Controls on Land-Air Carbon and Water Exchange in Semiarid Ecosystems

Abstract

Warming and rainfall intensification linked to climate change will alter water availability in drylands—arid and semiarid regions that cover 41% of the land surface, house over two billion people, and impact global carbon and water cycling. In these water-limited systems, episodic rainfall drives pulses of plant and soil activity that regulate ecosystem exchanges of water and carbon. Despite the outsize importance of pulse events on dryland carbon and water dynamics, prior frameworks have not considered how rising atmospheric demand and the intensification of rainfall modify pulse responses. Moreover, models of soil carbon emissions are largely based on studies from temperature-limited, mesic regions which may not reflect how soil moisture regulates respiration processes. These knowledge gaps limit our ability to assess how changing moisture availability in a warming climate will impact dryland carbon and water cycling. My dissertation addresses these gaps by using observations to confront models and advance the pulse framework of water-limited ecosystems. In Appendix A, I identify the dominant drivers of soil CO2 efflux in drylands and present a model that can capture efflux dynamics by considering interactions among temperature, moisture, and photosynthesis. In Appendix B, I integrate flux tower datasets to show that high atmospheric demand suppresses ecosystem photosynthesis more than respiration or evapotranspiration, which decreases the carbon uptake and water efficiency of rain pulses. In Appendix C, I use a rainfall manipulation experiment to conclude that precipitation intensification toward fewer, larger storms decreases seasonal soil CO2 efflux in a semiarid grassland. Together, these findings advance the pulse paradigm and indicate the potential for climate-mediated shifts in dryland carbon and water cycling.