Edge influence on vegetation communities in the Sylvania Wilderness, Upper Peninsula of Michigan

Background/Question/Methods

Significant issues facing land managers of natural areas include landscape fragmentation and its influence on effective preserve size. This study quantifies the influence of adjacent land use on a wilderness area, the Sylvania Wilderness (Sylvania), an old-growth hemlock-hardwood temperate forest in Michigan’s Upper Peninsula. Old-growth forests are rare, discontinuous, unlogged stands that are significant reservoirs of biodiversity. Changes along these borders impact stand development, habitat complexity, and abundance of native species. This research estimates the magnitude and pacing of edge effects on the Sylvania border as a result of changes in adjacent land use.

To quantify edge effects, we conducted forest surveys in regular intervals in ten, 1.5-km long transects crossing the Sylvania border. We collected plot data on vegetation (e.g., provenance, growth habit, density, percent cover, basal area, diversity) and structure (e.g., non-vegetated area, coarse woody habitat, windfalls, snags, canopy height, canopy strata). To quantify the spatial extent of edge effect, we fit linear mixed models with different possible depths and rates of influence. We selected the depth and rate that minimize an information criterion using likelihood ratio tests. To quantify uncertainty in the mean level of each variable, we constructed pointwise 95% confidence bands for the regression curve.

Results/Conclusions

Preliminary results from the linear mixed model fitting process indicate significant trends using a p-value threshold of 0.05 and minimized AIC values. Edge effects ranged from 50 m to 1000 m into Sylvania. Significant shifts occurring near the border (within 500 m) and internal to Sylvania largely included structural components such as canopy cover, basal area, non-vegetated area cover, windfall abundance, and coarse woody habitat. Farther into the interior of Sylvania (500 – 1000 m), seedling density, deciduous tree abundance, and canopy height significantly shifted with increasing trends towards the Sylvania interior. Effects external to Sylvania occurred rapidly (within 250 m) and included increases (tree density, evergreen tree abundance, tree richness) and decreases (graminoid cover, invasive cover) toward Sylvania. These results shed light on previous underestimations of edge effects and may also suggest a positive influence on external land use from wilderness areas.   

This project augments the understanding of land use effects on preserves and aims to benefit applied efforts in conservation management. Quantifying the conditions of highest and most abrupt changes near wilderness borders can highlight conservation priorities and timeframes. Consequently, spatial estimates of edge effects can determine the extent of necessary buffer zones surrounding wilderness preserves.