Forest Management in a Warming World: Enhencing Insights into Compounding Hydrologic Impacts

Montane watersheds of California’s Sierra Nevada are critical to sustaining local water security and economic wellbeing. However, decades of fire suppression have led to overgrown forests that are highly vulnerable to drought and wildfire risks. Moreover, climate change is further compounding the negative impacts of increased forest density. Land managers are implementing active forest management to restore source watersheds and build climate resilience. In this study, we investigated the individual and compounding impact of forest thinning and warming on watershed hydrologic response using a process-based model (i.e., SWAT+) in a large (3,998 km²) Sierra Nevada watershed. The model was parameterized using a multi-objective calibration of streamflow, snow water equivalent, and evapotranspiration. We conducted multiple numerical experiments with forest treatments (25% and 40% reduction in leaf area index implemented in a wet and a dry year) and warming (ambient temperature, +1.5 ^oC, and +3.0 ^oC) to evaluate the variability of the hydrological response across a water-energy gradient and the extent to which forest treatments can offset the response to warming. Results indicate that warming increased evapotranspiration in energy-limited forests, while a reduction in evapotranspiration was observed in water-limited forests due to an increase in water stress. The water made available through forest thinning was directed towards increasing streamflow or sustaining the remaining trees, depending on water and energy availability and forest regrowth. We found that large-scale forest restoration in the upper Kings River Basin has the potential to partially mitigate warming impacts on streamflow by a maximum of 48% and 36% for +1.5 ^◦C and +3.0 ^◦C temperature increase, respectively, thus reducing the severity of warming impacts on streamflow and forest water stress.