Historical changes in flowering phenology are governed by temperature x precipitation interactions in a widespread perennial herb in western North America

For most species, a precise understanding of how climatic parameters determine the timing of seasonal life cycle stages is constrained by limited long-term data. Further, most long-term studies of plant phenology that have examined relationships between phenological timing and climate have been local in scale or have focused on single climatic parameters. Herbarium specimens, however, can expand the temporal and spatial coverage of phenological datasets. Using Trillium ovatum specimens collected over > 100 yr across its native range, we analyzed how seasonal climatic conditions (mean minimum temperature (T-min), mean maximum temperature and total precipitation (PPT)) affect flowering phenology. We then examined long-term changes in climatic conditions and in the timing of flowering across T. ovatum's range. Warmer T-min advanced flowering, whereas higher PPT delayed flowering. However, T-min and PPT were shown to interact: the advancing effect of warmer T-min was strongest where PPT was highest, and the delaying effect of higher PPT was strongest where T-min was coldest. The direction of temporal change in climatic parameters and in the timing of flowering was dependent on geographic location. T-min, for example, decreased across the observation period in coastal regions, but increased in inland areas. Our results highlight the complex effects of climate and geographic location on phenology.