Montane Forest Water Use Under Increased Episodic and Earlier Snowmelt

Western U.S. montane forests rely disproportionately on snowmelt, which potentially makes them sensitive to changes in snowpack to alter large-scale water and carbon budgets. We used an aspect transect of sap flow and meteorological measurements in Sagehen Creek watershed, in the Sierra Nevada mountains, to empirically determine controls on the timing of water use by conifer forests. We determine which environmental variables (temperature, humidity, shortwave radiation, and soil moisture) are limiting sap flow across the growing season using a boundary lines analysis. We find that all sites exhibit transitions from energy limitations in spring, to water limitations in summer, and back to energy limitations in fall. Response of sap flow to predictors was more variable during spring than fall, which is consistent of more episodic temperature limitations in the spring. Later snowmelt leads to substantial transpiration prior to snow disappearance and trees were air temperature limited less of the time. Shifts from rain to snow under climate change might diminish existing forest refugia as snowmelt becomes more similar across slope aspects. Future efforts focused on modeling the relationship between snow water inputs and transpiration will be important to understanding the potential effects of climate change.