Spatial and Temporal Patterns and Associated Drivers of Tree Regeneration Density and Diversity in Northwoods Forests

Tree regeneration density and species diversity foretell future forest resilience in the face of changing climate, disturbance regimes, and pest and pathogen outbreaks. However, studies characterizing regeneration composition patterns and their multiple interacting drivers, particularly at regional scales, are scarce. Here, we characterize tree regeneration and associated driving factors for the Northwoods ecoregion, encompassing northern Michigan, Wisconsin, and Minnesota, USA. With data from the USDA Forest Service Forest Inventory and Analysis, we modeled current species richness and density of tree regeneration (stems taller than 30.48 cm (hardwoods) or 15.24 cm (conifers) and <2.54 cm diameter at 1.37 m height) as a function of climate, stand composition, ownership, harvest history, and deer use for the six most common forest groups in the Northwoods. By forest group, we also analyzed recent 20-year changes in tree regeneration richness, total density, and density of key species. Our results suggest tree regeneration abundance and richness are generally associated positively with canopy diversity, recent harvesting, and total annual precipitation; negatively with temperature and canopy basal area; and have few significant associations with deer use and ownership. Although species richness has increased over the past two decades, total abundance and abundance of several key species (such as Acer saccharum Marsh. and Quercus rubra L.) have declined, while subcanopy species (e.g., Amelanchier species Medik.) have proliferated, suggesting potential shifts in stand dynamics including future composition and ecosystem services. Our results suggest that harvest may be a viable tool to promote a wider range of regenerating tree species via increasing understory light availability. Declining regeneration of key ecological species at a regional scale confirms managers’ concerns of insufficient regeneration and highlights the potential for future forest compositional change.