Nancy Grulke Pacific Southwest Research Station USDA Forest Service 4955 Canyon Crest Drive Riverside, CA 92507 ngrulke@fs.fed.us
Components of global change such as changing precipitation patterns, duration of summer droughts, and warming are expected to significantly increase tree drought stress via changes in soil moisture availability and evapotranspirational demands. The recent chronic drought (1999-2001) followed by acute drought (2002) in the southwestern U.S., with subsequent 40% (southern California; Grulke et al., in press) to 80% (southern Arizona) pine mortality highlights the catastrophic responses that are to be experienced with changing climate and multiple environmental stressors. Such climate-change-driven stresses on the tree scale will translate to ecosystem-level changes in water, carbon and nitrogen balances. With increasing variability in climate, understanding the translation of multiple stressors at the tree level to total watershed water flow and quality will be critical for modeling the nation’s water supply under various climate change scenarios. In the southwestern U.S. example, we are witnessing feedbacks in biogeochemical and ecohydrological processes that are currently resulting in type conversions (pine dominated forests to oak woodlands) and the resulting effects on biodiversity and invasibility. Increased susceptibility to pests and disease, both native and exotic, is highly related to the level of drought stress experienced by individual trees (as a single point of infection) and to forest stands (with contagion). Understanding and forecasting ecosystem effects of these complex interactions requires knowledge of how stresses at the individual tree level translate to ecosystem-level balances in water, carbon, and nitrogen. How individual tree drought stress and climatic variability interacts at the national level provides a means to identify threshold points of response, and to develop reliable regional and continental forecasts.
We propose a simple National Experiment on Tree Drought (NETD) that will allow ecologists, hydrologists, biometeorologists and earth system modelers to directly measure effects of increased tree drought on stand and watershed scale processes in a network of 13 sites across the nation. Approximately one-third of major roots of dominant forest species will be cut in one third of the trees in experimental watersheds that are adjacent or near control wildland forest sites across the nation. Most of the selected sites have already been recommended to NEON as observational studies. This experiment extends the observational efforts proposed to induce a much greater level of drought stress than would be experienced under current conditions, in order to expand our capability to model forest response to future, unprecedented, environmental conditions. We propose detailed evaluation of the effects of drought on tree, stand and watershed scale carbon, water, and nutrient fluxes in paired control and root-severed trees within experimental watersheds as well as between the experimental and control watersheds. The network of 13 forest sites will provide significant power to distinguish interactions of the drought responses with different pollutant loading, tree species, and site characteristics. The 15 year experiment is designed to provide a cost effective approach to modifying forest water status that will be needed to obtain physiological, soil, and watershed level parameters for models that link biosphere stress functions to climatic drivers and water shed outputs. Such information is essential for forecasting the responses of our nation’s forest and watersheds to climate change.