Abstract: Spring phenology is one of the most sensitive ecological indicators of forest responses to climate warming. Understanding the precise climatic drivers of bud break in keystone species is crucial for developing robust phenological models and predicting future ecological and economic impacts. In this study, spring bud phenology was recorded 2020–2022 of 42 provenances of sugar maple (Acer saccharum Marsh.) originating from the northern range of the species in Quebec, Canada. The effect of temperature on budburst timing was assessed, and based on the observed linear relationship, we reconstructed the budburst timings of maple forests located between 45° and 49°N latitude and –70° and –76°W longitude over the past two decades. In the common garden the entire bud break process lasted between 20 and 40 d. Bud swelling occurred mid-April to mid-May, on average 5 d earlier in the southern and warmer stands. A strong correlation was observed between bud swelling dates and mean temperatures in the last two weeks of April, with temperature explaining 90% of the variance. An increase of 1 °C in mean temperature during this period advanced budburst by 4 d. At the northern limit of sugar maple, late April had an average temperature between 1.6 and 8.7 ℃ during 2003–2022, resulting in an estimated variability of 28 d in bud swelling from early April to early May. Our findings confirm that late April temperatures play a major role in the reactivation of sugar maple at its northern range. The earlier onset of leaf development under warming conditions could increase the risk of late frost damage, with consequences for maple syrup production and species distribution.

Key words: Bud break 、 Bud swelling, Temperature,  , Climate change, Acer saccharum Marsh