Canada yew (Taxus canadensis) populations currently browsed by white-tailed deer (Odocoileus virginianus) or browsed by deer in the past had significantly lower production of male strobili, female strobili, and seeds than unbrowsed yew populations. Exclosure studies showed that protected yews produced significantly more male and female strobili than unprotected yews, but only after several years of protection. Seed production did not respond as readily to protection from deer perhaps because of reduced pollination levels in browsed yew populations. Previously unbrowsed yews were clipped at different levels of removal of available browse (control (no removal), 25%, 50%, 75%, and 100% removal) to simulate deer browsing. Reduction in male strobilus production was linearly related to clipping intensity in three years of observation. Female strobilus production was significantly reduced only at the 100% level of removal. Intermediate levels of clipping may have even stimulated production of female strobili. Analysis of covariance, with previous year's branch production as the covariate, showed no significant effect of clipping on male strobilus production except in the 100% removal group. Female strobilus production showed no such covariance with branch production. Effects of clipping on seed production could not be reliably assessed in 1984 and 1985 due to low seed production. Seed production in 1986 was significantly reduced only in the 100% removal group. Field observations of deer browsing of Canada yew indicate that 100% levels of removal are typical of natural levels of browsing.
Hand-pollination significantly increased seed production in a Canada yew (Taxus canadensis) population that had been browsed previously by white-tailed deer (Odocoileus virginianus) indicating that pollen availability limited seed production in this population. Hand-pollination did not significantly increase seed production in a population never browsed by deer. Deer-browsing significantly reduces yew density and pollen production, which in turn results in reduced pollination and seed production. Pollen limitation of yew seed production is an indirect effect of deer browsing on Canada yew and results from an interaction between the direct effect of deer browsing and the pollination syndrome of Canada yew. Artificially created female yews had lower seed set than monoecious yews when yew seed production was pollen-limited, but not when pollen did not limit seed production.