As the CO2 content of the air continues to rise, nearly all of earth's plants should exhibit increases in photosynthesis and biomass production; but climate alarmists periodically 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) concluded that water stress will not negate the CO2-induced stimulation of plant productivity. In fact, they found the CO2-induced percentage increase in plant productivity was nearly always greater under water-stressed conditions than it was when plants were well-watered. More recently, Poorter and Perez-Soba (2001) conducted a similar literature review and reached the same conclusions. In this summary, we thus highlight some of the more recent work in this area.
Elevated levels of atmospheric CO2 tend to reduce the area of open stomatal pore space on leaf surfaces, thus reducing plant stomatal conductance. This phenomenon, in turn, effectively reduces the amount of water lost to the atmosphere via transpiration. In the study of Leymarie et al. (1999), for example, twice-ambient levels of atmospheric CO2 caused significant reductions in the stomatal conductance of water-stressed 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. Thus, atmospheric CO2 enrichment clearly reduces stomatal conductance and, hence, plant transpiration and soil water depletion in grassland ecosystems.
CO2-induced increases in root development together with CO2-induced reductions in stomatal conductance often combine to maintain a more favorable plant water status during times of drought. In the case of four grassland species comprising a pasture characteristic of New Zealand, for example, 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 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 water-stressed 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% 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 twice-ambient concentrations of atmospheric CO2 for eight years only exhibited significant increases in above- and below-ground biomass during years of less-than-average 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%, regardless of soil water content. Moreover, 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% 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% CO2-induced increases in whole-community biomass under well-watered and water-stressed conditions, respectively.
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 environmental conditions imposed by less-than-optimum soil moisture contents.
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.
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.
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.
Poorter, H. and Perez-Soba, M. 2001. The growth response of plants to elevated CO2 under non-optimal environmental conditions. Oecologia 129: 1-20.
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.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.