Sex, Size and Interyear Variation in Flowering Among Dioecious Trees of the Malayan Rain Forest

Abstract

In dioecious plants, intersex differences in the frequency and size dependence of flowering may often be important proximate determinants of a population's flowering sex ratio. Such differences have been little investigated among Southeast Asian rain forest trees, where dioecy is perhaps best represented in the world's flora. In this study we recorded flowering activity and sex of reproductive individuals in two separate flowering seasons for °2600 trees representing three species of Aporusa and two species of Baccaurea (all Euphorbiaceae) in a primary rain forest in peninsular Malaysia. We found neither sexually mixed trees nor sex switching of trees between years. Flowering sex ratios for the species of smallest stature. A. microstachya and B. parviflora, consistently exhibited a significant degree of male bias, which was greater in years with lower overall levels of flowering in the population. Two—year cumulative sex ratios were significantly male biased in these two species and in a second relatively small—statured species of Aporusa. The size distributions for male trees broadly overlapped those of female trees in all species. In the smaller statured species, male trees displayed a significantly greater degree of relative size variation than did female trees, suggesting that male trees begin flowering at a smaller size, but also grow to a larger size, than to females. Also, males were significantly more likely to flower in both years than were females in the smaller species studied. Sexual dimorphism in the frequency and size dependence of flowering has previously been explained as a result of higher energy costs of reproduction in females than in males. We suggest that this physiological constraint is most likely to play a strong role in energy limited environments. We therefore predict that, as found in this study, male—biased sex ratios and associated patterns of sexual dimorphism may generally be most pronounced among diminutive treelets of the rain forest understory.