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Climate warming is modifying the movement of air masses over Northern latitudes, producing warming and cooling events across the boreal regions. These new conditions changes may mismatch plant phenology from weather conditions, and affect the growing period of trees. Understanding the processes of local adaptation in bud phenology can help to predict the response of plants to these rapid and unexpected environmental changes.Our study monitored bud burst and bud set weekly during four growing seasons in black spruce [Picea mariana (Mill.) B.S.P.] saplings planted in a common garden and originating from five provenances representing the whole latitudinal distribution of the closed boreal forest in Quebec, Canada. We compared the variance in bud phenology among populations and years, and analysed the relationships with temperatures at the origin sites.Bud burst and bud set occurred in mid‐May and mid‐July, respectively, with a large variability among provenances and between the study years. A delayed bud phenology was observed in the provenances from warmer sites, with bud burst and bud set being 1.1 and 1.4 days later for every additional degree in mean annual temperature at the origin site respectively. Populations with earlier bud bursts also showed earlier bud sets, thus the growing season was similar among provenances. The heritability of bud set was higher than that of bud burst, with estimates of 0.26 and 0.21 respectively. On average, variance in bud phenology among provenances reached 5.3%, which was higher than that within provenances (2.6%). The factor year explained 37.7%–69.7% of the variance in bud phenology.Synthesis. The findings demonstrate the evolutionary response to climate variations in the boreal biome. Endogenous and environmental components have different effects on the phases of bud burst and bud set. This complex interaction among drivers of bud phenology may prevent reliable predictions of changes in the growing season under changing climate. An earlier growth reactivation due to higher spring temperatures could result in higher frost risks to the developing buds. Despite a longer period for photosynthesis expected for the evergreen black spruce under warmer conditions, the period spent for bud development could remain unchanged. [ABSTRACT FROM AUTHOR]

Late frost can cause damage to trees, especially to the developing bud of broadleaf species in spring. Through long-term adaptation, plants adjust leaf phenology to achieve an optimal trade-off between growing season length and frost avoidance. In this study, we aim to assess ecotypic differentiation in leaf development of sugar maple populations planted in a common garden. A total of 272 sugar maple seedlings from 29 Canadian provenances were planted at the northern boundary of the natural range, and the phenological phases of bud and leaf development were monitored during spring 2019. The wide geographical area under evaluation showed a complex seasonal pattern of temperature, with spring warming occurring later in the north and close to the sea. Overall, leaf development lasted between 20 and 36 days, from the end of May to end of June. We observed different timings and rates of leaf development among provenances, demonstrating the occurrence of ecotypes in this species. Minimum April temperatures of the original sites were able to explain such differences, while maximum April temperatures were not significant. Seedlings from sites with colder minimum April temperatures completed leaf development earlier and faster. On average, leaf development diverged by up to 6 days among provenances, with minimum April temperatures ranging from −3 to 3 °C. Our results demonstrated that the avoidance of late spring frost is a driving force of leaf development in sugar maple populations. In the colder sites, the growing season is a limiting factor for tree growth. Thus, when thermal conditions become favorable in spring, an earlier growth reactivation and high metabolic activity ensure a fast leaf emission, which maximizes the period available for photosynthesis and growth. These patterns demonstrate the long-term phenological adaptation of sugar maple populations to local climatic conditions and suggest the importance of frost events for leaf development. [ABSTRACT FROM AUTHOR]