Phenology – the study of the patterns, causes, and consequences of the timing of biological activities – is among the most sensitive bioindicators of environmental change. Rapid warming of winter temperatures is advancing the onset of spring and changing growing season lengths worldwide. Although there is a wealth of data on earlier flowering associated with warming climates, less is known about how advancing the initiation of reproduction may change other phenological components that also contribute to reproductive success. To evaluate the integration of phenological components and the ecological and evolutionary consequences of a warmer climate on reproductive phenology, we measured the phenology and performance of five populations of the American Bellflower, /Campanulastrum americanum/, sampled across an elevation gradient and reciprocally transplanted into common gardens at high and low elevations. Plants from higher elevations were transplanted to low elevation to create an expanded growing season; plastic responses to this new environment reveal potential short-term response to climate change. By transplanting in the complementary direction, it was possible to determine the extent of adaptive differentiation among populations from different seasonal environments. Such differentiation provides insight into potential long-term evolutionary response to changes in growing season length. We used this design to determine the effects of a longer growing season on reproductive phenology and whether plastic response to a warmer climate is adaptive.

Results/Conclusions

The low elevation planting site had an expanded growing season that induced the advance of a suite of phenological characters including the initiation of bolting and flowering, average flower date, and time to fruit maturity relative to the high elevation site. Each successive stage of reproduction was accelerated more than the previous one resulting in a compressed phenology in the warmer environment.

Comparison of both planting sites revealed fruit production was locally adapted and phenology was ecotypically differentiated across the elevation gradient. Phenotypic selection was stronger on phenological characters in the expanded growing season at the low elevation planting site, favoring delayed bolting and advanced flowering. Adaptive plasticity advanced flowering times, however maladaptive plasticity advanced bolting time suggesting evolutionary change will be required for this trait to enhance fitness under projected warmer conditions.

This study reveals phenological traits expressed throughout reproduction, not just the onset, will play a central role in plant reproductive success as the global climate continues to warm. We must consider the ecological and potential evolutionary changes of multiple phenological components to forecast response to climate change.