Floral sex ratios in monoecious species-why are trees more male-biased than herbs?

Abstract

Bulmer and Taylor suggested that in geographically structured populations, differential sibling competition generated through sexes is an important factor in determining the equilibrium sex ratios. They showed that reproductive resource allocation would be biased in favour of that sex which is dispersed to farther distances, and thus reduces sibling competition for local resources. This suggestion has important implications for explaining the evolution of resource allocation to the sexes across habits in plants. In this paper, we argue that the effective dispersal of pollen grains compared to that of seeds is relatively more in trees than in herbs; pollen grains hence generate less competition for maternal resources in trees compared to that in herbs. We suggest that such differential dispersal of sexes should bias the resource allocation relatively to males in trees than in herbs. We provide a test of this argument using a set of 53 monoecious species.