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

Abstract

• Premise of the study: 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 several ecological factors that could bias secondary sex ratios of Salix sitchensis colonizing Mount St. Helens after the 1980 eruption. • Methods: We determined whether S. sitchensis secondary sex ratios varied across disturbance zones created by the eruption and across mesic and hydric habitats within each zone. For one population, we tracked adult mortality, whole‐plant reproductive allocation, the number of stems, and plant size for 2 years. In a field experiment, we created artificial streams to test whether vegetative reproduction via stem fragments was sex‐biased. • Key results: We found a consistent 2:1 female bias in S. sitchensis secondary sex ratios across all disturbance zones and habitats. Despite female plants sometimes allocating more resources (in terms of carbon, nitrogen, and phosphorus) 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. • Conclusions: We hypothesize that S. sitchensis secondary sex ratios depend on either early‐acting genetic factors affecting the seed sex ratio or sex‐specific germination or survival rates before maturity, as opposed to factors associated with reproduction in adult plants.