DIOECISM IN TROPICAL FOREST TREES

Abstract

The existence of dioecism among angiosperms is universally acknowledged, yet is poorly understood both from the ecological and evolutionary points of view. The reason for this hiatus might be that some have believed the relative proportion of dioecious taxa to be low (Yamplosky and Yamplosky, 1922; Lewis, 1942), while others have felt that since the number of seed-bearing individuals is halved in a dioecious population its evolution is an unlikely and perhaps inconsequential event (HeslopHarrison, 1972). However, recent studies have reported a large proportion of dioecious tree species in tropical forests (Ashton, 1969; Bawa, 1974; see also Tomlinson, 1974); this indicates that the incidence and importance of dioecism may have been underestimated in the past. These new data are significant in at least one other respect. Dioecious (and monoecious) flowers in temperate plants are presumed to have evolved in response to selective pressures favoring wind-pollination (Grant, 1951; Stebbins, 1951), yet windpollination is probably either absent or uncommon in tropical forests (Whitehead, 1969; see also Daubenmire, 1972). Here we present an overview of the adaptive and evolutionary significance of dioecism in general and its expression in certain tropical forests in particular. We first present data on the frequency of dioecious species in a lowland tropical semi-deciduous forest in Costa Rica, then proceed to an examination of available information on