Questions: Do functional traits explain individual tree species’ responses to environmental filters and dispersal limitations following stand-replacing fire? Can post-fire conditions initiate alternate trajectories of community assembly? Location: Glacier National Park, Montana, USA. Methods: We characterized the species composition of tree re-establishment following nine recent fires in the lower subalpine zone of Glacier National Park, a region with multiple co-dominant tree species that has experienced extensive fire in recent decades. We used GLMMs to describe individual species’ responses to spatial and temporal heterogeneity of post-fire conditions and seed availability, and used multivariate approaches to examine whether post-fire conditions can initiate alternate trajectories of forest community assembly. Results: Post-fire regeneration responses were strongly species-dependent and consistent with functional trait differences. We found that there was a brief opportunity for establishment during the first several years after fire, during which tree recruitment for all species was contingent upon the availability of nearby seeds and the occurrence of favourable weather conditions. There was a remarkable amount of compositional change following fire, and the majority of our study sites transitioned from a mixed forest to a tree assemblage dominated by a single species. Conclusions: Differential species-specific responses to climate and disturbance can initiate alternate trajectories of community assembly, triggering substantial changes in forest composition. The composition of post-fire tree assemblages reflects the differential responses of individual species to climate and disturbance, providing support for the interacting role of dispersal and environmental filtering in shaping post-disturbance communities and highlighting the importance of incorporating functional traits into predictive models of ecosystem dynamics.