UC Santa Barbara

Climate change can reshape the flowering season by shifting species distributions and phenology, leading to cascading ecological impacts through changes in temporal and spatial synchrony among species. However, data limitations and incomplete knowledge of species-level flowering responses to climate within and among communities have hindered our understanding of how environmental changes impact the structure of the flowering season across biomes. Here, I used millions of herbarium specimens and community-science records to assess: i) whether variation in flowering time reflects local adaptation to long-term climatic conditions or rapid plastic responses, and ii) how species distribution shifts and plastic phenological responses scale to alter the flowering season across North American biomes. I first validated estimates of phenology-climate relationships from herbarium specimens against field observations, confirming their reliability at vast taxonomic and spatiotemporal scales. Then, I measured flowering time sensitivity to temperature for 1,605 well-sampled species across the United States to infer how adaptation and plasticity historically influenced phenology along temperature gradients. Results suggest plasticity is the primary driver of temperature-mediated variation in flowering phenology, with adaptation playing a more context-dependent role. Simulations showed that interspecific variation in plastic responses to temperature in flowering onset and duration could significantly redistribute floral resources and restructure patterns of flowering synchrony among co-occurring species. To evaluate this, I modeled the distribution and flowering phenology of 2,837 species across the U.S. under historical, current, and projected climate and land cover conditions, scaling duration, and termination responses from species to communities and from local to continental levels. Within species, flowering onset, duration, and termination responded differently to climate, with substantial variation in sensitivity among species both within and between communities. At the community level, climate change altered species composition and the timing and duration of flowering seasons, with ecoregion-specific shifts in co-flowering species diversity and flowering overlap networks that are projected to intensify with ongoing climate and land use changes. Together, these studies demonstrate that climate change is reassembling the flowering season across North America through plastic species-level responses, with sharp differences in the severity of changes across biomes. In doing so, these analyses highlight the emerging opportunities afforded by community-science records to study the impacts of climate change on plant phenology at unprecedented taxonomic, spatiotemporal, and ecological scales.