Water Balance and Plant Ecophysiology in Coastal California: Linking Models and Mechanisms to Project Winners and Losers under Future Climate Scenarios

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The geographic ranges of plant and animal species are closely tied to environmental conditions, including the seasonal patterns of temperature and rainfall. In California, almost all rain falls in winter, and plants experience severe water deficits during the hot, dry summer months. The summer dry season can have very different effects on different species, depending on how deep their roots grow to access water and differences in the degree of drought tolerance. Understanding how these factors vary across a landscape is critical to understand and predict the vulnerability of trees to a warmer and drier climate. This project will address this important problem in tree species of California oak woodlands, currently experiencing a severe, four-year drought. Isotopic and physiological measurements of trees will be combined with a landscape scale model that captures the effects of climate and soils on water deficits. By connecting these two approaches it will improve the ability to predict how different climatic conditions will impact the performance of tree species in different parts of their geographic range, and their vulnerability to warmer and drier conditions. The project will also provide training to young professionals at several career stages, including recruitment of undergraduate assistants from under-represented groups.The study of species distributions along environmental gradients is central to ecological research, and plays a critical role projecting species responses to a rapidly changing environments. This project addresses the physiological mechanisms underlying the responses of tree species in California oak woodlands to spatial and temporal variability in water deficits. A central goal is to understand how species differences in landscape position, rooting depth and the physiological regulation of stomata (isohydry vs. anisohdry) influence the intensity of seasonal water deficits, and the implications for vulnerability to a warmer, drier future climate. Eight tree species will be studied across four sites that span gradients of temperature, precipitation and climatic water balance. Oxygen isotopes and predawn water potentials will be used to assess functional rooting depth. Comparative hydraulic strategies, including measures of xylem vulnerability to embolism and leaf water relations, will capture species' responses to dry season water deficits. The results will be integrated into a high-resolution, landscape-scale water balance model to develop novel projections of how water deficits will impact species with different hydraulic strategies in responses to varying climatic conditions. These calibrated models will be further combined with projections of future environments to generate mechanistic predictions of species vulnerability to environmental change. The results of this project will be directly incorporated into local conservation planning and resource management, through collaborations with regional NGOs. Training will be provided for one post-doc and several graduate students; undergraduate research assistants will be recruited through the Berkeley Scholars Program, an award winning program supporting research and career development for students from under-represented groups, and through Pepperwood Preserve's collaborative programs with the Santa Rosa Junior College, an Hispanic-Serving Institution located close to the primary field site.