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

Rear-edge populations, relicts of glacial refugia, often persist in marginal habitats at the warmer edge of species distributions. Persistence in these habitats likely required adapting to postglacial climate warming. The resulting local adaptation may affect success in changing climates depending on whether adaptative strategies align or conflict with future conditions. We explored how rear-edge populations persist in their marginal habitats, and whether any adaptations affect responses to future warming in the North American herb Campanula americana. We raised plants from 23 rear-edge and central populations in a common garden experiment spanning their climatic gradient. We evaluated performance across sites and life stages to identify key traits underlying adaptation to rear-edge conditions. We then predicted the expression of key traits under future climates. Local adaptation at the rear edge was mainly driven by whether plants were able to transition from vegetative growth to reproduction, i.e. bolting. 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 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 adaptation to marginal rear-edge habitats can hinge on a single trait, and this adaptation may buffer rear-edge populations against future climates.