The Phenology of Tropical Trees
Photoperiodic control of tropical tree phenology
As discussed above, in many deciduous trees bud break is induced by the first rains of the wet season and its timing varies widely from year to year. In contrast, other tree species flush or flower synchronously at the same time each year throughout an entire region. This synchronous development appears to be induced by seasonal variation in day length, i.e., it is under photoperiodic control (Fig. 2). In the northern Neotropics many species leaf out between February and March, others flower synchronously in autumn. Some species flush or flower twice a year at the Equator, but do so annually further north (11, 14, 21, 22. 25). Synchronous development near the Equator, where day length is constant, raises the question, which environmental signal other than day length controls phenology at the Equator?
Figure 2. Flushing of tropical trees.
At all tropical latitudes, the seasonal course of tree development parallels that of daily insolation above the earth atmosphere (17, 18). Daily insolation is a geophysical variable calculated from day length and the intensity of irradiation, as determined by the angle at which the sun's rays strike the earth (24). The annual course of insolation varies widely with latitude. North of the Tropic of Cancer (23.5° N), insolation—like day length—peaks at the summer solstice (Fig. 1e, 20°N). Further South, insolation has two annual maxima, at the times when the sun passes directly overhead. Between the Tropics and the Equator these maxima shift from the summer solstice to the equinoxes. With declining latitude the two maxima become more distinct and separated by a growing second minimum (Fig. 1e, Equator). At the Equator day length is a constant 12 h and the seasonal change in insolation is therefore a function of irradiation intensity alone. In the southern hemisphere, seasonal changes in daily insolation are six months out of phase with those in the northern hemisphere. Because of cloud cover, the intensity of insolation is lower on the earth surface than above its atmosphere, but its seasonal course is the same.
Evergreen trees flushing in synchrony are common in equatorial rainforests and along the rivers of monsoon forests, where roots reach the ground water table and trees remain well hydrated throughout the year. In the northern hemisphere, most trees leaf out synchronously in February (winter flushing: Figs. 1c, green arrows; 2d), in the southern hemisphere they do so in August. At the Equator, where day length is constant, trees flush during two annual periods, which coincide with the periods of increasing insolation (Fig. 1c, e). These distinct correlations indicate that the seasonal rise in insolation induces winter flushing both at the Equator and in tropical forests throughout the world (18). Lateral or terminal flowers often appear on new shoots during shoot elongation, i.e., flower differentiation is induced by endogenous causes (Fig. 1c,).
Spring-flushing deciduous trees predominate in monsoon forests around the world. They stand leafless from a few days to many weeks (20, 22). Increasing insolation induces synchronous flushing and flowering during the late dry season, around the spring equinox (Fig. 1b, green arrow; 2b). Bud break during the dry season relies on the absorption of sub-soil water by deep roots.
Deciduous stem-succulent trees occupy the driest sites of tropical deciduous forests. They store large quantities of water in the extensive parenchyma of their low-density wood and remain well hydrated throughout the year (11, 15). Declining insolation induces rapid leaf abscission a few weeks before the winter solstice, and trees remain leafless for several months (Fig. 1a). Rain or irrigation do not cause bud break between the winter solstice and the spring equinox, i.e., vegetative buds are "winter-dormant" like those of cold-temperate trees (2, 11). Increasing insolation induces synchronous bud break and flowering around the spring equinox, several weeks before the first rains, but shoot growth remains arrested until the first rains of the wet season cause rapid shoot elongation and leaf expansion. Thus, in marked contrast to other deciduous trees (Fig. 1f), the phenology of stem succulents is not determined by the seasonality of rainfall, but by seasonal variation in photoperiod (Fig. 1a, green arrow)(11).
In a few Central American autumn-flushing trees declining, rather than increasing, insolation induces bud break and flowering towards the end of the wet season, in November (Fig. 1d, orange arrow)(18).
In sum, photoperiodic control adapts the phenology of tropical trees to climatic seasonality in various ways. In many evergreen, brevideciduous and succulent trees increasing insolation induces bud break within 2–4 months after the winter-solstice. In others declining insolation induces flowering, dormancy or, rarely, bud break in autumn.