Effects of altered precipitation regimes on North American desert plant physiology

Climate change will significantly impact deserts since precipitation drives biological activity in these ecosystems. Further, ecosystem responses to precipitation may be non-linear due to differential species responses to variation in the timing and magnitude of precipitation. Since plants impact total ecosystem function, it is critical to evaluate physiological responses to precipitation across multiple spatial and temporal scales. As such, this dissertation focuses on the physiological responses of desert grasses and shrubs to altered precipitation, spanning from the leaf- to the ecosystem-level. First, I examined the effects of increased precipitation on ecosystem fluxes of CO2 and H2O in Big Bend National Park (BIBE), Texas in the Chihuahuan Desert. To partition ecosystem fluxes, I conducted field measurements of plant, soil, and ecosystem fluxes after precipitation pulses throughout the summer. Increased summer precipitation altered soil carbon and water, and plant water fluxes over shorter time scales and carbon fluxes of grasses over longer time scales, which may lead to increased ecosystem carbon storage. Second, I focused on the leaf-level by measuring short-term physiological responses to precipitation events. I collected photosynthesis data for dominant plants in BIBE to examine the effects of natural and supplemental seasonal precipitation on carbon fluxes. I also measured environmental and biological variables to determine regulators of photosynthesis. Leaf-level fluxes of shrubs were more sensitive to both the amount of deep soil water and nitrogen, while grasses were only affected by leaf nitrogen. Further, increased precipitation in the summer and winter could have significant impacts on plant carbon gain and utilization due to soil nitrogen dynamics. Lastly, I collected photosynthetic data for seven dominant desert plants across North America to determine if the responses seen in BIBE were common across all deserts. I developed a Bayesian model of photosynthesis to analyze these data. Photosynthetic responses were similar across deserts and species, but different from temperate forest trees, leading to a unified understanding of desert plant physiology. When photosynthesis parameters were examined in relation to altered precipitation, data indicated that increased summer precipitation altered maximum electron transport and the temperature sensitivity of enzymatic reactions in C3 shrubs. Altered seasonal precipitation regimes also decoupled relationships between limitations to photosynthesis and nitrogen across all deserts and species studied.