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 fifteen
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.