The Phenology of Tropical Trees
Figure 1. Control of tropical tree phenology by seasonal variation in rainfall or daily insolation.
Vegetative phenology and rainfall seasonality
Periods of low rainfall, during which evapotranspiration exceeds precipitation and soil water content declines, constitute the principal climatic constraint in tropical forests, where variation in monthly temperatures is small. With increasing distance from the equator, annual rainfall decreases and seasonal drought becomes longer and more severe. Along this gradient, tropical forests change from evergreen rainforests to semi-deciduous or deciduous forests referred to as monsoon forests. In such forests, the 4-6 months-long dry season generally starts around the winter solstice, and less than 5% of the annual precipitation of 700–1500 mm may fall during its duration (Fig. 1e).
Seasonal development of shallow-rooted deciduous trees growing at dry sites is determined by the seasonal rainfall pattern (Fig. 1f). As the top soil dries out after the last rains of the wet season, trees experience increasing water stress (i.e., shoot water potentials decline)(15). This results in leaf abscission (Fig. 1f, brown arrow), shrinking of the tree trunk, arrest of cambium growth and the formation of distinct annual rings in the wood (9). Rehydration after the first heavy rains of the wet season causes bud break and rapid shoot growth (Fig.1f, blue arrows)(4, 5, 6).
The time course of seasonal tree development in monsoon forests varies widely with site water availability and the timing of the first rains of the wet season. Bud break of deciduous trees will occur as soon as leafless trees have become fully hydrated. In tropical lowlands, the leafless period may range from a few weeks in trees growing near a river to 5–6 months in trees at dry upland sites. Exceptional rain showers or irrigation during the dry season induce rapid flushing of leafless trees (4, 5, 6, 12). In Costa Rica, the 1997 El-Niņo event caused a severe abnormal drought period lasting from June to August. In most deciduous trees the drought induced leaf shedding followed by flushing in September, 4–5 months earlier than normal (12). With increasing altitude and the corresponding decline in water stress during the dry season, the leafless period becomes progressively shorter (1, 20).