The Phenology of Tropical Trees
Rolf Borchert


From the Equator to temperate latitudes, tree development is characterized by distinct periods of shoot growth (flushing), flowering and rest (dormancy), collectively referred to as phenology. In temperate climates, rising spring temperatures induce synchronous bud break, shoot growth and flowering of trees; in autumn, declining temperature and day length arrest shoot development. In the tropics, where temperatures are always favorable for tree growth, seasonal development is often not well correlated with climate. In tropical rain forests many trees flush and flower at the same species-specific time each year, but others do so at irregular intervals (Fig. 1a-d). In monsoon forests with a distinct dry season, the timing of tree development varies widely with soil water availability. Where tree roots reach the ground water table, trees are evergreen and may flush repeatedly each year (Fig. 1c). Elsewhere, moderate water stress induces leaf shedding during the dry season and trees refoliate during the late dry season (Fig. 1b, e). Where the upper soil layers dry out completely, deciduous trees stand leafless for several months and flush after the first rains of the wet season (Fig. 1e, f). The flowers of tropical trees may appear throughout the year at species-specific times on leaf-bearing or leafless, dormant shoots (Fig. 1, flower symbols).

The large variation in the timing of synchronous flushing and flowering among tropical tree species raises the question: which environmental cues determine the timing of tree development? To answer this question, the phenology of a tree species must be monitored over two years For well-collected neotropical tree species, flowering periods may be deduced from the herbarium records in the 'Tropicos' data base of the Missouri Botanical Garden. Establishing simple correlations between climate and flowering or fruiting at the community level cannot answer the question for lack of adequate resolution (see "Phenological field observations").