Shifts in vernalization and phenology at the rear edge hold insight into adaptation of temperate plants to future milder winters

Temperate plants often regulate reproduction through winter cues, such as vernalization, that may decrease under climate change. Studies of rear-edge populations, glacial relicts that persist in environments that have warmed since last glaciation, can provide insight into adaptive potential to milder winters. We studied how rear-edge populations have adapted to shorter winters and compared them to the rest of the range in the herb Campanula americana. Using citizen science, climate data and experimental climate manipulation, we characterize variation in vernalization requirements and reproductive phenology across the range and their potential climatic drivers. Rear-edge populations experienced little to no vernalization in nature. In climate manipulation experiments, these populations also had a reduced vernalization requirement, weaker response to changes in vernalization length, and flowered later compared to the rest of the range. Our results suggest shifts in phenology and its underlying regulation at the rear edge to compensate for unreliable vernalization cues. Thus, future milder winters may be less detrimental to these populations than more northern ones. Furthermore, our results showcase strong adaptive shifts at the rear edge of temperate plants’ ranges, highlighting the importance of these areas in studies of predicted future climates.温带植物通常通过春化作用（vernalization）这类冬季环境信号调控繁殖过程，而此类信号的有效性可能随气候变化而减弱。针对分布区后缘种群——即在末次冰期后环境持续变暖的条件下仍存续的冰期孑遗种群——的研究，能够为解析物种应对更温和冬季的适应潜力提供重要参考。本研究以草本植物美洲风铃草（Campanula americana）为研究对象，探究其后缘种群如何适应更短的冬季，并将其与该物种分布区内的其他种群进行对比。研究结合公民科学数据、气候观测资料与控制性气候实验，刻画了该物种分布区内春化需求与生殖物候的变异特征及其潜在气候驱动因子。结果显示，后缘种群在自然环境中几乎无需春化作用，甚至完全不需要。在控制性气候实验中，相较于分布区内的其他种群，这类种群的春化需求显著降低，对春化时长变化的响应也更弱，且开花时间更晚。本研究结果表明，分布区后缘的种群在物候及其内在调控机制上发生了适应性改变，以应对不可靠的春化信号。因此，未来更温和的冬季对这类种群的负面影响可能弱于更北部的种群。此外，本研究揭示了温带植物分布区后缘存在显著的适应性演化改变，凸显了这类区域在未来气候预测研究中的重要价值。