A review and brief evaluation of the few available investigations is presented by Frankie, Baker & Opler (1974). In most of these studies, a partial analysis of leafing, flowering or fruiting data is offered for a limited number of species and for limited time periods only. The results of a comparative phenological investigation of the trees in Wet and Dry forest sites in lowland Costa Rica are reported here. An attempt has been made to provide a unified analysis of the leafing, flowering, and fruiting periodicities of most species at both sites. The Wet forest site, Finca La Selva, is situated in the Atlantic watershed, while the Dry forest site, Comelco Property, is situated in the Pacific watershed. The study, which began in 1968, is part of a larger investigation aimed at analysing the biological organization of Wet and Dry forest ecosystems from the standpoint of plant reproductive biology. In this study the definition of a tree put forward by Lindley & Moore (1884, p. 1161) as 'any woody plant of perennial duration which rises from the ground with a trunk' has been adopted in part. To this we have added a minimum height qualification of approximately 3 m. We were unable to recognize distinct layers within the forest canopy at either site. Stratification, in which three or more tiers are generally designated, has been described by investigators working in various Tropical Rain forests (Beard 1955; Jones 1955; Lundell 1937; Richards 1952; Webb 1959). Other workers have been unable to discern definite layers clearly (Cain & Castro 1959; Grubb et al. 1963; Paijmans 1970; Schulz 1960). However, because trees are subject to different environmental conditions depending upon their position in the canopy and also for ease in recording data, tree species at the Wet forest site were arbitrarily divided into two categories: overstorey and understorey species. The first group consisted of canopy and subcanopy species, with the understorey comprising trees generally less than 15 m in height. Croat (1969) also divided trees at Barro Colorado Island into two layers for similar reasons. No layering effect was observed in the Dry forest and, because of the lesser tree heights, all trees in this forest were considered to belong to a single layer.

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Research on urban insect pollinators is changing views on the biological