Decreased Snow Cover and Seedling Fate: Microsite- and Species-Specific Responses in Seedling Growth and Survival

Decreased snow cover can reduce the insulating effect, exposing seedlings to lower temperatures and increased cold stress. While previous studies have shown that reduced snow cover generally has negative effects on seedling survival and growth in forest ecosystems, a lack of understanding remains regarding how environmental heterogeneity within forests can modify these responses In this study, we focused on tree canopy-induced environmental differences in cold stress response, specifically the contrast between under tree canopy and canopy gap, to examine how tree canopy structure can influence seedling responses to decreased snow cover. To achieve this goal, we conducted a two-year snow removal experiment in both under the tree canopy and in the canopy gap microsites within a spruce (Picea glehnii) plantation forest in northern Hokkaido, Japan. We targeted each of two evergreen conifer and deciduous broad-leaved species. By measuring near-ground air temperature directly experienced by seedlings, we found that the number of temperature crossings of the freezing point (0°C) was greatest in the snow removal plots under the tree canopy. Through the two years of observation, we found that seedling mortality of both oak and spruce was significantly higher in the snow removal plots than in the control plots, but only under the tree canopy. Additionally, the shoot and basal diameter growth of spruce were significantly reduced by snow removal only under the tree canopy, but not in the canopy gap. Overall, these findings indicate that the tree canopy can intensify cold stress under reduced snow cover, resulting in higher mortality and lower growth under the tree canopy compared to the canopy gaps for certain species.