The challenging past and precarious future of Canada yew (Taxus canadensis) in the Apostle Islands National Lakeshore

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

doi:10.36967/nrr-2285336

Forest understories on many of the Apostle Islands are unique due to the extensive distribution and abundance of Canada yew (Taxus canadensis), a robust evergreen shrub. When available, “yew” is a preferred food source for the white-tailed deer (Odocoileus virginiana). Historically, deer were less abundant in the “north woods” region near the Apostle Islands National Lakeshore (APIS) until widespread logging and fires that occurred with Euro-American settlement opened up habitat favorable to deer. APIS, a Lake Superior archipelago, has partially served as a refuge for yew, isolating it from the impacts of browsing deer. Yet, not all islands have escaped the influence of this herbivore. Variations in the spatial coverage of yew among the islands are reflective of deer histories and other elements of past land uses. Some islands experienced irruptive deer densities in the 1940s and 1950s, and others more recently in the 2000s. The impacts of deer on yew have been well-documented throughout its range by numerous studies, including several studies focused on the Apostle Islands and its varied deer history. To sustain yew and other fragile ecological components of the islands, resource managers have actively harvested deer on islands that experienced high abundance of deer (i.e., Rocky and Stockton in the 1950s and more intensive culling activities on Sand and York in the 2000s). As outlined by the park’s Harvestable Species Plan (National Park Service 2014), managers divided the islands into zones so that a subset of islands are managed to maintain historic low presence or absence of deer (Zone 1 islands), and other islands (Zone 2) are managed to allow for deer at or below pre-Euro-American settlement densities (ca. 10/mi2). In 1991–1992, a study of island vegetation was conducted by Emmet Judziewicz, and he recorded the abundance and distribution of yew among 18 islands. We used his data to determine the 22-year changes and current status in the abundance and distribution of yew on these islands, and to assess yew changes in the context of island history, related to deer and their management since the 1950s. The varied and active management of deer on the islands provides us with a natural “living laboratory” in which we can study yew dynamics. We observed steep declines in average cover of yew on several Zone 1 islands including Sand (−28.4%), York (−26.8%), Ironwood (−30.9%), and Cat islands (−25.1%). Despite these declines, yew cover on these islands is still currently higher than on some of the Zone 2 islands with a record of heavy deer activity in the 1950s. Declines on Sand and York (both Zone 1 islands) 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. A notable area of recovery included Outer Island (+13.3%), a Zone 1 island with a history of fire on the southern half of the island. Recovery of yew also occurred on Zone 2 islands including throughout South Twin Island (+15.9%), on Bear Island (+6.9%) especially at the north tip, and in scattered patches on Manitou (+6.3%), Stockton (+2.2%), Hermit (+2.7%), and Rocky Island (+3.5%). Except for Outer Island, where deer have been absent or transient in low numbers, cover of yew on these “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.

The challenging past and precarious future of Canada yew (Taxus canadensis) in the Apostle Islands National Lakeshore

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