One of the major biological mysteries still to be explained is the maintenance of the enormous local tree species diversity in tropical rain forests .This review explores the relationship between the dynamics of natural regeneration and the evolutionary and ecological processes and mechanisms involved in the origin and maintenance of such extraordinary diversity. First, 1 review ideas on the origin of tree species diversity in the tropics. This review suggests that: i) historical, evolutionary and biogeographical phenomena have a paramount influence on local species richness, and ii) tropical rain forest tree communities are species unsaturated, suggesting that newly originated species may freely migrate across a regional landscape. Second, I describe the forest regeneration process. Gap dynamics, promoted by branch and tree falls, is a fundamental component of the forest canopy renewal. Small gaps (caused by branch falls) facilitate the establishment and survival of seedlings and saplings in the shaded understory (advanced regeneration), whereas large gaps (caused by tree falls) enable trees to reach mature sizes. Gap creation and tree maturation are the extremes of a process of tree and species replacement in the forest canopy. Third, I explore relationships between the tree replacement process and the population and community mechanisms that facilitate maintenance of species diversity at a local scale of a few hectares. I argue and document that factors that promote high species diversity in the advanced regeneration favor high probabilities of heterospecific replacements among canopy trees. Hence, these factors facilitate the maintenance of species diversity in the forest canopy. Frugivores, by promoting diversity in the seed rain community, and biotic agents of seed, seedling and sapling mortality by operating mainly on abundant species, are key factors in facilitating diversity. Furthermore, the existence of trade offs in tree life history attributes (such as seed dispersal capacity, survivorship in the shade and growth under gap conditions) contributes to diversity maintenance by promoting heterospecific replacements. This review does not support ú1e idea that maintenance of tree species diversity in tropical rain forest depends on random processes, as some authors have claimed. instead, I conclude that ecological phenomena have a paran1ounl role on the possibility that a species gains a membresy in such highly diverse forests.
LONG-TERM SURVIVORSHIP AND CROWN AREA DYNAMICS OF TROPICAL RAIN FOREST CANOPY TREES
