Forest health monitoring at Isle Royale National Park: 2019 field season

Sanders, Suzanne; Kirschbaum, Jessica

doi:10.36967/2294732

Isle Royale National Park (ISRO), situated in western Lake Superior, supports 53,823 ha of forested land, most of which is designated wilderness. In 2019, the Great Lakes Inventory and Monitoring Network (GLKN) resampled permanent forest monitoring sites in the park, marking the second assessment of these sites. The goal of this long-term monitoring project is to provide managers with routine updates on the state of island forest communities; these data can also be used to tease apart changes in forest communities and elucidate causal agents when novel problems or situations arise. We initiated a comprehensive forest monitoring program at Isle Royale in 2010, establishing 52 sites at that time. We resampled those sites in 2019 and established an additional eight plots. Sampled and derived metrics included trees (density and basal area of live trees, seedlings, and snags), understory (herb and shrub frequency), browse (bite marks on woody species and presence and height of herbaceous species), richness, and earthworm impacts. We first classified sites into five broad forest types using the initial (2010) dataset (Sugar Maple-Yellow Birch, Cedar-mixed, Paper Birch-mixed, Mixed Conifer-Hardwood, and Balsam Fir), then examined changes since that time. Analysis of 2019 data shows regeneration of several overstory species remains impaired, particularly in forest types common on the western end of the island, where high moose browse pressure has resulted in neither balsam fir nor trembling aspen being common. While sugar maple was still abundant, density of small saplings fell 33% between the two sampling events. In paper birch forests, we saw little regeneration of hardwoods. Senescing paper birch dominates the forest, while, in some areas, few other individuals of paper birch or other species are present, either in the subcanopy or understory. In Mixed Conifer-Hardwood and Balsam Fir sites, concentrated on the central and eastern parts of the island, trembling aspen has increased in the smallest size classes. This is likely a release from browsing pressure from the period of 2006–2011 when moose density was lower. Higher recent moose density was reflected in seedling data in forest types common on the west side. Pooling species together, seedling density fell 26.2% at Sugar Maple-Yellow Birch sites and 25.8% at Cedar-Mixed sites. Browse on woody species was markedly higher in 2019. Mean site-level proportion browse (i.e., frequency of the 30 direct-browse circles in each site showing bite marks on woody individuals regardless of species) was highest in Sugar Maple-Yellow Birch sites, where 24.5% of direct browse circles had bite marks in 2019 (compared with 2.1% in 2010). At the species level, among trees, parkwide proportion browse was greatest on red maple (0.40) and sugar maple (0.35), with six species having a proportion browse value greater than 0.2. For shrubs, parkwide proportion browse was greatest on four species of the genus Cornus (dogwood), ranging from 0.41–0.80. In total, seven shrub species had proportion browse values greater than 0.2. Changes in the herbaceous layer between the two sampling events were notable. Site-level species richness fell 20% in Sugar Maple-Yellow Birch sites and 6.6% in Paper Birch-mixed sites. Species known to be preferred by moose also experienced declines, both in terms of site-level richness and site-level abundance. Declines in abundance were most notable in the forest types common on the west side of the island, where abundance of preferred herbs declined 38% in Sugar Maple-Yellow Birch sites and 22% in Cedar-Mixed sites.

Forest health monitoring at Isle Royale National Park: 2019 field season

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