Internal heterogeneity of gaps and species richness in Costa Rican tropical wet forest

Abstract

In Costa Rican tropical wet forest the distribution of seedlings of most tree species colonizing 51 tree-fall gaps is clumped, probably because of differences in the proximity of gaps to fruiting trees. Different gap zones (root, bole and crown) are more similar to one another, in terms of their species composition, than they are to other zones in the same gap. This nonrandom pattern is established soon after gap formation, indicating that mortality of young seedlings is both high and related to gap zones. Some tree species are strongly positively associated with one of the gap zones, whereas others are associated with the age of the gap at the time the census was taken or the species of tree whose fall caused the gap. Root zones are dominated by fewer species than are bole or crown zones. Results are consistent with the assumption that an initial random distribution of seedlings is quickly changed to a strongly non-random pattern by selective mortality of seedlings of different tree species in the different gap zones. The internal heterogeneity of gaps is probably one of the factors helping to maintain the high tree species richness characteristic of tropical wet forests.