Population structure, growth rates and spatial distribution of two dioecious tree species in a wet forest in Puerto Rico

Dioecious plants often exhibit male-biased sex ratios and sexual differences in life history traits such as plant size, growth rate and frequency of flowering, which arise from the different costs of reproduction for male and female plants. In tropical dioecious species sexual differences in reproductive costs have been demonstrated for several subcanopy species, but few canopy dioecious trees have been studied. We recorded the sexual expression of ≈ 2600 trees of Cecropia schreberiana and Dacryodes excelsa, two canopy dioecious species, during several censuses over 2 y in a 16-ha plot located in subtropical wet forest in the Luquillo Mountains, Puerto Rico. There were similar numbers of male and female trees of C. schreberiana but D. excelsa had a female-biased population. Cecropia schreberiana showed no differences in male and female diameter distributions or growth rates, suggesting that reproductive maturation and longevity are similar for both sexes. This lack of differences in size and growth rate in C. schreberiana may result from mechanisms to compensate for the higher cost of reproduction in females, no resource limitation related to its pioneer life-history, or similar male and female reproductive costs. In contrast, D. excelsa males were larger than females, probably because males grow slightly faster than females. This sexual difference in D. excelsa may reflect a higher cost of reproduction in females than in males. Spatial segregation of males and females into different habitats is not common in tropical forest and neither C. schreberiana nor D. excelsa males and females exhibited significant spatial segregation. The contrasting results for these two canopy species reflect their different life history strategies in this hurricane-affected forest.