Interacting effects of soil water, temperature and irradiance on CO2 exchange rates of two dominant grasses of the shortgrass prairie.
CO2 exchange rates of relatively undisturbed sods of Bouteloua gracilis (C4) and Agropyron smithii (C3) were investigated at 210, 780 and 1070 W/m2 visible irradiance, 20, 30 and 40 deg C and 0, -15 and -30 bar soil water potential. Light saturation of B. gracilis occurred at very high irradiances, accompanied by high temperatures and soil water stress. Light saturation of A. smithii occurred at relatively low irradiances. A. smithii was more sensitive to water stress than was B. gracilis. The optimum temperature for photosynthesis of B. gracilis was affected by water stress and ranged from about 26 to 33 deg C. The optimum temperature for A. smithii was lower than the lowest temperature included in the experiments. Dark respiration in both species increased with increasing temperature and decreased with increasing soil water stress. Respiration rates of B. gracilis were usually greater than those of A. smithii except at low temperature (20 deg C) and high soil water stress (-15 and -30 bar).