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Water Use Efficiency (Grassland Species) -- Summary
As the air's CO2 content rises, most plants exhibit increased rates of net photosynthesis and biomass production.  Moreover, on a per-unit-leaf-area basis, they often lose less water via transpiration, as they tend to display lower stomatal conductances at elevated CO2.  Hence, the amount of carbon gained per unit of water lost per unit leaf area - or water-use efficiency - should increase dramatically as the air's CO2 content rises.  In this review, we summarize the results of some recent studies of this phenomenon in 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% 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 water-use efficiency two C3 grasses and two broad-leaved species common to the loess grasslands of Budapest by 72 and 266%, 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% under well-watered and water-stressed 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 has also 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 gerardiiLeCain 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.

In summary, 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 also 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.  Thus, this phenomenon will likely contribute to a greater "greening of the globe."

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 CO2Oecologia 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 CO2Physiologia 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 thalianaPlant, 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 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.