Abstract

Undisturbed tallgrass prairie was exposed to ambient and elevated (twice-ambient) levels of atmospheric CO2 and experimental dry periods. Seasonal and diurnal midday leaf water potential (Ψleaf), net photosynthesis Anet, and stomatal conductance (gs) responses of three tallgrass prairie growth forms—a C4 grass, Andropogon gerardii; a broad-leaved woody C3 shrub, Symphiocarpos orbiculatus; and a C3 perennial forb, Salvia pitcheri—were assessed. Ψleaf in A. gerardii and S. orbiculatus was higher under elevated CO2, regardless of soil moisture, while Ψleaf in S. pitcheri responded only to drought. Elevated CO2 always stimulated Anet in the C3 species, while A. gerardii Anet increased only under dry conditions. However, Anet under elevated CO2 in the C3 species declined with drought but not in the C4 grass. Under wet conditions, gs reduced in elevated CO2 for all species. During dry periods, gs at elevated CO2 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.

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