Significance Understanding how species respond to climate change is crucial to the development of effective conservation strategies. We found directional and systematic changes in the composition of both adult and juvenile tree species in tropical north Andean forests at rates consistent with concurrent temperature increases, supporting the hypothesis of upward species migrations resulting from global warming. Our results indicate that compositional shifts occur primarily via range retractions. This suggests elevated risk for species extinctions and local biodiversity loss with ongoing warming. Other environmental filters, such as species-soil adaptations, as well as anthropogenic disturbances (e.g., habitat loss and hunting) may limit future species migrations and decrease the ability of forest communities and their constituent species to respond to climate change.