Plasticity and not adaptation is the primary source of temperature-mediated variation in flowering phenology in North America

Abstract Phenology varies widely over space and time because of its sensitivity to climate. However, whether phenological variation is primarily generated by rapid organismal responses (i.e., plasticity) or local adaptation remains unresolved. Here, we used 1,038,027 herbarium specimens representing 1,605 species to measure flowering time sensitivity to temperature over time (‘S time ’) and space (‘S space ’). By comparing these estimates, we inferred how adaptation and plasticity historically influenced phenology along temperature gradients and how their contributions vary among species with different phenology and native climates, and among ecoregions differing in species composition. S space and S time were highly positively correlated ( r = 0.87), of similar magnitude, and more frequently consistent with plasticity than adaptation. Apparent plasticity and adaptation generated earlier flowering in spring, limited responsiveness in summer, and delayed flowering in fall in response to temperature increases. Nonetheless, ecoregions differed in the relative contributions of adaptation and plasticity, from consistently greater importance of plasticity (e.g., Southeastern USA Plains) to their nearly equal importance throughout the season (e.g., Western Sierra Madre Piedmont). Our results support the hypothesis that plasticity is the primary driver of flowering time variation along climatic gradients, with local adaptation having a widespread but comparatively limited role.