Late autumn warming can both delay and advance spring budburst through contrasting effects on bud dormancy depth in Fagus sylvatica L.

This article investigates the effects of mild autumn warming (+2.5–3.5 °C) during the endodormancy phase on bud dormancy depth and spring phenology in saplings of European beech (Fagus sylvatica L.). Conducted over two consecutive years, the study found that warming reduced chilling accumulation but had contrasting effects on dormancy depth and budburst timing: in one year, warming increased dormancy depth and delayed budburst, while in the other, it decreased dormancy depth and advanced budburst. These differing responses appear influenced by factors such as the timing of leaf senescence and the intensity of heat accumulation (forcing) during endodormancy, suggesting that chilling and forcing jointly regulate bud development. The findings highlight the complexity of tree phenological responses to climate warming and underscore the need to incorporate dormancy depth and concurrent chilling and forcing effects into phenological and climate models.