm, respectively) and O3 (35-60 ppb and 52-90 ppb, respectively) on the foliar chemistry of more mature aspen trees of two different genotypes (216 and 259) growing
out-of-doors at the Aspen Free Air CO2 Enrichment (FACE) facility near Rhinelander, Wisconsin, USA, as well as the impacts of these effects on the host plant preferences of forest tent
caterpillar larvae. In reporting the results of the study, Agrell et al. say that “the only chemical component showing a somewhat consistent covariation with larval preferences was condensed
tannins,” noting that “the tree becoming relatively less preferred as a result of CO2 or O3 treatment was in general also the one for which average levels of condensed tannins were most
positively (or least negatively) affected by that treatment.” The mean mean condensed tannin concentrations of the aspen 216 and 259 genotypes were 25 percent and 57 percent higher,
respectively, under the elevated CO2 and O3 combination treatment compared to the ambient CO2 and O3 combination treatment. In light of these findings, it is logical to presume that
as atmospheric concentrations of CO2 and O3 continue to rise, the increase in condensed tannin concentration likely to occur in the foliage of aspen trees should lead to their leaves
becoming less preferred for consumption by the forest tent caterpillar. Additional information on this topic, including reviews of newer publications as they become available, can be
found at http://www.co2science.org/subject/t/tanninsaspen.php. References Agrell, J., Kopper, B., McDonald, E.P. and Lindroth, R.L. 2005. CO2 and O3 effects on host plant preferences
of the forest tent caterpillar (Malacosoma disstria). Global Change Biology 11: 588-599. King, J.S., Pregitzer, K.S., Zak, D.R., Kubiske, M.E., Ashby, J.A. and Holmes, W.E. 2001. Chemistry
and decomposition of litter from Populus tremuloides Michaux grown at elevated atmospheric CO2 and varying N availability. Global Change Biology 7: 65-74. McDonald, E.P., Agrell, J.,
and Lindroth, R.L. 1999. CO2 and light effects on deciduous trees: growth, foliar chemistry, and insect performance. Oecologia 119: 389-399.
Climate Change Reconsidered 536 7.8.7.2. Birch Trees How do condensed tannin concentrations in the leaves and roots of paper birch (Betula papyrifera Marsh.) and silver birch (Betula
pendula Roth) trees respond to atmospheric CO2 enrichment with and without concomitant increases in atmospheric temperature and ozone concentrations? We here briefly summarize the findings
of several studies that have broached one or more parts of this question. McDonald et al. (1999) grew paper birch seedlings in controlled environment greenhouses that were maintained
at either ambient (387 ppm) or elevated (696 ppm) CO2 concentrations under conditions of either low or high light availability (half and full sunlight, respectively) for 31 days after
the mean date of bud break. In doing so, they determined that under low light conditions the CO2-enriched seedlings exhibited an increase of approximately 15 percent in leaf condensed
tannin concentration, while under high light conditions the CO2-induced tannin increase was a whopping 175 percent. Peltonen et al. (2005) studied the impacts of doubled atmospheric
CO2 and O3 concentrations on the accumulation of 27 phenolic compounds, including soluble condensed tannins, in the leaves of two European silver birch clones in seven-year-old soil-grown
trees that were exposed in open-top chambers for three growing seasons to ambient and twice-ambient atmospheric CO2 and O3 concentrations singly and in combination. This work, which
was carried out in central Finland, revealed that elevated CO2 increased the concentration of soluble condensed tannins in the leaves of the trees by 19 percent. In addition, they found
that the elevated CO2 protected the leaves from elevated O3 because, as they describe it, “all the O3-derived effects on the leaf phenolics and traits were prevented by elevated CO2.”
Kuokkanen et al. (2003) grew two-year-old silver birch seedlings in ambient air of 350 ppm CO2 or air enriched to a CO2 concentration of 700 ppm under conditions of either ambient temperature
or ambient temperature plus 3°C for one full growing season in the field in closed-top chambers at the Mekrijarvi Research Station of the University of Joensuu in eastern Finland. Then,
during the middle of the summer, when carbon-based secondary compounds of birch leaves are fairly stable, they picked several leaves from each tree and determined their condensed tannin
concentrations, along with the concentrations of a number of other physiologically important substances. This work revealed that the concentration of total phenolics, condensed tannins
and their derivatives significantly increased in the leaves produced in the CO2-enriched air, as has also been observed by Lavola and Julkunen-Titto (1994), Williams et al. (1994), Kinney
et al. (1997), Bezemer and Jones (1998) and Kuokkanen et al. (2001). In fact, the extra 350 ppm of CO2 nearly tripled condensed tannin concentrations in the ambienttemperature air, while
it increased their concentrations in the elevated-temperature air by a factor in excess of 3.5. In a study of roots, Parsons et al. (2003) grew two-year-old paper birch saplings in well-watered
and fertilized 16-L pots from early May until late August in glasshouse rooms maintained at either 400 or 700 ppm CO2. This procedure revealed that the concentration of condensed tannins
in the fine roots of the saplings was increased by 27 percent in the CO2-enriched treatment; and in regard to this finding, the researchers say “the higher condensed tannin concentrations
that were present in the birch fine roots may offer these tissues greater protection against soil-borne pathogens and herbivores.” Parsons et al. (2004) collected leaf litter samples
from early September to mid-October beneath paper birch trees growing in ambient and CO2-enriched (to 200 ppm above ambient) FACE plots in northern Wisconsin, USA, which were also maintained
under ambient and O3-enriched (to 19 ppb above ambient) conditions, after which the leaf mass produced in each treatment was determined, sub-samples of the leaves were assessed for a
number of chemical constituents. The researchers learned that condensed tannin concentrations were 64 percent greater in the CO2-enriched plots. Under CO2-and O3-enriched conditions,
condensed tannin concentrations were 99 percent greater. In conclusion, it appears that elevated concentrations of atmospheric CO2 tend to increase leaf and fine-root tannin concentrations
of birch trees, and that this phenomenon tends to protect the trees’ foliage from predation by voracious insect herbivores and protect the trees’ roots from soil-borne pathogens and
herbivores. Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/t/tanninsbirch.php.
Biological Effects of Carbon Dioxide Enrichment 537 References Bezemer, T.M. and Jones, T.H. 1998. Plant-insect herbivore interactions in elevated atmospheric CO2, quantitative analyses
and guild effects. Oikos 82: 212-222. Kinney, K.K., Lindroth, R.L., Jung, S.M. and Nordheim, E.V. 1997. Effects of CO2 and NO3 availability on deciduous trees, phytochemistry and insect
performance. Ecology 78: 215-230. Kuokkanen, K., Julkunen-Titto, R., Keinanen, M., Niemela, P. and Tahvanainen, J. 2001. The effect of elevated CO2 and temperature on the secondary chemistry
of Betula pendula seedlings. Trees 15: 378-384. Kuokkanen, K., Yan, S. and Niemela, P. 2003. Effects of elevated CO2 and temperature on the leaf chemistry of birch Betula pendula (Roth)
and the feeding behavior of the weevil Phyllobius maculicornis. Agricultural and Forest Entomology 5: 209-217. Lavola, A. and Julkunen-Titto, R. 1994. The effect of elevated carbon dioxide
and fertilization on primary and secondary metabolites in birch, Betula pendula (Roth). Oecologia 99: 315-321. McDonald, E.P., Agrell, J., and Lindroth, R.L. 1999. CO2 and light effects
on deciduous trees: growth, foliar chemistry, and insect performance. Oecologia 119: 389-399. Parsons, W.F.J., Kopper, B.J. and Lindroth, R.L. 2003. Altered growth and fine root chemistry
of Betula papyrifera and Acer saccharum under elevated CO2. Canadian Journal of Forest Research 33: 842-846. Parsons, W.F.J., Lindroth, R.L. and Bockheim, J.G. 2004. Decomposition of
Betula papyrifera leaf litter under the independent and interactive effects of elevated CO2 and O3. Global Change Biology 10: 1666-1677. Peltonen, P.A., Vapaavuori, E. and Julkunen-Tiitto,
R. 2005. Accumulation of phenolic compounds in birch leaves is changed by elevated carbon dioxide and ozone. Global Change Biology 11: 1305-1324. Williams, R.S., Lincoln, D.E. and Thomas,
R.B. 1994. Loblolly pine grown under elevated CO2 affects early instar pine sawfly performance. Oecologia 98: 64-71. 7.8.7.3. Oak Trees Dury et al. (1998) grew four-year-old old pedunculate
oak trees (Quercus robur L.) in pots within greenhouses maintained at ambient and twice-ambient atmospheric CO2 concentrations in combination with ambient and elevated (ambient plus
3°C) air temperatures for approximately one year. This work revealed that elevated CO2 had only minor and contrasting direct effects on leaf palatability: a temporary increase in foliar
phenolic concentrations and decreases in leaf toughness and nitrogen content. The elevated temperature treatment, on the other hand, significantly reduced leaf palatability, because
oak leaf toughness increased as a consequence of temperature-induced increases in condensed tannin concentrations. As a result, the researchers concluded that “a 3°C rise in temperature
might be expected to result in prolonged larval development, increased food consumption, and reduced growth” for herbivores feeding on oak leaves in a CO2-enriched and warmer world of
the future. Cornelissen et al. (2003) studied fluctuating asymmetry in the leaves of two species of schlerophyllous oaks—myrtle oak (Quercus myrtifolia) and sand live oak (Quercus geminata)—
that dominate a native scrub-oak community at the Kennedy Space Center, Titusville, Florida (USA). Fluctuating asymmetry is the term used to describe small variations from perfect symmetry
in otherwise bilaterally symmetrical characters in an organism (Moller and Swaddle, 1997), which asymmetry is believed to arise as a consequence of developmental instabilities experienced
during ontogeny that may be caused by various stresses, including both genetic and environmental factors (Moller and Shykoff, 1999). Based on measurements of (1) distances from the leaf
midrib to the left and right edges of the leaf at its widest point and (2) leaf areas on the left and right sides of the leaf midrib, Cornelissen et al. determined that “asymmetric leaves
were less frequent in elevated CO2, and, when encountered, they were less asymmetric than leaves growing under ambient CO2.” In addition, they found that “Q. myrtifolia leaves under
elevated CO2 were 15.0% larger than in ambient CO2 and Q. geminata leaves were 38.0 percent larger in elevated CO2 conditions.” They also determined that “elevated CO2 significantly
increased tannin concentration for both Q. myrtifolia and Q. geminata leaves” and that “asymmetric leaves contained significantly lower concentrations of tannins than symmetric leaves
for both Q. geminata and Q. myrtifolia.” Specifically, they found induced incfreases in tanning concentrations of approximately 35 percent for Q. myrtifolia and 43 percent for Q. geminata.
In commenting on their primary findings of reduced percentages of leaves experiencing asymmetry in the presence of elevated levels of atmospheric CO2 and the lesser degree of asymmetry
Climate Change Reconsidered 538 exhibited by affected leaves in the elevated CO2 treatment, Cornelissen et al. say that “a possible explanation for this pattern is the fact that, by
contrast to other environmental stresses, which can cause negative effects on plant growth, the predominant effect of elevated CO2 on plants is to promote growth with consequent reallocation
of resources (Docherty et al., 1996).” Another possibility they discuss “is the fact that CO2 acts as a plant fertilizer,” and, as a result, that “elevated CO2 ameliorates plant stress
compared with ambient levels of CO2.” In a subsequent study conducted at the Kennedy Space Center’s scrub-oak community, Hall et al. (2005b) evaluated foliar quality and herbivore damage
in three oaks (Q. myrtifolia, Q. chapmanii and Q. geminata) plus the nitrogen-fixing legume Galactia elliottii at three-month intervals from May 2001 to May 2003, at which times samples
of undamaged leaves were removed from each of the four species in all chambers and analyzed analyzed for various chemical constituents, while 200 randomly selected leaves of each species
in each chamber were scored for the presence of six types of herbivore damage. Analyses of the data thereby obtained indicated that for condensed tannins, hydrolyzable tannins, total
phenolics and lignin, in all four species there were always greater concentrations of all four leaf constituents in the CO2-enriched leaves, with acrossspecies mean increases of 6.8
percent for condensed tannins, 6.1 percent for hydrolyzable tannins, 5.1 percent for total phenolics and 4.3 percent for lignin. In addition, there were large CO2-induced decreases in
all leaf damage categories among all species: chewing (-48 percent), mines (-37 percent), eye spot gall (-45 percent), leaf tier (-52 percent), leaf mite (-23 percent) and leaf gall
(-16 percent). Hall et al. thus concluded that the changes they observed in leaf chemical constituents and herbivore damage “suggest that damage to plants may decline as atmospheric
CO2 levels continue to rise.” Last, and largely overlapping the investigation of Hall et al. (2005b), was the study of Hall et al. (2005a), who evaluated the effects of the Kennedy Space
Center experiment’s extra 350 ppm of CO2 on litter quality, herbivore activity, and their interactions, over the three-year-period 2000-2002. This endeavor indicated, in their words,
that “changes in litter chemistry from year to year were far larger than effects of CO2 or insect damage, suggesting that these may have only minor effects on litter decomposition.”
The one exception to this finding was that “condensed tannin concentrations increased under elevated CO2 regardless of species, herbivore damage, or growing season,” rising by 11 percent
in 2000, 18 percent in 2001 and 41 percent in 2002 as a result of atmospheric CO2 enrichment, as best we can determine from the researchers’ bar graphs. Also, the five scientists report
that “lepidopteran larvae can exhibit slower growth rates when feeding on elevated CO2 plants (Fajer et al., 1991) and become more susceptible to pathogens, parasitoids, and predators
(Lindroth, 1996; Stiling et al., 1999),” noting further that at their field site, “which hosts the longest continuous study of the effects of elevated CO2 on insects, herbivore populations
decline markedly under elevated CO2 (Stiling et al., 1999, 2002, 2003; Hall et al., 2005b).” In conclusion, it would appear CO2-enriched air produces a large and continuous enhancement
of condensed tannin concentrations in oak tree foliage, which causes marked declines in herbivore populations observed in CO2-enriched open-topchamber studies. Additional information
on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/t/tanninsoak.php. References Cornelissen, T., Stiling,
P. and Drake, B. 2003. Elevated CO2 decreases leaf fluctuating asymmetry and herbivory by leaf miners on two oak species. Global Change Biology 10: 27-36. Docherty, M., Hurst, D.K.,
Holopainem, J.K. et al. 1996. Carbon dioxide-induced changes in beech foliage cause female beech weevil larvae to feed in a compensatory manner. Global Change Biology 2: 335-341. Dury,
S.J., Good, J.E.G., Perrins, C.M., Buse, A. and Kaye, T. 1998. The effects of increasing CO2 and temperature on oak leaf palatability and the implications for herbivorous insects. Global
Change Biology 4: 55-61. Fajer, E.D., Bowers, M.D. and Bazzaz, F.A. 1991. The effects of enriched CO2 atmospheres on the buckeye butterfly, Junonia coenia. Ecology 72: 751-754. Hall,
M.C., Stiling, P., Hungate, B.A., Drake, B.G. and Hunter, M.D. 2005a. Effects of elevated CO2 and herbivore damage on litter quality in a scrub oak ecosystem. Journal of Chemical
Ecology 31: 2343-2356. Hall, M.C., Stiling, P., Moon, D.C., Drake, B.G. and Hunter, M.D. 2005b. Effects of elevated CO2 on foliar
Biological Effects of Carbon Dioxide Enrichment 539 quality and herbivore damage in a scrub oak ecosystem. Journal of Chemical Ecology 31: 267-285. Lindroth, R.L. 1996. CO2-mediated
changes in tree chemistry and tree-Lepidoptera interactions. In: Koch, G.W. and Mooney, H,A. (Eds.). Carbon Dioxide and Terrestrial Ecosystems. Academic Press, San Diego, California,
USA, pp. 105-120. Moller, A.P. and Shykoff, P. 1999. Morphological developmental stability in plants: patterns and causes. International Journal of Plant Sciences 160: S135-S146. Moller,
A.P. and Swaddle, J.P. 1997. Asymmetry, Developmental Stability and Evolution. Oxford University Press, Oxford, UK. Stiling, P., Cattell, M., Moon, D.C., Rossi, A., Hungate, B.A., Hymus,
G. and Drake, B.G. 2002. Elevated atmospheric CO2 lowers herbivore abundance, but increases leaf abscission rates. Global Change Biology 8: 658-667. Stiling, P., Moon, D.C., Hunter,
M.D., Colson, J., Rossi, A.M., Hymus, G.J. and Drake, B.G. 2003. Elevated CO2 lowers relative and absolute herbivore density across all species of a scrub-oak forest. Oecologia 134:
82-87. Stiling, P., Rossi, A.M., Hungate, B., Dijkstra, P., Hinkle, C.R., Knot III, W.M., and Drake, B. 1999. Decreased leafminer abundance in elevated CO2: Reduced leaf quality and
increased parasitoid attack. Ecological Applications 9: 240-244. 7.8.8. Transgenic Plants Toxins produced by Bacillus thuringiensis (Bt) supplied to crops via foliar application have
been used as a means of combating crop pests for well over half a century. More recently, the Bt gene for producing the toxin has been artificially inserted in some species of plants,
producing transgenic plants that are pest resistant. The effectiveness of this management technique depends primarily upon the amount of Bt-produced toxins that are ingested by targeted
insects. Another kind of transgenic plant is wheat that has been made heat resistant by the introduction into its gene code of heat shock protein (HSP) or plastidial EF-Tu (protein synthesis
elongation factor). How does atmospheric CO2 enrichment affect transgenic plants? If soil nitrogen levels are low, foliar nitrogen concentrations in plants grown in enhanced CO2 environments
are generally reduced from what they are at the current atmospheric CO2 concentration, which suggests that insects would have to eat more foliage to get their normal requirement of nitrogen
for proper growth and development in CO2-enriched air. But by eating more foliage, the insects would also ingest more Bt-produced toxins, and they would be more severely impacted by
those substances. To test this hypothesis, Coviella and Trumble (2000) grew cotton plants in each of six Teflon-film chambers in a temperature-controlled greenhouse, where three of the
chambers were maintained at an atmospheric CO2 concentration of 370 ppm, and three were maintained at 900 ppm CO2. In addition, half of the plants in each chamber received high levels
of nitrogen (N) fertilization, while half received low levels (30 vs. 130 mg N/kg soil/week). After 45 days days of growth under these conditions, leaves were removed from the plants
and dipped in a Bt solution, after which known amounts of treated leaf material were fed to Spodoptera exigua larvae and their responses measured and analyzed. By these means, the two
researchers determined that the plants grown in the elevated CO2 chambers did indeed have significantly lower foliar nitrogen concentrations than the plants grown in the ambient CO2
chambers under the low N fertilization regime; but this was not the case under the high N regime. They also discovered that older larvae fed with foliage grown in elevated CO2 with low
N fertilization consumed significantly more plant material than insects fed with foliage grown in ambient CO2; but, again, no differences were observed with high N fertilization. Last,
and “consistent with the effect of higher Bt toxin intake due to enhanced consumption,” they found that “insects fed on low N plants had significantly higher mortality in elevated CO2.”
Yet, again, no such effect was evident in the high N treatment. Consequently, with respect to pest management using Bt-produced toxins supplied to crops via foliar application, Coviella
and Trumble concluded that “increasing atmospheric CO2 is making the foliar applications more efficacious.” Coviella et al. (2000), in an analogous experiment to that of Coviella and
Trumble, grew cotton plants in 12 Teflon-film chambers in a temperature-controlled greenhouse, where six chambers were maintained at an atmospheric CO2 concentration of 370 ppm and six
were maintained at 900 ppm CO2. Half of the cotton plants in each of these chambers were of a transgenic line containing the Bt gene for the production of the Cry1Ac toxin, which is
mildly toxic for Spodoptera exigua, while the other half were of a near isogenic line without the Bt gene. In addition, and as before,
Climate Change Reconsidered 540 half of the plants in each chamber received the same low and high levels of N fertilization; and between 40 and 45 days after emergence, leaves were removed
from the plants and fed to the S. exigua larvae, after which a number of larval responses were measured and analyzed, along with various leaf properties. This work revealed that the
low-N plants in the elevated CO2 treatment had lower foliar N concentrations than did the low-N plants in the ambient CO2 treatment, and that the transgenic plants from the low-N, high
CO2 treatment produced lower levels of Bt toxin than did the transgenic plants from the low-N, ambient CO2 treatment. In addition, the high level of N fertilization only partially compensated
for this latter high-CO2 effect. In the ambient CO2 treatment there was also a significant increase in days to pupation for insects fed transgenic plants; but this difference was not
evident in elevated CO2. In addition, pupal weight in ambient CO2 was significantly higher in non-transgenic plants; and, again, this difference was not observed in elevated CO2. In
discussing their findings, the three researchers wrote that “these results support the hypothesis that the lower N content per unit of plant tissue caused by the elevated CO2 will result
in lower toxin production by transgenic plants when nitrogen supply to the plants is a limiting factor.” They also note that “elevated CO2 appears to eliminate differences between transgenic
and non-transgenic plants for some key insect developmental/fitness variables including length of the larval stage and pupal weight.” These findings suggest that in the case of inadvertent
Bt gene transference to wild relatives of transgenic crop lines, elevated levels of atmospheric CO2 will tend to negate certain of the negative effects the wayward genes might otherwise
inflict on the natural world. Hence, the ongoing rise in the air’s CO2 content could be said to constitute an “insurance policy” against this potential outcome. On the other hand, hand,
Coviella et al.’s results also suggest that transgenic crops designed to produce Bttype toxins may become less effective in carrying out the objectives of their design as the air’s CO2
content continues to rise. Coupling this possibility with the fact that the foliar application of Bacillus thuringiensis to crops should become even more effective in a higher-CO2 world,
as found by Coviella and Trumble, one could argue that the implantation of toxin-producing genes in crops is not the way to go in the face of the ongoing rise in the air’s CO2 content,
which reduces that technique’s effectiveness at the same time that it increases the effectiveness of direct foliar applications. In a study of three different types of rice—a wild type
(WT) and two transgenic varieties, one with 65 percent wild-type Rubisco (AS-77) and one with 40 percent wild-type Rubisco (AS-71)—Makino et al. (2000) grew plants from seed for 70 days
in growth chambers maintained at 360 and 1000 ppm CO2, after which they harvested the plants and determined their biomass. In doing so, they found that the mean dry weights of the WT,
AS-77 and AS-71 varieties grown in air of 360 ppm were, respectively, 5.75, 3.02 and 0.83 g, while in air of 1000 ppm CO2, corresponding mean dry weights were 7.90, 7.40 and 5.65 g.
Consequently, although the growth rates of the genetically engineered rice plants were inferior to that of the wild type when grown in normal air of 360 ppm CO2 (with AS-71 producing
less than 15 percent as much biomass as the wild type), when grown in air of 1000 ppm CO2 they experienced greater CO2-induced increases in growth: a 145 percent increase in the case
of AS-77 and a 581 percent increase in the case of AS-71. Hence, whereas the transgenic plants were highly disadvantaged in normal air of 360 ppm CO2 (with AS-71 plants attaining a mean
dry weight of only 0.83 g while the WT plants attained a mean dry weight of 5.75 g), they were found to be pretty much on an equal footing in highly CO2-enriched air (with AS-71 plants
attaining a mean dry weight of 5.65 g while the WT plants attained a mean dry weight of 7.90 g). This finding bodes well for the application of this type of technology to rice crops
in a future world of higher atmospheric CO2 content. Returning to cotton, Chen et al. (2005) grew well watered and fertilized plants of two varieties—one expressing Cry1A (c) genes from
Bacillus thurigiensis and a non-transgenic cultivar from the same recurrent parent—in pots placed within opentop chambers maintained at either 375 or 750 ppm CO2 in Sanhe County, Hebei
Province, China, from planting in mid-May to harvest in October, throughout which period several immature bolls were collected and analyzed for various chemical characteristics, while
others were stored under refrigerated conditions for later feeding to cotton bollworm larvae, whose growth characteristics were closely were monitored. In pursuing this protocol, the
five researchers found that the elevated CO2 treatment increased immature boll concentrations of condensed tannins by approximately 22 percent and 26 percent in transgenic and non-transgenetic
cotton, respectively (see Tannins in Section 7.8.7. for a
Biological Effects of Carbon Dioxide Enrichment 541 discussion of the significance of this observation). In addition, they found that elevated CO2 slightly decreased the body biomass
of the cotton bollworms and reduced moth fecundity. The Bt treatment was even more effective in this regard, and in the combined Bt-high-CO2 treatment, the negative cotton bollworm responses
were expressed most strongly of all. Chen et al. concluded that the expected higher atmospheric CO2 concentrations of the future will “either not change or only slightly enhance the
efficacy of Bt technology against cotton bollworms.” Two years later, Chen et al. (2007) reported growing the same two cultivars under the same conditions from the time of planting on
10 May 2004 until the plants were harvested in October, after which egg masses of the cotton bollworms were reared in a growth chamber under ambient-CO2 conditions, while three successive
generations of them were fed either transgenic or non-transgenic cotton bolls from plants grown in either ambient or twice-ambient atmospheric CO2 concentrations, during which time a
number of physiological characteristics of the cotton bollworms were periodically assessed. This work revealed, in the words of Chen et al., that “both elevated CO2 and transgenic Bt
cotton increased larval lifespan,” but that they decreased “pupal weight, survival rate, fecundity, frass output, relative and mean relative growth rates, and the efficiency of conversion
of ingested and digested food.” As a result, they say that “transgenic Bt cotton significantly decreased the population-trend index compared to non-transgenic cotton for the three successive
bollworm generations, especially at elevated CO2 [our italics].” Based on these findings, the four researchers concluded that the negative effects of elevated CO2 on cotton bollworm
physiology and population dynamics “may intensify through successive generations,” in agreement with the findings of Brooks and Whittaker (1998, 1999) and Wu et al. (2006). They additionally
additionally concluded that “both elevated CO2 and transgenic Bt cotton are adverse environmental factors for cotton bollworm long-term population growth,” and that the combination of
the two factors may intensify their adverse impact on the population performance of the cotton bollworm, which would be good news for cotton growers. Fu et al. (2008) note that “heat
stress is a major constraint to wheat production and negatively impacts grain quality, causing tremendous economic losses, and may become a more troublesome factor due to global warming.”
Consequently, as they describe it, they “introduced into wheat the maize gene coding for plastidal EF-Tu [protein synthesis elongation factor],” in order to assess “the expression of
the transgene, and its effect on thermal aggregation of leaf proteins in transgenic plants,” as well as “the heat stability of photosynthetic membranes (thylakoids) and the rate of CO2
fixation in young transgenic plants following exposure to heat stress.” These operations led, in their words, “to improved protection of leaf proteins against thermal aggregation, reduced
damage to thylakoid membranes and enhanced photosynthetic capability following exposure to heat stress,” which results “support the concept that EF-Tu ameliorates negative effects of
heat stress by acting as a molecular chaperone.” Fu et al. describe their work as “the first demonstration that a gene other than HSP [heat shock protein] gene can be used for improvement
of heat tolerance,” noting it also indicates that the improvement is possible in a species that has a complex genome, such as hexaploid wheat. They conclude by stating their results
“strongly suggest that heat tolerance of wheat, and possibly other crop plants, can be improved by modulating expression of plastidal EF-Tu and/or by selection of genotypes with increased
endogenous levels of this protein.” In summary, genetic alterations to crop plants enable them to better withstand the assaults of insects pests or increases in seasonal maximum air
temperates. temperates. Elevated CO2 either improves or does not change the effectiveness of genetic alternatives to achieve these objectives, while it simultaneously reduces the severity
of possible negative effects that could arise from the escape of transplanted genes into the natural environment. Additional information on this topic, including reviews of newer publications
as they become available, can be found at http://www.co2science.org/subject/t/summaries/transgenicplants.php. References Brooks, G.L. and Whittaker, J.B. 1998. Responses of multiple
generations of Gastrophysa viridula, feeding on Rumex obtusifolius, to elevated CO2. Global Change Biology 4: 63-75. Brooks, G.L. and Whittaker, J.B. 1999. Responses of three generations
of a xylem-feeding insect, Neophilaenus lineatus (Homoptera), to elevated CO2. Global Change Biology 5: 395-401.
Climate Change Reconsidered 542 Chen, F., Wu, G., Ge, F., Parajulee, M.N. and Shrestha, R.B. 2005. Effects of elevated CO2 and transgenic Bt cotton on plant chemistry, performance, and
feeding of an insect herbivore, the cotton bollworm. Entomologia Experimentalis et Applicata 115: 341-350. Chen, F., Wu, G., Parajulee, M.N. and Ge, F. 2007. Longterm impacts of elevated
carbon dioxide and transgenic Bt cotton on performance and feeding of three generations of cotton bollworm. Entomologia Experimentalis et Applicata 124: 27-35. Coviella, C.E., Morgan,
D.J.W. and Trumble, J.T. 2000. Interactions of elevated CO2 and nitrogen fertilization: Effects on production of Bacillus thuringiensis toxins in transgenic plants. Environmental Entomology
29: 781-787. Coviella, C.E. and Trumble, J.T. 2000. Effect of elevated atmospheric carbon dioxide on the use of foliar application of Bacillus thuringiensis. BioControl 45: 325-336.
Fu, J., Momcilovic, I., Clemente, T.E., Nersesian, N., Trick, H.N. and Ristic, Z. 2008. Heterologous expression of a plastid EF-Tu reduces protein thermal aggregation and enhances CO2
fixation in wheat (Triticum aestivum) following heat stress. Plant Molecular Biology 68: 277-288. Makino, A., Harada, M., Kaneko, K., Mae, T., Shimada, T. and Yamamoto, N. 2000. Whole-plant
growth and N allocation in transgenic rice plants with decreased content of ribulose-1,5-bisphosphate carboxylase under different CO2 partial pressures. Australian Journal of Plant Physiology
27: 1-12. Wu, G., Chen, J.F. and Ge, F. 2006. Response of multiple generations of cotton bollworm Helicoverpa armigera Hubner, feeding on spring wheat, to elevated CO2. Journal of Applied
Entomology 130: 2-9. 7.8.9. Isoprene Isoprene (C5H8 or 2-methyl-1,3-butadiene) is a highly reactive non-methane hydrocarbon (NMHC) that is emitted in copious quantities by vegetation
and is responsible for the production of vast amounts of tropospheric ozone (Chameides et al., 1988; Harley et al., 1999), which is a debilitating scourge of plant and animal life alike.
Poisson et al. (2000) calculate that current levels of NMHC emissions—the vast majority of which are isoprene, accounting for more than twice as much as all other NMHCs combined—may
increase surface ozone concentrations by up to 40 percent in the marine boundary-layer and 50-60 percent over land. They further estimate that the current tropospheric ozone content
extends the atmospheric lifetime of methane—one of the world’s most powerful greenhouse gases—by approximately 14 percent. Consequently, it can be appreciated that reducing isoprene
emissions from vegetation is to be desired. Although a few experiments conducted on certain plant species have suggested that elevated concentrations of atmospheric CO2 have little to
no effect on their emissions of isoprene (Buckley, 2001; Baraldi et al., 2004; Rapparini et al., 2004), a much larger number of other experiments are suggestive of substantial CO2-induced
reductions in isoprene emissions, as demonstrated by the work of Monson and Fall (1989), Loreto and Sharkey (1990), Sharkey et al. (1991) and Loreto et al. (2001). Rosentiel et al. (2003)
studied three 50-tree cottonwood plantations growing in separate mesocosms within the forestry section of the Biosphere 2 facility near Oracle, Arizona, USA, one of which mesocosms was
maintained at an atmospheric CO2 concentration of 430 ppm, while the other two were enriched to concentrations of 800 and 1200 ppm for one entire growing season. Integrated over that
period, the total above-ground biomass of the trees in the latter two mesocosms was increased by 60 percent and 82 percent, respectively, while their production of isoprene was decreased
by 21 percent and 41 percent, respectively. Scholefield et al. (2004) measured isoprene emissions from Phragmites australis plants (one of the world’s most important natural grasses)
growing at different distances from a natural CO2 spring in central Italy. At the specific locations they chose to make their measurements, atmospheric CO2 concentrations of approximately
350, 400, 550 and 800 ppm had likely prevailed for the entire lifetimes of the plants. Across this CO2 gradient, plant isoprene emissions dropped ever lower as the air’s CO2 concentration
rose ever higher. Over the first 50-ppm CO2 increase, isoprene emissions were reduced to approximately 65 percent of what they were at ambient CO2, while for CO2 increases of 200 and
450 ppm, they were reduced to only about 30 percent and 7 percent of what they were in the 350-ppm-CO2 air. The researchers note that these reductions were likely caused by reductions
in leaf isoprene synthase, which was observed to be highly correlated with isoprene emissions, leading them to conclude that “elevated CO2 generally inhibits the expression of isoprenoid
synthesis genes and isoprene synthase activity which may, in turn, limit formation of every chloroplastderived isoprenoid.” They state that the “basal
Biological Effects of Carbon Dioxide Enrichment 543 emission rate of isoprene is likely to be reduced under future elevated CO2 levels.” Centritto et al. (2004) grew hybrid poplar saplings
for one full growing season in a FACE facility located at Rapolano, Italy, where the air’s CO2 concentration was increased by approximately 200 ppm. Their study demonstrated that “isoprene
emission is reduced in elevated CO2, in terms of both maximum values of isoprene emission rate and isoprene emission per unit of leaf area averaged across the total number of leaves
per plant,” which in their case amounted to a reduction of approximately 34 percent. When isoprene emission was summed over the entire plant profile, however, the reduction was not nearly
so great (only 6 percent), because of the greater number of leaves on the CO2-enriched saplings. “However,” as they state, “Centritto et al. (1999), in a study with potted cherry seedlings
grown in open-top chambers, and Gielen et al. (2001), in a study with poplar saplings saplings exposed to FACE, showed that the stimulation of total leaf area in response to elevated
CO2 was a transient effect, because it occurred only during the first year of growth.” Hence, they concluded “it may be expected that with similar levels of leaf area, the integrated
emission of isoprene would have been much lower in elevated CO2.” Indeed, they say that their data, “as well as that reported by Scholefield et al. (2004), in a companion experiment
on Phragmites growing in a nearby CO2 spring, mostly confirm that isoprene emission is inversely dependent on CO2 [concentration] when this is above ambient, and suggests that a lower
fraction of C will be re-emitted in the atmosphere as isoprene by single leaves in the future.” Working at another FACE facility, the Aspen FACE facility near Rhinelander, Wisconsin,
USA, Calfapietra et al. (2008) measured emissions of isoprene from sun-exposed upper-canopy leaves of an O3-tolerant clone and an O3-sensitive clone of trembling aspen (Populus tremuloides
Michx.) trees that were growing in either normal ambient air, air enriched with an extra 190-200 ppm CO2, air with 1.5 times the normal ozone concentration, or air simultaneously enriched
with the identical concentrations of both of these atmospheric trace gases. Results of their analysis showed that for the trees growing in air of ambient ozone concentration, the extra
190 ppm of CO2 decreased the mean isoprene emission rate by 11.7 percent in the O3-tolerant aspen clone and by 22.7 percent in the O3-sensitive clone, while for the trees growing in
air with 1.5 times the ambient ozone concentration, the extra CO2 also decreased the mean isoprene emission rate by 10.4 percent in the O3-tolerant clone and by 32.7 percent in the O3-sensitive
clone. At the same time, and in the same order, net photosynthesis rates were increased by 34.9 percent, 47.4 percent, 31.6 percent and 18.9 percent. Possell et al. (2004) grew seedlings
of English oak (Quercus robur), one to a mesocosm (16 cm diameter, 60 cm deep), in either fertilized or unfertilized soil in solardomes maintained at atmospheric CO2 concentrations of
either ambient or ambient plus 300 ppm for one full year, at the conclusion of which period they measured rates of isoprene emissions from the trees’ foliage together with their rates
of photosynthesis. In the unfertilized trees, this work revealed that the 300-ppm increase in the air’s CO2 concentration reduced isoprene emissions by 63 percent on a leaf area basis
and 64 percent on a biomass basis, while in the fertilized trees the extra CO2 reduced isoprene emissions by 70 percent on a leaf area basis and 74 percent on a biomass basis. In addition,
the extra CO2 boosted leaf photosynthesis rates by 17 percent in the unfertilized trees and 13 percent in the fertilized trees. Possell et al. (2005) performed multiple threeweek-long
experiments with two known isopreneemitting herbaceous species (Mucuna pruriens and Arundo donax), which they grew in controlled environment chambers that were maintained at two different
sets of day/night temperatures (29/24°C and 24/18°C) and atmospheric CO2 concentrations characteristic of glacial (180 ppm), pre-industrial (280 ppm) and current (366 ppm) conditions,
where canopy isoprene emission rates were measured on the final day of each experiment. They obtained what they describe as “the first empirical evidence for the enhancement of isoprene
production, on a unit leaf area basis, by plants that grew and developed in [a] CO2-depleted atmosphere,” which results, in their words, “support earlier findings from short-term studies
with woody species (Monson and Fall, 1989; Loreto and Sharkey, 1990).” Then, combining their emission rate data with those of Rosenstiel et al. (2003) for Populus deltoides, Centritto
et al. (2004) for Populus x euroamericana and Scholefield et al. (2004) for Phragmites australis, they developed a single downward-trending isoprene emissions curve that stretches all
the way from 180 to 1200 ppm CO2, where it asymptotically approaches a value that is an order of magnitude less than what it is at 180 ppm. Working at the Biosphere 2 facility near Oracle,
Arizona, USA, in enclosed ultraviolet light-depleted
Climate Change Reconsidered 544 mesocosms (to minimize isoprene depletion by atmospheric oxidative reactions such as those involving OH), Pegoraro et al. (2005) studied the effects of
atmospheric CO2 enrichment (1200 ppm compared to an ambient concentration of 430 ppm) and drought on the emission of isoprene from cottonwood (Populus deltoides Bartr.) foliage and its
absorption by the underlying soil for both wellwatered and drought conditions. In doing so, they found that “under well-watered conditions in the agriforest stands, gross isoprene production
(i.e., the total production flux minus the soil uptake) was inhibited by elevated CO2 and the highest emission fluxes of isoprene were attained in the lowest CO2 treatment.” In more
quantitative terms, it was determined that the elevated CO2 treatment resulted in a 46 percent reduction in gross isoprene production. In addition, it was found that drought suppressed
the isoprene sink capacity of the soil beneath the trees, but that “the full sink capacity of dry soil was recovered within a few hours upon rewetting.” Putting a slightly negative slant
on their findings, Pegoraro et al. suggested that “in future, potentially hotter, drier environments, higher CO2 may not mitigate isoprene emission as much as previously suggested.”
However, we note that climate models generally predict an intensification of the hydrologic cycle in response to rising atmospheric CO2 concentrations, and that the anti-transpirant
effect of atmospheric CO2 enrichment typically leads to increases in the moisture contents of soils beneath vegetation. Also, we note that over the latter decades of the twentieth century,
when the IPCC claims the earth warmed at a rate and to a level that were unprecedented over the past two millennia, soil moisture data from all around the world tended to display upward
trends. Robock et al. (2000), for example, developed a massive collection of soil moisture data from over 600 stations spread across a variety of climatic regimes, including the former
Soviet Union, China, Mongolia, India and the United States, determining that “In contrast to predictions of summer desiccation with increasing temperatures, for the stations with the
longest records, summer soil moisture in the top 1 m has increased while temperatures have risen.” And in a subsequent study of “45 years of gravimetrically observed plant available
soil moisture for the top 1 m of soil, observed every 10 days for April-October for 141 stations from fields with either winter or spring cereals from the Ukraine for 1958-2002,” Robock
et al. (2005) discovered that these real-world observations “show a positive soil moisture trend for the entire period of observation,” noting that “even though for the entire period
there is a small upward trend in temperature and a downward trend in summer precipitation, the soil moisture still has an upward trend for both winter and summer cereals.” Consequently,
in a CO2-enriched world of the future, we likely will have the best of both aspects of isoprene activity: less less production by vegetation and more consumption by soils. Finally, we
address the issue of how well models predict the response of isoprene emission to future
global change. According to Monson et al. (2007) such predictions “probably contain large errors,” which clearly need to be corrected. The reason for the errors, write the twelve researchers
who conducted the study, is that “the fundamental logic of such models is that changes in NPP [net primary production] will produce more or less biomass capable of emitting isoprene,
and changes in climate will stimulate or inhibit emissions per unit of biomass.” They continue, “these models tend to ignore the discovery that there are direct effects of changes in
the atmospheric CO2 concentration on isoprene emission that tend to work in the opposite direction to that of stimulated NPP,” as has been indicated in the research studies described
above. Their results showed, in their words, “that growth in an atmosphere of elevated CO2 inhibited the emission of isoprene at levels that completely compensate for possible increases
in emission due to increases in aboveground NPP.” In lamenting this sorry state of global-change modeling, Monson et al. say that, “to a large extent, the modeling has ‘raced ahead’
of our mechanistic understanding of how isoprene emissions will respond to the fundamental drivers of global change,” and that “without inclusion of these effects in the current array
of models being used to predict changes in atmospheric chemistry due to global change, one has to question the relevance of the predictions.” A year later, Arneth et al. (2008) used
a mechanistic isoprene-dynamic vegetation model of European woody vegetation to “investigate the interactive effects of climate and CO2 concentration on forest productivity, species
composition, and isoprene emissions for the periods 1981-2000 and 2081-2100,” which included a parameterization of the now-well-established direct CO2-isoprene inhibition phenomenon
we have described in the papers above. The study found that “across the model domain,” the CO2-isoprene inhibition effect “has the potential to
Biological Effects of Carbon Dioxide Enrichment 545 offset the stimulation of [isoprene] emissions that could be expected from warmer temperatures and from the increased productivity
and leaf area of emitting vegetation.” In view of these findings, it appears the ongoing rise in the atmosphere’s CO2 concentration will lead to ever greater reductions in atmospheric
isoprene concentrations. As noted in the introductory paragraph of this section, such a consequence would be welcome news for man and nature. Additional information on this topic, including
reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/i/isoprene.php. References Arneth, A., Schurgers, G., Hickler, T. and Miller,
P.A. 2008. Effects of species composition, land surface cover, CO2 concentration and climate on isoprene emissions from European forests. Plant Biology 10: 150-162. Baraldi, R., Rapparini,
F., Oechel, W.C., Hastings, S.J., Bryant, P., Cheng, Y. and Miglietta, F. 2004. Monoterpene emission responses to elevated CO2 in a Mediterraneantype ecosystem. New Phytologist 161:
17-21. Buckley, P.T. 2001. Isoprene emissions from a Florida scrub oak species grown in ambient and elevated carbon dioxide. Atmospheric Environment 35: 631-634. Calfapietra, C., Scarascia-Mugnozza,
G., Karnosky, D.F., Loreto, F. and Sharkey, T.D. 2008. Isoprene emission rates under elevated CO2 and O3 in two field-grown aspen clones differing in their sensitivity to O3. New Phytologist
179: 55-61. Centritto, M., Lee, H. and Jarvis, P. 1999. Interactive effects of elevated [CO2] and water stress on cherry (Prunus avium) seedlings. I. Growth, total plant water use efficiency
and uptake. New Phytologist 141: 129-140. Centritto, M., Nascetti, P., Petrilli, L., Raschi, A. and Loreto, F. 2004. Profiles of isoprene emission and photosynthetic parameters in hybrid
poplars exposed to free-air CO2 enrichment. Plant, Cell and Environment 27: 403-412. Chameides, W.L., Lindsay, R.W., Richardson, J. and Kiang, C.S. 1988. The role of of biogenic hydrocarbons
in urban photochemical smog: Atlanta as a case study. Science 241: 1473-1475. Gielen, B., Calfapietra, C., Sabatti, M. and Ceulemans, R. 2001. Leaf area dynamics in a poplar plantation
under freeair carbon dioxide enrichment. Tree Physiology 21: 1245-1255. Harley, P.C., Monson, R.K. and Lerdau, M.T. 1999. Ecological and evolutionary aspects of isoprene emission from
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J., Hewitt, C.N., Ayres, E. and Kerstiens, G. 2004. Interactive effects of elevated CO2 and soil fertility on isoprene emissions from Quercus robur. Global Change Biology 10: 1835-1843.
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01312). Scholefield, P.A., Doick, K.J., Herbert, B.M.J., Hewitt, C.N.S., Schnitzler, J.-P., Pinelli, P. and Loreto, F. 2004. Impact of rising CO2 on emissions of volatile organic compounds:
isoprene emission from Phragmites australis growing at elevated CO2 in a natural carbon dioxide spring. Plant, Cell and Environment 27: 393-401. Sharkey, T.D., Loreto, F. and Delwiche,
C.F. 1991. High carbon dioxide and sun/shade effect on isoprene emissions from oak and aspen tree leaves. Plant, Cell and Environment 14: 333-338. 7.8.10. Microorganisms Plants grown
in CO2-enriched atmospheres nearly always exhibit increased photosynthetic rates and biomass biomass production. Due to this productivity enhancement, more plant material is typically
added to soils from root growth, turnover and exudation, as well as from leaves and stems following their abscission and falling to the ground during senescence. Such additions of carbon
onto and into soils often serve as the only carbon source for supporting the development and growth of microorganisms in terrestrial habitats. Thus, it is important to understand how
CO2-induced increases in plant growth affect microorganisms, a topic omitted from discussion by the IPCC. Several studies have shown that atmospheric CO2 enrichment does not significantly
impact soil microorganisms. Zak et al. (2000), for example, observed no significant differences in soil microbial biomass beneath aspen seedlings grown at 350 and 700 ppm CO2 after 2.5
years of differential treatment. Likewise, in the cases of Griffiths et al. (1998) and Insam et al. (1999), neither research team reported any changes in microbial community structure
beneath ryegrass and artificial tropical ecosystems, respectively, after subjecting them to atmospheric CO2 enrichment. Other studies, however, have found that elevated CO2 can significantly
affect soil microorganisms. Van Ginkel and Gorissen (1998) observed that three months of elevated CO2 exposure (700 ppm) increased soil microbial biomass beneath ryegrass plants by 42
percent relative to that produced under ambient CO2 conditions, as did Van Ginkel et al. (2000). Likewise, soil microbial biomass was reported to increase by 15 percent beneath agricultural
fields subjected to a two-year wheatsoybean crop rotation (Islam et al., 2000). In a study by Marilley et al. (1999), atmospheric CO2 enrichment significantly increased bacterial numbers
in the rhizospheres beneath ryegrass and white clover monocultures. Similarly, Lussenhop et al. (1998) reported CO2-induced increases in the amounts of bacteria, protozoa, and microarthropods
in soils that had supported regenerating poplar tree cuttings for five months. In addition, Hungate et al. (2000) reported that twice-ambient CO2 concentrations significantly increased
the biomass of active fungal organisms and flagellated protozoa beneath serpentine and sandstone grasslands after four years of treatment exposure. In taking a closer look at the study
of Marilley et al. (1999), it is evident that elevated CO2 caused shifts in soil microbial populations. In soils beneath their leguminous white clover, for example, elevated CO2 favored
shifts towards Rhizobium bacterial species, which likely increased nitrogen availability— via nitrogen fixation—to support enhanced plant growth. However, in soils beneath non-leguminous
ryegrass monocultures, which do not form symbiotic relationships with Rhizobium species, elevated CO2 favored shifts towards Pseudomonas species, which likely acquired nutrients to support
enhanced plant growth through mechanisms other than nitrogen fixation. Nonetheless, in both situations, the authors observed CO2-induced shifts in bacterial populations that would likely
optimize nutrient acquisition for specific host plant species. In an unrelated study, Montealegre et al. (2000) reported that elevated CO2 acted as a selective agent among 120 different
isolates of Rhizobium growing beneath white clover plants. Specifically, when bacterial strains favored by ambient and elevated CO2 concentrations were mixed together and grown with
white clover at an atmospheric CO2 concentration of 600 ppm, 17 percent more root nodules were formed by isolates previously determined to be favored by elevated CO2. Hu et al. (2001)
subjected fertile sandstone grasslands to five years of twice-ambient CO2 concentrations and found they exhibited increased soil microbial biomass while simultaneously enhancing plant
nitrogen uptake. The net effect of these phenomena reduced nitrogen availability for
Biological Effects of Carbon Dioxide Enrichment 547 microbial use, which consequently decreased microbial respiration and, hence, microbial decomposition. Consequently, these ecosystems
displayed CO2-induced increases in net carbon accumulation. Similarly, Williams et al. (2000) reported that microbial biomass carbon increased by 4 percent in a tallgrass prairie after
five years exposure to twice-ambient CO2 concentrations, which contributed to a total soil carbon enhancement of 8 percent. In summation, as the CO2 content of the air continues to rise,
earth’s vegetation will likely respond with increasing photosynthetic rates and biomass production, returning more organic carbon to the soil where it will be utilized by microbial organisms
to maintain or increase their population numbers, biomass and heterotrophic activities (Weihong et al., 2000; Arnone and Bohlen, 1998). Shifts in microbial community structure may occur
that will favor the intricate relationships that currently exist between leguminous and and non-leguminous plants and the specific microorganisms upon which they depend. Additional information
on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/m/microorganisms.php. References Arnone, J.A., III
and Bohlen, P.J. 1998. Stimulated N2O flux from intact grassland monoliths after two growing seasons under elevated atmospheric CO2. Oecologia 116: 331-335. Griffiths, B.S., Ritz, K.,
Ebblewhite, N., Paterson, E. and Killham, K. 1998. Ryegrass rhizosphere microbial community structure under elevated carbon dioxide concentrations, with observations on wheat rhizosphere.
Soil Biology and Biochemistry 30: 315-321. Hu, S., Chapin III, F.S., Firestone, M.K., Field, C.B. and Chiariello, N.R. 2001. Nitrogen limitation of microbial decomposition in a grassland
under elevated CO2. Nature 409: 88-191. Hungate, B.A., Jaeger III, C.H., Gamara, G., Chapin III, F.S. and Field, C.B. 2000. Soil microbiota in two annual grasslands: responses to elevated
atmospheric CO2. Oecologia 124: 589-598. Insam, H., Baath, E., Berreck, M., Frostegard, A., Gerzabek, M.H., Kraft, A., Schinner, F., Schweiger, P. and Tschuggnall, G. 1999. Responses
of the soil microbiota to elevated CO2 in an artificial tropical ecosystem. Journal of Microbiological Methods 36: 45-54. Islam, K.R., Mulchi, C.L. and Ali, A.A. 2000. Interactions of
tropospheric CO2 and O3 enrichments and moisture variations on microbial biomass and respiration in soil. Global Change Biology 6: 255-265. Lussenhop, J., Treonis, A., Curtis, P.S.,
Teeri, J.A. and Vogel, C.S. 1998. Response of soil biota to elevated atmospheric CO2 in poplar model systems. Oecologia 113: 247-251. Marilley, L., Hartwig, U.A. and Aragno, M. 1999.
Influence of an elevated atmospheric CO2 content on soil and rhizosphere bacterial communities beneath Lolium perenne and Trifolium repens under field conditions. Microbial Ecology 38:
39-49. Montealegre, C.M., Van Kessel, C., Blumenthal, J.M., Hur, H.G., Hartwig, U.A. and Sadowsky, M.J. 2000. Elevated atmospheric CO2 alters microbial population structure in a pasture
ecosystem. Global Change Biology 6: 475-482. Van Ginkel, J.H. and Gorissen, A. 1998. In situ decomposition of grass roots as affected by elevated atmospheric carbon dioxide. Soil Science
Society of America Journal 62: 951-958. Van Ginkel, J.H., Gorissen, A. and Polci, D. 2000. Elevated atmospheric carbon dioxide concentration: effects of increased carbon input in a Lolium
perenne soil on microorganisms and decomposition. Soil Biology & Biochemistry 32: 449-456. Weihong, L., Fusuo, Z. and Kezhi, B. 2000. Responses of plant rhizosphere to atmospheric CO2
enrichment. Chinese Science Bulletin 45: 97-101. Williams, M.A., Rice, C.W. and Owensby, C.E. 2000. Carbon dynamics and microbial activity in tallgrass prairie exposed to elevated CO2
for 8 years. Plant and Soil 227: 127-137. Zak, D.R., Pregitzer, K.S., Curtis, P.S. and Holmes, W.E. 2000. Atmospheric CO2 and the composition and function of soil microbial communities.
Ecological Applications 10: 47-59. 7.8.11. Worms Perhaps the best known worm in the world is the common earthworm. How will it be affected as the air’s CO2 content continues to climb,
and how will its various responses affect the biosphere? What about other worms? How will they fare in a CO2-enriched world of the future, and what will be the results of their responses?
Climate Change Reconsidered 548 “Earthworms,” in the words of Edwards (1988), “play a major role in improving and maintaining the fertility, structure, aeration and drainage of agricultural
soils.” As noted by Sharpley et al. (1988), “by ingestion and digestion of plant residue and subsequent egestion of cast material, earthworms can redistribute nutrients in a soil and
enhance enzyme activity, thereby increasing plant availability of both soil and plant residue nutrients,” as others have also demonstrated (Bertsch et al., 1988; McCabe et al., 1988;
Zachmann and Molina, 1988). Kemper (1988) describes how “burrows opened to the surface by surface-feeding worms provide drainage for water accumulating on the surface during intense
rainfall,” noting that “the highly compacted soil surrounding the expanded burrows has low permeability to water which often allows water to flow through these holes for a meter or so
before it is sorbed into the surrounding soil.” Hall and Dudas (1988) report that the presence of earthworms appears to mitigate the deleterious effects of certain soil toxins. Logsdon
and Linden (1988) describe a number of other beneficial effects of earthworms, including (1) enhancement of soil aeration, since under wet conditions earthworm channels do not swell
shut as many soil cracks do, (2) enhancement of soil water uptake, since roots can explore deeper soil layers by following earthworm channels, and (3) enhancement of nutrient uptake,
since earthworm casts and channel walls have a more neutral pH and higher available nutrient level than bulk soil. Hence, we should care about what happens to earthworms as the air’s
CO2 content rises because of the many important services they provide for earth’s plant life. Edwards (1988) says “the most important factor in maintaining good earthworm populations
in agricultural soils is that there be adequate availability of organic matter,” while Hendrix et al. (1988) and Kladivko (1988) report that greater levels of plant productivity promote
greater levels of earthworm activity. Consequently, since the most ubiquitous and powerful effect of atmospheric CO2 enrichment is its stimulation of plant productivity, which leads
to enhanced organic matter delivery to soils, it logically follows that this aerial fertilization effect of the ongoing rise in the air’s CO2 content should increase earthworm populations
and amplify the many beneficial services they provide for plants. The second most significant and common effect of atmospheric CO2 enrichment on plants is its antitranspirant effect,
whereby elevated levels of atmospheric CO2 reduce leaf stomatal apertures and slow the rate of evaporative water loss from the vast bulk of earth’s vegetation. Both growth chamber studies
and field experiments that have studied this phenomenon provide voluminous evidence that it often leads to increased soil water contents in many terrestrial ecosystems, which also benefits
earthworm populations. Zaller and Arnone (1997) fumigated open-top and -bottom chambers in a calcareous grassland near Basal, Switzerland with air of either 350 or 600 ppm CO2 for an
entire growing season. They found that the mean annual soil moisture content in the CO2-enriched chambers was 10 percent greater than that observed in the ambient-air chambers, and because
rates of surface cast production by earthworms are typically positively correlated with soil moisture content, they found that cumulative surface cast production after only one year
was 35 percent greater in the CO2-enriched chambers than in the control chambers. In addition, because earthworm casts are rich in organic carbon and nitrogen, the cumulative amount
of these important nutrients on a per-landarea basis was found to be 28 percent greater in the CO2-enriched chambers than it was in the ambient-air chambers. In a subsequent study of
the same grassland, Zaller and Arnone (1999) found that plants growing in close proximity to the earthworm casts produced more biomass than similar plants growing further away from them.
They also found that the CO2-induced growth stimulation experienced by the various grasses was also greater for those plants growing nearer the earthworm casts. These various observations
suggest that atmospheric CO2 enrichment sets in motion a selfenhancing cycle of positive biological phenomena, whereby increases in the air’s CO2 content (1) stimulate plant productivity
and (2) reduce plant evaporative water loss, which results in (3) more organic matter entering the soil and (4) a longer soil moisture retention time and/or greater soil water contents,
all of which factors lead to (5) the development of larger and more active earthworm populations, which (6) enhance many important soil properties, including fertility, structure, aeration
and drainage, which improved properties (7) further enhance the growth of the plants whose CO2-induced increase in productivity was the factor that started the whole series of processes
in the first place. More earthworms also can increase soil’s ability to sequester carbon. As Jongmans et al. (2003) point out, “the rate of organic matter decomposition can be
Biological Effects of Carbon Dioxide Enrichment 549 decreased in worm casts compared to bulk soil aggregates (Martin, 1991; Haynes and Fraser, 1998).” On the basis of these studies and
their own micromorphological investigation of structural development and organic matter distribution in two calcareous marine loam soils on which pear trees had been grown for 45 years
(one of which soils exhibited little to no earthworm activity and one of which exhibited high earthworm activity, due to different levels of heavy metal contamination of the soils as
a consequence of the prior use of different amounts of fungicides), they concluded that “earthworms play an important role in the intimate mixing of organic residues and fine mineral
soil particles and the formation of organic matter-rich micro-aggregates and can, therefore, contribute to physical protection of organic matter, thereby slowing down organic matter
turnover and increasing the soil’s potential for carbon sequestration.” Put more simply, atmospheric CO2 CO2 enrichment that stimulates the activity of earthworms also leads to more—and
more secure—sequestration of carbon in earth’s soils. Don et al. (2008) studied the effects of anecic earthworms—which generally inhabit a single vertical burrow throughout their entire
lives that can be as much as five meters in depth, but is generally in the range of one to two meters—on soil carbon stocks and turnover via analyses of enzyme activity, stable isotopes,
nuclear magnetic resonance spectroscopy, and the 14C age of their burrow linings. The results of their study indicated that “the carbon distribution in soils is changed by anecic earthworms’
activity with more carbon stored in the subsoil where earthworms slightly increase the carbon stocks.” In this regard they also state that “the translocation of carbon from [the] organic
layer to the subsoil will decrease the carbon vulnerability to mineralization,” since “carbon in the organic layer and the surface soil is much more prone to disturbances with rapid
carbon loss than subsoil carbon.” Bossuyt et al. (2005) conducted a pair of experiments designed to investigate “at what scale and how quickly earthworms manage to protect SOM [soil
organic matter].” In the first experiment, soil aggregate size distribution together with total C and 13C were measured in three treatments—control soil, soil + 13C-labeled sorghum leaf
residue, and soil + 13C-labeled residue + earthworms—after a period of 20 days incubation, where earthworms were added after the eighth day. In the second experiment, they determined
the protected C and 13C pools inside the newly formed casts and macro-and micro-soilaggregates. Results indicated that the proportion of large water-stable macroaggregates was on average
3.6 times greater in the soil-residue samples that contained earthworms than in those that lacked earthworms, and that the macroaggregates in the earthworm treatment contained approximately
three times more sequestered carbon. What is more, the earthworms were found to form “a significant pool of protected C in microaggregates within large macroaggregates after 12 days
of incubation,” thereby demonstrating the rapidity with which earthworms perform their vital function of sequestering carbon in soils when plant residues become available to them. Cole
et al. (2002) report that “in the peatlands of northern England, which are classified as blanket peat, it has been suggested that the potential effects of global warming on carbon and
nutrient dynamics will be related to the activities of dominant soil fauna, and especially enchytraeid worms.” In harmony with these ideas, Cole et al. say they “hypothesized” that warming
would lead to increased enchytraeid worm activity, which would lead to higher grazing pressure on microbes in the soil; and since enchytraeid grazing has been observed to enhance microbial
activity (Cole et al., 2000), they further hypothesized that more carbon would be liberated in dissolved organic form, “supporting the view that global warming will increase carbon loss
from blanket peat ecosystems.” The scientists next describe how they constructed small microcosms from soil and litter they collected near the summit of Great Dun Fell, Cumbria, England.
Subsequent to “defaunating” this material by reducing its temperature to -80°C for 24 hours, they thawed and inoculated it with native soil microbes, after which half of the microcosms
were incubated in the dark at 12°C and half at 18°C, the former of which temperatures was approximately equal to mean August soil temperature at a depth of 10 cm at the site of soil
collection, while the latter was said by them to be “close to model predictions for soil warming that might result from a doubling of CO2 in blanket peat environments.” Ten seedlings
of an indigenous grass of blanket peat were then transplanted into each of the microcosms, while 100 enchytraeid worms were added to each of half of the mini-ecosystems. These procedures
resulted in the creation of four experimental treatments: ambient temperature, ambient temperature + enchytraeid worms, elevated temperature, and elevated temperature + enchytraeid worms.
The resulting 48 microcosms—sufficient to destructively harvest three replicates of each
Climate Change Reconsidered 550 treatment four different times throughout the course of the 64-day experiment—were arranged in a fully randomized design and maintained at either 12 or
18°C with alternating 12-hour light and dark periods. In addition, throughout the entire course of the study, the microcosms were given distilled water every two days to maintain their
original weights. So what did the researchers find? First, and contrary to their hypothesis, elevated temperature reduced the ability of the enchytraeid worms to enhance the loss of
carbon from the microcosms. At the normal ambient temperature, for example, the presence of the worms enhanced dissolved organic carbon (DOC) loss by 16 percent, while at the elevated
temperature expected for a doubling of the air’s CO2 content, the worms had no effect at all on DOC. In addition, Cole et al. note that “warming may cause drying at the soil surface,
forcing enchytraeids to burrow to deeper subsurface horizons.” Hence, since the worms are known to have little influence on soil carbon dynamics below a depth of 4 cm (Cole et al., 2000),
they concluded that this additional consequence of warming would further reduce the ability of enchytraeids to enhance carbon loss from blanket peatlands. In summarizing their findings,
Cole et al. say that “the soil biotic response to warming in this study was negative.” That is, it was of such a nature that it resulted in a reduced loss of carbon to the atmosphere,
which would tend to slow the rate of rise of the air’s CO2 content, just as was suggested by the results of the study of Jongmans et al. Yeates et al. (2003) report results from a seasonlong
FACE study of a 30-year-old New Zealand pasture, where three experimental plots had been maintained at the ambient atmospheric CO2 concentration of 360 ppm and three others at a concentration
of 475 ppm (a CO2 enhancement of only 32 percent) for a period of four to five years. The pasture contained about twenty species of plants, including C3 and C4 grasses, legumes and forbs.
Nematode,
or “roundworm,” populations increased significantly in response to the 32 percent increase in the air’s CO2 concentration. Of the various feeding groups studied, Yeates et al. report
that the relative increase “was lowest in bacterial-feeders (27%), slightly higher in plant (root) feeders (32%), while those with delicate stylets (or narrow lumens; plantassociated,
fungal-feeding) increased more (52% and 57%, respectively).” The greatest nematode increases were recorded among omnivores (97 percent) and predators (105 percent). Most dramatic of
all, rootfeeding populations of the Longidorus nematode taxon rose by 330 percent. Also increasing in abundance were earthworms: Aporrectodea caliginosa by 25 percent and Lumbricus rubellus
by 58 percent. Enchytraeids, on the other hand, decreased in abundance, by approximately 30 percent. With respect to earthworms, Yeates et al. note that just as was found in the studies
cited in the first part of this review, the introduction of lumbricids has been demonstrated to improve soil conditions in New Zealand pastures (Stockdill, 1982), which benefits pasture
plants. Hence, the CO2-induced increase in earthworm numbers observed in Yeates et al.’s study would be expected to do more of the same, while the reduced abundance of enchytraeids they
documented in the CO2-enriched pasture would supposedly lead to less carbon being released to the air from the soil, as per the known ability of enchytraeids to promote carbon loss from
British peat lands under current temperatures. In summary, the lowly earthworm and still lowlier soil nematodes respond to increases in the air’s CO2 content via a number of plant-mediated
phenomena in ways that further enhance the positive effects of atmospheric CO2 enrichment on plant growth and development, while at the same time helping to sequester more carbon more
securely in the soil. Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subject/e/earthworms.php.
References Bertsch, P.M., Peters, R.A., Luce, H.D. and Claude, D. 1988. Comparison of earthworm activity in long-term notillage and conventionally tilled corn systems. Agronomy Abstracts
80: 271. Bossuyt, H., Six, J. and Hendrix, P.F. 2005. Protection of soil carbon by microaggregates within earthworm casts. Soil Biology & Biochemistry 37: 251-258. Cole, L., Bardgett,
R.D. and Ineson, P. 2000. Enchytraeid worms (Oligochaeta) enhance mineralization of carbon in organic upland soils. European Journal of Soil Science 51: 185-192. Cole, L., Bardgett,
R.D., Ineson, P. and Hobbs, P.J. 2002. Enchytraeid worm (Oligochaeta) influences on microbial community structure, nutrient dynamics and plant growth in blanket peat subjected to warming.
Soil Biology & Biochemistry 34: 83-92.
Biological Effects of Carbon Dioxide Enrichment 551 Don, A., Steinberg, B., Schoning, I., Pritsch, K., Joschko, M., Gleixner, G. and Schulze, E.-D. 2008. Organic carbon sequestration
in earthworm burrows. Soil Biology & Biochemistry 40: 1803-1812. Edwards, C.A. 1988. Earthworms and agriculture. Agronomy Abstracts 80: 274. Hall, R.B. and Dudas, M.J. 1988. Effects
of chromium loading on earthworms in an amended soil. Agronomy Abstracts 80: 275. Haynes, R.J. and Fraser, P.M. 1998. A comparison of aggregate stability and biological activity in earthworm
casts and uningested soil as affected by amendment with wheat and lucerne straw. European Journal of Soil Science 49: 629-636. Hendrix, P.F., Mueller, B.R., van Vliet, P., Bruce, R.R.
and Langdale, G.W. 1988. Earthworm abundance and distribution in agricultural landscapes of the Georgia piedmont. Agronomy Abstracts 80: 276. Jongmans, A.G., Pulleman, M.M., Balabane,
M., van Oort, F. and Marinissen, J.C.Y. 2003. Soil structure and characteristics of organic matter matter in two orchards differing in earthworm activity. Applied Soil Ecology 24: 219-232.
Kemper, W.D. 1988. Earthworm burrowing and effects on soil structure and transmissivity. Agronomy Abstracts 80: 278. Kladivko, E.J. 1988. Soil management effects on earthworm populations
and activity. Agronomy Abstracts 80: 278. Logsdon, S.D. and Linden, D.L. 1988. Earthworm effects on root growth and function, and on crop growth. Agronomy Abstracts 80: 280. Martin,
A. 1991. Short-and long-term effects of the endogenic earthworm Millsonia anomala (Omodeo) (Megascolecidae, Oligochaeta) of tropical savannas on soil organic matter. Biology and Fertility
of Soils 11: 234-238. McCabe, D., Protz, R. and Tomlin, A.D. 1988. Earthworm influence on soil quality in native sites of southern Ontario. Agronomy Abstracts 80: 281. Sharpley, A.N.,
Syers, J.K. and Springett, J. 1988. Earthworm effects on the cycling of organic matter and nutrients. Agronomy Abstracts 80: 285. Stockdill, S.M.J. 1982. Effects of introduced earthworms
on the productivity of New Zealand pastures. Pedobiologia 24: 29-35. Yeates, G.W., Newton, P.C.D. and Ross, D.J. 2003. Significant changes in soil microfauna in grazed pasture under
elevated carbon dioxide. Biology and Fertility of Soils 38: 319-326. Zachmann, J.E. and Molina, J.A. 1988. Earthwormmicrobe interactions in soil. Agronomy Abstracts 80: 289. Zaller,
J.G. and Arnone III, J.A. 1997. Activity of surfacecasting earthworms in a calcareous grassland under elevated atmospheric CO2. Oecologia 111: 249-254. Zaller, J.G. and Arnone III, J.A.
1999. Interactions between plant species and earthworm casts in a calcareous grassland under elevated CO2. Ecology 80: 873-881. 7.9. Greening of the Earth More than two decades ago,
Idso (1986) published a small item in Nature advancing the idea that the aerial fertilization effect of the CO2 that is liberated by the burning of coal, gas and oil was destined to
dramatically enhance the productivity of earth’s vegetation. In a little book he had published four years earlier earlier (Idso, 1982), he had predicted that “CO2 effects on both the
managed and unmanaged biosphere will be overwhelmingly positive.” In a monograph based on a lecture he gave nine years later (Idso, 1995), he said “we appear to be experiencing the initial
stages of what could truly be called a rebirth of the biosphere, the beginnings of a biological rejuvenation that is without precedent in all of human history.” In light of the fact
that Idso’s worldview is nearly the exact opposite of the apocalyptic vision promulgated by the IPCC, it is instructive to see what real-world observations reveal about the matter. In
this section we review studies that show a CO2-induced greening of Africa, Asia, Europe, North America, the oceans, and the entire globe. Additional information on this topic, including
reviews not discussed here, can be found at http://www.co2science.org/subject/g/subject_g.php under the heading Greening of the Earth. References Idso, S.B. 1982. Carbon Dioxide: Friend
or Foe? IBR Press, Tempe, AZ. Idso, S.B. 1986. Industrial age leading to the greening of the Earth? Nature 320: 22. Idso, S.B. 1995. CO2 and the Biosphere: The Incredible Legacy of the
Industrial Revolution. Department of Soil, Water & Climate, University of Minnesota, St. Paul, Minnesota, USA.
Climate Change Reconsidered 552 7.9.1. Africa In an article by Fred Pearce that was posted on the website of New Scientist magazine on 16 September 2002 titled “Africa’s deserts are
in ‘spectacular’ retreat,” we were told the story of vegetation reclaiming great tracts of barren land across the entire southern edge of the Sahara. This information likely came as
a bit of a surprise to many, since the United Nations Environment Program had reported to the World Summit on Sustainable Development in Johannesburg, South Africa in August of that
year that over 45 percent of the continent was experiencing severe desertification. The world of nature, however, told a vastly different story. Pearce began by stating “the southern
Saharan desert is in retreat, making farming viable again in what were some of the most arid parts of Africa,” noting that “Burkina Faso, one of the West African countries devastated
by drought and advancing deserts 20 years ago, is growing so much greener that families who fled to wetter coastal regions are starting to go home.” The good news was not confined to
Burkina Faso. “Vegetation,” according to Pearce, “is ousting sand across a swathe of land stretching from Mauritania on the shores of the Atlantic to Eritrea 6000 kilometers away on
the Red Sea coast.” Besides being widespread in space, the greening was widespread in time, having been happening since at least the mid-1980s. Quoting Chris Reij of the Free University
of Amsterdam, Pearce wrote that “aerial photographs taken in June show ‘quite spectacular regeneration of vegetation’ in northern Burkina Faso.” The data indicated the presence of more
trees for firewood and more grassland for livestock. In addition, a survey that Reij was collating showed, according to Pearce, “a 70% increase in yields of local cereals such as sorghum
and millet in one province in recent years.” Also studying the area was Kjeld Rasmussen of the University of Copenhagen, who reported that since the 1980s there had been a “steady reduction
in bare ground” with with “vegetation cover, including bushes and trees, on the increase on the dunes.” Pearce also reported on the work of a team of geographers from Britain, Sweden
and Denmark that had spent much of the prior summer analyzing archived satellite images of the Sahel. Citing Andrew Warren of University College London as a source of information on
this study, he said the results showed “that ‘vegetation seems to have increased significantly’ in the past 15 years, with major regrowth in southern Mauritania, northern Burkina Faso,
north-western Niger, central Chad, much of Sudan and parts of Eritrea.” Should these findings take us by surprise? Not in the least, as Nicholson et al. (1998) reported in a study of
a series of satellite images of the Central and Western Sahel that were taken from 1980 to 1995, they could find no evidence of any overall expansion of deserts and no drop in the rainfall
use efficiency of native vegetation. In addition, in a satellite study of the entire Sahel from 1982 to 1990, Prince et al. (1998) detected a steady rise in rainfall use efficiency,
suggesting that plant productivity and coverage of the desert had increased during this period. That the greening phenomenon has continued apace is borne out by the study of Eklundh
and Olsson (2003), who analyzed Normalized Difference Vegetation Index (NDVI) data obtained from the U.S. National Oceanic and Atmospheric Administration’s satellite-borne Advanced Very
High Resolution Radiometer whenever it passed over the African Sahel for the period 1982-2000. As they describe their findings, “strong positive change in NDVI occurred in about 22%
of the area, and weak positive change in 60% of the area,” while “weak negative change occurred in 17% of the area, and strong negative change in 0.6% of the area.” In addition, they
report that “integrated NDVI has increased by about 80% in the areas with strong positive change,” while in areas with weak negative change, “integrated NDVI has decreased on average
by 13%.” The primary story told by these data, data, therefore, is one of strong positive trends in NDVI for large areas of the African Sahel over the last two decades of the twentieth
century. Eklundh and Olsson conclude that the “increased vegetation, as suggested by the observed NDVI trend, could be part of the proposed tropical sink of carbon.” Due to the increase
in vegetation over the past quarter-century in the Sahel, the African region was recently featured in a special issue of the Journal of Arid Environments titled “The ‘Greening’ of the
Sahel.” Therein, Anyamba and Tucker (2005) describe their development of an NDVI history of the region for the period 1981-2003. Comparing this history with the precipitation history
of the Sahel developed by Nicholson (2005), they found that “the persistence and spatial coherence of drought conditions during the 1980s is well represented by the NDVI anomaly patterns
and corresponds with the documented rainfall anomalies across the region during this time period.” In addition, they report that
Biological Effects of Carbon Dioxide Enrichment 553 “the prevalence of greener than normal conditions during the 1990s to 2003 follows a similar increase in rainfall over the region
during the last decade.” In another analysis of NDVI and rainfall data in the same issue of the Journal of Arid Environments, Olsson et al. (2005) report finding “a consistent trend
of increasing vegetation greenness in much of the region,” which they describe as “remarkable.” They say increasing rainfall over the last few years “is certainly one reason” for the
greening phenomenon. However, they find the increase in rainfall “does not fully explain” the increase in greenness. For one thing, the three Swedish scientists note that “only eight
out of 40 rainfall observations showed a statistically significant increase between 1982-1990 and 1991-1999.” In addition, they report that “further analysis of this relationship does
not indicate an overall relationship between rainfall increase and vegetation trend.” So what else could be driving the increase in greenness? Olsson et al. suggest that “another potential
explanation could be improved land management, which has been shown to cause similar changes in vegetation response elsewhere (Runnstrom, 2003).” However, in more detailed analyses of
Burkina Faso and Mali, where production of millet rose by 55 percent and 35 percent, respectively, since 1980, they could find “no clear relationship” between agricultural productivity
and NDVI, which argues against the land management explanation. A third speculation of Olsson et al. is that the greening of the Sahel could be caused by increasing rural-to-urban migration.
In this scenario, widespread increases in vegetation occur as a result of “reduced area under cultivation,” due to a shortage of rural laborers, and/or “increasing inputs on cropland,”
such as seeds, machinery and fertilizers made possible by an increase in money sent home to rural households by family members working in cities. However, Olsson et al. note that “more
empirical research is needed to verify this [hypothesis].” About the only thing left is what Idso (1982, 1986, 1995) has suggested, i.e., that the aerial fertilization effect of the
ongoing rise in the air’s CO2 concentration (which greatly enhances vegetative productivity) and its anti-transpiration effect (which enhances plant water-use efficiency and enables
plants to grow in areas that were once too dry for them) are the major players in the greening phenomenon. Whatever was the reason for the greening of the Sahel over the past quarter-century,
it is clear that in spite of what the IPCC claims were unprecedented increases in anthropogenic CO2 emissions and global temperatures, the Sahel experienced an increase in vegetative
cover that was truly, as Olsson et al. write, “remarkable.” Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2scien
e.org/subject/g/africagreen.php. References Anyamba, A. and Tucker, C.J. 2005. Analysis of Sahelian vegetation dynamics using NOAA-AVHRR NDVI data from 1981-2003. Journal of Arid Environments
63: 596-614. Eklundh, L. and Olssson, L. 2003. Vegetation index trends for the African Sahel 1982-1999. Geophysical Research Letters 30: 10.1029/2002GL016772. Idso, S.B. 1982. Carbon
Dioxide: Friend or Foe? IBR Press, Tempe, AZ. Idso, S.B. 1986. Industrial age leading to the greening of the Earth? Nature 320: 22. Idso, S.B. 1995. CO2 and the Biosphere: The Incredible
Legacy of the Industrial Revolution. Department of Soil, Water & Climate, University of Minnesota, St. Paul, Minnesota, USA. Nicholson, S. 2005. On the question of the ‘recovery’ of
the rains in the West African Sahel. Journal of Arid Environments 63: 615-641. Nicholson, S.E., Tucker, C.J. and Ba, M.B. 1998. Desertification, drought, and surface vegetation: An example
from the West African Sahel. Bulletin of the American Meteorological Society 79: 815-829. Olsson, L., Eklundh, L. and Ardo, J. 2005. A recent greening of the Sahel—trends, patterns and
potential causes. Journal of Arid Environments 63: 556-566. Prince, S.D., Brown De Colstoun, E. and Kravitz, L.L. 1998. Evidence from rain-use efficiencies does not indicate extensive
Sahelian desertification. Global Change Biology 4: 359-374. Runnstrom, M. 2003. Rangeland development of the Mu Us Sandy Land in semiarid China: an analysis using Landsat and NOAA remote
sensing data. Land Degradation & Development Studies 14: 189-202.
Climate Change Reconsidered 554 7.9.2. Asia We begin a review of Asia with the modeling work of Liu et al. (2004), who derived detailed estimates of the economic impact of predicted
climate change on agriculture in China, utilizing county-level agricultural, climate, social, economic and edaphic data for 1275 agriculture-dominated counties for the period 1985-1991,
together with the outputs of three general circulation models of the atmosphere that were based on five different scenarios of anthropogenic CO2-induced climate change that yielded a
mean countrywide temperature increase of 3.0°C and a mean precipitation increase of 3.9 percent for the 50-year period ending in AD 2050. In doing so, they determined that “all of China
would benefit from climate change in most scenarios.” In addition, they state that “the effects of CO2 fertilization should [also] be included, for some studies indicate that this may
produce a significant increase in yield.” The significance of these findings is readily grasped when it is realized, in Liu et al.’s words, that “China’s agriculture has to feed more
than one-fifth of the world’s population, and, historically, China has been famine prone.” They report that “as recently as the late 1950s and early 1960s a great famine claimed about
thirty million lives (Ashton et al., 1984; Cambridge History of China, 1987).” Moving from agro-ecosystems to natural ones, Su et al. (2004) used an ecosystem process model to explore
the sensitivity of the net primary productivity (NPP) of an oak forest near Beijing (China) to the global climate changes projected to result from a doubling of the atmosphere’s CO2
concentration from 355 to 710 ppm. The results of this work suggested that the aerial fertilization effect of the specified increase in the air’s CO2 content would raise the forest’s
NPP by 14.0 percent, that a concomitant temperature increase of 2°C would boost the NPP increase to 15.7 percent, and that adding a 20 percent increase in precipitation would push the
NPP increase to 25.7 percent. They calculated that a 20 percent increase in precipitation and a 4°C increase in temperature would also boost the forest’s NPP by 25.7 percent. Grunzweig
et al. (2003) tell the tale of the Yatir forest, a 2,800-hectare stand of Aleppo and other pine trees, that had been planted some 35 years earlier at the edge of the Negev Desert in
Israel. An intriguing aspect of this particular forest, which they characterize as growing in poor soil of only 0.2 to 1.0 meter’s depth above chalk and limestone, is that although it
is located in an arid part of Asia that receives less annual precipitation than all of the other scores of FluxNet stations in the global network of micrometeorological tower sites that
use eddy covariance methods to measure exchanges of CO2, water vapor and energy between terrestrial ecosystems and the atmosphere (Baldocchi et al., 2001), the forest’s annual net ecosystem
CO2 exchange was just as high as that of many highlatitude boreal forests and actually higher than that of most temperate forests. Grunzweig et al. note that the increase in atmospheric
CO2 concentration that has occurred since pre-industrial times should have improved water use efficiency (WUE) in most plants by increasing the ratio of CO2 fixed to water lost via evapotranspiration
They report that “reducing water loss in arid regions improves soil moisture conditions, decreases water stress and extends water availability,” which “can indirectly increase carbon
sequestration by influencing plant distribution, survival and expansion into water-limited environments.” That this hypothesis is correct has been demonstrated by Leavitt et al. (2003)
within the context of the long-term atmospheric CO2 enrichment experiment of Idso and Kimball (2001) on sour orange trees. It has also been confirmed in nature by Fang (2003), who obtained
identical (to the study of Leavitt et al.) CO2-induced WUE responses for 23 groups of naturally occurring trees scattered across western North America over the period 1800-1985, which
response, Fang concludes, “would have caused natural trees in arid environments to grow more rapidly, acting as a carbon sink for anthropogenic CO2,” which is exactly what Grunzweig
et al. found to be happening in the Yatir forest on the edge of the Negev Desert. Based primarily on satellite-derived Normalized Difference Vegetation Index (NDVI) data, Zhou et al.
(2001) found that from July 1981 to December 1999, between 40 and 70° N latitude, there was a persistent increase in growing season vegetative productivity in excess of 12 percent over
a broad contiguous swath of Asia stretching from Europe through Siberia to the Aldan plateau, where almost 58 percent of the land is forested. And in a companion study, Bogaert et al.
(2002) determined that this productivity increase occurred at a time when this vast Asian region showed an overall warming trend “with negligible occurrence of cooling.” In another study
that included a portion of Europe, Lapenis et al. (2005) analyzed trends in forest biomass in all 28 ecoregions of the Russian
Biological Effects of Carbon Dioxide Enrichment 555 territory, based on data collected from 1953 to 2002 within 3196 sample plots comprised of about 50,000 entries, which database, in
their words, “contains all available archived and published data.” This work revealed that over the period 1961-1998, as they describe it, “aboveground wood, roots, and green parts increased
by 4%, 21%, and 33%, respectively,” such that “the total carbon density of the living biomass stock of the Russian forests increased by ~9%.” They also report there was a concomitant
increase of ~11 percent in the area of Russian forests. In addition, the team of U.S., Austrian and Russian scientists reported that “within the range of 50-65° of latitude [the range
of 90 percent of Russian forests], the relationship between biomass density and the area-averaged NDVI is very close to a linear function, with a slope of ~1,” citing the work of Myneni
et al. (2001). Therefore, as they continue, “changes in the carbon density of live biomass in Russian forests occur at about the same rate as the increase in the satellite-based estimate
in the seasonally accumulated NDVI,” which observation strengthens the findings of all satellite-based NDVI studies. Returning to China for several concluding reports, we begin with
the work of Brogaard et al. (2005), who studied the dry northern and northwestern regions of the country—including the Inner Mongolia Autonomous Region (IMAR)— which had been thought
to have experienced declining vegetative productivity over the past few decades due to “increasing livestock numbers, expansion of cultivated land on erosive soils and the gathering
of fuel wood and herb digging,” which practices were believed to have been driven by rising living standards, which in combination with a growing population were assumed to have increased
the pressure on these marginal lands. In the case of increasing grazing, for example, Brogaard et al. note that the total number of livestock in the IMAR increased from approximately
46 million head in 1980 to about 71 million in 1997. To better assess the seriousness of this supposedly “ongoing land degradation process,” as they describe it, the researchers adapted
a satellitedriven parametric model, originally developed for Sahelian conditions, to the central Asian steppe region of the IMAR by including “additional stress factors and growth efficiency
computations.” The applied model, in their words, “uses satellite sensoracquired reflectance in combination with climate data to generate monthly estimates of gross primary production.”
To their great surprise, this work revealed that “despite a rapid increase in grazing animals on the steppes of the IMAR for the 1982-1999 period,” their model estimates did “not indicate
declining biological production.” Clearly, some strong positive influence compensated for the increased human and animal pressures on the lands of the IMAR over the period of Brogaard
et al.’s study. In this regard, they mention the possibility of increasing productivity on the agricultural lands of the IMAR, but they note that crops are grown on “only a small proportion
of the total land area.” Other potential contributing factors they mention are “an increase in precipitation, as well as afforestation projects.” Two things they do not mention are the
aerial fertilization effect and the transpiration-reducing effect of the increase in the air’s CO2 concentration that was experienced over the study period. Applied together, the sum
of these positive influences (and possibly others that remain unknown) was demonstrably sufficient to keep plant productivity from declining in the face of greatly increasing animal
and human pressures on the lands of the IMAR from 1982 to 1999. Piao et al. (2005a) used a time series of NDVI data from 1982 to 1999, together with precipitation and temperature data,
to investigate variations of desert area in China by “identifying the climatic boundaries of arid area and semiarid area, and changes in NDVI in these areas.” In doing so, they discovered
that “average rainy season NDVI in arid and semiarid regions both increased significantly during the period 1982-1999.” Specifically, they found that the NDVI increased for 72.3 percent
of total arid regions and for 88.2 percent of total semiarid regions, such that the area of arid regions decreased by 6.9 percent and the area of semiarid regions decreased by 7.9 percent.
They also report that by analyzing Thematic Mapper satellite images, “Zhang et al. (2003) documented that the process of desertification in the Yulin area, Shannxi Province showed a
decreased trend between 1987 and 1999,” and that “according to the national monitoring data on desertification in western China (Shi, 2003), the annual desertification rate decreased
from 1.2% in the 1950s to -0.2% at present.” Further noting that “variations in the vegetation coverage of these regions partly affect the frequency of sand-dust storm occurrence (Zou
and Zhai, 2004),” Piao et al. concluded that “increased vegetation coverage in these areas will likely fix soil, enhance its anti-wind-erosion ability, reduce the possibility of released
dust, and consequently cause a mitigation of
Climate Change Reconsidered 556 sand-dust storms.” They also reported that “recent studies have suggested that the frequencies of strong and extremely strong sand-dust storms in northern
China have significantly declined from the early 1980s to the end of the 1990s (Qian et al., 2002; Zhao et al., 2004).” Piao et al. (2006) investigated vegetation net primary production
(NPP) derived from a carbon model (Carnegie-Ames-Stanford approach, CASA) and its interannual change in the Qinghai-Xizang (Tibetan) Plateau using 1982-1999 NDVI data and paired ground-based
information on vegetation, climate, soil, and solar radiation. This work revealed that over the entire study period, NPP rose at a mean annual rate of 0.7 percent. However, Piao et al.
report that “the NPP trends in the plateau over the two decades were divided into two distinguished periods: without any clear trend from 1982 to 1990 and significant increase from 1991
to 1999.” The three researchers say their findings suggest that “vegetation growth on the plateau in the 1990s has been much enhanced compared to that in [the] 1980s, consistent with
the trend in the northern latitudes indicated by Schimel et al. (2001).” In addition, they say that “previous observational and NPP modeling studies have documented substantial evidence
that terrestrial photosynthetic activity has increased over the past two to three decades in the middle and high latitudes in the Northern Hemisphere,” and that “satellite-based NDVI
data sets for the period of 1982-1999 also indicate consistent
trends of NDVI increase,” citing multiple references in support of each of these statements. Piao et al.’s findings, therefore, add to the growing body of evidence that reveals a significant
“greening of the earth” is occurring. Applying the same techniques, Fang et al. (2003) looked at the whole of China, finding that its terrestrial NPP increased by 18.7 percent between
1982 and 1999. Referring to this result as “an unexpected aspect of biosphere dynamics,” they say that this increase “is is much greater than would be expected to result from the fertilization
effect of elevated CO2, and also greater than expected from climate, based on broad geographic patterns.” From 1982 to 1999, the atmosphere’s CO2 concentration rose by approximately
27.4 ppm. The aerial fertilization effect of this CO2 increase could be expected to have increased the NPP of the conglomerate of forest types found in China by about 7.3 percent. (See
the procedures and reasoning described in a CO2 Science editorial, September 18, 2002, http://www.co2science.org/articles/V5/N38/EDIT.php). But this increase is only a part of the total
NPP increase we could expect, for Fang et al. note that “much of the trend in NPP appeared to reflect a change towards an earlier growing season,” which was driven by the 1.1°C increase
in temperature they found to have occurred in their region of study between 1982 and 1999. Following this lead, we learn from the study of White et al. (1999)—which utilized 88 years
of data (1900-1987) that were obtained from 12 different locations within the eastern U.S. deciduous forest that stretches from Charleston, SC (32.8°N latitude) to Burlington, VT (44.5°N
latitude)—that a 1°C increase in mean annual air temperature increases the length of the forest’s growing season by approximately five days. In addition, White et al. determined that
a oneday extension in growing season length increased the mean forest NPP of the 12 sites they studied by an average of 1.6 percent. Hence, we could expect an additional NPP increase
due to the warming-induced growing season expansion experienced in China from 1982 to 1999 of 1.6 percent/day x 5 days = 8.0 percent, which brings the total CO2-induced plus warming-induced
increase in NPP to 15.3 percent. Last, we note there is a well-documented positive synergism between increasing air temperature and CO2 concentration (Idso and Idso, 1994), such that
the 1°C increase in temperature experienced in China between 1982 and 1999 could easily boost the initial CO2-induced 7.3 percent NPP enhancement to the 10.7 percent enhancement that
when combined with the 8.0 percent enhancement caused by the warminginduced increase in growing season length would produce the 18.7 percent increase in NPP detected in the satellite
data. In view of these observations, the findings of Fang et al. are seen to be right in line with what would be expected to result from the increases in air temperature and atmospheric
CO2 concentration that occurred between 1982 and 1999 in China: a stimulated terrestrial biosphere that is growing ever more productive with each passing year. This is the true observed
consequence of rising CO2 and temperature, and it is about as far removed as one can get from the negative scenarios offered by the IPCC. Analyzing the same set of data still further,
Piao et al. (2005b) say their results suggest that “terrestrial NPP in China increased at a rate of 0.015 Pg C yr-1 over the period 1982-1999, corresponding to a total increase of 18.5%,
or 1.03% annually.” They also found that “during the past 2 decades the amplitude of
Biological Effects of Carbon Dioxide Enrichment 557 the seasonal curve of NPP has increased and the annual peak NPP has advanced,” which they say “may indirectly explain the enhanced
amplitude and advanced timing of the seasonal cycle of atmospheric CO2 concentration (Keeling et al., 1996),” the former of which phenomena they further suggest “was probably due to
the rise in atmospheric CO2 concentration, elevated temperature, and increased atmospheric N and P deposition,” while the latter phenomenon they attribute to “advanced spring onset and
extended autumn growth owing to climate warming.” We are in basic agreement on most of these points, but note that the advanced onset of what may be called biological spring is also
fostered by the enhancement of early spring growth that is provided by the ongoing rise in the air’s CO2 concentration. Citing a total of 20 scientific papers at various places in the
following sentence from their research report, Piao et al. conclude that “results from observed atmospheric CO2 and O2 concentrations, inventory data, remote sensing data, and carbon
process models have all suggested that terrestrial vegetation NPP of the Northern Hemisphere has increased over the past 2 decades and, as a result, the northern terrestrial ecosystems
have become important sinks for atmospheric CO2.” In conclusion, the historical increases in the atmosphere’s CO2 concentration and temperature have fostered a significant greening of
the earth, including that observed throughout the length and breadth of Asia. It would appear that the climatic change claimed by the IPCC to have been experienced by the globe over
the latter part of the twentieth century either did not occur or was dwarfed by opposing phenomena that significantly benefited China, as its lands grew ever greener during this period
and its increased vegetative cover helped to stabilize its soils and throw feared desertification into reverse. Additional information on this topic, including reviews of newer publications
as they become available, can be found at http://www.co2science.org/subject/g/asiagreen.php. References Ashton, B., Hill, K., Piazza, A. and Zeitz, R. 1984. Famine in China, 1958-1961.
Population and Development Review 10: 613-615. Baldocchi, D., Falge, E., Gu, L.H., Olson, R., Hollinger, D., Running, S., Anthoni, P., Bernhofer, C., Davis, K., Evans, R., Fuentes, J.,
Goldstein, A., Katul, G., Law B., Lee, X.H., Malhi, Y., Meyers, T., Munger, W., Oechel, W., Paw U, K.T., Pilegaard, K., Schmid, H.P., Valentini, R., Verma, S., Vesala, T., Wilson, K.
and Wofsy, S. 2001. FLUXNET: A new tool to study the temporal and spatial variability of ecosystem-scale carbon dioxide, water vapor, and energy flux densities. Bulletin of the American
Meteorological Society 82: 2415-2434. Bogaert, J., Zhou, L., Tucker, C.J, Myneni, R.B. and Ceulemans, R. 2002. Evidence for a persistent and extensive greening trend in Eurasia inferred
from satellite vegetation index data. Journal of Geophysical Research 107: 10.1029/2001JD001075. Brogaard, S., Runnstrom, M. and Seaquist, J.W. 2005. Primary production of Inner Mongolia,
China, between 1982 and 1999 estimated by a satellite data-driven light use efficiency model. Global and Planetary Change 45: 313-332. Fang, J., Piao, S., Field, C.B., Pan, Y., Guo,
Q., Zhou, L., Peng, C. and Tao, S. 2003. Increasing net primary production in China from 1982 to 1999. Frontiers in Ecology and the Environment 1: 293-297. Feng, X. 1999. Trends in intrinsic
water-use efficiency of natural trees for the past 100-200 years: A response to atmospheric CO2 concentration. Geochimica et Cosmochimica Acta 63: 1891-1903. Grunzweig, J.M., Lin, T.,
Rotenberg, E., Schwartz, A. and Yakir, D. 2003. Carbon sequestration in arid-land forest. Global Change Biology 9: 791-799. Idso, K.E. and Idso, S.B. 1994. Plant responses to atmospheric
CO2 enrichment in the face of environmental constraints: A review of the past 10 years' research. Agricultural and Forest Meteorology 69: 153-203. Idso, S.B. and Kimball, B.A. 2001.
CO2 enrichment of sour orange trees: 13 years and counting. Environmental and Experimental Botany 46: 147-153. Keeling, C.D., Chin, J.F.S. and Whorf, T.P. 1996. Increased activity of
northern vegetation inferred from atmospheric CO2 measurements. Nature 382: 146-149. Lapenis, A., Shvidenko, A., Shepaschenko, D., Nilsson, S. and Aiyyer, A. 2005. Acclimation of Russian
forests to recent changes in climate. Global Change Biology 11: 2090-2102. Leavitt, S.W., Idso, S.B., Kimball, B.A., Burns, J.M., Sinha, A. and Stott, L. 2003. The effect of long-term
atmospheric CO2 enrichment on the intrinsic water-use efficiency of sour orange trees. Chemosphere 50: 217-222. Liu, H., Li, X., Fischer, G. and Sun, L. 2004. Study on the impacts of
climate change on China’s agriculture. Climatic Change 65: 125-148.
Climate Change Reconsidered 558 Myneni, R.B., Dong, J., Tucker, C.J., Kaufmann, R.K., Kauppi, P.E., Liski, J., Zhou, L., Alexeyev, V. and Hughes, M.K. 2001. A large carbon sink in the
woody biomass of Northern forests. Proceedings of the National Academy of Sciences, USA: 98: 14,784-14,789. Piao, S., Fang, J. and He, J. 2006. Variations in vegetation net primary production
in the Qinghai-Xizang Plateau, China, from 1982-1999. Climatic Change 74: 253-267. Piao, S., Fang, J., Liu, H. and Zhu, B. 2005a. NDVIindicated decline in desertification in China in
the past two decades. Geophysical Research Letters 32: 10.1029/2004GL021764. Piao, S., Fang, J., Zhou, L., Zhu, B., Tan, K. and Tao, S. 2005b. Changes in vegetation net primary productivity
from 1982 to 1999 in China. Global Biogeochemical Cycles 19: 10.1029/2004GB002274. Qian, Z.A., Song, M.H. and Li, W.Y. 2002. Analysis on distributive variation and forecast of sand-dust
storms in recent 50 years in north China. Journal of Desert Research 22: 106-111. Schimel, D.S., House, J.I., Hibbard, J.I., Bousquet, P., Ciais, P., Peylin, P., Braswell, B.H., Apps,
M.J., Baker, D., Bondeau, A., Canadell, J., Churkina, G., Cramer, W., Denning, A.S., Field, C.B., Friedlingstein, P., Goodale, C., Heimann, M., Houghton, R.A., Melillo, J.M., Moore III,
B., Murdiyarso, D., Noble, I., Pacala, S.W., Prentice, I.C., Raupach, M.R., Rayner, P.J., Scholes, R.J., Steffen, W.L. and Wirth, C. 2001. Recent patterns and mechanisms of carbon exchange
by terrestrial ecosystems. Nature 414: 169-172. Shi, Y.F., Ed. 2003. An Assessment of the Issues of Climatic Shift from Warm-Dry to Warm-Wet in Northwest China. China Meteorology, Beijing.
Su, H.-X. and Sang, W.-G. 2004. Simulations and analysis of net primary productivity in Quercus liaotungensis forest of Donglingshan Mountain Range in response to different climate change
scenarios. Acta Botanica Sinica 46: 1281-1291. White, M.A., Running, S.W. and Thornton, P.E. 1999. The impact of growing-season length variability on carbon assimilation and evapotranspiration
over 88 years in the eastern US deciduous forest. International Journal of Biometeorology 42: 139-145. Zhang, L., Yue, L.P. and Xia, B. 2003. The study of land desertification in transitional
zones between the MU US desert and the Loess Plateau using RS and GIS—A case study of the Yulin region. Environmental Geology 44: 530-534. Zhao, C., Dabu, X. and Li, Y. 2004. Relationship
between climatic factors and dust storm frequency in Inner Mongolia of China. Geophysical Research Letters 31: 10.1029/2003GL018351. Zhou, L., Tucker, C.J., Kaufmann, R.K., Slayback,
D., Shabanov, N.V. and Myneni, R.B. 2001. Variations in northern vegetation activity inferred from satellite data of vegetation index during 1981-1999. Journal of Geophysical Research
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7.9.3. Europe Allen et al. (1999) analyzed sediment cores from a lake in southern Italy and from the Mediterranean Sea, developing high-resolution climate and vegetation data sets for
this region over the last 102,000 years. These materials indicated that rapid changes in vegetation were well correlated with rapid changes in climate, such that complete shifts in natural
ecosystems would sometimes occur over periods of less than 200 years. Over the warmest portion of the record (the Holocene), the total organic carbon content of the vegetation reached
its highest level, more than doubling values experienced over the rest of the record, while other proxy indicators revealed that during the more productive woody-plant period of the
Holocene, the increased vegetative cover also led to less soil erosion. The results of this study demonstrate that the biosphere can successfully respond to rapid changes in climate.
As the 15 researchers involved in the work put it, “the biosphere was a full participant in these rapid fluctuations, contrary to widely held views that vegetation is unable to change
with such rapidity.” Furthermore, their work revealed that warmer was always better in terms of vegetative productivity. Osborne et al. (2000) used an empirically based mechanistic model
of Mediterranean shrub vegetation to address two important questions: (1) Has recent climate change, especially increased drought, negatively impacted Mediterranean shrublands? and (2)
Has the historical increase in the air’s CO2 concentration modified this impact? The data-based model they employed suggests that the warming and reduced precipitation experienced in
the Mediterranean area over the past century should have had negative impacts on net primary production and leaf area index. When the measured increase in
Biological Effects of Carbon Dioxide Enrichment 559 atmospheric CO2 concentration experienced over the period was factored into the calculation, however, these negative influences were
overpowered, with the net effect that both measures of vegetative prowess increased: net primary productivity by 25 percent and leaf area index by 7 percent. These results, in their
words, “indicate that the recent rise in atmospheric CO2 may already have had significant impacts on productivity, structure and water relations of sclerophyllous shrub vegetation, which
tended to offset the detrimental effects of climate change in the region.” How can we relate this observation to climate change predictions for the earth as a whole? For a nominal doubling
of the air’s CO2 concentration from 300 to 600 ppm, earth’s mean surface air temperature is predicted by current climate models to rise by approximately 3°C, which equates to a temperature
rise of 0.01°C per ppm CO2. In the case of the Mediterranean region here described, the temperature rise over the past century was quoted by Osborne et al. as being 0.75°C, over which
period of time the air’s CO2 concentration rose by approximately 75 ppm, for an analogous climate response of exactly the same value: 0.01°C per ppm CO2. With respect to model-predicted
changes in earth’s precipitation regime, a doubling of the air’s CO2 content is projected to lead to a modest intensification of the planet’s hydrologic cycle. In the case of the Mediterranean
region over the last century, however, there has been a recent tendency toward drier conditions. Hence, the specific case investigated by Osborne et al. represents a muchworse-case scenario
than what is predicted by current climate models for the earth as a whole. Nevertheless, the area’s vegetation has done even better than it did before the climatic change, thanks to
the overpowering beneficial biological effects of the concurrent rise in the air’s CO2 content. Cheddadi et al. (2001) employed a standard biogeochemical model (BIOME3)—which uses monthly
temperature and precipitation data, certain soil characteristics, cloudiness, and atmospheric CO2 concentration as inputs—to simulate the responses of various biomes in the region surrounding
the Mediterranean Sea to changes in both climate (temperature and precipitation) and the air’s CO2 content. Their first step was to validate the model for two test periods: the present
and 6000 years before present (BP). Recent instrumental records provided actual atmospheric CO2, temperature and precipitation data for the present period; while pollen data were used
to reconstruct monthly temperature and precipitation values for 6000 years BP, and ice core records were used to determine the atmospheric CO2 concentration of that earlier epoch. These
efforts suggested that winter temperatures 6000 years ago were about 2°C cooler than they are now, that annual rainfall was approximately 200 mm less than today, and that the air’s CO2
concentration averaged 280 ppm, which is considerably less than the value of 345 ppm the researchers used to represent the present, i.e., the mid-point of the period used for calculating
30-year climate normals at the time they wrote their paper. Applying the model to these two sets of conditions, they demonstrated that “BIOME3 can be used to simulate ... the vegetation
distribution under ... different climate and [CO2] conditions than today,” where [CO2] is the abbreviation they use to represent “atmospheric CO2 concentration.” Cheddadi et al.’s next
step was to use their validated model to explore the vegetative consequences of an increase in anthropogenic CO2 emissions that pushes the air’s CO2 concentration to a value of 500 ppm
and its mean annual temperature to a value 2°C higher than today’s mean value. The basic response of the vegetation to this change in environmental conditions was “a substantial southward
shift of Mediterranean vegetation and a spread of evergreen and conifer forests in the northern Mediterranean.” More specifically, in the words of the researchers, “when precipitation
is maintained at its present-day level, an evergreen forest spreads in the eastern Mediterranean and a conifer forest in Turkey.” Current xerophytic woodlands in this scenario become
“restricted to southern Spain and southern Italy and they no longer occur in southern France.” In northwest Africa, on the other hand, “Mediterranean xerophytic vegetation occupies a
more extensive territory than today and the arid steppe/desert boundary shifts southward,” as each vegetation zone becomes significantly more verdant than it is currently. What is the
basis for these positive developments? Cheddadi et al. say “the replacement of xerophytic woodlands by evergreen and conifer forests could be explained by the enhancement of photosynthesis
due to the increase of [CO2].” Likewise, they note that “under a high [CO2] stomata will be much less open which will lead to a reduced evapotranspiration and lower water loss, both
for C3 and C4 plants,” adding that “such mechanisms may
Climate Change Reconsidered 560 help plants to resist long-lasting drought periods that characterize the Mediterranean climate.” Contrary to what is often predicted for much of the world’s
moisture-challenged lands, therefore, the authors were able to report that “an increase of [CO2], jointly with an increase of ca. 2°C in annual temperature would not lead to desertification
on any part of the Mediterranean unless annual precipitation decreased drastically,” where they define a drastic decrease as a decline of 30 percent or more. Equally important in this
context is the fact that Hennessy et al. (1997) have indicated that a doubling of the air’s CO2 content would in all likelihood lead to a 5 to 10 percent increase in annual precipitation
at Mediterranean latitudes, which is also what is predicted for most of the rest of the world. Hence, the results of the present study—where precipitation was held constant—may validly
be considered to be a worst-case scenario, with the true vegetative response being even even better than the good-news results reported by Cheddadi et al., even when utilizing what we
believe to be erroneously inflated global warming predictions. Julien et al. (2006) “used land surface temperature (LST) algorithms and NDVI [Normalized Difference Vegetation Index]
values to estimate changes in vegetation in the European continent between 1982 and 1999 from the Pathfinder AVHRR [Advanced Very High Resolution Radiometer] Land (PAL) dataset.” This
program revealed that arid and semi-arid areas (Northern Africa, Southern Spain and the Middle East) have seen their mean LST increase and NDVI decrease, while temperate areas (Western
and Central Europe) have suffered a slight decrease in LST but a more substantial increase in NDVI, especially in Germany, the Czech Republic, Poland and Belarus. In addition, parts
of continental and Northern Europe have experienced either slight increases or decreases in NDVI while LST values have decreased. Considering the results in their totality, the Dutch
and Spanish researchers concluded that, over the last two decades of the twentieth century, “Europe as a whole has a tendency to greening,” and much of it is “seeing an increase in its
wood land proportion.” Working in the Komi Republic in the northeast European sector of Russia, Lopatin et al. (2006) (1) collected discs and cores from 151 Siberian spruce trees and
110 Scots pines from which they developed ring-width chronologies that revealed yearly changes in forest productivity, (2) developed satellite-based time series of NDVI for the months
of June, July, August over the period 1982-2001, (3) correlated their site-specific ring-width-derived productivity histories with same-site NDVI time series, (4) used the resulting
relationship to establish six regional forest productivity histories for the period 1982-2001, and (5) compared the six regional productivity trends over this period with corresponding-region
temperature and precipitation trends. For all six vegetation zones of the Komi Republic, this work indicated that the 1982-2001 trends of integrated NDVI values from June to August were
positive, and that the “increase in productivity reflected in [the] NDVI data [was] maximal on the sites with increased temperature and decreased precipitation.” In discussing their
findings, the three scientists state that “several studies (Riebsame et al., 1994; Myneni et al., 1998; Vicente-Serrano et al., 2004) have shown a recent increase in vegetation cover
in different world ecosystems.” What is special about their study, as they describe it, is that “in Europe, most forests are managed, except for those in northwestern Russia [the location
of their work], where old-growth natural forests are dominant (Aksenov et al., 2002).” Consequently, and because of their positive findings, they say we can now conclude that “productivity
during recent decades also increased in relatively untouched forests,” where non-management-related “climate change with lengthening growing season, increasing CO2 and nitrogen deposition”
are the primary determinants of changes in forest productivity. In conclusion, this brief review of pertinent studies conducted in Europe strongly contradicts today’s obsession with
the ongoing rise in the atmosphere’s CO2 content, as well as the many environmental catastrophes it has been predicted to produce. The results of rising CO2 concentrations and temperatures
in the twentieth century were overwhelmingly positive. Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.or
/subject/g/europegreen.php. References Aksenov, D., Dobrynin, D., Dubinin, M., Egorov, A., Isaev, A., Karpachevskiy, M., Laestadius, L., Potapov, P., Purekhovskiy, P., Turubanova, S.
and Yaroshenko, A. 2002. Atlas of Russia’s Intact Forest Landscapes. Global Forest Watch Russia, Moscow.
Biological Effects of Carbon Dioxide Enrichment 561 Allen, J.R.M., Brandt, U., Brauer, A., Hubberten, H.-W., Huntley, B., Keller, J., Kraml, M., Mackensen, A., Mingram, J., Negendank,
J.F.W., Nowaczyk, N.R., Oberhansli, H., Watts, W.A., Wulf, S. and Zolitschka, B. 1999. Rapid environmental changes in southern Europe during the last glacial period. Nature 400: 740-743.
Cheddadi, R., Guiot, J. and Jolly, D. 2001. The Mediterranean vegetation: what if the atmospheric CO2 increased? Landscape Ecology 16: 667-675. Hennessy, K.J., Gregory, J.M. and Mitchell,
J.F.B. 1997. Changes in daily precipitation under enhanced greenhouse conditions. Climate Dynamics 13: 667-680. Julien, Y., Sobrino, J.A. and Verhoef, W. 2006. Changes in land surface
temperatures and NDVI values over Europe between 1982 and 1999. Remote Sensing of Environment 103: 43-55. Lopatin, E., Kolstrom, T. and Spiecker, H. 2006. Determination of forest growth
trends in Komi Republic (northwestern Russia): combination of tree-ring analysis and remote sensing data. Boreal Environment Research 11: 341-353. Myneni, R.B., Tucker, C.J., Asrar,
G. and Keeling, C.D. 1998. Interannual variations in satellite-sensed vegetation index data from 1981 to 1991. Journal of Geophysical Research 103: 6145-6160. Osborne, C.P., Mitchell,
P.L., Sheehy, J.E. and Woodward, F.I. 2000. Modellng the recent historical impacts of atmospheric CO2 and climate change on Mediterranean vegetation. Global Change Biology 6: 445-458.
Riebsame, W.E., Meyer, W.B. and Turner, B.L. 1994. Modeling land-use and cover as part of global environmental-change. Climatic Change 28: 45-64. Vicente-Serrano, S.M., Lasanta, T. and
Romo, A. 2004. Analysis of spatial and temporal evolution of vegetation cover in the Spanish central Pyrenees: Role of human management. Environmental Management 34: 802-818. 7.9.4.
North America In a paper titled “Variations in northern vegetation activity inferred from satellite data of vegetation index during 1981-1999,” Zhou et al. (2001) determined that the
magnitude of the the satellite-derived normalized difference vegetation index (NDVI) rose by 8.44 percent in North America over this period. Noting that the NDVI “can be used to proxy
the vegetation’s responses to climate changes because it is well correlated with the fraction of photosynthetically active radiation absorbed by plant canopies and thus leaf area, leaf
biomass, and potential photosynthesis,” they went on to suggest that the increases in plant growth and vitality implied by their NDVI data were primarily driven by concurrent increases
in near-surface air temperature, although temperatures may have actually declined throughout the eastern part of the United States over the period of their study. Zhou et al.’s attribution
of this “greening” of the continent to increases in near-surface air temperature was challenged by Ahlbeck (2002), who suggested that the observed upward trend in NDVI was primarily
driven by the increase in the air’s CO2 concentration, and that fluctuations in temperature were primarily responsible for variations about the more steady upward trend defined by the
increase in CO2. In replying to this challenge, Kaufmann et al. (2002) claimed Ahlbeck was wrong and reaffirmed their initial take on the issue. We believe it was Ahlbeck who was “clearly
the ‘more correct’ of the two camps.” (See the discussion in CO2 Science at www.co2science.org/articles/V5/N38/EDIT.php.) About the same time, Hicke et al. (2002) computed net primary
productivity (NPP) over North America for the years 1982-1998 using the Carnegie-Ames-Stanford Approach (CASA) carbon cycle model, which was driven by a satellite NDVI record at 8-km
spatial resolution. This effort revealed that NPP increases of 30 percent or more occurred across the continent from 1982 to 1998. During this period, the air’s CO2 concentration rose
by 25.74 ppm, as calculated from the Mauna Loa data of Keeling and Whorf (1998), which amount is 8.58 percent of the 300 ppm increase often used in experiments on plant growth responses
to atmospheric CO2 enrichment. Consequently, for herbaceous plants that display NPP increases of 30-40 percent in response to a 300-ppm increase in atmospheric CO2 concentration, the
CO2-induced NPP increase experienced between 1982 and 1998 would be expected to have been 2.6-3.4 percent. Similarly, for woody plants that display NPP increases of 60-80 percent in
response to a 300-ppm increase in atmospheric CO2 (Saxe et al., 1998; Idso and Kimball, 2001), the expected increase in productivity between 1982 and 1998 would have been 5.1-6.9 percent.
Since both of these NPP increases are considerably less that the 30 percent or more observed by Hicke et al., additional factors must have helped to stimulate NPP over this period, some
of which may have been concomitant increases in precipitation and air temperature, the tendency for warming to lengthen growing seasons and enhance
Climate Change Reconsidered 562 the aerial fertilization effect of rising CO2 concentrations, increasingly intensive crop and forest management, increasing use of genetically improved
plants, the regrowth of forests on abandoned cropland, and improvements in agricultural practices such as irrigation and fertilization. Whatever the mix might have been, one thing is
clear: Its effect was overwhelmingly positive. In a study based on a 48-year record derived from
an average of 17 measurements per year, Raymond and Cole (2003) demonstrated that the export of alkalinity, in the form of bicarbonate ions, from the Mississippi River to the Gulf of
Mexico had increased by approximately 60 percent since 1953. “This increased export,” as they described it, was “in part the result of increased flow resulting from higher rainfall in
the Mississippi basin,” which had led to a 40 percent increase in annual Mississippi River discharge to the Gulf of Mexico over the same time period. The remainder, however, had to have
been due to increased rates of chemical weathering of soil minerals. What factors might have been responsible for this phenomenon? The two researchers noted that potential mechanisms
included “an increase in atmospheric CO2, an increase [in] rainwater throughput, or an increase in plant and microbial production of CO2 and organic acids in soils due to biological
responses to increased rainfall and temperature.” Unfortunately, they forgot to mention the increase in terrestrial plant productivity that is produced by the increase in the aerial
fertilization effect provided by the historical rise in the air’s CO2 content, which also leads to “an increase in plant and microbial production of CO2 and organic acids in soils.”
This phenomenon should have led to an increase in Mississippi River alkalinity equivalent to that which they had observed since 1953. In a study using data obtained from dominant stands
of loblolly pine plantations growing at 94 locations spread across the southeastern United States, Westfall and Amateis (2003) employed mean height measurements made at three-year intervals
over a period of 15 years to calculate a site index related to the mean growth rate for each of five threeyear periods, which index would be expected to increase monotonically if growth
rates were being enhanced above normal by some monotonically increasing factor that promotes growth. This work revealed, in their words, that “mean site index over the 94 plots consistently
increased at each remeasurement period,” which would suggest, as they further state, that “loblolly pine plantations are realizing greater than expected growth rates,” and, we would
add, that the growth rate increases are growing larger and larger with each succeeding three-year period. As to what could be causing the monotonically increasing growth rates of loblolly
pine trees over the entire southeastern United States, Westfall and Amateis named increases in temperature and precipitation in addition to rising atmospheric CO2 concentrations. However,
they report that a review of annual precipitation amounts and mean ground surface temperatures showed no trends in these factors over the period of their study. They also suggested that
if increased nitrogen deposition were the cause, “such a factor would have to be acting on a regional scale to produce growth increases over the range of study plots.” Hence, they tended
to favor the ever-increasing aerial fertilization effect of atmospheric CO2 enrichment as being responsible for the accelerating pine tree growth rates. Returning to satellite studies,
Lim et al. (2004) correlated the monthly rate of relative change in NDVI, which they derived from advanced very high resolution radiometer data, with the rate of change in atmospheric
CO2 concentration during the natural vegetation growing season within three different ecoregion zones of North America (Arctic and Sub-Arctic Zone, Humid Temperate Zone, and Dry and
Desert Zone, which they further subdivided into 17 regions) over the period 1982-1992, after which they explored the temporal progression of annual minimum NDVI over the period 1982-2001
throughout the eastern humid temperate zone of North America. The result of these operations was that in all of the regions but one, according to the researchers, “dCO2 was positively
correlated with the rate of change in vegetation greenness in the following month, and most correlations were high,” which they say is “consistent with a CO2 fertilization effect” of
the type observed in “experimental manipulations of atmospheric CO2 that report a stimulation of photosynthesis and above-ground productivity at high CO2.” In addition, they determined
that the yearly “minimum vegetation greenness increased over the period 1982-2001 for all the regions of the eastern humid temperate zone in North America.” As for the cause of this
phenomenon, Lim et al. say that rising CO2 could “increase minimum greenness by stimulating photosynthesis at the beginning of the growing season,” citing the work of Idso et al. (2000).
In a somewhat similar study, but one that focused more intensely on climate change, Xiao and Moody (2004) examined the responses of the normalized
Biological Effects of Carbon Dioxide Enrichment 563 difference vegetation index integrated over the growing season (gNDVI) to annual and seasonal precipitation, maximum temperature (Tmax)
and minimum temperature (Tmin) over an 11-year period (1990-2000) for six biomes in the conterminous United States (Evergreen Needleleaf Forest, Deciduous Broadleaf Forest, Mixed Forest,
Open Shrubland, Woody Savanna and Grassland), focusing on within-and across-biome variance in long-term average gNDVI and emphasizing the degree to which this variance is explained by
spatial gradients in longterm average seasonal climate. The results of these protocols indicated that the greatest positive climatechange impacts on biome productivity were caused by
increases in spring, winter and fall precipitation, as well as increases in fall and spring temperature, especially Tmin, which has historically increased at roughly twice the rate of
Tmax in the United States. Hence, “if historical climatic trends and the biotic responses suggested in this analysis continue to hold true, we can anticipate further increases in productivity
for both forested and nonforested ecoregions in the conterminous US, with associated implications for carbon budgets and woody proliferation.” Goetz et al. (2005) transformed satellite-derived
NDVI data obtained across boreal North America (Canada and Alaska) for the period 1982-2003 into photosynthetically active radiation absorbed by green vegetation and treated the result
as a proxy for relative June-August gross photosynthesis (Pg), stratifying the results by vegetation type and comparing them with spatially matched concomitant trends in surface air
temperature data. Over the course of the study, this work revealed that area-wide tundra experienced a significant increase in Pg in response to a similar increase in air temperature;
and Goetz et al. say “this observation is supported by a wide and increasing range of local field measurements characterizing elevated net CO2 uptake (Oechel et al., 2000), greater depths
of seasonal thaw (Goulden et al., 1998), changes in the composition and density of herbaceous vegetation (Chapin et al., 2000; Epstein et al., 2004), and increased woody encroachment
in the tundra areas of North America (Sturm et al., 2001).” In the case of interior forest, on the other hand, there was no significant increase in air temperature and essentially no
change in Pg, with the last data point of the series being essentially indistinguishable from the first. This latter seemingly aberrant observation is in harmony with the fact that at
low temperatures the growth-promoting effects of increasing atmospheric CO2 levels are often very small or even non-existent (Idso and Idso, 1994), which is what appears to have been
the case with North American boreal forests over the same time period. As a result, Canada’s and Alaska’s tundra ecosystems exhibited increasing productivity over the past couple of
decades, while their boreal forests did not. Also working in Alaska, Tape et al. (2006) analyzed repeat repeat photography data from a photo study of the Colville River conducted between
1945 and 1953, as well as 202 new photos of the same sites that were obtained between 1999 and 2002, to determine the nature of shrub expansion in that region over the past half-century.
This approach revealed, in their words, that “large shrubs have increased in size and abundance over the past 50 years, colonizing areas where previously there were no large shrubs.”
In addition, they say their review of plot and remote sensing studies confirms that “shrubs in Alaska have expanded their range and grown in size” and that “a population of smaller,
intertussock shrubs not generally sampled by the repeat photography, is also expanding and growing.” Taken together, they conclude that “these three lines of evidence allow us to infer
a general increase in tundra shrubs across northern Alaska.” Tape et al. attribute this to largescale pan-Arctic warming. From analyses of logistic growth curves, they estimate that
the expansion began about 1900, “well before the current warming in Alaska (which started about 1970).” Hence, they conclude that “the expansion predates the most recent warming trend
and is perhaps associated with the general warming since the Little Ice Age.” These inferences appear reasonable, although we would add that the 80-ppm increase in the atmosphere’s CO2
concentration since 1900 likely played a role in the shrub expansion as well. If continued, the researchers say the transition “will alter the fundamental architecture and function of
this ecosystem with important ramifications,” the great bulk of which, in our opinion, will be positive. Working at eight different sites within the Pacific Northwest of the United States,
Soule and Knapp (2006) studied ponderosa pine trees to see how they may have responded to the increase in the atmosphere’s CO2 concentration that occurred after 1950. The two geographers
say the sites they chose “fit several criteria designed to limit potential confounding influences associated with anthropogenic disturbance.” In addition, they selected locations with
“a variety of climatic and topo-edaphic conditions, ranging from extremely water-limiting
Climate Change Reconsidered 564 environments ... to areas where soil moisture should be a limiting factor for growth only during extreme drought years.” They also say that all sites
were located in areas “where ozone concentrations and nitrogen deposition are typically low.” At all eight of the sites that met all of these criteria, Soule and Knapp obtained core
samples from about 40 mature trees that included “the potentially oldest trees on each site,” so that their results would indicate, as they put it, “the response of mature, naturally
occurring ponderosa pine trees that germinated before anthropogenically elevated CO2 levels, but where growth, particularly post-1950, has occurred under increasing and substantially
higher atmospheric CO2 concentrations.” Utilizing meteorological evaluations of the Palmer Drought Severity Index, they thus compared ponderosa pine radial growth rates during matched
wet and dry years pre-and post-1950. Overall, the two researchers found a post-1950 radial growth enhancement that was “more pronounced during drought years compared with wet years,
and the greatest response occurred at the most stressed site.” As for the magnitude of the response, they determined that “the relative change in growth [was] upward at seven of our
[eight] sites, ranging from 11 to 133%.” With respect to the significance of their observations, Soule and Knapp say their results show that “radial growth has increased in the post-1950s
period ... while climatic conditions have generally been unchanged,” which further suggests that “nonclimatic driving forces are operative.” In addition, they say the “radial growth
responses are generally consistent with what has been shown in long-term open-top chamber (Idso and Kimball, 2001) and free-air CO2 enrichment (FACE) studies (Ainsworth and Long, 2005).”
They conclude their findings “suggest that elevated levels of atmospheric CO2 are acting as a driving force for increased radial growth of ponderosa pine, but that the overall influence
of this effect may be enhanced, reduced or obviated by site-specific conditions.” Wang et al. (2006) examined ring-width development in cohorts of young and old white spruce trees in
a mixed grass-prairie ecosystem in southwestern Manitoba, Canada, where a 1997 wildfire killed most of the older trees growing in high-density spruce islands, but where younger trees
slightly removed from the islands escaped the ravages of the flames. There, “within each of a total of 24 burned islands,” in the words of the three researchers, “the largest dominant
tree (dead) was cut down and a disc was then sampled from the stump height,” while “adjacent to each sampled island, a smaller, younger tree (live) was also cut down, and a disc was
sampled from the stump height.” After removing size-, age-and climate-related trends in radial growth from the ring-width histories of the trees, Wang et al. plotted the residuals as
functions of time for the 30-year periods for which both the old and young trees would have been approximately the same age: 1900-1929 for the old trees and 1970-1999 for the young trees.
During the first of these periods, the atmosphere’s CO2 concentration averaged 299 ppm; during the second it averaged 346 ppm. Also, the mean rate-of-rise of the atmosphere’s CO2 concentration
was 0.37 ppm/year for first period and 1.43 ppm/year for the second. The results of this exercise revealed that the slope of the linear regression describing the rate-of-growth of the
ring-width residuals for the later period (when the air’s CO2 concentration was 15 percent greater and its rate-of-rise 285 percent greater) was more than twice that of the linear regression
describing the rateof-growth of the ring-width residuals during the earlier period. As the researchers describe it, these results show that “at the same developmental stage, a greater
growth response occurred in the late period when atmospheric CO2 concentration and the rate of atmospheric CO2 increase were both relatively high,” and they say that “these results are
consistent with expectations for CO2-fertilization effects.” In fact, they say “the response of the studied young trees can be taken as strong circumstantial evidence for the atmospheric
CO2-fertilization effect.” Another thing Wang et al. learned was that “postdrought growth response was much stronger for young trees (1970-1999) compared with old trees at the same development
stage (1900-1929),” and they add that “higher atmospheric CO2 concentration in the period from 1970-1999 may have helped white spruce recover from severe drought.” In a similar vein,
they also determined that young trees showed a weaker relationship to precipitation than did old trees, noting that “more CO2 would lead to greater water-use efficiency, which may be
dampening the precipitation signal in young trees.” In summary, Wang et al.’s unique study provides an exciting real-world example of the benefits the historical increase in the air’s
CO2 content has likely conferred on nearly all of earth’s plants, and especially its long-lived woody species. Together with the results of the other North American studies we have reviewed,
this body of research paints a
Biological Effects of Carbon Dioxide Enrichment 565 picture of a significant greening of North America in the twentieth century. Additional information on this topic, including reviews
of newer publications as they become available, can be found at http://www.co2science.org/subject/g/namergreen.php. References Ahlbeck, J.R. 2002. Comment on “Variations in northern
vegetation activity inferred from satellite data of vegetation index during 1981-1999” by L. Zhou et al. Journal of Geophysical Research 107: 10.1029/2001389. Ainsworth, E.A. and Long,
S.P. 2005. What have we learned from 15 years of free-air CO2 enrichment (FACE)? A meta-analytic review of the responses of photosynthesis, canopy properties and plant production to
rising CO2. New Phytologist 165: 351-372. Chapin III, F.S., McGuire, A.D., Randerson, J., Pielke, R., Baldocchi, D., Hobbie, S.E., Roulet, N., Eugster, W., Kasischke, E., Rastetter,
E.B., Zimov, S.A., and Running, S.W. 2000. Arctic and boreal ecosystems of western North America as components of the climate system. Global Change Biology 6: 211-223. Epstein, H.E.,
Calef, M.P., Walker, M.D., Chapin III, F.S. and Starfield, A.M. 2004. Detecting changes in arctic tundra plant communities in response to warming over decadal time scales. Global Change
Biology 10: 1325-1334. Goetz, S.J., Bunn, A.G., Fiske, G.J. and Houghton, R.A. 2005. Satellite-observed photosynthetic trends across boreal North America associated with climate and
fire disturbance. Proceedings of the National Academy of Sciences 102: 13,521-13,525. Goulden, M.L., Wofsy, S.C., Harden, J.W., Trumbore, S.E., Crill, P.M., Gower, S.T., Fries, T., Daube,
B.C., Fan, S.M., Sutton, D.J., Bazzaz, A. and Munger, J.W. 1998. Sensitivity of boreal forest carbon balance to soil thaw. Science 279: 214-217. Hicke, J.A., Asner, G.P., Randerson,
J.T., Tucker, C., Los, S., Birdsey, R., Jenkins, J.C. and Field, C. 2002. Trends in North American net primary productivity derived from satellite observations, 1982-1998. Global Biogeochemical
Cycles 16: 10.1029/2001GB001550. Idso, C.D., Idso, S.B., Kimball, B.A., Park, H., Hoober, J.K. and Balling Jr., R.C. 2000. Ultra-enhanced spring branch growth in CO2-enriched trees:
can it alter the phase of the atmosphere’s seasonal CO2 cycle? Environmental and Experimental Botany 43: 91-100. Idso, K.E. and Idso, S.B. 1994. Plant responses to atmospheric CO2 enrichment
in the face of environmental constraints: a review of the past 10 years’ research. Agricultural and Forest Meteorology 69: 153-203. Idso, S.B. and Kimball, B.A. 2001. CO2 enrichment
of sour orange trees: 13 years and counting. Environmental and Experimental Botany 46: 147-153. Kaufmann, R.K., Zhou, L., Tucker, C.J., Slayback, D., Shabanov, N.V. and Myneni, R.B.
2002. Reply to Comment on “Variations in northern vegetation activity inferred from satellite data of vegetation index during 1981-1999: by J.R. Ahlbeck. Journal of Geophysical Research
107: 10.1029/1001JD001516. Keeling, C.D. and Whorf, T.P. 1998. Atmospheric CO2 Concentrations—Mauna Loa Observatory, Hawaii, 1958-1997 (revised August 2000). NDP-001. Carbon Dioxide
Information Analysis Center, Oak Ridge National Laboratory, Oak Ridge, Tennessee. Lim, C., Kafatos, M. and Megonigal, P. 2004. Correlation between atmospheric CO2 concentration and vegetation
greenness in North America: CO2 fertilization effect. Climate Research 28: 11-22. Oechel, W.C., Vourlitis, G.L., Verfaillie, J., Crawford, T., Brooks, S., Dumas, E., Hope, A., Stow,
D., Boynton, B., Nosov, V. and Zulueta, R. 2000. A scaling approach for quantifying the net CO2 flux of the Kuparuk River Basin, Alaska. Global Change Biology 6: 160-173. Raymond, P.A.
and Cole, J.J. 2003. Increase in the export of alkalinity from North America’s largest river. Science 301: 88-91. Saxe, H., Ellsworth, D.S. and Heath, J. 1998. Tree and forest functioning
in an enriched CO2 atmosphere. New Phytologist 139: 395-436. Soule, P.T. and Knapp, P.A. 2006. Radial growth rate increases in naturally occurring ponderosa pine trees: a late-twentieth
century CO2 fertilization effect? New Phytologist doi: 10.1111/j.1469-8137.2006.01746.x. Sturm, M., Racine, C. and Tape, K. 2001. Increasing shrub abundance in the Arctic. Nature 411:
546-547. Tape, K., Sturm, M. and Racine, C. 2006. The evidence for shrub expansion in Northern Alaska and the Pan-Arctic. Global Change Biology 12: 686-702. Wang, G.G., Chhin, S. and
Bauerle, W.L. 2006. Effect of natural atmospheric CO2 fertilization suggested by opengrown white spruce in a dry environment. Global Change Biology 12: 601-610. Westfall, J.A. and Amateis,
R.L. 2003. A model to account for potential correlations between growth of loblolly pine
Climate Change Reconsidered 566 and changing ambient carbon dioxide concentrations. Southern Journal of Applied Forestry 27: 279-284. Xiao, J. and Moody, A. 2004. Photosynthetic activity
of US biomes: responses to the spatial variability and seasonality of precipitation and temperature. Global Change Biology 10: 437-451. Zhou, L., Tucker, C.J., Kaufmann, R.K., Slayback,
D., Shabanov, N.V. and Myneni, R.B. 2001. Variations in northern vegetation activity inferred from satellite data of vegetation index during 1981-1999. Journal of Geophysical Research
106: 20,069-20,083. 7.9.5. Oceans Rising air temperatures and atmospheric CO2 concentrations in the twentieth century also have affected the productivity of earth’s seas. We begin with
a study that takes a much longer view of the subject. Elderfield and Rickaby (2000) note that the typically low atmospheric CO2 concentrations of glacial periods have generally been
attributed to an increased oceanic uptake of CO2, “particularly in the southern oceans.” However, the assumption that intensified phytoplanktonic photosynthesis may have stimulated CO2
uptake rates during glacial periods has always seemed at odds with the observational fact that rates of photosynthesis are generally much reduced in environments of significantly lower-thancurrent
atmospheric CO2 concentrations, such as typically prevail during glacial periods. The two scientists provide a new interpretation of Cd/Ca systematics in sea water that helps to resolve
this puzzle, as it allows them to more accurately estimate surface water phosphate conditions during glacial times and thereby determine the implications for concomitant atmospheric
CO2 concentrations. What they found, in their words, is that “results from the Last Glacial Maximum [LGM] show similar phosphate utilization in the subantarctic to that of today, but
much smaller utilization in the polar Southern Ocean,” which implies, according to Delaney (2000), that Antarctic productivity was lower at that time than it is now, but that subantarctic
productivity was about the same as it has been in modern times, due perhaps to greater concentrations of bio-available iron compensating for the lower atmospheric CO2 concentrations
of the LGM. So what caused the much smaller utilization of phosphate in the polar Southern Ocean during the LGM? Noting that the area of sea-ice cover in the Southern Ocean during glacial
periods may have been as much as double that of modern times, Elderfield and Rickaby suggest that “by restricting communication between the ocean and atmosphere, sea ice expansion also
provides a mechanism for reduced CO2 release by the Southern Ocean and lower glacial atmospheric CO2.” Hence, it is possible that phytoplanktonic productivity in the glacial Southern
Ocean may have been no higher than it is at the present time, notwithstanding the greater supply of bio-available iron typical of glacial epochs. In the case of the interglacial period
in which we currently live, Dupouy et al. (2000) say it has long been believed that N2 fixation in the world’s oceans is unduly low, in consequence of the present low supply of wind-blown
iron compared to that of glacial periods, and that this state of affairs leads to low phytoplanktonic productivity, even in the presence of higher atmospheric CO2 concentrations. The
evidence they acquired, however, suggests that marine N2 fixation may be much greater than what has generally been thought to be the case. In particular, they note that several Trichodesmium
species of N2-fixing cyanobacteria have “a nearly ubiquitous distribution in the euphotic zone of tropical and subtropical seas and could play a major role in bringing new N to these
oligotrophic systems.” And this feat, in their words, “could play a significant role in enhancing new production.” The importance of these findings is perhaps best appreciated in light
of the findings of Pahlow and Riebesell (2000), who in studying data obtained from 1173 stations in the Atlantic and Pacific Oceans, covering the years 1947 to 1994, detected changes
in Northern Hemispheric deep-ocean Redfield ratios that are indicative of increasing nitrogen availability there, which increase was concomitant with an increase in export production
that has resulted in ever-increasing oceanic carbon sequestration. These investigators further suggest that the growing supply of nitrogen has its origin in anthropogenic activities
that release nitrous oxides to the air. In addition, the increased carbon export may be partly a consequence of the historical increase in the air’s CO2 concentration, which has been
demonstrated to have the ability to enhance phytoplanktonic productivity (see Section 6.1.3.4. in this document), analogous to the way in which elevated concentrations of atmospheric
CO2 enhance the productivity of terrestrial plants, including their ability to fix nitrogen. Further elucidating the productivity-enhancing power of the ongoing rise in the air’s CO2
content is
Biological Effects of Carbon Dioxide Enrichment 567 the study of Pasquer et al. (2005), who employed a complex model of growth regulation of diatoms, pico/nano phytoplankton, coccolithophorids
and Phaeocystis spp. by light, temperature and nutrients (based on a comprehensive analysis of literature reviews focusing on these taxonomic groups) to calculate changes in the ocean
uptake of carbon in response to a sustained increase in atmospheric CO2 concentration of 1.2 ppm per year for three marine ecosystems where biogeochemical time-series of the data required
for model initialization and comparison of results were readily available. These systems were (1) the ice-free Southern Ocean Time Series station KERFIX (50°40’S, 68°E) for the period
1993-1994 (diatom-dominated), (2) the sea-ice associated Ross Sea domain (76°S, 180°W) of the Antarctic Environment and Southern Ocean Process Study AESOPS in 1996-1997 (Phaeocystis-dominated),
and (3) the North Atlantic Bloom Experiment NABE (60°N, 20°W) in 1991 (coccolithophorids). Their results, in their words, “show that at all tested latitudes the prescribed increase of
atmospheric CO2 enhances the carbon uptake by the ocean.” Indeed, we calculate from their graphical presentations that (1) at the NABE site a sustained atmospheric CO2 increase of 1.2
ppm per year over a period of eleven years increases the air-sea CO2 flux in the last year of that period by approximately 17 percent, (2) at the AESOPS site the same protocol applied
over a period of six years increases the air-sea CO2 flux by about 45 percent, and (3) at the KERFIX site it increases the air-sea CO2 flux after nine years by about 78 percent. Although
the results of this interesting study based on the complex SWAMCO model of Lancelot et al. (2000), as modified by Hannon et al. (2001), seem overly large, they highlight the likelihood
that the ongoing rise in the air’s CO2 content may be having a significant positive impact on ocean productivity and the magnitude of the ocean carbon sink. But what about increasing
temperatures? Sarmiento et al. (2004) conducted a massive computational study that employed six coupled climate model simulations to determine the biological response of the global ocean
to the climate warming they simulated from the beginning of the Industrial Revolution to the year 2050. Based on vertical velocity, maximum winter mixed-layer depth and seaice cover,
they defined six biomes and calculated how their surface geographies would change in response to their calculated changes in global climate. Next, they used satellite ocean color and
climatological observations to develop an empirical model for predicting surface chlorophyll concentrations from the final physical properties of the world’s oceans as derived from their
global warming simulations, after which they used three primary production algorithms to estimate the response of oceanic primary production to climate warming based on their calculated
chlorophyll concentrations. When all was said and done, the thirteen scientists from Australia, France, Germany, Russia, the United Kingdom and the United States arrived at a global
warming-induced increase in global ocean primary production that ranged from 0.7 to 8.1 percent. So what do real-world measurements of oceanic productivity reveal? Goes et al. (2005)
analyzed seven years (1997-2004) of satellite-derived ocean color data pertaining to the Arabian Sea, as well as associated sea surface temperatures (SSTs) and winds. They report that
for the region located between 52 to 57°E and 5 to 10°N, “the most conspicuous observation was the consistent year-by-year increase in phytoplankton biomass over the 7-year period.”
This phenomenon was so dramatic that by the
summer of 2003, in their words, “chlorophyll a concentrations were >350% higher than those observed in the summer of 1997.” They also report that the increase in chlorophyll a was “accompanied
by an intensification of sea surface winds, in particular of the zonal (east-to-west) component,” noting that these “summer monsoon winds are a coupled atmosphere-land-ocean phenomenon,
whose strength is significantly correlated with tropical SSTs and Eurasian snow cover anomalies on a year-to-year basis.” More specifically, they say that “reduced snow cover over Eurasia
strengthens the spring and summer land-sea thermal contrast and is considered to be responsible for the stronger southwest monsoon winds.” In addition, they state that “the influence
of southwest monsoon winds on phytoplankton in the Arabian Sea is not through their impact on coastal upwelling alone but also via the ability of zonal winds to laterally advect newly
upwelled nutrient-rich waters to regions away from the upwelling zone.” They conclude that “escalation in the intensity of summer monsoon winds, accompanied by enhanced upwelling and
an increase of more than 350 percent in average summertime phytoplankton biomass along the coast and over 300 percent offshore, raises the possibility that the current warming trend
of the Eurasian landmass is making the Arabian Sea more productive.” To the north and west on the other side of Eurasia, Marasovic et al. (2005) analyzed monthly
Climate Change Reconsidered 568 observations of basic hydrographic, chemical and biological parameters, including primary production, that had been made since the 1960s at two oceanographic
stations, one near the coast (Kastela Bay) and one in the open sea. They found that mean annual primary production in Kastela Bay averaged about 430 mg C m-2 d-1 over the period 1962-72,
exceeded 600 mg C m-2 d-1 over the period 1972-82, and rose to over 700 mg C m-2 d-1 over the period 1982-96, accompanied by a similar upward trend in percent oxygen saturation of the
surface water. The initial value of primary production in the open sea was much less (approximately 150 mg C m-2 d-1), but it began to follow the upward trend of the Kastela Bay data
after about one decade. Marasovic et al. thus concluded that “even though all the relevant data indicate that the changes in Kastela Bay are closely related to an increase of anthropogenic
nutrient loading, similar changes in the open sea suggest that primary production in the Bay might, at least partly, be due to global climatic changes,” which, in their words, are “occurring
in the Mediterranean and Adriatic Sea open waters” and may be directly related to “global warming of air and ocean,” since “higher temperature positively affects photosynthetic processes.”
Raitsos et al. (2005) investigated the relationship between Sea-viewing Wide Field-of-view Sensor (SeaWiFS) chlorophyll-a measurements in the Central Northeast Atlantic and North Sea
(1997-2002) and simultaneous measurements of the Phytoplankton Color Index (PCI) collected by the Continuous Plankton Recorder survey, which is an upper-layer plankton monitoring program
that has operated in the North Sea and North Atlantic Ocean since 1931. By developing a relationship between the two data bases over their five years of overlap, they were able to produce
a Chl-a history for the Central Northeast Atlantic and North Sea for the period 1948-2002. Of this record they say that “an increasing trend is apparent in mean Chl-a for the the area
of study over the period 1948-2002.” They also say “there is clear evidence for a stepwise increase after the mid-1980s, with a minimum of 1.3mg m-3 in 1950 and a peak annual mean of
2.1 mg m-3 in 1989 (62% increase).” Alternatively, it is possible that the data represent a more steady long-term upward trend upon which is superimposed a decadal-scale oscillation.
In a final comment on their findings, they note that “changes through time in the PCI are significantly correlated with both sea surface temperature and Northern Hemisphere temperature,”
citing Beaugrand and Reid (2003). In a contemporaneous study, Antoine et al. (2005) applied revised data-processing algorithms to two ocean-sensing satellites, the Coastal Zone Color
Scanner (CZCS) and SeaWiFS, over the periods 1979-1986 and 1998-2002, respectively, to provide an analysis of the decadal changes in global oceanic phytoplankton biomass. Results of
the analysis showed “an overall increase of the world ocean average chlorophyll concentration by about 22%” over the two decades under study. Dropping down to the Southern Ocean, Hirawake
et al. (2005) analyzed chlorophyll a data obtained from Japanese Antarctic Research Expedition cruises made by the Fuji and Shirase ice-breakers between Tokyo and Antarctica from 15
November to 28 December of nearly every year between 1965 and 2002 in a study of interannual variations of phytoplankton biomass, calculating results for the equatorial region between
10°N and 10°S, the Subtropical Front (STF) region between 35°S and 45°S, and the Polar Front (PF) region between 45°S and 55°S. They report that an increase in chl a was “recognized
in the waters around the STF and the PF, especially after 1980 around the PF in particular,” and that “in the period between 1994 and 1998, the chl a in the three regions exhibited rapid
gain simultaneously.” They also say “there were significant correlations between chl a and year through all of the period of observation around the STF and PF, and the rates of increase
are 0.005 0.005 and 0.012 mg chl a m-3 y-1, respectively.” In addition, they report that the satellite data of Gregg and Conkright (2002) “almost coincide with our results.” In commenting
on these findings, the Japanese scientists say that “simply considering the significant increase in the chl a in the Southern Ocean, a rise in the primary production as a result of the
phytoplankton increase in this area is also expected.” Also working in the Southern Hemisphere, Sepulveda et al. (2005) presented “the first reconstruction of changes in surface primary
production during the last century from the Puyuhuapi fjord in southern Chile, using a variety of parameters (diatoms, biogenic silica, total organic carbon, chlorins, and proteins)
as productivity proxies.” Noting that the fjord is located in “a still-pristine area,” they say it is “suitable to study changes in past export production originating from changes in
both the paleo-Patagonian ice caps and the globally important Southern Ocean.”
Biological Effects of Carbon Dioxide Enrichment 569 The analysis revealed that the productivity of the Puyuhuapi fjord “was characterized by a constant increase from the late 19th century
to the early 1980s, then decreased until the late 1990s, and then rose again to present-day values.” For the first of these periods (1890-1980), they additionally report that “all proxies
were highly correlated (r > 0.8, p < 0.05),” and that “all proxies reveal an increase in accumulation rates.” From 1980 to the present, however, the pattern differed among the various
proxies; and the researchers say that “considering that the top 5 cm of the sediment column (~10 years) are diagenetically active, and that bioturbation by benthic organisms may have
modified and mixed the sedimentary signal, paleo-interpretation of the period 1980-2001 must be taken with caution.” Consequently, there is substantial solid evidence that, for the first
90 years of the 111-year record, surface primary production in the Puyuhuapi fjord rose dramatically, while with lesser confidence it appears to have leveled out over the past two decades.
In spite of claims that the “unprecedented” increases in mean global air temperature and CO2 concentration experienced since the inception of the Industrial Revolution have been bad
for the biosphere, Sepulveda et al. presented yet another case of an ecosystem apparently thriving in such conditions. Still, claims of impending ocean productivity declines have not
ceased, and some commentators single out the study of Behrenfeld et al. (2006) in support of their claims. Working with NASA’s Seaviewing Wide Field-of-view Sensor (Sea WiFS), the team
of 10 U.S. scientists calculated monthly changes in net primary production (NPP) from similar changes in upper-ocean chlorophyll concentrations detected from space over the past decade.
(See Figure 7.9.5.) They report that this period was dominated by an initial NPP increase of 1,930 teragrams of carbon per year (Tg C yr-1), which they attributed to the significant
cooling of “the 1997 to 1999 El Niño to La Niña transition,” and they note that this increase was “followed by a prolonged decrease averaging 190 Tg C yr-1,” which they attributed to
subsequent warming. The means by which changing temperatures were claimed by the researchers to have driven the two sequential linear-fit trends in NPP is based on their presumption
that a warming climate increases the density contrast between warmer surface waters and cooler underlying nutrient-rich waters, so that the enhanced stratification that occurs with warming
“suppresses nutrient exchange through vertical mixing,” which decreases NPP by reducing the supply of nutrients to the surface waters where photosynthesizing phytoplankton predominantly
live. By contrast, the ten scientists suggest that “surface cooling favors elevated vertical exchange,” which increases NPP by enhancing the supply of nutrients to the ocean’s surface
waters, which are more frequented by phytoplankton than are under-lying waters, due to light requirements for photosynthesis. It is informative to note, however, that from approximately
the middle of 2001 to the end of the data series in early 2006 (which interval accounts for more than half of the data record), there has been, if anything, a slight increase in global
NPP. (See again Figure 7.9.5.) Does this observation mean there has been little to no net global warming since mid-2001? Or does it mean the global ocean’s mean surface temperature actually
cooled a bit over the last five years? Neither alternative is what one would expect if global warming were a real problem. On the other hand, the relationship between global warming
and oceanic productivity may not be nearly as strong as what Behrenfeld et al. have suggested; and they themselves say “modeling studies suggest that shifts in ecosystem structure from
climate variations may be as [important as] or more important than the alterations in bulk integrated properties reported here,” noting that some “susceptible ecosystem characteristics”
that might be so so shifted include “taxonomic composition, physiological status, and light absorption by colored dissolved organic Figure 7.9.5. Monthly anomalies of global NPP (green
line) plus similar results for the permanently stratified ocean regions of the world (grey circles and black line), adapted from Behrenfeld et al. (2006).
Climate Change Reconsidered 570 material.” It is possible that given enough time, the types of phenomena Behrenfeld et al. describe as possibly resulting in important “shifts in ecosystem
structure” could compensate for or even overwhelm what might initially appear to be negative warminginduced consequences. Another reason for not concluding too much from the oceanic
NPP data set of Behrenfeld et al. is that it may be of too short a duration to reveal what might be occurring on a much longer timescale throughout the world’s oceans, or that its position
in time may be such that it does not allow the detection of greater short-term changes of the opposite sign that may have occurred a few years earlier or that might occur in the near
future. Consider, for example, the fact that the central regions of the world’s major oceans were long thought to be essentially vast biological deserts (Ryther, 1969), but that several
studies of primary photosynthetic production conducted in those regions over the 1980s (Shulenberger and Reid, 1981; Jenkins, 1982; Jenkins and Goldman, 1985; Reid and Shulenberger,
1986; Marra and Heinemann, 1987; Laws et al., 1987; Venrick et al., 1987; Packard et al., 1988) yielded results that suggested marine productivity at that time was twice or more as great
as it likely was for a long time prior to 1969, causing many of that day to speculate that “the ocean’s deserts are blooming” (Kerr, 1986). Of even greater interest, perhaps, is the
fact that over this particular period of time (1970-1988), the data repository of Jones et al. (1999) indicates the earth experienced a (linear-regression-derived) global warming of
0.333°C, while the data base of the Global Historical Climatology Network indicates the planet experienced a similarly calculated global warming of 0.397°C. The mean of these two values
(0.365°C) is nearly twice as great as the warming that occurred over the post-1999 period studied by Behrenfeld et al.; yet this earlier much larger warming (which according to the ten
researchers’ way of thinking should have produced a major decline in ocean productivity) was concomitant with a huge increase in ocean productivity. Consequently, it would appear that
just the opposite of what Behrenfeld et al. suggest about global warming and ocean productivity is likely to be the more correct of the two opposing cause-and-effect relationships. Moving
closer to the present, Levitan et al. (2007) published a study of major significance that addresses the future of oceanic productivity under rising atmospheric CO2 concentrations. In
their paper the authors note that “among the principal players contributing to global aquatic primary production, the nitrogen (N)-fixing organisms (diazotrophs) are important providers
of new N to the oligotrophic areas of the oceans,” and they cite several studies which demonstrate that “cyanobacterial (phototrophic) diazotrophs in particular fuel primary production
and phytoplankton blooms which sustain oceanic food-webs and major economies and impact global carbon (C) and N cycling.” These facts compelled them to examine how the ongoing rise in
the air’s CO2 content might impact these relationships. They began by exploring the response of the cyanobacterial diazotroph Trichodesmium to changes in the atmosphere’s CO2 concentration,
choosing this particular diazotroph because it dominates the world’s tropical and sub-tropical oceans in this regard, contributing over 50 percent of total marine N fixation. The eight
Israeli and Czech researchers grew Trichodesmium IMS101 stock cultures in YBCII medium (Chen et al., 1996) at 25°C and a 12-hour:12-hour light/dark cycle (with the light portion of the
cycle in the range of 80-100 µmol photons m-2 s-1) in equilibrium with air of three different CO2 concentrations (250, 400 and 900 ppm, representing low, ambient and high concentrations,
respectively), which was accomplished by continuously bubbling air of the three CO2 concentrations through the appropriate culture vessels throughout various experimental runs, each
of which lasted a little over three weeks, during which time they periodically monitored a number of diazotrophic physiological processes and properties. So what did the scientists learn?
Levitan et al. report that Trichodesmium in the high CO2 treatment “displayed enhanced N fixation, longer trichomes, higher growth rates and biomass yields.” In fact, they write that
in the high CO2 treatment there was “a three-to four-fold increase in N fixation and a doubling of growth rates and biomass,” and that the cultures in the low CO2 treatment reached a
stationary growth phase after only five days, “while both ambient and high CO2 cultures exhibited exponential growth until day 15 before declining.” In discussing possible explanations
for what they observed, the researchers suggest that “enhanced N fixation and growth in the high CO2 cultures occurs due to reallocation of energy and resources from carbon concentrating
mechanisms required under low and ambient CO2.” Consequently, they conclude, in their words, that “in oceanic regions, where light and
Biological Effects of Carbon Dioxide Enrichment 571 nutrients such as P and Fe are not limiting, we expect the projected concentrations of CO2 to increase N fixation and growth of Trichodesmium,”
and that “other diazotrophs may be similarly affected, thereby enhancing inputs of new N and increasing primary productivity in the oceans.” And to emphasize these points, they write
in the concluding sentence of their paper that “Trichodesmium’s dramatic response to elevated CO2 may consolidate its dominance in subtropical and tropical regions and its role in C
and N cycling, fueling subsequent primary production, phytoplankton blooms, and sustaining oceanic foodwebs.” Arrigo et al. (2008) introduce their work by writing that “between the late
1970s and the early part of the 21st century, the extent of Arctic Ocean sea ice cover has declined during all months of the year, with the largest declines reported in the boreal summer
months, particularly in September (8.6 ± 2.9% per decade),” citing the work of Serreze et al. (2007). In an effort to “quantify the change in marine primary productivity in Arctic waters
resulting from recent losses of sea ice cover,” the authors “implemented a primary productivity algorithm that accounts for variability in sea ice extent, sea surface temperature, sea
level winds, downwelling spectral irradiance, and surface chlorophyll a concentrations,” and that “was parameterized and validated specifically for use in the Arctic (Pabi et al., 2008)
and utilizes forcing variables derived either from satellite data or NCEP reanalysis fields.” Arrigo et al. determined that “annual primary production in the Arctic increased yearly
by an average of 27.5 Tg C per year since 2003 and by 35 Tg C per year between 2006 and 2007,” 30 percent of which total increase was attributable to decreased minimum summer ice extent
and 70 percent of which was due to a longer phytoplankton growing season. Arrigo et al. thus conclude that if the trends they discovered continue, “additional loss of ice during Arctic
spring could boost productivity >3-fold above 1998-2002 levels.” Hence, they additionally state that if the 26 percent increase in annual net CO2 fixation in the Arctic Ocean between
2003 and 2007 is maintained, “this would represent a weak negative feedback on climate change.” On the other side of the globe and working in the Southern Ocean, Smith and Comiso (2008)
employed phytoplankton pigment assessments, surface temperature estimates, modeled irradiance, and observed sea ice concentrations—all of which parameters were derived from satellite
data—and incorporated them into a vertically integrated production model to estimate primary productivity trends according to the technique of Behrenfeld et al. (2002). Of this effort,
the two authors say that “the resultant assessment of Southern Ocean productivity is the most exhaustive ever compiled and provides an improvement in the quantitative role of carbon
fixation in Antarctic waters.” So what did they find? During the nine years (1997-2006) analyzed in the study, “productivity in the entire Southern Ocean showed a substantial and significant
increase,” which increase can be calculated from the graphical representation of their results as ~17 percent per decade. In commenting on their findings, the two researchers note that
“the highly significant increase in the productivity of the entire Southern Ocean over the past decade implies that long-term changes in Antarctic food webs and biogeochemical cycles
are presently occurring,” which changes we might add are positive. In light of these several real-world observations, we find no indications of a widespread decline in oceanic productivity
over the twentieth century in response to increases in air temperature and CO2 concentration. In fact, we see evidence that just the opposite is occurring, that environmental changes
are occurring that are proving to be beneficial. Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/subj
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Hayward, T.L. 1987. Climate and chlorophyll a: Long-term trends in the central North Pacific Ocean. Science 238: 70-72. 7.9.6. Global How have earth’s terrestrial plants responded—on
average and in their entirety—to the atmospheric temperature and CO2 increases of the past quartercentury? In this subsection we report the results of studies that have looked at either
the world as a whole or groups of more than two continents at the same time. In one of the earlier studies of the subject, Joos and Bruno (1998) used ice core and direct observations
of atmospheric CO2 and 13C to reconstruct the histories of terrestrial and oceanic uptake of anthropogenic carbon over the past two centuries. This project revealed, in their words,
that “the biosphere acted on average as a source [of CO2] during the last century and the first decades of this century ... Then, the biosphere turned into a [CO2] sink,” which implies
a significant increase in global vegetative productivity over the last half of the twentieth century. More recently, Cao et al. (2004) derived net primary production (NPP) values at
8-km and 10-day resolutions for the period 1981-2000 using variables based almost entirely on satellite observations, as described in the Global Production Efficiency Model (GLO-PEM),
which consists, in their words, “of linked components that describe the processes of canopy radiation absorption, utilization, autotrophic respiration, and the regulation of these processes
by environmental factors (Prince and Goward, 1995; Goetz et al., 2000).” They learned that over the last two decades of the twentieth century, when temperatures were rising, “there was
an increasing trend toward enhanced terrestrial NPP,” which they say was “caused mainly by increases in atmospheric carbon dioxide and precipitation.” A year later, Cao et al. (2005)
used the CEVSA (Carbon Exchanges in the Vegetation-Soil-Atmosphere system) model (Cao and Woodward, 1998;
Cao et al., 2002), forced by observed variations in climate and atmospheric CO2, to quantify changes in NPP, soil heterotrophic respiration (HR) and net ecosystem production (NEP) from
1981 to 1998. As an independent check on the NPP estimate of CEVSA, they also estimated 10-day NPP from 1981-2000 with the GLO-PEM model that uses data almost entirely from remote sensing,
including both the normalized difference vegetation index (NDVI) and meteorological variables (Prince and Goward, 1995; Cao et al., 2004). This protocol revealed, in Cao et al.’s words,
that “global terrestrial temperature increased by 0.21°C from the 1980s to the 1990s, and this alone increased HR more than NPP and hence reduced global annual NEP.” However, they found
that “combined changes in temperature and precipitation increased global NEP significantly,” and that “increases in atmospheric CO2 produced further increases in NPP and NEP.” They also
discovered that “the CO2 fertilization effect [was] particularly strong in the tropics, compensating for the negative effect of warming on NPP.” Enlarging on this point, they write that
“the response of photosynthetic biochemical reactions to increases in atmospheric CO2 is greater greater in warmer conditions, so the CO2 fertilization effect will increase with warming
in cool regions and be high in warm environments.” The end result of the application of these models and measurements was their finding that global NEP increased “from 0.25 Pg C yr-1
in the 1980s to 1.36 Pg C yr-1 in the 1990s.” Commenting on their findings, Cao et al. note that “the NEP that was induced by CO2 fertilization and climatic variation accounted for 30
percent of the total terrestrial carbon sink implied by the atmospheric carbon budget (Schimel et al., 2001), and the fraction changed from 13 percent in the 1980s to 49 percent in the
1990s,” which indicates the growing importance of the CO2 fertilization effect. Also, they say “the increase in the terrestrial carbon sink from the 1980s to the 1990s was a continuation
of the trend
Climate Change Reconsidered 574 since the middle of the twentieth century, rather than merely a consequence of short-term climate variability,” which suggests that as long as the air’s
CO2 content continues its upward course, so too will its stimulation of the terrestrial biosphere likely continue its upward course. Using a newly developed satellite-based vegetation
index (Version 3 Pathfinder NDVI) in conjunction with a gridded global climate dataset (global monthly mean temperature and precipitation at 0.5° resolution from New et al., 2000), Xiao
and Moody (2005) analyzed trends in global vegetative activity from 1982 to 1998. The greening trends they found exhibited substantial latitudinal and longitudinal variability, with
the most intense greening of the globe located in high northern latitudes, portions of the tropics, southeastern North America and eastern China. Temperature was found to correlate strongly
with greening trends in Europe, eastern Eurasia and tropical Africa. Precipitation, on the other hand, was not found to be a significant driver of increases in greenness, except for
isolated and spatially fragmented regions. Some decreases in greenness were also observed, mainly in the Southern Hemisphere in southern Africa, southern South America and central Australia,
which trends were associated with concomitant increases in temperature and decreases in precipitation. There were also large regions of the globe that showed no trend in greenness over
the 17-year period, as well as large areas that underwent strong greening that showed no association with trends of either temperature or precipitation. These greening trends, as they
concluded, must have been the result of other factors, such as “CO2 fertilization, reforestation, forest regrowth, woody plant proliferation and trends in agricultural practices,” about
which others will have more to say as we continue. Working with satellite observations of vegetative activity over the period 1982 to 1999, Nemani et al. (2003) discovered that the productivity
of earth’s terrestrial vegetation rose significantly over this period. More specifically, they determined that terrestrial net primary production (NPP) increased by 6.17 percent, or
3.42 PgC, over the 18 years between 1982 and 1999. What is more, they observed net positive responses over all latitude bands studied: 4.2 percent (47.5-22.5°S), 7.4 percent (22.5°S-22.5°N),
3.7 percent (22.5-47.5°N), and 6.6 percent (47.5-90.0°N). The eight researchers mention a number of likely contributing factors to these significant NPP increases: nitrogen deposition
and forest regrowth in northern mid and high latitudes, wetter rainfall regimes in water-limited regions of Australia, Africa, and the Indian subcontinent, increased solar radiation
reception over radiation-limited parts of Western Europe and the equatorial tropics, warming in many parts of the world, and the aerial fertilization effect of rising atmospheric CO2
concentrations everywhere. With respect to the latter factor, Nemani et al. say “an increase in NPP of only 0.2% per 1-ppm increase in CO2 could explain all of the estimated global NPP
increase of 6.17% over 18 years and is within the range of experimental evidence.” However, they report that terrestrial NPP increased by more than 1 percent per year in Amazonia alone,
noting that “this result cannot be explained solely by CO2 fertilization.” We tend to agree with Nemani et al. on this point, but also note that the aerial fertilization effect of atmospheric
CO2 enrichment is most pronounced at higher temperatures, rising from next to nothing at a mean temperature of 10°C to a 0.33 percent NPP increase per 1-ppm increase in CO2 at a mean
temperature of 36°C for a mixture of plants comprised predominantly of herbaceous species (Idso and Idso, 1994). For woody plants, we could possibly expect this number to be two (Idso,
1999) or even three (Saxe et al., 1998; Idso and Kimball, 2001; Leavitt et al., 2003) times larger, yielding a 0.7 percent to 1 percent NPP increase per 1-ppm increase in atmospheric
CO2, which would represent the lion’s share of the growth stimulation observed by Nemani et al. in tropical Amazonia. The message of Nemani et al.’s study is that satellite-derived observations
indicate the planet’s terrestrial vegetation significantly increased its productivity over the last two decades of the twentieth century, in the face of a host of both real and imagined
environmental stresses, chief among the latter of which was what the IPCC claims to be unprecedented CO2-induced global warming. Perhaps the most striking evidence for the significant
twentieth century growth enhancement of earth’s forests by the historical increase in the air’s CO2 concentration was provided by the study of Phillips and Gentry (1994). Noting that
turnover rates of mature tropical forests correlate well with measures of net productivity (Weaver and Murphy, 1990), the two scientists assessed the turnover rates of 40 tropical forests
from around the world in order to test the hypothesis that global forest productivity was increasing in situ. In doing so, they found that the
Biological Effects of Carbon Dioxide Enrichment 575 turnover rates of these highly productive forests had indeed been rising ever higher since at least 1960, with an apparent pan-tropical
acceleration since 1980. In discussing what might be causing this phenomenon, they stated that “the accelerating increase in turnover coincides with an accelerating buildup of CO2,”
and as Pimm and Sugden (1994) stated in a companion article, it was “the consistency and simultaneity of the changes on several continents that lead Phillips and Gentry to their conclusion
that enhanced productivity induced by increased CO2 is the most plausible candidate for the cause of the increased turnover.” Four years later, a group of eleven researchers headed by
Phillips (Phillips et al., 1998), working with data on tree basal area (a surrogate for tropical forest biomass) for the period 1958-1996, which they obtained from several hundred plots
of mature tropical trees scattered about the world, found that average forest biomass for the the tropics as a whole had increased substantially. In fact, they calculated that the increase
amounted to approximately 40 percent of the missing terrestrial carbon sink of the entire globe. They suggested that “intact forests may be helping to buffer the rate of increase in
atmospheric CO2, thereby reducing the impacts of global climate change,” as Idso (1991a,b) had earlier suggested, and they identified the aerial fertilization effect of the ongoing rise
in the air’s CO2 content as one of the factors responsible for this phenomenon. Other contemporary studies also supported their findings (Grace et al., 1995; Malhi et al., 1998), verifying
the fact that neotropical forests were indeed accumulating ever more carbon. Phillips et al. (2002) continued to state that this phenomenon was occurring “possibly in response to the
increasing atmospheric concentrations of carbon dioxide (Prentice et al., 2001; Malhi and Grace, 2000).” As time progressed, however, it became less popular (i.e., more “politically
incorrect”) to report positive biological consequences of the ongoing rise in the air’s CO2 concentration. The conclusions of Phillips and others began to be repeatedly challenged (Sheil,
1995; Sheil and May, 1996; Condit, 1997; Clark, 2002; Clark et al., 2003). In response to those challenges, CO2 Science published an editorial rebuttal (see http://www.co2science.org/articles/V6/N25/
DIT.php), after which Phillips, joined by 17 other researchers (Lewis et al., 2005b), including one who had earlier criticized his and his colleagues’ conclusions, published a new analysis
that vindicated Phillips et al.’s earlier thoughts on the subject. One of the primary concerns of the critics of Phillips et al.’s work was that their meta-analyses included sites with
a wide range of tree census intervals (2-38 years), which they claimed could be confounding or “perhaps even driving conclusions from comparative studies,” as Lewis et al. (2005b) describe
it. However, in Lewis et al.’s detailed study of this potential problem, which they concluded was indeed real, they found that re-analysis of Phillips et al.’s published results “shows
that the pan-tropical increase in stem turnover rates over the late twentieth century cannot be attributed to combining data with differing census intervals.” Or as they state more obtusely
in another place, “the conclusion that turnover rates have increased in tropical forests over the late twentieth century is robust to the charge that this is an artifact due to the combination
of data that vary in census interval (cf. Sheil, 1995).” Lewis et al. (2005b) additionally noted that “Sheil’s (1995) original critique of the evidence for increasing turnover over the
late twentieth century also suggests that the apparent increase could be explained by a single event, the 1982-83 El Niño Southern Oscillation (ENSO), as many of the recent data spanned
this event.” However, as they continued, “recent analyses from Amazonia have shown that growth, recruitment and mortality rates have simultaneously increased within the same plots over
the 1980s and 1990s, as has net above-ground biomass, both in areas largely unaffected, and in those strongly affected, by ENSO events (Baker et al., 2004; Lewis et al., 2004a; Phillips
et al., 2004).” In a satellite study of the world’s tropical forests, Ichii et al. (2005) “simulated and analyzed 1982-1999 Amazonian, African, and Asian carbon fluxes using the Biome-BGC
prognostic carbon cycle model driven by National Centers for Environmental Prediction reanalysis daily climate data,” after which they “calculated trends in gross primary productivity
(GPP) and net primary productivity (NPP).” This work revealed that solar radiation variability was the primary factor responsible for interannual variations in GPP, followed by temperature
and precipitation variability, while in terms of GPP trends, Ichii et al. report that “recent changes in atmospheric CO2 and climate promoted terrestrial GPP increases with a significant
linear trend in all three tropical regions.” In the Amazonian region, the rate of GPP increase was 0.67 PgC year-1 decade-1, while in Africa and Asia it was about 0.3 PgC year-1 decade-1.
Likewise, they report that “CO2 fertilization effects strongly increased recent NPP trends in regional totals.”
Climate Change Reconsidered 576 In a review of these several global forest studies, as well as many others (which led to their citing 186 scientific journal articles), Boisvenue and
Running (2006) examined reams of “documented evidence of the impacts of climate change trends on forest productivity since the middle of the twentieth century.” In doing so, they found
that “globally, based on both satellite and ground-based data, climatic changes seemed to have a generally positive impact on forest productivity when water was not limiting,” which
was most of the time, because they report that “less than 7% of forests are in strongly water-limited systems.” Last, Young and Harris (2005) analyzed, for the majority of earth’s land
surface, a near 20-year time series (1982-1999) of NDVI data, based on measurements obtained from the Advanced Very High Resolution Radiometer (AVHRR) carried aboard U.S. National Oceanic
and Atmospheric Administration satellites. In doing so, they employed two different datasets derived derived from the sensor: the Pathfinder AVHRR Land (PAL) data set and the Global
Inventory Modeling and Mapping Studies (GIMMS) dataset. Based on their analysis of the PAL data, the two researchers determined that “globally more than 30% of land pixels increased
in annual average NDVI greater than 4% and more than 16% persistently increased greater than 4%,” while “during the same period less than 2% of land pixels declined in NDVI and less
than 1% persistently declined.” With respect to the GIMMS dataset, they report that “even more areas were found to be persistently increasing (greater than 20%) and persistently decreasing
(more than 3%).” All in all, they report that “between 1982 and 1999 the general trend of vegetation change throughout the world has been one of increasing photosynthesis.” As for what
has been responsible for the worldwide increase in photosynthesis—which is the ultimate food source for nearly all of the biosphere— the researchers mention global warming (perhaps it’s
not so bad after all), as well as “associated precipitation change and increases in atmospheric carbon dioxide,” citing Myneni et al. (1997) and Ichii et al. (2002). In addition, they
say that “many of the areas of decreasing NDVI are the result of human activity,” primarily deforestation (Skole and Tucker, 1993; Steininger et al., 2001) and urbanization (Seto et
al. (2000)). In conclusion, the results of these many studies demonstrate there has been an increase in plant growth rates throughout the world since the inception of the Industrial
Revolution, and that this phenomenon has been gradually accelerating over the years, in concert with the historical increases in the air’s CO2 content and its temperature. Additional
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