Collaborative Research: Impacts of Dynamic, Climate-Driven Water Availability on Tree Water Use and Health in Mediterranean Riparian Forests

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Forests along rivers are under threat due to climate-driven changes in water availability to trees. When water in the rooting zone is limited, trees undergo physiological changes that affect their overall growth and health. This problem is particularly acute within forests in river floodplains in regions with warm, dry summers (e.g., large areas of the USA). Such forests provide a range of ecosystem services, but they are limited in area, species diversity, and are challenging to manage under a changing climate. This project aims to build new understanding of the ?ecohydrological? links between water availability and forest health by employing an interdisciplinary set of research methods. It involves extracting tree cores from floodplain trees to investigate annual growth and will further investigate the ?isotopic signatures? of source water used by trees as recorded within each annual ring. By also monitoring climate variations and collecting water from various contributing sources, the project will provide insight into variations in potential source waters to forest trees. To generalize this research, the research will employ a numerical model to assess how climate controls water access to forests under plausible scenarios of regional climate change. The project will generate: new tools and information for practitioners of forest/water/basin management, international educational opportunities for underrepresented groups in the hydrologic sciences, and a new ecohydrology course for undergraduates. This research project combines: 1) field-based measurements of climate and hydrology and laboratory analysis of oxygen isotopes from all potential tree water sources; (2) contemporary and retrospective analysis of oxygen and carbon isotopes in annual tree-rings to investigate recent climate-driven fluctuations in tree water use and water use efficiency; (3) seasonal (intra-annual) analysis of oxygen isotopes via high-resolution ?micro-slicing? of annual tree rings to assess seasonal fluctuations in tree water source use; and 4) improvement and application of a climate-driven numerical ecohydrology model that includes dynamic water fluxes into the floodplain, isotopic fractionation/mixing, and tree water uptake. It will compare the ecohydrologic responses to climate in water availability at forest sites along a strong climatic gradient. There is currently limited ability to link tree/forest response to decadal climate shifts. Generalizable understanding linking regional climate to water availability to water use by riparian trees across forests stands is currently lacking, which limits predictive capability of forest response to drought stress over decadal timescales. This research will enable: i) identification of signatures of water stress within riparian forests; ii) predictive capability of forest response to climate change; iii) a clearer picture of regional variations in the expression of climate within floodplain water availability; and iv) improvement of dendro-paleoclimate reconstructions by providing better constraints on water availability and use under different conditions of climatic forcing. This research project will provide managers with a new perspective and tools for anticipating and mitigating the risks of climate change on vulnerable riparian forest resources along major rivers in temperate and Mediterranean climatic regions. The project will host a 2-day workshop to disseminate our findings to practitioners and stakeholders in in the study region. It also includes a Research Experiences for Undergraduates (REU) supplement to enable two female and/or minority students to travel to France as their first international trip to participate on the first field campaign. The REU will provide these students will an exciting international research experience, a world-class learning opportunity, and an introduction to a potential career in environmental/hydrologic sciences. Additionally, data and models from this research will be integrated into a new upper division course on dryland ecohydrology at UCSB.