The functional ecology of dioecy in {\it Siparuna grandiflora}, a tropical understory shrub

Date of Completion

January 1997


Biology, Botany|Biology, Ecology




In tropical wet forests about 20% of woody plant species are dioecious. Because staminate (male) and pistillate (female) flowers are borne on separate individuals in dioecious species, differences in secondary sex characters (sexual dimorphisms) and ecological divergence between the sexes may evolve. However, the ecological importance of differentiation at the leaf, plant and population level is poorly understood for any species. I examined the ecological implications of dioecy in Siparuna grandiflora (Siparunaceae), a neo-tropical understory shrub. Populations were surveyed for sex-ratio bias and to determine whether individuals were distributed non-randomly with regard to space or light availability. At the whole-plant level, I examined whether the sexes differed in reproductive allocation, delayed costs of reproduction, and patterns of growth or carbon allocation under field or experimental conditions. At the leaf level, I examined differentiation in efficiency of carbon uptake, net assimilation and nitrogen use efficiency.^ Population sex-ratios of reproductively mature Siparuna grandiflora are significantly male-biased sex as a result of sex-based differentiation in life-history traits. The sexes are not spatially segregated. The sex ratio bias arises primarily because males begin reproduction at a smaller size and reproduce more frequently than females. Reproductive allocation in females exceeds that of males, a difference which is reflected in lower probability of reproduction and decreased growth in the year following reproduction in females, but not males. Pre-reproductive females grow faster than pre-reproductive males, and this capacity enables reproductively mature females to achieve nearly equivalent growth to males under natural conditions, despite greater reproductive costs. Field gas-exchange measurements indicate that both sexes have reduced photosynthetic rates during reproduction, and that males have greater maximal photosynthetic rates than females, regardless of leaf age. Thus, the ability of female plants to achieve equivalent or greater growth rates than males is not directly related to photosynthetic capacity. Instead, the sexes appear to differ in patterns of stem extension growth and leaf allocation in such a way that they achieve both equivalent carbon assimilation and equivalent growth rates on the whole plant level. ^