As the air’s CO2 content continues to rise, most plants will exhibit increased rates of photosynthesis and biomass production, including those of various grassland ecosystems. This increase in productivity should increase the amount of forage available for grazing animals and possibly reduce the land area occupied by bare soil in certain environments. However, some individuals have predicted that global warming may negate the growth-promoting effects of atmospheric CO2 enrichment and actually stimulate the process of desertification. Thus, there is some question as to whether pasture and rangeland plants will continue to respond positively to increases in the air’s CO2 content if ambient air temperatures rise in the future. In this summary, we seek to develop an answer to this important question by reviewing the photosynthetic and growth responses of grassland plants to atmospheric CO2 enrichment when exposed to higher-than-normal temperatures.
In order to better understand the issues addressed in this review, it is important to remember that the optimum growth temperatures of several plants have already 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. Subsequently, Cowling and Sykes (1999) produced a literature review demonstrating that this was indeed the case for a number of different plants. One would thus expect plant photosynthetic rates to rise in response to concomitant increases in the air’s CO2 concentration and temperature, as previously documented by Idso and Idso (1994). Hence, we here proceed to see if these previously-determined positive
CO2 x temperature interactions are continuing to appear in the recent scientific literature pertaining to 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% at ambient air temperature. At elevated air temperature, however, plants grown at ambient CO2 exhibited a 28% reduction in foliage growth, while CO2-enriched plants still displayed a growth enhancement of 8%. Similarly, Morgan et al. (2001) reported that twice-ambient levels of atmospheric CO2 increased aboveground biomass in native shortgrass steppe ecosystems by an average of 38%, 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%, respectively, 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% 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% at 18°C to 95% at 28°C. Thus, the beneficial effects of elevated CO2 on grassland productivity is often significantly enhanced by elevated 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 Norton et al. (1999), where 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, the recent scientific literature continues to indicate that as the air’s CO2 content rises, grassland plants 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 does rise, the growth-promoting effects of atmospheric CO2 enrichment will likely rise right along with it, becoming more and more robust in agreement with the experimental observations reviewed by Idso and Idso (1994). The future ability of grasslands to produce increasingly greater amounts of forage, and perhaps reclaim areas of barren ground in certain environments, thus looks promising indeed, as long as the air’s CO2 content continues to rise.
References
Berry, J. and Bjorkman, O. 1980. Photosynthetic response and adaptation to temperature in higher plants. Annual Review of Plant Physiology 31: 491-543.
Cowling, S.A. and Sykes, M.T. 1999. Physiological significance of low atmospheric CO2 for plant-climate interactions. Quaternary Research 52: 237-242.
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.
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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.
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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.
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.