ALBERT

Description: The extensive forests that cover the mountains of the Pacific Northwest, USA, modify snow processes and therefore affect snow water storage as well as snow disappearance timing. However, forest influences on snow accumulation and ablation vary with climate, topography, and land cover and are therefore subject to substantial temporal and spatial variability. We utilize multiple years of snow observations from across the region to assess forest-snow interactions in the relatively warm winter conditions characteristic of the maritime and maritime-continental climates. We (1) quantify the difference in snow magnitude and disappearance timing between forests and open areas and (2) assess how forest modifications of snow accumulation and ablation processes combine to determine whether snow is retained longer in the forest or in the open. We find that snow disappearance timing generally ranges from synchronous in the forest and open, to snow persisting up to 13 weeks longer in the open relative to a forested area. However, in locations subject to high wind speeds, differential snow disappearance timing is reversed: snow persists 2-5 weeks longer in the forest. Analysis of snow accumulation and loss through the winter and spring indicate that forest canopy-modulated snow deposition, rather than ablation processes, sets up the difference in snow duration between forests and open. The difference in snow accumulation between open areas and forests is larger than the difference in snow ablation, except at windy sites. Subsequently, differential snow disappearance timing tends toward longer snow retention in the open. The time lag in snow disappearance between the forest and open is then a consequence of the amount of snow remaining and the ablation rate in the open. These findings suggest that improved quantification of forest effects on snow accumulation processes are needed to accurately predict the effect of forest canopy change via harvest or natural disturbance on snow water resources of the Pacific Northwest.