Atmospheric CO2 enrichment generally reduces leaf stomatal conductances in most plants, which reduces the amount of water they transpire. At the same time, it often stimulates the production of more leaf area, which tends to increase the amount of water they convey to the atmosphere; but the extra leaf area shades more of the underlying ground from the direct rays of the sun, which reduces the amount of water that evaporates from the soil. Hence, it is not immediately clear what the net effect of these several competing phenomena would be with respect to evapotranspiration, or the loss of water from the entire soil-plant system. In this summary we thus review what has been learned about the subject from several studies that have directly measured this latter parameter.
In a two-year study of winter wheat conducted in open-top chambers and sunlit climate-controlled enclosures, Dijkstra et al. (1999) determined that an extra 350 ppm of CO2 reduced evapotranspiration rates by 10% to 21%; while in a two-year FACE study of spring wheat, Hunsaker et al. (2000) found that an extra 200 ppm of CO2 reduced evapotranspiration rates by 1% and 4% under conditions of low and high soil nitrogen, respectively. Likewise, in a two-year FACE study of sorghum, Conley et al. (2001) found that an extra 200 ppm of CO2 reduced cumulative crop evapotranspiration by 4% and 10% under water-stressed and well-watered conditions, respectively. Finally, in the last of the herbaceous plant experiments we have reviewed - a five-month study of model grasslands typical of the Negev of Israel, which were maintained in growth chambers supplied with air of 280, 440 and 600 ppm CO2 - Grunzweig and Korner (2001) determined that the model ecosystem maintained at 440 ppm lost 2% less water than the one maintained at 280 ppm over the first four months of the study; while the ecosystem maintained at 600 ppm lost 11% less than the one maintained at 280 ppm. Over the last month of the study, however, these differences largely disappeared.
Moving on to woody plants, Bucher-Wallin et al. (2000) grew saplings of Norway spruce and beech on calcareous and acidic soils for four years in open-top chambers receiving either ambient air or air enriched with an extra 200 ppm CO2, finding that the elevated CO2 concentration had no effect on the evapotranspiration rate of the trees growing on calcareous soils, but that it reduced this parameter by 7% in the trees growing on acidic soils. Likewise, in a FACE study conducted within a 12-year-old stand of sweetgum trees growing on nutrient-rich soils, Wullschleger et al. (2002) found that an extra 150 ppm of CO2 reduced stand evaporation by 7%. Last of all, in an eight-month study of scrub-oak communities growing in open-top chambers, Hungate et al. (2002) determined that an extra 320 ppm of CO2 reduced the mean daily rate of evapotranspiration by 19%.
These experimental observations from a variety of agricultural and natural ecosystems, including woody species, indicate that evaporative water losses from terrestrial surfaces will likely experience a small decline as the air's CO2 content continues to rise, all else being equal. For a 300 ppm increase in atmospheric CO2 concentration, for example, the results reported in this summary suggest a mean evapotranspiration reduction on the order of 10%.
But what if "all else" does not remain equal? What if the globe continues to warm, as it has over the past two centuries, as indicated by the work of Esper et al. (2002)? Will the CO2-induced water savings be lost?
The study of Moonen et al. (2002) provides an interesting perspective on this question. Based on 122 years (1878-1999) of meteorological data collected at a site near Pisa, Italy, they found that although annual air temperatures rose over this period, evapotranspiration actually declined, possibly in response to enhanced daytime cloud cover, such that there were "no significant changes in soil water surplus or deficit on an annual basis." Hence, it would appear that a climate change of the type about which most people worry, i.e., CO2-induced global warming, would not necessarily negate the direct water-savings produced by the rise in the air's CO2 content.
References
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.
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.
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.
Esper, J., Cook, E.R. and Schweingruber, F.H. 2002. Low-frequency signals in long tree-ring chronologies for reconstructing past temperature variability. Science 295: 2250-2253.
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.
Hungate, B.A., Reichstein, M., Dijkstra, P., Johnson, D., Hymus, G., Tenhunen, J.D., Hinkle, C.R. and Drake, B.G. 2002. Evapotranspiration and soil water content in a scrub-oak woodland under carbon dioxide enrichment. Global Change Biology 8: 289-298.
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 evapotranspiration and water use efficiency. Agricultural and Forest Meteorology 104: 85-105.
Moonen, A.C., Ercoli, L., Mariotti, M. and Masoni, A. 2002. Climate change in Italy indicated by agrometeorological indices over 122 years. Agricultural and Forest Meteorology 111: 13-27.
Wullschleger, S.D., Gunderson, C.A., Hanson, P.J., Wilson, K.B. and Norby, R.J. 2002. Sensitivity of stomatal and canopy conductance to elevated CO2 concentration - interacting variables and perspectives of scale. New Phytologist 153: 485-496.