What makes the sex ratio female-biased in the dioecious tree Salix sachalinensis?

1. The female-biased sex ratio in the genus Salix is widely documented, but little is known about the ecological mechanisms causing this bias. We investigated the flowering sex ratio, population dynamics, clonality and spatial segregation of the sexes in three Salix sachalinensis populations of different ages (young, intermediate and old) for a total of 2922 trees over 3 years. 2. Spatial segregation of the sexes did not occur in any of the populations. 3. Flowering sex ratios were female biased (2 : 1) in both the intermediate and old populations, but were slightly male biased in the young population. The female-biased sex ratios in the mature populations were due to the increasing number of flowering females with increasing plant size, but not to differences in demographic parameters; neither mortality nor growth rate differed between the sexes in any population. 4. Amplified fragment length polymorphism analysis revealed that there were very few clonal individuals in any population, suggesting little effect of clonality on the biased sex ratios. 5. These results suggest an initially and inherently determined female-biased sex ratio in mature S. sachalinensis populations.