Effects of elevated CO2 concentrations on drought response in three prairie species

Soil water availability can modulate the response of plants to changing atmospheric CO₂ concentrations and may induce any of a spectrum of growth and morphological changes in prairie species. The degree of photosynthetic responsiveness to increased CO₂ varies by species, thereby complicating the calculation of the effect of CO₂ on plant level gas exchange. The objective of this study was to evaluate the response of prairie species growing under two different CO₂ concentrations (ambient: ~360 ppm, elevated: 560 ppm) to a significant dry-down event in the Upper Midwest during the summer of 2006. For this study 3 species were observed: Andropogon gerardi (C4 grass), Lespedeza capitata (legume), and Solidago rigida (non-legume forb). The photosynthetic rate of all 3 species decreased significantly during the dry-down in both elevated and ambient CO₂ concentrations, but the decrease was less drastic in plants exposed to the elevated CO₂ concentration (P < 0.1). With decreased rainfall, there was also a decrease in stomatal conductance in both elevated and ambient CO₂ concentrations for Solidago rigida and Lespedeza capitata. In many cases, a reduced stomatal conductance under elevated CO₂ concentrations led to increased water-use efficiency under drought conditions. Interspecific differences in these responses may have implications for individual species’ fitness and ultimate community responses in the face of environmental conditions predicted for the future.