Flowering phenology

Many tropical trees flower during species-specific phases of the annual vegetative cycle (Fig. 1g). Flowering may be induced endogenously or by environmental cues. Flower expansion during shoot growth triggered by increasing or declining insolation is under endogenous control (Fig. 1a-d, green arrows). Inflorescences may arise either from the axillary buds of expanding leaves (Figs. 1; 3c; title page: Plumeria) or by transformation of the apical meristem (Figs. 1; 3a, b)(18). During the late rainy season, the autumnal decline of insolation may induce expansion of lateral or terminal inflorescences on shoots with mature leaves (Figs. 1,, red arrow; 3a, b). In Mexico, the autumnal decline in insolation starts in August (Fig. 1e, 20°N), several months earlier than near the Equator (Fig. 1e, Equator), and flowering times of wide-ranging species vary correspondingly with latitude (17, 21). In many deciduous trees, flower buds differentiate during shoot growth, but remain dormant until trees have shed their leaves. Expansion of dormant flower buds may be triggered by rehydration after leaf fall (Fig. 1f, brown arrow; title page: Erythrina, Tabebuia)(1, 20), increasing insolation (Fig. 1a, f, ) (11, 16) dry-season rain showers, or the first heavy rains of the wet season (Fig. 1f,, blue arrows) (4, 5, 6, 12, 20) In some species, dormant flower buds open during the expansion of new shoots, as they do in many temperate trees (Fig. 2c). In general, fruit growth begins soon after flowering, but in Guazuma ulmifolia it is triggered by declining insolation several months after flowering (Fig. 3c, d) (18).

Figure 3. Inflorescence and fruit development in tropical trees.
a, b: Terminal inflorescences of Calicophyllum candidissimum and Cordia alliodora.
c, d: Axillary flowers and fruits of Guazuma ulmifolia.