Low-Diversity Tropical Rain Forests: Some Possible Mechanisms for Their Existence

Abstract

Tropical rain forests do not always have a high diversity in tree species. In some cases, most of the trees in the canopy layer are a single species. This results in a low diversity among the other canopy tree species (fig. 1). Increasing dominance by one canopy species has little effect on the diversity of subcanopy trees (fig. 2). Such single-dominant forests are of two types: the dominant either persists at the site beyond one generation or it does not. In forests with a persistent dominant species (type I), one species may achieve dominance either by colonizing most of a large open patch and persisting thereafter or by gradually replacing the existing residents. The latter could take place either because the dominant is the species most resistant to deleterious physical or biotic conditions or because it is superior in competition to all others. One possible mechanism enabling one species to replace and exclude many others is the possession of an ectomycorrhizal (EM) association. Most tropical tree species sampled in the Neotropics and Africa have a different type, the vesicular-arbuscular mycorrhizal (VAM) association. Various characteristics of the EM association, such as its greater host specificity, the greater protection it gives its host from natural enemies and deleterious physical factors, and its ability to secure nutrients in both organic and inorganic form before they are available to a VAM association, may confer an advantage on its host tree species that allows it to replace itself and to displace or exclude VAM host trees. This "mycorrhiza hypothesis" needs to be tested; various types of surveys and experiments are suggested as appropriate tests. In forests where the VAM type is more common, most tree species are associated with the same set of fungal species and thus may be nearly equivalent in competitive ability for securing resources. Such equivalence promotes diversity, as suggested by earlier work. Examples of nonpersistent dominants (type II) are those that first colonize large open patches (>1 ha). Dominance probably results when few species are available for colonization during the short period open to invasion after a disturbance and few are adapted to the conditions in recently disturbed large patches. We suggest that after a single EM tree species achieves dominance in a type-I forest, other species in the same family are more likely to invade than are those of a different family. Thus, a many-species VAM rain forest might gradually shift to one with a single EM species dominant, leading to a forest of higher diversity dominated by several species from a few families that associate with EM fungi (e.g., as found in dipterocarp forests in southeastern Asia). The latter forests should tend to maintain their diversity because, like VAM forests, EM species in the same family may be nearly equivalent in competitive ability.