Forest health monitoring at Apostle Islands National Lakeshore: 2021 field season

Sanders, Suzanne; Kirschbaum, Jessica

doi:10.36967/2296650

Apostle Islands National Lakeshore (APIS), situated in western Lake Superior, is composed of 21 islands and a mainland unit. The park supports 17,037 ha of forested land, with much of that (80%) designated as wilderness. In 2021, the Great Lakes Inventory and Monitoring Network (GLKN) resampled permanent forest monitoring sites in the park, marking the second assessment of these sites, which were established and initially sampled in 2011. The goal of this long-term monitoring project is to provide managers with routine updates on which to base management decisions; these data can also be used to tease apart impacts and elucidate causal agents when novel problems or situations arise. We initiated the comprehensive forest monitoring program at APIS in 2011, establishing 48 sites at that time; in 2021, we resampled those sites and established an additional two. Our sampled and derived metrics included trees (density and basal area of live trees, seedlings, and snags), understory (herb and shrub frequency), deer browse (bite marks on woody species and presence and height of herbaceous species), coarse woody material, and species richness. Following the 2011 field season, we classified sites into five broad forest types (hemlock-hardwood, aspen-birch, cedar, mountain maple-black ash, and balsam fir); following the 2021 sampling season, we examined changes over the 10-year interval. In addition to the five forest types, we recognized three deer management zones within the park. Zone 1 is composed of 11 islands managed for few to no deer and includes 18 of our sampling sites. Within Zone 2, nine islands are managed for deer density at or below 3.86 deer/km2 (10 deer/mi2); here there are 27 sampling sites. Five permanent monitoring sites are located within the Mainland zone, which is part of a larger deer management unit that extends beyond park boundaries. Deer density goals in this third zone are established by the Wisconsin DNR. Analysis of the 2021 data shows some forest types are transitioning away from aspen-birch. Many of these stands are now approximately 75 years old and are becoming maple (Acer spp. L.)-dominated. Regeneration of both hemlock (Tsuga canadensis (L.) Carrière) and yellow birch (Betula alleghaniensis Britton) continues in hemlock-hardwood sites, but these species have experienced small declines in density between sampling events. Similarly, yellow birch continues to regenerate, but density in balsam-dominated sites has declined over the past 10 years. We found broadscale declines in taxa richness and abundance throughout the park but particularly in balsam fir forests (7.5 fewer taxa/site across all zones) and hemlock-hardwood sites (1.8 fewer taxa/site across all zones). We also observed notable declines at Zone 1 sites, where deer impacts are minimal: hemlock-hardwood sites lost 3.6 taxa/site, while mountain maple-black ash sites declined 7.0 taxa/site. It appears that there is no single cause for these declines, but likely several contributing factors. Work by others cites deer browse, fire history, Canada yew (Taxus candensis Marshall) cover, island size, and location within the archipelago as possible causal factors. Here, we also tie in the role of browse by snowshoe hare, as well as the influence of canopy closure. Hare were particularly abundant on Devils Island, where one of the four balsam fir sites is located, partially contributing to the sharp declines observed in that forest type. Canopy closure is a slow process, gradually limiting light availability on the forest floor. As stands age, conditions for ruderal species, typically more common at open sites, become less favorable, and therefore, these species can slowly become less common. This may explain some of the losses in hemlock-hardwood sites.

Forest health monitoring at Apostle Islands National Lakeshore: 2021 field season

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