What causes female bias in the secondary sex-ratios of the dioecious woody shrub Salix sitchensis colonizing a primary successional landscape?

Background/Question/Methods

Females often outnumber males in Salix populations, although the mechanisms behind female-bias are not well understood, and could be caused by both genetic and ecological factors. We investigated post-germination factors that could bias secondary sex ratios of Salix sitchensis< colonizing Mount St. Helens after the 1980 eruption. We determined whether S. sitchensissecondary sex ratios varied across disturbance zones created by the eruption and mesic and hydric habitats within each zone. For one population, we tracked adult mortality, whole-plant reproductive allocation (in terms of carbon, nitrogen, and phosphorus), the number of stems, and plant size for two years. We performed a field experiment, creating artificial streams to test whether vegetative reproduction via stem fragments was sex-biased.

Results/Conclusions

We found a consistent 2:1 female bias in S. sitchensis secondary sex ratios across all disturbance zones and habitats, with no evidence that this bias is caused by historical contingency associated with the colonization of these areas, or spatial differences in the distribution of the sexes in response to water or stress gradients. Despite female plants sometimes allocating more nutrients that are broadly limiting in this system to reproduction than males, we found no evidence of sex-biased mortality. The establishment rate of S. sitchensis experimental stems did not differ between the sexes, indicating that vegetative reproduction was not distorting secondary sex ratios. We hypothesize that S. sitchensis secondary sex ratios depend on early-acting genetic factors affecting the seed sex ratio, or by sex-specific germination or survival rates prior to maturity, as opposed to factors associated with reproduction in adult plants.