Forest patch geometry and climate regulate the impact of forest thinning on snowpack in the Southwest United States

Authors: BROXTON, P.D - University Of Arizona; Biederman, Joel; DWIVEDI, R. - University Of Arizona; VAN LEEUWEN, W.J.D. - University Of Arizona; SANKEY, T. - Northern Arizona University; WOOLLEY, T. - The Nature Conservancy; SVOMA, B. - Salt River Project

Submitted to: Ecohydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/29/2025
Publication Date: 9/16/2025
Citation: Broxton, P., Biederman, J.A., Dwivedi, R., Van Leeuwen, W., Sankey, T., Woolley, T., Svoma, B. 2025. Forest patch geometry and climate regulate the impact of forest thinning on snowpack in the Southwest United States. Ecohydrology. 18(6): Article e70111. https://doi.org/10.1002/eco.70111.
DOI: https://doi.org/10.1002/eco.70111

Interpretive Summary: As water becomes increasingly scarce in the western US, it is critical to understand how forests regulate snowpack water supplies. Here, we studied a forested site in northern Arizona where forest management thinned the trees from about 30% cover to 15% cover. The study used an ultra-high precision snow model (1-m spatial resolution) to investigate the relationship between the height, location, and canopy density of trees and the surrounding snowpack and snowmelt. The model was trained with data from 5 years of field measurements including airborne laser maps of terrain, canopy, and snow, handheld radiation sensors, and Snowtography (snow-photography), which uses automated cameras to record snow depth daily at dozens of locations. We found that the forest thinning reduced snow sublimation losses from the tree canopy and therefore increased snowmelt volumes in both long, cold winters and shorter, warmer winters. Thinning had no impacts on the duration of snow cover in warmer winters, but thinning shortened the snow cover duration during longer, colder winters, due to greater sunshine exposure, which is more important later in the winter. These results imply a potential trade-off for colder, higher-elevation watersheds: thinning may increase streamflow but advance snowmelt timing, putting pressure on water resources infrastructure and prolonging the summer dry period during which forests are most susceptible to wildfire.

Technical Abstract: Despite having important implications for water resources, the climatic dependence of forest thinning impacts on snowpack ispoorly quantified. In this study, we used a high-resolution snow model to understand the impact of forest thinning on snowpackin Arizona under contrasting climate conditions, leading to ephemeral vs. seasonal snowpack conditions. The model is evalu-ated using a spatiotemporally extensive set of snowpack measurements and is run for the same set of pre- and post-thinningforest patch geometry using two meteorological forcing datasets representing locally mid- and high-elevation climate conditions.Although the high-elevation climate is only 1°C cooler and has 20% more winter precipitation, it leads to markedly differentsnowpack conditions, i.e., twice as long-lasting snowpack, less mid-winter ablation events and ~60% larger at its peak. For bothclimates, forest thinning increased peak snow water equivalent (SWE) and liquid water input (LWI), but it decreased snow coverduration (SCD) only for the high-elevation climate. Total sublimation losses decreased from ~35% of wintertime precipitation pre-thinning to ~25% post-thinning for the high-elevation climate and from ~25% to ~15% for the mid-elevation climate. Generally, a10% reduction in canopy cover resulted in ~4.5% more snowfall reaching the ground, and a 10- day decrease in SCD reduced thefraction of winter precipitation lost to snowpack sublimation by ~2%. Post-thinning changes in forest patch geometry were alsoimportant as larger canopy gaps had more LWI, and areas with warmer canopy edges had lower peak SWE and SCD.