Snow Disappearance Timing in Warm Winter Climates is Dominated by Forest Effects On Snow Accumulation

Forests modify snow processes and affect snow water storage as well as snow disappearance timing. However, forest influences on snow accumulation and ablation vary with climate and topography, and are therefore subject to substantial temporal and spatial variability. Prediction of where and how forest cover will accelerate versus delay snow disappearance timing has valuable forest management applications, particularly in regions of extensive forest cover and intensive timber harvest such as the Pacific Northwest (PNW), USA. However, an improved understanding of how different forest-snow processes combine to contribute to snow disappearance timing is needed before applying empirical models (Varhola et al., 2010; Lundquist et al., 2013), or physically-based hydrologic models (Ellis et al., 2013; Du et al., 2016) to management practices. We therefore utilize multiple years of snow observations from across the PNW to assess forest-snow interactions in the relatively warm winter conditions characteristic of maritime and transitional maritime-continental climates (Figure 1). 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 combine to determine whether snow disappears later in the forest or in the open.