Differential snow depth in warm edges versus cold edges of forest gaps, and its potential implications for tree growth in a Sierra Nevada conifer forest

Abstract

Forest canopy gaps affect the timing and amount of snowmelt, because canopies act as key controls over snowpack dynamics. Overlying canopy can decrease snowpack by intercepting snowfall, but it can also reduce ablation rates via shading. Changes in forest structure and canopy gaps may therefore affect the amount, timing, and duration of snowmelt and potentially forest response to water limitations. We examine opposing edges of gaps, testing how higher energy-input (warm) edges differ from lower energy-input (cool) edges with respect to snow depth, snowmelt timing, and tree growth in a snow-dominated forest in the Western US. We use multiple dates of LiDAR-based measurement to assess springtime snow depths in warm and cool gap edges in Sagehen Creek Basin, CA. Then we use paired tree sampling and ring width chronologies to ascertain moisture sensitivity of trees adjacent to warm and cool edges. Pre-ablation snow depths in cool gap edges exceeded those in warm gap edges by 9–18 %, with effect size depending on elevation and aspect. Snowpack also persisted longer in cool edges than in warm edges. Growth variations in warm-edge-adjacent trees were more correlated with interannual variations in snow depth those of cool edge trees, although neither had strong correlations. These findings suggest that forest structures that maximize cool edge area may benefit snow depth and persistence, and may lead to cool-edge trees being less sensitive to interannual hydroclimatic variability than warm edge trees, despite this effect being small relative to other controls over growth.