SOIL- AND PLANT-WATER DYNAMICS IN A C3/C4 GRASSLAND EXPOSED TO A SUBAMBIENT TO SUPERAMBIENT CO2 GRADIENT

Authors: Polley, Herbert; Johnson, Hyrum; Derner, Justin

Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/16/2002
Publication Date: 4/16/2002
Citation: Polley, H.W., Johnson, H.B., Derner, J.D. Soil- and plant-water dynamics in a C3/C4 grassland exposed to a subambient to superambient CO2 gradient. Global Change Biology. 2002. v. 8. p. 1118-1129.

Interpretive Summary: The concentration of carbon dioxide gas in air has doubled since the last ice age and is expected to increase by another 50% by mid-century. Rising carbon dioxide levels may increase plant growth on grasslands and in other water-limited habitats by reducing plant requirements for water. Uncertainty remains, however, because several environmental factors could lessen or even eliminate this benefit. For three years, we measured rates at which plants used soil water and quantified the efficiency with which this water was converted to plant growth in a Texas grassland that was exposed to carbon dioxide concentrations spanning low levels of the past to high concentrations expected during the next 50 years. Increasing carbon dioxide lessened water stress experienced by the dominant plant species, partly by slowing the rate at which plants depleted soil water during drought. These processes were equally sensitive to carbon dioxide at low levels of the past and at high concentrations predicted for the future. The efficiency with which plants converted water to growth also increased over both low and higher-than-present carbon dioxide concentrations in the nine plant species examined. These results indicate that plants in many water-limited environments grow larger today than they did in the past. Plant production on most grasslands will continue to increase as carbon dioxide levels climb during the next 50 years.

Technical Abstract: We measured soil water content and determined xylem pressure potentials and stable carbon isotope compositions of leaves of abundant species in a C3/C4 grassland exposed from 1997-1999 to a continuous gradient in atmospheric CO2 concentrations from 200-560 ppm. Our objectives were 1) to test the prediction that CO2 enrichment would lessen soil water depletion and increase xylem potentials more over subambient than superambient concentrations and 2) to test an hypothesized feedback of CO2-mediated changes in soil and plant water relations on intrinsic water use efficiency of C3 species (net assimilation rate/leaf conductance; A/g). Maximum net depletion of soil water to 1.35 m depth declined linearly with increasing CO2 concentration during droughts in each of three years. Mid-day xylem potentials of the dominant C4 grass and the dominant C3 perennial forb usually increased linearly with CO2 over subambient to superambient concentrations. Leaf A/g was relatively insensitive to feedbacks from CO2 effects on soil and plant water. Among most C3 species sampled, including annual grasses, perennial grasses, and perennial forbs, A/g increased linearly and by a similar relative amount as did CO2 across subambient concentrations. Carbon isotope values were too unstable at superambient CO2 concentrations to reliably determine A/g. Significant changes in soil and plant water relations over subambient to superambient concentrations and in leaf A/g over subambient CO2 concentrations generally were linearly related to CO2. The predominance of linear, responses to CO2 indicates that periodically water-limited grasslands may remain sensitive to rising CO2 concentration.