Collaborative Research: Determining the role of hydraulic redistribution regimes in the critical zone - an experimental and modeling synthesis

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Understanding and predicting interactions between aboveground and belowground ecohydrologic processes is challenging. Roots play an important role in linking aboveground plant ecophysiological processes, such as carbon, water, and energy exchange with the atmosphere, and the belowground processes associated with soil moisture and carbon, and microbial and nutrient dynamics. Interestingly, root water movement is not always unidirectional; plants create pathways for preferential water movement through a phenomenon called "hydraulic redistribution", by which water moves downward through the roots during wet periods but upward during drier periods. This project will study such hydraulic redistribution. The study will both provide insights for dryland regions and enable broader generalizations regarding the interaction between groundwater, roots, and aboveground assemblages and their role in whole-ecosystem performance. The salient aspect of this study is the coupling between field research and modeling to enable hypotheses testing and provide an upscaling framework from site-based studies. Given projections of a warmer climate, there has been a rapid expansion in scientific interest in how ecosystems will respond to periods of prolonged stress. This research will help address this critical issue.The scientific objective of the project is to systematically examine the role of hydraulic redistribution by roots as a mechanism for facilitating aboveground-belowground linkages that drive water and carbon dynamics, and the development of emergent spatial patterns of soil moisture and vegetation distribution. Field-based activities will be conducted at two sites in southern Arizona: a riparian site where precipitation inputs are augmented with belowground water sources and an upland site that lacks this secondary water pool. This research will advance the theory for understanding the interaction of aboveground and belowground ecohydrological processes using a model that mechanistically resolves the related processes. Coupled modeling and field experimentation and monitoring of root and stem sap flux, soil, carbon, whole-plant, and ecosystem-scale (via eddy covariance tower) water and carbon fluxes will mutually inform each other to provide detailed insights at a range of scales. Results from this study will be presented in kiosks on the tour route of Biosphere 2 that sees over 100,000 visitors per year that include K-12 students to retirees.