Flowering, growth and defence in the two sexes:: consequences of herbivore exclusion for Salix polaris

1. For a long time, dioecious plants have been a model system for understanding the interactions between plants and herbivores. Differences in growth rate and, consequently, investment in defence between sexes may lead to skewed sex ratios due to differential herbivory. 2. In this study we evaluated the applicability of this idea to polar willow (Salix polaris ), which in the study site, Svalbard, displays a female-biased sex ratio. 3. Excluding reindeer for 3 years increased the abundance of male flowers in one of two vegetation types investigated. Growth rates differed only slightly between the sexes, with females investing more in inflorescences. 4. The concentration of chemical defence compounds (phenolics and condensed tannins) did not differ between the sexes. 5. On the basis of these findings, the idea that growth rate-dependent herbivory caused the unbalanced sex ratio in S. polaris has to be rejected. Possibly an interaction of niche differentiation between male and female willows, in combination with reindeer grazing, produced the observed female-biased sex ratio, but the mechanism remains unclear.