Tree regeneration layers are presently understocked or conspicuously absent in many managed northern hardwood forests. These forests have historically been managed under a low-intensity, frequent harvesting regime, which relies on small canopy openings to recruit advance regeneration. Notably, there have been declines in the recruitment of canopy dominant sugar maple (Acer saccharum) as well as overall tree species diversity. Currently, we lack an understanding of how various patterns and processes, such as overabundant white-tailed deer, are driving these trends, particularly at broad scales. The objective of our study is to quantify drivers of low sugar maple regeneration in northern hardwood forests. Our goal is to both further basic understanding of ecosystem processes and guide management toward increased sustainability and resiliency. This study draws on extensive stand and site data from 141 selection-managed northern hardwood stands in northern Michigan, distributed over gradients of site quality, deer density, climate, and harvest history. Using a hierarchical Bayesian framework, we developed a model to explore predictors of regeneration for sugar maple in three key regeneration size classes: seedlings (<25 cm tall), browse-zone saplings (25-137 cm tall), and browsing-escaped saplings (>137 cm tall and < 5 cm diameter).
Results/Conclusions
Preliminary results indicate that density of sugar maple regeneration classes is related to a variety of complex biotic and abiotic factors. At the broadest geographic scale, we found evidence for regional trends in sugar maple regeneration density, with the three broad geographic regions our study encompasses each having different effects on average sugar maple density. Within these regions, northern hardwood stands of intermediate nutrient-moisture regimes tended to have more sugar maple regeneration than either high or low sites. For biotic factors, density of woody shrubs was negatively associated with browse-zone sapling density, suggesting that the interaction may be more competitive than facilitative (e.g. protection from deer browse). Winter deer usage had a positive relationship with seedling and browse-zone sapling density, but a negative effect on browsing-escaped sapling density. Density of canopy trees was negatively related to regeneration density for all size classes. Overall, we found that deer, regional geography, site quality, stand density and shrub competition likely influence sugar maple regeneration patterns. Understanding where and when sugar maple can be regenerated at densities adequate for sustaining gap-dominated stand dynamics could guide site selection of areas which require alternative management regimes. This ultimately promotes forest resilience and continued ecosystem functioning.