FLAME · Future ResiLient Forest in a ChAnging ClimatE: isotope observations and mechanistic modeling of soil water residence time and vegetation water uptake dynamics

The time that precipitated water resides in soil (residence time) varies from a few days to several months or even years, and increases with soil depth. How much of the water used by plants originates from the growing season precipitation and how much of it comes from previous events or seasons largely depends on storage capacity, permeability, and residence time of precipitation in soil. It is unclear how the dynamics of subsurface water storage and release, the seasonal origins and turnover time of water used by plants, and plant water uptake depths will change when environmental conditions change (e.g. receding groundwater, more frequent droughts). Yet, they are the most crucial in predicting vegetation resilience in response to drought. Studying the resilience of different plant species to climate change will facilitate promotion of climate-smart forest as conservation, afforestation, and restoration practices at several scales.Previous studies have attempted to improve the mechanistic understanding of ecosystem response to dry conditions or climate change by focusing either on vegetation water availability1 or plant physiological adaptation strategies2-4, but the combined effects of shifting terrestrial water availability and atmospheric demand have not been mechanistically investigated. In order to understand terrestrial ecosystems’ response to a changing climate, it is crucial to characterize precipitation partitioning in terrestrial systems, species-specific water uptake strategies, and plants' adaptive water use efficiency, all in a coupled framework. FLAME will use a newly developed high-frequency in-situ measurements of stable water isotopes (18O and 2H) in soil and xylem as a unique natural signature to trace the origin of vegetation water uptake and its residence time in subsurface. It will combine these observations with a high resolution physically-based water and vegetation uptake model to track water.