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An index of canopy closure was used to estimate closure above the crowns of all trees ≥ 10 cm dbh in 11.11 ha of undisturbed lowland tropical forest at La Selva, Costa Rica. To correct for the efFects of tree size on canopy closure, we used the residual of the regression of the canopy closure index on tree size. Analyses were carried out for the 104 species which had ≥ individuals; a total of 3224 trees were included. Nine species were found to have their crowns in significantly more open conditions than expected by chance and five species were found to have their crowns in significantly more closed conditions than expected by chance (P < 0.05). The remaining 90 species (86.5% of the assemblage) were distributed at random with respect to canopy closure, occupying the available light conditions indiscriminately. Species occurring under higher light levels did not show a narrower range of tolerance than did other species. Most species were found to occur over a substantial proportion of the canopy closure continuum present in the stand; overlap among the great majority of species in the assemblage is extensive. The results support the view that tropical forests comprise assemblages of generalist tree species, and raise questions about the classic notions of gap-phase dynamics. [ABSTRACT FROM PUBLISHER]