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Added Upon: Indirect Effects of Atmospheric CO2 Enrichment on the Productivity of Grasslands and Deserts
Morgan, J.A., Pataki, D.E., Korner, C., Clark, H., Del Grosso, S.J., Grunzweig, J.M., Knapp, A.K., Mosier, A.R., Newton, P.C.D., Niklaus, P.A., Nippert, J.B., Nowak, R.S., Parton, W.J., Polley, H.W. and Shaw, M.R.  2004.  Water relations in grassland and desert ecosystems exposed to elevated atmospheric CO2Oecologia 140: 11-25.

In a review of the scientific literature pertaining to the role of water relations in the response of grassland and desert ecosystems to elevated levels of atmospheric CO2, the authors note that "atmospheric CO2 enrichment may stimulate plant growth directly through (1) enhanced photosynthesis or indirectly, through (2) reduced plant water consumption and hence slower soil moisture depletion, of the combination of both."

What was done
Morgan et al. "describe gas exchange, plant biomass and species responses of five native or semi-native temperate and Mediterranean grasslands and three semi-arid systems to CO2 enrichment, with an emphasis on water relations."

What was learned
In the words of the authors, "increasing CO2 led to decreased leaf conductance for water vapor, improved plant water status, altered seasonal evapotranspiration dynamics, and in most cases, periodic increases in soil water content," such that "across the grasslands of the Kansas tallgrass prairie, Colorado shortgrass steppe and Swiss calcareous grassland, increases in aboveground biomass from CO2 enrichment were relatively greater in dry years."  In contrast, they report that "CO2-induced aboveground biomass increase in the Texas C3/C4 grassland and the New Zealand pasture seemed little or only marginally influenced by yearly variation in soil water, while plant growth in the Mojave Desert was stimulated by CO2 in a relatively wet year."  Also, they say that "Mediterranean grasslands sometimes failed to respond to CO2-related increased late-season water, whereas semiarid Negev grassland assemblages profited."

What it means
Although Morgan et al. remark that "vegetative and reproductive responses to CO2 were highly varied among species and ecosystems, and did not generally follow any predictable pattern in regard to function groups," they say that, considered in their entirety, the literature results they reviewed, which they themselves had been instrumental in collecting, "suggest that the indirect effects of CO2 on plant and soil water relations may contribute substantially to experimentally induced CO2-effects."

Reviewed 4 August 2004