Warming-induced changes in seasonal priority effects drive shifts in community composition

Shifting community assembly dynamics are an underappreciated mechanism by which warming will alter plant community composition. Germination timing (which can determine the order in which seedlings emerge within a community) will likely shift unevenly across species in response to warming. In seasonal environments where communities reassemble at the beginning of each growing season, changes in germination timing could lead to changes in seasonal priority effects, and ultimately community composition. We test this expectation by assembling mesocosms of 15 species in one of two orders-"ambient" assembly order or "warmed" assembly order-based on the order in which the constituent species germinated under ambient and warmed conditions. Community composition differed significantly between mesocosms assembled in ambient versus warmed orders. The impact of assembly order on species mean biomass was largely explained by how much earlier (or later) a species arrived in the warmed-order treatment relative to the ambient-order treatment. Species whose germination phenology advanced more under warmed conditions relative to ambient conditions showed greater relative increases in biomass under the warmed assembly treatment. These findings demonstrate that warming can drive community assembly and shape community composition by reordering the relative timing of germination among species. These findings enhance our ability to predict which species are likely to benefit from warming and which may decline based on how warming may shift assembly order, ultimately informing how warming may alter plant communities.