Shifts in vernalization and phenology at the rear edge hold insight into the 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 the last glaciation, can provide insight into the 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, a 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.