Chilling and forcing proceed in parallel to regulate spring leaf unfolding in temperate trees

Aim: Temperature is the main driver of growth reactivation in plants of extratropical regions. Accumulations of chilling and forcing units during dormancy co-regulate spring phenology. Here, we aimed to answer whether chilling and forcing proceed in parallel or sequentially to regulate spring phenology in temperate trees.Location: Europe.Time Period: 1951-2016.Major Taxa Studied: Nine temperate woody species.Methods: Using long-term and large-scale records of in situ leaf unfolding dates of temperate tree species at more than 2300 sites, we analysed the rolling partial correlations between leaf unfolding dates and chilling and forcing in winter and spring using a weekly smoothing window. Through process-based modelling, we further identified the start of forcing accumulation and the end of chilling accumulation using the Unified model and compared the model efficiency of the Parallel and the Sequential models.Results: We observed negative responses of leaf unfolding dates to accumulations of both chilling and forcing units for most of winter and spring across successional types of species (early- and late-successional taxa), elevations and periods. Using the Unified model, we also found overlapping windows for chilling and forcing accumulations. Moreover, the Parallel model performed better than the Sequential model. These findings suggested that chilling and forcing requirements may be fulfilled simultaneously in temperate trees.Main Conclusions: Our study not only provides a guideline for identifying the effective periods of chilling and forcing, but also a general and robust perspective that accumulations of chilling and forcing act in parallel to regulate spring leaf unfolding in temperate trees, promoting more precise and reasonable predictions of temperature-driven phenological shifts under future climate change.