Population structure and growth rates of three dioecious tree species in a sub-tropical wet forest in Puerto Rico

Background/Question/Methods Dioecious plants frequently exhibit male biased sex ratios and sexual differences in life history traits such as plant size and growth rate. Theory indicates that sexual dimorphisms in life history traits evolve as a result of differential reproductive costs between male and female plants; because the resources required per gamete are greater in females than males. Higher costs of reproduction in females have been mainly inferred from observations that males  grow faster, begin reproduction at smaller sizes or grow to larger sizes than females. In this study, we recorded the sexual expression of all the potentially reproductive individuals ≈ 2700 trees of three canopy dioecious species, Cecropia schreberiana, Tetragastris balsamifera and Dacryodes excelsa, conducting multiple censuses during two years in the 16-ha Luquillo Forest Dynamics Plot located in a subtropical wet forest in Puerto Rico. The main objective was to determine if the populations exhibit a male biased sex ratio and, if so, explain whether this bias is the result of sexual differences in life history traits. To examine evidence of higher costs of reproduction in females we compared size distributions and growth rates of the sexes using fifteen years of tree growth data.

Results/Conclusions The cumulative sex ratios of C. schreberiana and T. balsamifera were not significantly different, while D. excelsa exhibited a female biased sex ratio. Neither Cecropia schreberiana nor T. balsamifera exhibited sexual differences in their size distributions or growth rates, which suggested that reproductive maturation and longevity are similar for both sexes in these species. In contrast, D. excelsa, females were more abundant than males in the intermediate size classes, but the largest individuals were males. The sexual differences in size for D. excelsa may be because males grew slightly faster than females. In the tropics male biased sex ratios and associated patterns of sexual dimorphism have been frequently reported for subcanopy species living in light limited environments. The relatively few differences between the sexes found in this study suggest that adult trees of these canopy species are less limited by light availability, and therefore they are less likely to exhibit sexual differences in costs of reproduction.