Declines and recovery of Canada yew (Taxus canadensis) in the Apostle Islands National Lakeshore: a challenging past and precarious future

Johnson, Sarah; Mead, Jordan; Widen, Matthew; Leonard, Emily

Forest understories on many of the Apostle Islands have a rather unique quality due to the extensive distribution and abundance of a robust evergreen shrub called Canada yew or ‘yew’ (Taxus canadensis). When available, yew is a preferred food source for the white-tailed deer (Odocoileus virginiana). Deer were historically not very common in the ‘northwoods’ region surrounding the Apostle Islands National Lakeshore until habitat began to open up with the widespread and intensive logging and fires that occurred with Euro-American settlement. This Lake Superior archipelago has partially served as a refuge for the yew, isolating it from the impacts of browsing deer. Yet, not all islands have escaped the influence of this yew-loving omnivore. Variations in the spatial coverage of yew among the islands are reflective of the deer histories and other elements of past land uses. Some islands experienced irruptive densities in deer in the 1940s and 50s and others more recently in the 2000s. The radical impacts of deer on yew have been well-documented throughout its range by numerous research studies, including several focused on the Apostle Islands with its varied deer history. To sustain yew and other fragile ecological components of the islands, resource managers have actively harvested deer on islands that have had experienced unsustainable numbers of deer (i.e., Rocky and Stockton in the 1950s and more intensive culling activities on Sand and York in the 2000s). Islands are divided into zones so that a subset of islands are managed to maintain their historic low or no deer presences (Zone 1 islands) and other islands (Zone 2) are managed to maintain deer densities at or below historic densities for the region. Zone 2 islands have a recorded history of high to low deer numbers. This varied and active management of deer on the islands provides us with a natural ‘living laboratory’ in which we can study yew dynamics. In 1991-92, a study of island vegetation was conducted and the abundance and distribution of yew was recorded among 18 islands. We used those data to determine the status and 22-year changes in the abundance and distribution of yew on these islands and assessed yew changes in the context of island history, most specifically deer history and their management since the 1950s. We observed steep declines in yew cover on several Zone 1 islands including Sand (-51.3%), York (-39.4%), Ironwood (-51.3%) and Cat Islands (-47.4%). Despite these declines, yew cover on these islands is still currently higher than islands with a record of heavy deer activity in the 1950s. Declines on Sand and York are explainable by the high numbers of deer recorded there in the 2000s. Observed signs of browsing activity may explain declines of yew on Cat and Ironwood, but this needs to be further investigated to ascertain deer presence. Our results suggest that yew may be recovering, albeit very slowly, in patches among some islands. The most notable areas of recovery include the south end of Outer Island, throughout South Twin Island, the north tip of Bear Island and scattered patches on Manitou, Stockton, Hermit and Rocky. Accept for Outer Island (which historically and currently is not known to have a deer population), cover of yew on these other ‘recovering’ islands remains at levels lower than islands that have not historically had deer populations. Where deer have persisted on Oak and Basswood Islands, yew does not appear to be recovering.