Dioecism in tropical forest trees.

In tropical lowland forests, one fourth to one half of all species have unisexual flowers, and a majority of such species are dioecious. Almost all dioecious species have relatively small (? 1.0 cm in length and breadth)pale yellow to pale green flowers. Field observations of selected species indicate that a large number of dioecious species are pollinated by small bees such as various species of Anthophoridae, Halictidae, Megachilidae and Meliponinae, while a small number is pollinated by moths. In the light of the high incidence of dioecism and the prevalence of entomophily in tropical trees, it is suggested that as far as tropical trees are concerned, anemophily cannot be invoked to explain the origin of dioecism (and monoecism)as has been done for temperate plants. In order to explain the evolution of dioecism in tropical trees, it is postulated that, with the exception of a few taxa where dioecism has evolved from heterostyly, in a majority of investigated species dioecism has probably evolved from a self-compatible breeding system in response to selective pressure for outcrossing. The main question on which attention is focused is: When selective pressure for outcrossing arises, what favors the evolution of dioecy over self-incompatibility? Four specific hypotheses are proposed which provide possible answers to the question. These are based on the assumptions of relative ease with which dioecism can arise, increased pollination success allowed by dioecism under certain conditions, enhanced escape from seed predators under a dioecious breeding system and the ability of dioecious species to exploit a wider variety of microhabitats than hermaphroditic species.