Tropical forests frequently experience the opening and closing of canopy gaps as part of their natural dynamics. When fragmented, the forests are also exposed to edges, which (at least in the lowlands) cause increased mortality on the fragment's periphery and—depending on the fragment size—also on the forest interior. Here we explore if the presence of edges increase the rate at which the canopy opens in a montane forest and if and how that interaction affects the vegetation composition and structure. At Las Cruces Biological Station (SW Costa Rica), we surveyed four 150 m edge-to-interior transects to evaluate changes in vegetation structure (stem density and basal area for all plants with DBH > 5 cm) and species richness and composition as a function of both distance to the edge and canopy openness. Edge proximity did not increase canopy openness. Canopy openness, however, did affect stem density, species richness, and composition. Consequently, heterogeneity in forest structure, diversity, or composition could not be attributed to indirect edge effects. Forest edges, however, had a direct effect on species composition, but with a negligible effect on species diversity and forest structure. Overall, these results reveal that the structure, diversity, and composition of tree communities in this tropical premontane forest fragment are more strongly influenced by canopy openness than by the proximity of edges.