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.

中文翻译：

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起源地的最低春季温度推动了糖枫种群的叶片物候。

后期霜冻会损坏树木，特别是春季的阔叶树种。通过长期适应，植物可以调节叶片的物候特性，从而在生长季节长度与避免霜冻之间取得最佳平衡。在这项研究中，我们旨在评估在共同花园种植的糖枫种群叶片发育中的生态型分化。在自然范围的北边界共种植了来自29个加拿大种源的272棵枫树幼苗，并在2019年春季监测了芽和叶发育的物候期。受评估的广阔地理区域显示出温度的复杂季节性模式，春季变暖发生在北部和海边。总体而言，从5月底到6月底，叶片发育持续了20到36天。我们观察到种源之间叶片发育的时间和速率不同，证明了该物种中生态型的发生。原始站点的最低四月温度可以解释这种差异，而最高四月的温度并不重要。四月份最低温度较低的地方的幼苗较早且更快地完成了叶片发育。平均而言，种源之间的叶片发育差异最多可达6天，4月最低温度范围为−3至3°C。我们的结果表明，避免春季晚霜是糖枫种群叶片发育的驱动力。在寒冷的地方，生长季节是树木生长的限制因素。因此，当春季的热条件变得有利时，较早的生长恢复和高代谢活性可确保叶片快速排出，从而最大程度延长了光合作用和生长的时间。这些模式证明了糖枫种群对当地气候条件的长期物候适应性，并表明霜冻事件对叶片发育的重要性。