Isolating plant physiological responses to a changing climate of elevated temperatures and more variable water availability

The semiarid southwest has and is predicted to continue to experience multiple concomitant changes in the coming decades. Climatic warming and more variable precipitation with long periods of drought are projected for the region. Vegetative change, through the widespread expansion of woody plants into grasslands, is also expected to continue. Due to differences in plant physiological responses to varying conditions of temperature and moisture availability, these climatic and vegetative shifts could mean big changes in how ecosystems sequester carbon dioxide (CO₂) from or respire CO₂ to our atmosphere. In this study, we compared photosynthetic rates of CO₂ assimilation (A) in C₃ woody mesquites and C₄ grasses in response to a large rainfall event after a prolonged drought. Paired treatments were maintained at two temperatures differing by 4°C to provide insight into responses under current and expected future climate conditions. Preliminary results indicate greater A within grasses than mesquites at both dry and post-rainfall time points, suggesting a possible movement toward less carbon sequestration in the future under the influence of woody plant encroachment. Time to reach peak A after an initial rainfall event was greater in grasses demonstrating a difference in growth strategies for precipitation response. Furthermore, grasses tended to have higher optimum temperatures, but mesquites were able to maintain A across a broader range of temperatures under dry, drought conditions. The effects of deeper rooting and groundwater access in woody plants during longer drought periods could complicate these results and would warrant future study.