Abstract:

Research on the ecological life history (or life cycle) of tropical trees has lagged behind that of other plants, particularly trees of the temperate zone. In the tropics, life-history research has followed two lines of inquiry: demography and succession. Compared to temperate trees, tropical trees (sensu latus) appear to have both a shorter timespan to first reproduction and shorter life spans. Elucidating life-history strategies in species-rich tropical forests is daunting because the traditional dependence on seed and seedling behavior is inadequate and complete life-history analysis is required for each species to define appropriate groupings. Analysis of the behavior of seeds, seedlings, and saplings of lowland tropical moist, wet, and rain forest trees suggests two groups of species: pioneer and nonpioneer (or climax). For most trees (nonpioneers), the survival of adults contributes most to fitness, not the number of seeds or seedlings they produce or how fast the trees grow. For pioneers, the seed bank and the growth of juveniles affects fitness most. More research is needed to validate these trends, however, because the tree species and environmental conditions in the tropics are diverse. Further division of nonpioneer life histories by functional grouping has proved difficult because a continuum of traits appears to exist among all species.

Formation of tree-fall gaps has been emphasized as the main disturbance event in the life cycle of tropical moist to rain forest trees and gaps are where most tree life histories experience their critical stages of development. Incident light and, to a lesser extent, nutrients are the factors most responsible for the growth of individuals through gaps or inside a closed canopy. In dry forests, water is more important than light as the factor that determine the success of individuals. Some life-history traits in dry forest trees are small seeds, moisture-related seed dormancy, reliance on root or stem sprouts for regeneration, synchronous growth and reproduction processes, low abundance of seedlings, reduced seed banks, and high density of stems.

Large and infrequent disturbances such as hurricanes instantaneously introduce extreme conditions that significantly alter the stages of the life-cycles of some tropical trees. Sprouting, formation of tree unions, small individuals, short life spans, rapid changes in sun and shade adaptation, explosive establishment of seedling populations, accelerated rates of primary productivity and nutrient cycling, and increased abundance of gap-depending canopy species, are examples of life-history traits in frequently disturbed areas. In degraded sites, such as those with frequent landslides or human-affected systems, different groups of species---including different life forms and a greater fraction of alien species---achieve success in regeneration. These differences suggest that another set of life-history characteristics are required to overcome these extreme environments. Much research is needed before a clear picture of the diversity of life-history strategies of tropical trees emerges.

Most plant communities are successional and each species is doomed to local extinction; the two strategies of "escape to somewhere else"or "wait until the right habitat reappears"are alternative ways of meeting the deterioration of the local habitat (J.L. Harper 1977, p 82).

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KeyWords Plus:
land-use and land-cover change, Puerto Rico, secondary succession, exotic species, globalization.

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