SPATIAL RELATIONSHIPS BETWEEN STAMINATE AND PISTILLATE PLANTS OF DIOECIOUS TROPICAL FOREST TREES.

Abstract

Alternate floral morphs in the form of dioecious, gynodioecious, distylous and tristylous plants within the same species are of widespread occurrence among flowering plants. The spatial distribution of different floral morphs, and therefore floral resources, within a population is likely to influence the foraging behavior of pollinators and concomitantly, the level of pollination. This distribution might also indicate the extent to which plants with different floral morphs exploit different microhabitats, particularly in dioecious species in which sexual dimorphism is most pronounced. Recently, Levin (1974) and Ornduff and Weller (1975) have analyzed the spatial distribution of pins and thrums among distylous Hedyotis nigricans (Rubiaceae) and Jepsonia heterandra (Saxifragaceae), respectively. However, to the best of our knowledge, no such analysis exists for a dioecious species despite the fact that dioecy is much more common than heterostyly, especially in species-rich tropical forests (see Ashton, 1969; Bawa, 1974; Bawa and Opler, 1975). Herein, we report the dispersion patterns of staminate and pistillate plants of several dioecious tree species that occur in tropical lowland forests of Costa Rica. The objective of the study was to investigate: a) if staminate and pistillate plants of the same species display spatial segregation, b) if the dispersion pattern indicates asexual reproduction by one or both sexes, and c) if the distribution of one sex might be more correlated with a particular environmental factor than the other. The study is part of a wider investigation dealing with ecological aspects of dioecism in tropical forests (Bawa and Opler, 1975, 1977; Opler and Bawa, 1977).