Adaptation to marginal habitats at the rear edge provides resilience to future warming

Rear-edge populations are typically relicts of glacial refugia that persist in marginal habitats at the warmer edge of species distributions. Persistence likely required adapting to postglacial climate warming. The resulting climate adaptation may affect success under future climate change if adaptative strategies align with future conditions. We explored how rear-edge populations persist in their marginal climates, and whether any adaptations affect responses to expected future warming in the North American herb Campanula americana. We raised plants from 23 rear-edge and central populations in three common gardens spanning their natural climatic gradient. We evaluated performance across the life cycle in each garden to identify key life stages underlying climate adaptation at the rear-edge. We then predicted population performance at the key stage under future climates. Local climate adaptation at the rear edge was mainly driven by whether plants were able to initiate bolting, thereby transitioning from vegetative growth to reproduction. Rear-edge populations exhibited high bolting frequency under local warm winters, while more northern populations failed to bolt in these conditions. Bolting is cued by winter cold (vernalization) in the species, but rear-edge climates provide only a few days sufficiently cold for effective vernalization. We found that rear-edge populations have adapted to these conditions by evolving reduced vernalization requirement and sensitivity. Under projected warmer winters, bolting is expected to decline at mid-latitudes, whereas rear-edge populations are predicted to maintain high bolting. Our findings highlight that adaptations that likely allowed rear-edge populations to persist under past warming may also buffer these populations against future warming.