Dioecy in a lowland terrace rimu (Dacrydium cupressinum, Podocarpaceae) forest, South Westland, New Zealand

Abstract

Females and males of dioecious plants have been shown to have dissimilar resource allocation patterns. Females are thought to invest more in reproduction and less in growth and maintenance due to differences in reproductive function. This differential investment between the sexes could result in distinct growth patterns and contrasting survival rates. As a consequence of putting more effort into reproduction, females are expected to grow slower, mature later, die earlier, occur in lower numbers than males, and occupy less stressful sites. Females and males within a population may therefore differ in size and age distribution, and in sex ratio and may be spatially segregated. To verify these predictions, age and size distributions, spatial relationships, and sex ratios of females and males were compared for the dioecious tree Dacrydium cupressinum, rimu (Podocarpaceae). These comparisons were made in relation to the disturbance dynamics of D. cupressinum within a one hectare stand of virgin forest in Okarito Forest, South Westland, New Zealand. The results of this study and future research on females and males of this species may have important implications for forest management and the preservation of frugivorous avifauna.

Four groups of relatively even-aged cohorts were identified by spatial analysis of D. cupressinum tree age distribution. No significant differences were found between females and males in size and age distributions, growth rate, age of first reproduction, mortality, or spatial distribution for the entire plot or within the identified age cohorts. The sex ratios of the total population and the identified cohort groups were consistently female biased. The expected ecological consequences of presumed greater investment in reproduction for females were not apparent even within the competitive conditions of this high density stand. The consistency of finding no differences in stand structure between the sexes, except for the sex ratio, suggests that there is little difference in resource allocation between females and males.

Many factors could reduce differences in the cost of reproduction between females and males, including the amount of biomass being contributed to reproduction annually by individuals of each sex, differences in the timing of investment between females and males, and possible photosynthetic contribution of female cones. Alternatively, there may be no differences between the sexes in their reproductive investment. Explanations for the biased sex ratio are discussed, however further studies of D. cupressinum across environmental gradients and between differing disturbances are needed for interpretations to go beyond the confines of this study area.