For managed forests which rely on natural tree regeneration for canopy recruitment, abundance and composition of tree regeneration portend future forest structure and diversity. For northern hardwood forests, a geographically widespread forest type in North America, typical single-tree selection (STS) management relies on natural regeneration to promote new cohorts of canopy trees. Harvesting dispersed, select trees every 15 – 20 years, STS generates low light levels intended to promote sugar maple (Acer saccharum) and other shade-tolerant tree species in an uneven-aged system. However, following 60 + years of STS implementation in the Great Lakes region, concerning regeneration trends have emerged, namely low densities of sugar maple and low tree species diversity. Additionally, few studies have analyzed age structure under this system to assess its past efficacy in generating uneven-aged forests. The research presented here characterizes regeneration and recruitment outcomes of STS, analyzing data from a uniquely detailed and geographically widespread research project of 141 northern hardwood stands across northern Michigan. Given the silvicultural focus on regenerating sugar maple, the first two chapters focus on management outcomes for this key species. First, a flexible Bayesian hierarchical model offers insight on patterns of sugar maple regeneration for key size classes as a function of plot and stand level predictors. Our results indicate that sugar maple regeneration is sparse to absent, particularly for size classes actively browsed by deer and recently escaped from the deer browsing zone. The second analysis characterizes age structure for a subset of 51 stands, drawing on 1499 sugar maple trees > 5 cm diameter sampled via basal discs from recently harvested stumps; this analysis provides insight to past patterns of recruitment and establishment. The results suggest little evidence of sugar maple seedling regeneration and canopy ingrowth over the past 60 + years of STS management; instead, stands have highly suppressed saplings plus aging poletimber and sawtimber classes, which are at or quickly approaching economic maturity. Given declines in sugar maple dominance as evidenced by the first two research analyses, the third analysis assesses stand-level tree species diversity and individual species abundance as a function of landscape predictors and size class to shed light on projected future canopy composition. On average, there are approximately three effective common species for seedlings, saplings, and canopy stems at the stand level, and species less desirable for management are occupying growing space in the sapling layer. Together, these results indicate that STS has been unsuccessful in regenerating or recruiting sugar maple over the past 60 + years, and stands are characterized by a paucity of tree species. Our results support several potential alternative management strategies, including decreasing basal area via more intense harvests, prohibiting deer browsing via natural browsing barriers, or introducing greater diversity of tree species via direct seeding or planting. These results should be considered to improve current management of northern hardwood forests in the Great Lakes region.
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