Abstract: Phenology is crucial for assessing the effect of climate change on the survival and growth dynamics of temperate and boreal plants. Warmer temperatures induce earlier budbreak, possibly increasing the risk of late frost, while warmer winters may fail to fulfill the chilling requirement delay budbreak. In our study, we simulate early spring warming on the seedlings and branch cuttings of sugar maple (Acer saccharum Marsh.) from two provenances (Cantley, more southern, and Duchesnay, more northern) originating from different bioclimatic zones in Quebec, Canada. We assessed budbreak in seedlings and branch cuttings after transfer to controlled forcing temperatures (15 or 20 °C) on two dates (DOY 61 and 115). We also calculated chilling accumulation using three commonly applied models including the Chilling Hours, Utah, and Dynamic models. We tested either direct transfer from natural conditions or transfer after a period in artificial chilling temperatures (4 or 7 °C). Seedlings transferred to 20 °C on DOY 61 required 12 additional days to complete budbreak compared to those transferred to the same temperature on DOY 115. The northern provenance (Duchesnay) completed budbreak 11 d faster than the southern provenance (Cantley). Seedlings exposed to 7 °C chilling and 20 °C forcing performed budbreak 7 d faster than those submitted to 4 °C chilling and 15 °C forcing, and 4 d faster than seedlings at 4 °C chilling and 20 °C forcing. The tested chilling metrics models were not able to fully explain the difference in budbreak timing between the treatments. No difference in budbreak was found between branch cuttings and seedlings, validating the branch cuttings as a reliable proxy for phenological studies. Our findings demonstrate the role of chilling and forcing accumulation on budbreak during late winter and early spring. We also show that current chilling models need to be modified to incorporate subzero temperatures to better represent and predict budbreak in boreal and northern temperate species. Warming during winter and spring could advance the timing of budbreak in sugar maple, thus lengthening the growing season, but possibly exposing the trees to damage by late frosts. The warmer provenance (Cantley) showed later budbreak, suggesting a potential for spring frost avoidance that is relevant from a forest management perspective.

Key words: Phenology, Chilling requirements, Climate change, Budbreak timing, Late frost damage