Scale-Dependent Browsing Patterns on Canada Yew (Taxus canadensis) by White-Tailed Deer (Odocoileus virginianus)

Canada yew (Taxus canadensis) is a clonal shrub that forms discrete patches and was formerly an important component of forest understories in much of northeastern North America. Following Euro-American settlement, Canada yew has been extirpated or reduced in abundance throughout much of its former range, particularly in the USA; winter browsing by white-tailed deer (Odocoileus virginianus) has been implicated as responsible for much of its decline. Little is known about the factors affecting deer browsing intensity on Canada yew. We examined factors related to browsing intensity on Canada yew across three spatial scales in 29 forest stands in Michigan, USA. Browsing intensity on stems was related principally to two factors acting simultaneously across multiple spatial scales. Browsing intensity was negatively related to amount of Canada yew at the scale of the forest stand and negatively related to distance from the edge of Canada yew patches, effectively creating refugia from browsing. The browsing patterns we observed suggest that yew exists in two alternate stable states: (1) as loose aggregations of small stems or (2) large, dense patches of large stems. The implications of changes in deer density or snow cover to the probability of local persistence of Canada yew are discussed.

Resistance of Canada Yew (Taxus canadensis) Branch Wood to Two Wood Decay Fungi

Wood of the larger yews (Taxus spp.) is reported to be decay-resistant, but little is known about the decay resistance of Canada Yew (Taxus canadensis Marsh.) wood. Branch wood from Canada Yew was compared to branch wood from Northern Red Oak (Quercus rubra L.) and Eastern White Cedar (Thuja occidentalis L.) in a standard laboratory decay test to evaluate its resistance to decay by two decay fungi. Canada Yew was shown to be significantly more resistant to decay by Gloeophyllum trabeum (Pers.) Murr. (a brown rot fungus) and Trametes versicolor (L.: Fr.) Quél. (a white rot fungus) than Northern Red Oak (P ≤ 0.05). Canada Yew was shown to be equal to Eastern White Cedar in resistance to decay by G. trabeum and more than twice as resistant to decay by Trametes versicolor (P ≤ 0.05). These results may have relevance for survival of Canada Yew, which is under pressure from browsing by White-tailed Deer (Odocoileus virginianus).

Establishing a sustainable harvest for canada yew (Taxus canadensis marsh.) in Ontario

In 2003, commercial harvest of Canada yew (Taxus canadensis Marsh.) in Ontario began—but without a sustainable harvest policy. In 2005, we began to determine the sustainability of three harvest intensity treatments at three sites in central Ontario. Harvest treatments were labelled control (no initial harvest), light (two-year-old shoots removed), moderate (three-year-old shoots removed), and severe (seven-year-old shoots removed). We also looked at effects of harvest season and light levels on shoot regrowth. After three and four years, severe-harvest plants yielded less than half the biomass of the initial harvest, while biomass from moderate-harvest plants was about equal to the initial. Biomass from light-harvest plants generally increased. Moderate light levels stimulated more first-year regrowth in all plants than low light levels did but increased only Year 2 regrowth in severe-harvest plants. Spring harvest reduced first-year regrowth only. Comparing biomass of moderate-harvest plants after three or four years with initial moderate-harvest biomass suggested similar growth rate across time periods. Our results concur with Canada Yew Association sustainable harvest guidelines: Moderately harvesting three-year-old shoots plus allowing four years of regrowth before reharvest ensures sustainable harvest, at least through one harvest cycle.

The ecology of Canada Yew (Taxus canadensis Marsh.): A review

Canada yew (Taxus canadensis Marsh.) is a shade-tolerant evergreen shrub native to the understories of the boreal and deciduous forests of northeastern North America. Canada yew has a relatively unique growth form, with low sprawling branches capable of forming dense clusters of stems. Historic accounts suggest that before Euro-American settlement, Canada yew was a common plant across its range in many forest types and that it was locally abundant or dominant in 5%–20% of forest stands in the northern part of its range. Canada yew’s range has declined during the last century as a result of browsing by native ungulates, fire, intensive forest management, and clearing of land for agriculture and other development. It is considered uncommon throughout most of its present North American range, except for often isolated populations in areas that remain free from heavy browsing pressure by moose and deer (e.g., islands, areas with heavy snow cover, or areas naturally low in ungulate abundance). Its decline has likely resulted in changes to abiotic and biotic conditions, including structure and composition of understory vegetation and concomitant effects on understory vertebrates. 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. Suggestions for future research are discussed.

Allometric models for predicting the aboveground biomass of Canada yew (Taxus canadensis Marsh.) from visual and digital cover estimates

The objectives of this study were to evaluate visual and digital estimates of percent cover as source data and to develop cover-based allometric models for the prediction of aboveground biomass of Canada yew ( Taxus canadensis Marsh.). Cover was determined from visual assessment and digital images captured over 25 plots (1 m2) at a model training site near Timmins, Ontario. Linear and power functions were fit to the cover–biomass data to develop models of foliage, stem, and total aboveground biomass. Both sources of cover data produced models that explained between 70% and 85% of the variance in the training data, with root mean square error estimates ranging from 27 g·m–2 (foliage) to 85 g·m–2 (total). Models based on visual cover data performed consistently better and were tested on independent data. Stem and total biomass were underestimated in the model testing data set; however, prediction statistics indicated that the linear and power forms of foliage biomass models were validated by the testing data. Final models of foliage biomass were developed from the entire data set, with mean absolute errors of 18.3 and 18.7 g·m–2 for the linear and power forms, respectively. Additional variables (e.g., plant height, age) may be required to provide general predictions of the woody biomass of Canada yew.

Small Mammal Abundance and Diversity in Forests With and Without Canada Yew, Taxus canadensis

Canada Yew (Taxus canadensis) has been extirpated from much of its former range in northeastern North America possibly due to logging, fire, agriculture, and browsing by White-tailed Deer (Odocoileus virginianus). We compared the relative abundance and species diversity of small mammals in five northern hardwood stands containing Canada Yew to five adjacent stands without Canada Yew in the Upper Peninsula of Michigan, during October-November 2000. Overall, 72 individuals were captured (53 in yew, 19 in non-yew); dominant species were Short-tailed Shrew (Blarina brevicauda), Deer Mouse (Peromyscus maniculatus), and Red-backed Vole (Clethrionomys gapperi). Overall mean (+ sd) capture rate (individuals/100 adjusted trap nights) in sites with yew (5.5 + 2.2) was greater (P = 0.04) than mean capture rate in sites without yew (1.9 + 1.0). Three indices of species diversity suggested greater small mammal diversity in stands with Canada Yew understories in northern hardwood forests.