Ecohydrological dynamics of managed upslope and riparian forests: Insights from virtual fuel reduction treatments in the Sierra Nevada, CA

Wildfires pose an existential threat to forests throughout the western US. In the Sierra Nevada, CA, forest management is the primary tool to combat catastrophic wildfires. Fuel reduction treatments thin overly-dense stands and remove understory fuels with prescribed burns to reduce wildfire intensity, but they also modify forest structure, with potential impacts on ecohydrological processes including forest productivity and streamflow. The "Sagehen Fuels Reduction Project" recently implemented such novel management strategies in the central Sierra Nevada, CA. This study develops an ecohydrological conceptual model that considers the impact of fuel treatments on stand and hillslope processes. We simulate the effects of thinning on forest structure by combining a unique data set of lidar, flux tower data, vegetation surveys, and sap flow with the RHESSys ecohydrological model. Virtual management scenarios at hillslope and watershed scales quantify thinning impacts on carbon and water fluxes. Our results show that interactions between the overstory and understory can limit thinned tree regrowth, but the released water provides a 5% boost in transpiration for remaining trees. Riparian thinning increases hillslope-scale annual streamflow in the first year by 20%, low flows by 25%, and peak flows by 30%, while the influence of upslope thinning is small and delayed by soil storage for three years. Watershed-scale streamflow effects primarily depend on treatment effect size, while drought can eliminate any potential benefits. These results help to predict how fuel reduction treatments interact with complex ecohydrological processes and to inform forest management decisions for landscape-scale restoration in the Sierra Nevada.