Chilling Limits and impacts on leaf-out Timing and Ecosystem dynamics under climate change

Objective

The timing and duration of leaf emergence in temperate trees are critical for ecosystem functioning, biodiversity, and climate regulation. While phenologists have long studied the mechanisms driving leaf phenology and its response to warming climates, one key factor—winter chilling—remains poorly understood. Winter chilling influences not only when leaves emerge but whether they can emerge at all. Recognized since the 1920s, this process requires trees to experience sufficient cold conditions over winter to break dormancy and enable spring leaf emergence. Although spring leaf emergence is advancing with climate warming, insufficient winter chilling could eventually override spring warming effects, delaying or even halting leaf emergence. The project aims to: (1) experimentally quantify chilling temperature thresholds for over 200 woody species across temperate and boreal forests, including the relative importance of day versus night temperatures in dormancy release; (2) quantify biogeographic variation in chilling sensitivity linked to regional winter temperature variability; and (3) integrate experimental findings with 15–25 years of unique long-term common garden observations from species across the globe, as well as satellite observations. These insights will inform the development of refined phenology models, which can then be incorporated into dynamic global vegetation models to predict phenological shifts and their effects on forest productivity and ecosystem dynamics under future climate scenarios. By combining large-scale experiments, extensive observational datasets, and advanced modelling techniques, this interdisciplinary project bridges local empirical studies with global modelling. Its findings will advance phenology models, enhance carbon flux predictions, and support strategies to mitigate biodiversity loss and ecosystem disruptions in temperate and boreal forests under a rapidly changing climate.