PHOTOSYNTHETIC GAS-EXCHANGE AND WATER RELATION RESPONSES OF 3 TALLGRASS PRAIRIE SPECIES TO ELEVATED CARBON-DIOXIDE AND MODERATE DROUGHT

Undisturbed tallgrass prairie was exposed to ambient and elevated (twi ce-ambient) levels of atmospheric CO, and experimental dry periods. Se asonal and diurnal midday leaf water potential (Psi(leaf)), net photos ynthesis (A(net)), and stomatal conductance (g(s)) responses of three tallgrass prairie growth forms-a C-4 grass, Andropogon gerardii; a bro ad-leaved woody C, shrub, Symphiocarpos orbiculatus; and a C-3 perenni al forb, Salvia pitcheri-were assessed. Psi(leaf) in A. gerardii and S , orbiculatus was higher under elevated CO2, regardless of soil moistu re, 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 elevat ed 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 am bient 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.