Sex Ratio and Neighbor Effects in Monospecific Stands of Juniperus Virginiana

Sex ratio, sexual dimorphism, and neighbor effects were investigated in 20 monospecific stands involving 3500 trees of Juniperus virginiana, a dioecious gymnosperm, near its northern limit in eastern Ontario. We tested the hypothesis that the intensity of competition for a given tree will depend on the sex of its nearest neighbor because of differential costs of reproduction in male and female plants. Population sex ratio was significantly male biased in only one stand but was male biased when data from all stands were combined. Stand sex ratio was not related to age structure, stand density, or local competition intensity (based on separation distance/size regression). Male trees were significantly taller and had greater diameters in only three stands. In the combined data from all stands, however, there was a greater representation of male trees among the larger diameter trees and among the taller trees. Male trees were also significantly taller in the pooled data set. These results suggest that female trees pay a slightly but significantly greater cost for reproduction in terms of reduced vegetative growth. However, the effects of close neighbor proximity did not depend on a neighbor's sex, and male and female trees were randomly distributed relative to each other. Hence, males and females appear to compete with similar intensities against each other and are not spatially segregated into separate "niches." The overall greater height for males may be interpreted as a consequence of sexual selection, as greater height in male trees would confer greater success as a pollen donor, especially in dense monospecific stands characteristic of this species.