1. Climate change is causing the growing season to expand and many plants are flowering earlier. However we know less about whether other components of reproductive phenology are altered or whether these changes in phenology are adaptive.
2. We evaluated reproductive phenology and fitness components for populations of Campanulastrum americanum sampled across an elevation gradient and reciprocally transplanted into common gardens at high and low elevations.
3. The low-elevation planting site had an expanded growing season that induced the advance of bolting, flowering, average flower date, and time to fruit maturity relative to the high-elevation site for transplants. With the exception of flowering initiation, each successive stage of reproduction was advanced more than the previous one, resulting in a compressed phenology in the warmer environment.
4. In contrast, populations from low elevation had a longer reproductive cycle when grown at both sites, with each phenological component extended relative to populations from high elevation. Fruit production indicated populations were locally adapted to elevation, suggesting these differences in phenology are adaptive.
5. Selection on phenological characters was stronger on transplants in the expanded low-elevation growing season, favouring delayed bolting and advanced flowering. Plastic response to the longer growing season was adaptive for flowering time but maladaptive for bolt initiation.
6. Synthesis. The compressed reproductive phenology favoured in the expanded growing season expected under climate change will largely be achieved with adaptive plasticity of individual phenological traits. Traits under selection in the longer growing season were genetically differentiated between populations that currently differ in growing season length, suggesting evolutionary malleability and likely modification of reproductive phenology in response to climate change.