Photosynthetic gas exchange and water relation responses of three tallgrass prairie species to elevated carbon dioxide and moderate drought

Undisturbed tallgrass prairie was exposed to ambient and elevated (twice-ambient) levels of atmospheric CO, and experimental dry periods. Seasonal and diurnal midday leaf water potential (Psi(leaf)), net photosynthesis (A(net)), and stomatal conductance (g(s)) responses of three tallgrass prairie growth forms-a C-4 grass, Andropogon gerardii; a broad-leaved woody C, shrub, Symphiocarpos orbiculatus; and a C-3 perennial forb, Salvia pitcheri-were assessed. Psi(leaf) in A. gerardii and S, orbiculatus was higher under elevated CO2, regardless of soil moisture, while Psi(leaf) in S. pitcheri responded only to drought. Elevated CO2 always stimulated A(net) in the C-3 species, while A. gerardii A(net) increased only under dry conditions. However, A(net) under elevated CO2 in the C-3 species declined with drought but not in the C,grass, Under wet conditions, g(s) reduced in elevated CO2 for all species. During dry periods, g, at elevated CO, was sometimes higher than in ambient CO2. Our results support claims that elevated CO2 will stimulate tallgrass prairie productivity during dry periods and possibly reduce temporal and spatial variability in productivity in these grasslands.