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

The current state of knowledge on bud dormancy is limited. However, expanding such knowledge is crucial in order to properly model forest responses and feedback to future climate. Recent studies have shown that warming can decrease chilling accumulation and increase dormancy depth, thereby inducing delayed budburst in European beech (Fagus sylvatica L). Whether fall warming can advance spring phenology is unclear. To investigate the effect of warming on endodormancy of deciduous trees, we tested the impact of mild elevated temperature (+ 2.5-3.5 °C; temperature on average kept at 10 °C) in mid- and late autumn on bud dormancy depth and spring phenology of beech. We studied saplings by inducing periods of warming in greenhouses during two years. Even though warming reduced chilling in both years, we observed that the response of dormancy depth and spring budburst were year-specific. We found that warming during endodormancy peak could decrease bud dormancy depth and therefore advance spring budburst. This effect appears to be modulated by factors such as the date of senescence onset and forcing intensity during endodormancy. Results from this study suggest that not only chilling, but also forcing controls bud development during endodormancy, and that extra forcing in autumn can offset reduced chilling.