Shifts in Phenology and Species Ranges Synergistically Alter the Timing and Species Composition of the Flowering Season

Global change is altering the phenology and geographic ranges of flowering species, with potentially profound consequences for the timing and composition of floral resources and the seasonal structure of ecological communities. However, shifts in flowering phenology and species distributions have historically been studied in isolation due to disciplinary silos and limited data, leaving critical gaps in our understanding of their combined effects. To address this, we used millions of herbarium and occurrence records to model phenological and range shifts for 2837 plant species in the United States across historical, recent, and projected climate and land cover conditions, enabling us to scale responses from species to communities, and from local to continental geographies. Our analysis reveals that communities are shifting toward earlier, longer flowering seasons in most biomes, with co-flowering species richness increasing at the edges of the season and declining at historical peaks-trends projected to intensify under ongoing environmental trends. Although range and phenology shifts operate concurrently, they predominantly affect different aspects of the flowering season: phenological changes primarily alter seasonality-its start, end, and duration-and co-flowering diversity at the edges of the season, while range shifts more strongly influence co-flowering species richness during historical seasonal peaks and the identity and degree of flowering synchrony among co-occurring species pairs. Together, these results demonstrate that shifts in phenology and species ranges act synergistically to restructure the flowering seasons across the conterminous United States, revealing wide variation in the pace and direction of change among biomes.