Sugar maple communities and regeneration in the Upper Great Lakes region

Background/Question/Methods

In this work we were interested in investigating tree and seedling composition of hardwood communities dominated by sugar maple in the Upper Great Lakes region (Minnesota, Wisconsin, and Michigan). We divided this region into three distinctive zones spanning from southwest to northeast: the Prairie-Forest Border (PFB) forms the transition zone between the tall grass prairies and the northern forests (Zone 1), the forest interior extends beyond the PFB into northern Wisconsin (Zone 2), and the deep forest lies in Michigan’s Upper Peninsula (Zone 3). We used tree and seedling data from the Forest Inventory and Analysis (FIA) program of the U.S. Forest Service and the Bray-Curtis ordination of PC-ORD v. 5.10 to test the following hypothesis: (1) Hardwood forests dominated by sugar maple form a series of distinct communities via association of sugar maple with other tree species across the Upper Great Lakes Region, (2) Mesophication is occurring in sugar maple-red oak communities via the replacement of fire-dependent xerophytic species such as oak by sugar maple, and (3) Sugar maple dominates the understory category and succession to sugar maple is occurring under all of the communities—therefore the overstory communities identified are not stable. 

Results/Conclusions

We found that sugar maple forms unique communities with other species across the Upper Great Lakes region. Co-dominant species across the area include red oak and basswood in the PFB (Zone 1), red oak, quaking aspen, and red maple in the forest interior (Zone 2), and red maple, hemlock, and yellow birch in the deep forest (Zone 3). Mesophication is occurring in most sugar maple-red oak communities via the replacement of fire-dependent xerophytic species such as oak by sugar maple. Although sugar maple recruitment appears to be successful, the combination of increasing deer population, earthworm invasion, and increases in average global temperatures may contribute to the regeneration failure of sugar maple, explaining why some of our plots showed no sugar maple regeneration. Our seedling communities did not match the overstory communities, suggesting that overstory communities are not stable and that succession resulting from European settlement during the late 1800s to early 1900s is still widespread.  It is clear that sugar maple communities are undergoing compositional shifts as a result of complex interactions and that this will lead to a shift in species composition in the future.