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In many conspecific trees of >50 species highly synchronous bud break with low inter-annual variation was observed during the late dry season, around the spring equinox, in semideciduous tropical forests of Argentina, Costa Rica, Java and Thailand and in tropical savannas of Central Brazil. Bud break was 6 months out of phase between the northern and southern hemispheres and started about 1 month earlier in the subtropics than at lower latitudes. These observations indicate that "spring flushing", i.e., synchronous bud break around the spring equinox and weeks before the first rains of the wet season, is induced by an increase in photoperiod of 30 min or less. Spring flushing is common in semideciduous forests characterized by a 4-6 month dry season and annual rainfall of 800-1,500 mm, but rare in neotropical forests with a shorter dry season or lower annual precipitation. Establishment of new foliage shortly before the wet growing season is likely to optimize photosynthetic gain in tropical forests with a relatively short growing season.

In tropical rainforests, 30-65% of tree species grow at densities of less than one individual per hectare. At these low population densities, successful cross-pollination relies on synchronous flowering. In rainforests with low climatic seasonality, photoperiodic control is the only reliable mechanism for inducing synchronous flowering. This poses a problem because there is no variation in day length at the Equator. Here we propose a new mechanism of photoperiodic timekeeping based on the perception of variation in sunrise or sunset time, which explains and predicts the annually repeated, staggered, synchronous and bimodal flowering of many tree species in Amazonian rainforests near the Equator.

The seasonal time course of vegetative phenology and cambium growth is compared for tree species from Central America and Asia growing in tropical climates with a long, severe dry season. Although the inhibition of plant growth by water stress is weH established, responses to seasonal drought vary widely among such trees, and their annual development is not weH synchronized by climatic seasonality. In deciduous trees growing at microsites with low soil moisture storage, phenology and cambium growth are weH correlated with each other and with seasonal rainfall, and most trees have distinct annual rings. Phenology and cambium growth are progressively uncoupled from climatic seasonality in brevideciduous and evergreen trees growing at microsites with large soil water reserves which buffer trees against seasonal drought and thus may prevent the formation of distinct annual rings. There is some experimental evidence conceming the control of growth initiation in apical meristems and the cambium, but little is known about the mechanisms which arrest growth and deterrnine qualitative changes in organ development and cambium cell differentiation.