Modules of Production and Reproduction in a Dioecious Clonal Shrub, Rhus Typhina

Abstract

The hypothesis that male—biased sex ratios result from the greater costs of reproduction for females was examined at various modular levels in a study of the dioecious clonal shrub, staghorn sumac (Rhus typhina). The population of flowering trunks was male—biased, though male and female clones proved to be of similar ages. Trunks within apparent female clones were more likely to be vegetative or dead than in male clones, though the density of living plus dead trunks was comparable in male and female clones. Females of given ages had the same mean trunk diameter as similarly aged males. Overall, the average female trunk diameter was the same as the average male diameter. Differential costs and patterns of reproduction for males and females were revealed through destructive harvests and separate analysis of trunks and branches. An abrupt discontinuity n the performance (growth) of male trunks and branches. An abrupt discontinuity in the performance (growth) of male trunks was found; those with a diameter less than °4 cm had greater total numbers of leaves, branches, and inflorescences than females of comparable diameter, whereas larger males and females did not differ from each other. Flowering males branches had significantly fewer leaves than those of flowering females. The number of leaves per branch was highly correlated with the number of fruits produced in the corresponding terminal inflorescence. The average (±SE) male inflorescence had 4099 ± 312 flowers, whereas the average female inflorescence bore 1575 ± 67.2 fruits. There was a sixfold difference in the biomass of these terminal reproductive units (female infructescences being heavier.) This study suggests the importance in a clonal species of defining the modular level at which costs are assessed. The hypothesis that females suffer greater reproductive costs as a consequence of fruit production is supported by the finding of diminished survivorship of daughter trunks in female clones. However, it is not supported by data on annual trunk diameter increments. Reproductive costs in sumac are shown at the level of the clone (ramet survivorship) or the branch, rather than at the level of the flowering trunk module.