Phenology and control of flowering in tropical trees

Descriptions of structure and flowering phenology of the tropical tree species Cedrela mexicana, Cordia glabra, Erythrina poeppigiana. Tabebuia rosea, and Tabebuia ochrea ssp. neochrysantha are used to analyze and illustrate the complex relations between structure, vegetative development, and flowering in tropical trees. Because of the strong positional differentiation within tree crowns and the repetition of vegetative and reproductive growth cycles in trees, flower development and its control are more complex than in herbaceous plants. Formation of terminal inflorescences, as found in Cedrela, Cordia, and Tabebuja, regularly occurs at the end of a flush of determinate shoot growth; flower initation, like shoot growth cessation, is therefore controlled by internal, correlative factors rather than by environmental changes. Initiation of lateral inflorescences is usually preceded by growth cessation in the indeterminate, vegetative shoot, but the timing and control of flower induction in lateral buds are unknown. For many tropical trees, particularly those growing in seasonal climates, flower development is discontinuous, i.e., flower initiation and anthesis are separated by a prolonged rest period and must be controlled separately. Delayed anthesis is usually triggered by rehydration of previously water-stressed trees resulting from leaf shedding or, in dry habitats, by rainfall. Because of different degrees of seasonal water stress, leaf fall and subsequent anthesis in several species extend over three to four months. The observed periodicity of anthesis in tropical trees is thus largely determined by seasonal changes in tree water status and is unlikely to be the result of selection for optimum tree-pollinator interaction. IN MOST TROPICAL TREES, flowering, like vegetative growth is episodic, and seasonal peaks of flowering have been recorded for many tropical forests (e.g., Fournier and Salas 1966, Janzen 1967). During recent years, tropical ecologists have considered the temporal pattern of flowering the result of coevolution of trees with pollinators and seed predators. It has been argued that peaks of flowering occur during periods of high pollinator activity, and that overlapping, but different, flowering periods in related species reduce competition for pollinators and assure a continuous source of nectar to pollinators (Janzen 1967, Gentry 1976, Stiles 1978). If the temporal pattern of flowering is adaptive, then mechanisms for the specific timing of flowering must have evolved in tropical trees. The timing of flowering in tropical trees usually has been ascribed to environmental control mechanisms analogous to those that have evolved in temperate plants to adapt vegetative and reproductive development to a growing season of limited duration (Larcher 1980). Relations between tree architecture, vegetative growth and flowering of tropical trees have been rarely considered. In this paper, I shall use descriptions of the flowering behavior of five tropical tree species as a basis for discussing the relations between tree architecture, vegetative development, and flowering, as well as our scant knowledge of their internal and environmental controls. I will show that evolution of the temporal pattern of flowering in tropical trees cannot be adequately explained as resulting from biotic interactions alone, but must be seen in the context of overall tree development as determined by a given set of climatic, edaphic, and biotic factors. STUDY SITES AND METHODS Most of the phenological observations on flowering of tropical trees discussed in this paper were made on the Pacific slope of Costa Rica, Central America, along an altitudinal gradient extending from the deciduous lowland forest of Guanacaste Province (Janzen 1967, Daubenmire 1972, Frankie et al. 1974, Opler et al. 1976, Reich and Borchert 1982, 1984) through the lower montane forest at Villa Col6n (800 m altitude) and Santa Ana (900 m; Fournier and Salas 1966, Fournier 1969) to the Central Valley of Costa Rica (La Sabana, 1100 m; San Pedro Montes de Oca, 1200 m; Granadilla, 1350 m) (Fournier 1969, Borchert 1980). The climate in the entire area is characterized by a dry season lasting from late November to April/May and a wet season providing adequate rainfall (approx. 2000 mm) for the remainder of the year. With increasing altitude, the severity of the dry season diminishes and some species change from a deciduous to an evergreen habit (e.g., Erythrina poeppigiana, Tabebuia rosea; Fournier 1969, Borchert 1980). The discussion of tree morphology is based on the analysis of tropical tree architecture by Hall6 et al. 1978, where definitions of all morphological terms can be found. MORPHOLOGY AND PHENOLOGY OF FLOWERING IN FIVE TROPICAL TREE SPECIES TABEBUIA ROSEA (BERTOL.) DC, BIGNONIACEAE. -The shoot system of T. rosea is composed of many orthotropic, symBIOTROPICA 15(2): 81-89 1983 81 This content downloaded from 157.55. 39.17 on Fri, 02 Sep 2016 06:06:02 UTC All use subject to http://about.jstor. org/terms