Differential Effects of Wildfire and Forest Harvest on Snow Hydrology in the Oregon Cascades

Abstract:

Snow hydrology, climate, and forest ecosystems are intricately connected, with snow serving as a key moisture source for forests while forests fundamentally affect snow accumulation and ablation. These connections have important implications for western US water resources and forest management. Forests reduce snow accumulation via canopy interception while ablation is affected via changes in energy balance. We contrast the snow hydrology of undisturbed forests in the Oregon Cascades with those affected by wildfire and forest harvest. When the forest canopy is removed by wildfire or forest harvest it increases total snow accumulation and increases snowmelt rates. After a high-severity wildfire, canopy removal increases light transmission to the snow. The charred standing trees shed burned debris onto the snowpack surface decreasing snow albedo. The net result for the snowpack is much higher absorbed shortwave radiation and earlier/faster melt. Our work shows that the albedo effect can persist for several years after the fire. Forest harvest also reduces the forest canopy but unlike the post-wildfire environment, forest litter decreases. Our measurements and modeling results show that the effects of forest harvest on snow vary with elevation. At our lower-elevation warmer sites, snow persists longer in the open areas than in the forest while at the higher elevation colder sites, snow persists longer in the forest. In addition to our snow hydrology results, we present preliminary hydrologic modeling showing how these differences in snow accumulation and melt rates influence streamflow in watersheds dominated by surface runoff and in those dominated by groundwater flow.