Rainfall Variability and Carbon Cycling in Semi-Arid Ecosystems

Abstract

Shifting patterns of precipitation associated with climate change may affect water-limited ecosystems to a greater degree than atmospheric CO2 or temperature changes, yet we lack a mechanistic understanding of the effects of water in these ecosystems. In water-limited ecosystems, annual net primary productivity correlates strongly with total annual precipitation. However, precipitation in these ecosystems arrives in episodic events, suggesting that biophysical investigations should focus on the implications of discrete precipitation events. Further, examining dynamics of ecosystem processes over a period of days or weeks promises to link our leaf-level mechanistic understandings with larger scale patterns and temporal dynamics of ecosystem photosynthetic CO2 uptake, respiration and evapotranspiration.The objectives of this dissertation were to quantify: (1) the influence of biotic and abiotic features of an ecosystem (e.g., species composition and soil physical characteristics) on short-term patterns of resilience and resistance to a precipitation pulse; (2) the role of antecedent climatic conditions and the seasonal timing of rainfall in limiting ecosystem carbon exchange in response to precipitation events; and (3) the effect of changes in woody plant abundance on seasonal ecosystem carbon dynamics in relation to the North American Monsoon.Major findings and contributions of this research include defining the concepts of ecosystem functional resistance and resilience and their implications in the presence of a dominant nonnative bunchgrass in semi-arid grasslands (Appendix A); a better understanding of the influence of warm-season precipitation variability and the seasonal timing of rainfall on ecosystem carbon dynamics in a semi-arid grassland (Appendix B); the use of flux duration analysis, a novel approach to analyzing ecosystem carbon and water flux time-series data to distinguish between "pulse-driven" or "steady-state" ecosystems (Appendix C); and, finally, the application of flux duration analysis to quantify the sensitivity of ecosystem carbon exchange in response to seasonal rainfall in a riparian grassland and shrubland and the role that plant functional type diversity may play in constraining carbon exchange sensitivity (Appendix D).