Abstract
Plants may be more sensitive to carbon dioxide (CO 2 ) enrichment at subambient concentrations than at superambient concentrations, but field tests are lacking. We measured soil‐water content and determined xylem pressure potentials and δ 13 C values of leaves of abundant species in a C3/C4 grassland exposed during 1997–1999 to a continuous gradient in atmospheric CO 2 spanning subambient through superambient concentrations (200–560 µmol mol 2−1 ). We predicted that CO 2 enrichment would lessen soil‐water depletion and increase xylem potentials more over subambient concentrations than over superambient concentrations. Because water‐use efficiency of C3 species (net assimilation/leaf conductance; A/g) typically increases as soils dry, we hypothesized that improvements in plant‐water relations at higher CO 2 would lessen positive effects of CO 2 enrichment on A/g. Depletion of soil water to 1.35 m depth was greater at low CO 2 concentrations than at higher CO 2 concentrations during a mid‐season drought in 1998 and during late‐season droughts in 1997 and 1999. During droughts each year, mid‐day xylem potentials of the dominant C4 perennial grass ( Bothriochloa ischaemum (L.) Keng) and the dominant C3 perennial forb ( Solanum dimidiatum Raf.) became less negative as CO 2 increased from subambient to superambient concentrations. Leaf A/g—derived from leaf δ 13 C values—was insensitive to feedbacks from CO 2 effects on soil water and plant water. Among most C3 species sampled—including annual grasses, perennial grasses and perennial forbs—A/g increased linearly with CO 2 across subambient concentrations. Leaf and air δ 13 C values were too unstable at superambient CO 2 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 concentrations generally were not greater over low CO 2 than over higher CO 2 . The continuous response of these variables to CO 2 suggests that atmospheric change has already improved water relations of grassland species and that periodically water‐limited grasslands will remain sensitive to CO 2 enrichment.