Suppressed water availability in winter buds delays the bud burst of broad-leaved trees in a heavy snow forest.

Early snowmelt is known to accelerate budburst. Budburst and leaf expansion require water absorption, and current-year vessels, which function as water pathways, begin to mature in early spring. However, whether the limitation of xylem reactivation by snow affects budburst and leaf expansion remains unclear. The response of winter buds to seasonal changes in both air and soil temperatures under snow cover, as buds transition from dormancy to budburst, requires clarification. The aim of this study was to clarify how snow affects budburst and leaf expansion. We focused on whether limitations in xylem reactivation and the restrictions on water use in stems, twigs and buds due to snow affect budburst. We established two distinct sites with different snow depths in Japan. From winter to summer, we observed leaf phenology, current-year xylem reactivation and measured bud-water content in response to ambient temperatures in canopy trees of Quercus crispula and Fagus crenata. Water absorption in winter buds towards bud burst may not be suppressed by the limitation of xylem reactivation, because the maturation of current-year vessels is likely important for water use for new leaf expansion after budburst in both tree species and sites. We suggest that current-year vessels matured for water use for new leaf expansion and transpiration because vessel maturation timing was linked to the leaf growth period during early spring. From the results of water absorption rate in winter buds towards budburst, we elucidated that budburst timing was delayed because winter buds require substantial time to absorb water in a forest with a deep snow cover during spring than in a forest with less snow cover. This study concluded that soil temperature influences water absorption in winter buds towards budburst.