Interacting Effects of Soil Water, Temperature and Irradiance on CO 2 Exchange Rates of Two Dominant Grasses of the Shortgrass Prairie

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Abstract
CO2 exchange rates of relatively undisturbed sods of blue grama (Bouteloua gracilis (H.B.K.) Lag.), a C4 species, and western wheatgrass (Agropyon smithii Rydb.), a C3 species, were affected by soil water potential, temperature, visible irradiance and their interactions. Light saturation of blue grama occurred at very high irradiances, accompanied by high temperatures and soil water stress. Light saturation of western wheatgrass occurred at relatively low irradiances. Western wheatgrass was more sensitive to water stress than was blue grama. The optimum temperature for photosynthesis of blue grama was affected by water stress and ranged from about 26-33.degree. C. The optimum temperature for western wheatgrass was lower than the lowest temperature (20.degree. C) included in the experiments. Dark respiration for both species increased with increasing temperature and decreased with increasing soil water stress. Respiration rates of blue grama were usually greater than those of western wheatgrass, except at a low temperature (20.degree. C) and high soil water stress (-15 and -30 bars).

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