THE ECOLOGY OF CANADA YEW (TAXUS CANADENSIS MARSH.), A DECLINING SPECIES

Windels, Steve

Canada yew (Taxus canadensis Marsh.) is an evergreen shrub native to forests of northeastern North America. Anecdotal accounts suggest that before Euro-American settlement Canada yew was a common plant across its range in many forest types. However, confusion persists about the historic role of Canada yew in eastern forests and other aspects of its autecology. Here, I synthesized available information on the ecology and management of Canada yew, with particular focus on its historic range and abundance and the impacts of browsing mammals. Several unique life history traits, including high shade tolerance and a dense clonal growth form, allowed it to become locally abundant or dominant in 5-20% of mainland forest stands in the northern parts of its range, and to reach extreme understory dominance on ungulate-free islands in the Great Lakes and coastal Canada. The species’ range has declined significantly from its historic maximum as a result of a combination of several factors, including fire, intensive forest management, and clearing of land for agriculture and other development. Browsing by wild cervids, however, appears to be the most common and ubiquitous cause of its decline across its range. Canada yew appears incompatible with white-tailed deer (Odocoileus virginianus) and moose (Alces alces) except at low levels of animal abundance or in regions where sufficient snowfall protects stems. Increasing white-tailed deer populations and reduced snowfall as a result of climate change in eastern North America threaten to extirpate this species from additional parts of its range in the next century. Commercial harvest for this species for paclitaxel and other anti-cancer pharmaceuticals is another emerging threat. To explore mechanisms by which white-tailed deer can impact populations of Canada yew, I investigated patterns of deer browsing on Canada yew at multiple spatial scales in the Upper Peninsula of Michigan. Browsing on Canada yew at the scale of individual forest stands followed a classic Type II functional response whereby average intensity of browsing on stems decreased as abundance of yew increased. Simultaneously, browsing intensity was also affected by the position of stems in relation to the edges of Canada yew patches. Stems not associated with dense patches were browsed most heavily while browsing intensity decreased toward the interior of patches. I speculate that deer avoid traveling through the interior portions of dense patches because it is energetically expensive to do so. I believe this is the first example of refugia from browsing created by the physical obstruction of individuals of the same species. The decline of Canada yew from eastern forests has likely resulted in changes to several components of forested ecosystems, including structure and composition of understory vegetation and concomitant effects on understory vertebrates. I demonstrate that Canada yew does inhibit density of seedlings of deciduous woody plants but that it does not necessarily affect recruitment into sapling stages. I also show that an index of understory structure, foliage height density, is quadratically related to Canada yew density, such that maximum structure is attained at median values of Canada yew abundance in most forest stands. In older, closed canopy forests, however, Canada yew can provide structure that would not otherwise be provided by other species of woody or herbaceous plants. I also assessed the potential effects of the loss of Canada yew from eastern forests on understory vertebrates by analyzing patterns of individual species abundance and community-level indices of diversity. As has been reported for many of the focal species in my study, most individual species of small mammals and breeding songbirds appear to be more closely associated with physiognomy (structure) rather than floristics, i.e., most species were not closely associated with Canada yew