Effects of precipitation on photosynthesis and water potential in Andropogon gerardii and Schizachyrium scoparium in a southern mixed grass prairie

In grassland ecosystems, spatial and temporal variability in precipitation is a key driver of species distributions and population dynamics. We experimentally manipulated precipitation to understand the physiological basis for differences in responses of species to water availability in a southern mixed grass prairie. We focused on the performance of two dominant C4 grasses, Andropogon gerardii Vitman and Schizachyrium scoparium (Michx.) Nash, in treatments that received ambient rainfall, half of ambient rainfall (“drought” treatment), or approximately double ambient rainfall (“irrigated” treatment). Water potentials of S. scoparium were lower than A. gerardii, suggesting superior ability to adjust to water deficit in S. scoparium. Additionally, drought reduced photosynthesis to a greater extent in A. gerardii compared to S. scoparium. Leaf-level photosynthesis rates were similar in ambient and irrigated treatments, but were significantly lower in the drought treatment. Although stomatal conductance was reduced by drought, this was not limiting for photosynthesis. Leaf δ13C values were decreased by drought, caused by an increase in Ci/Ca. Chlorophyll fluorescence measures indicated light-harvesting rates were highest in irrigated treatments, and were lower in ambient and drought treatments. Moreover, drought resulted in a greater proportion of absorbed photon energy being lost via thermal pathways. Reductions in photosynthesis came as a result of non-stomatal limitations in the C4 cycle. Our results provide mechanistic support for the hypothesis that S. scoparium is more drought tolerant than A. gerardii.

Research highlights
► Physiological responses to precipitation were measured in a mixed grass prairie.
► Water potentials of Schizachyrium scoparium were lower than Andropogon gerardii.
► Drought reduced photosynthesis more in A. gerardii compared to S. scoparium.
► Leaf δ13C values were decreased by drought, caused by an increase in Ci/Ca.
► Reductions in photosynthesis came as a result of non-stomatal limitations, specifically in the C4 cycle.