Assessing the effects of forest biomass reductions on forest health and streamflow

Forest biomass reductions in overgrown forests have the potential to provide hydrologic benefits in the form of improved forest health and increased streamflow production in water‐limited systems. Biomass reductions may also alter evaporation. These changes are generated when water that previously would have been transpired or evaporated from the canopy of the removed vegetation is transferred to transpiration of the remaining vegetation, streamflow, and/or non‐canopy evaporation. In this study, we combined a new vegetation‐change water‐balance approach with lumped hydrologic modelling outputs to examine the effects of forest biomass reductions on transpiration of the remaining vegetation and streamflow in California's Sierra Nevada. We found that on average, 102 mm and 263 mm (8.0% and 20.6% of mean annual precipitation [MAP]) of water were made available following 20% and 50% forest biomass‐reduction scenarios, respectively. This water was then partitioned to both streamflow and transpiration of the remaining forest, but to varying degrees depending on post‐biomass‐reduction precipitation levels and forest biomass‐reduction intensity. During dry periods, most of the water (approximately 200 mm [15.7% on MAP] for the 50% biomass‐reduction scenario) was partitioned to transpiration of the remaining trees, while less than 50 mm (3.9% on MAP) was partitioned to streamflow. This increase in transpiration during dry periods would likely help trees maintain forest productivity and resistance to drought. During wet periods, the hydrologic benefits of forest biomass reductions shifted to streamflow (200 mm [15.7% on MAP]) and away from transpiration (less than 150 mm [11.8% on MAP]) as the remaining trees became less water stressed. We also found that streamflow benefits per unit of forest biomass reduction increased with biomass‐reduction intensity, whereas transpiration benefits decreased. By accounting for changes in vegetation, the vegetation‐change water balance developed in this study provided an improved assessment of watershed‐scale forest health benefits associated with forest biomass reductions.