Background/Question/Methods
Canada yew (Taxus canadensis) abundance has declined throughout eastern North America over the last several decades. This evergreen shrub provides winter browse for white-tailed deer (Odocoileus virginianus) and moose (Alces alces) and creates important ground-cover habitat for small mammals and shrub-nesting songbirds. To identify factors that influence the establishment and recruitment of this species, in the spring of 2004 we planted 1,080 Canada yew cuttings in a hemlock-hardwood forest located near Alberta in the Upper Peninsula of Michigan. Cuttings were planted across a gradient of light conditions; subplot arrays were established on the edges of group selection openings and within the continuous forest matrix. Half of the subplots were enclosed with mini-hoop deer exclosures. Exclosures were built with heavy gauge, wire mesh (15 cm²) that does not exclude snowshoe hare (Lepus americanus). In fall 2007, we measured survival and growth (height and diameter) for all planted yew. At each subplot, we also estimated the percent cover of competing vegetation by functional group (graminoids, forbs, woody species, Rubus spp., ferns, club mosses, and Equisetum spp.) and hemispherical photography was used to quantify the light environment.
Results/Conclusions
After four growing seasons, Canada yew survival in exclosures was 16.9% higher than in controls (χ2 = 55.43, df = 1, p < 0.001). Multiple logistic regression analysis revealed the presence of an exclosure had the most influence on probability of survival; all else being equal an individual was 5.22 times more likely to survive if planted inside an exclosure. High levels of forb and fern coverage were also associated with increased survival, but increased levels of graminoid coverage and elevated understory radiation (i.e., light uninterrupted by the canopy) were associated with reduced survival. Height growth also differed significantly between exclosure and control plots after four years (F_1,10=178.68, p < 0.001, RMSE = 14.78). Mean change in height for exclosures was 13.26 cm greater than in controls. This, in part, can be attributed to frequency of browse. After four growing seasons, 93.9% of alive control plot individuals exhibited signs of herbivore damage compared to just 3.8% of surviving yew in exclosures. Collectively, these results show that white-tailed deer herbivory is an extremely important factor affecting Canada yew survival and growth.