Effects of uneven-aged management on diameter distribution and species composition of northern hardwoods in Upper Michigan

Uneven-aged management using single-tree or group selection has been in practice for many decades, especially in northern hardwood forests. Use of stocking regulation tools is thought to produce and maintain specific stand structures that upon regulation, are sustainable over time. We evaluated stand structures in northern hardwoods in Upper Michigan across three ownerships that practice different approaches toward uneven-aged management. Industry land (MeadWestvaco - MWV) uses primarily diameter limit cutting combined with a sawyer-select harvest method, retaining a maximum diameter of 45.7 cm (18 in.) with a residual basal area of 16.1 m²/ha (70 ft²/ac) and a cutting cycle of 10 years. The Michigan Department of Natural Resources (DNR) uses a crop tree release technique with a residual basal area of 18.4 m ²/ha (80 ft²/ac) and a cutting cycle of 20 years. Both ownerships view regeneration of new cohorts as inevitable given the intensity of disturbance and the forest type. The third ownership (the Ford Forestry Center School Forest at Michigan Technological University - FFC) employs strict stocking regulation using the BDq method (residual basal area of 16.1 m ²/ha (70 ft²/ac), maximum diameter of 50.8-55.9 cm (20-22 in.), and a q-ratio of 1.3) with a cutting cycle ranging from 12 to 15 years. Stand structure on a total of 25 stands was characterized for these ownerships to assess the impact of management strategy on stand structure and species composition. Differences in species composition and lower diversity indices were found where increasing sugar maple dominance was an objective (FFC ownership). All ownerships showed reduced relative importance values of mid-tolerant species such as yellow birch in their stands as compared to values reported for old-growth or unmanaged stands. Diameter distributions were classified into one of three shape categories (negative exponential, increasing q-ratio, and rotated sigmoid) using the regression of DBH, DBH², and DBH³ on the log₁₀ of trees per hectare. The best model in terms of adjusted-R² and root mean square error (RMSE) was selected for each stand. All management strategies resulted in similar occurrences of distribution shapes, despite some evidence of a trend toward a rotated sigmoid distribution. These trends suggest that several different diameter distribution shapes in uneven-aged northern hardwoods in the Lake States are possible following a variety of management approaches, with sugar maple increasing in dominance with strict adherence to certain stocking regulation guidelines.