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Most tropical woody plants produce new leaves and flowers in bursts rather than continuously, and most tropical forest communities display seasonal variation in the presence of new leaves, flowers, and fruits. This patterning suggests that phonological changes represent adaptations to either biotic or abiotic factors. Biotic factors may select for either a staggering or a clustering of the phenological activity of individual plant species. We review the evidence for several hypotheses. The idea that plant species can reduce predation by synchronizing their phonological activity has the best support. However, because biotic factors are often arbitrary wit respect to the timing of these peaks, it is essential also to consider abiotic influences. A review of published studies demonstrates a major role for climate. Peaks in irradiance are accompanied by peaks in flushing and flowering except where water stress makes this impossible. Thus, in seasonally dry forests, many plants concentrate leafing and flowering around the start of the rainy season; they also tend to fruit at the same time, probably to minimize seedling mortality during the subsequent dry season. Phenological variation at the level of the forest community affects primary consumers who respond by dietary switching, seasonal breeding, changes in range use, or migration. During periods of scarcity, certain plant products, keystone resources, act as mainstays of the primary consumer community. [ABSTRACT FROM AUTHOR]