Complex drivers of sugar maple (Acer saccharum) regeneration reveal challenges to long-term sustainability of managed northern hardwood forests

Abstract

Single-tree selection silviculture management of northern hardwood forests relies on natural tree regeneration for long-term sustainability, yet current trends in tree regeneration and recruitment elicit concern. Low densities of economically valuable sugar maple (Acer saccharum Marsh.) in understories are often common, likely driven by many factors, including deer browsing, management-dictated stand structure, and site fertility/moisture regimes. However, landscape sugar maple regeneration patterns and relationships with underlying factors are largely unknown. We quantified associations of spatially varying factors with sugar maple regeneration using detailed vegetation and white-tailed deer winter fecal pellet surveys from 141 northern hardwood stands in Michigan, managed for decades with single-tree selection silviculture. We developed models of plot-level sugar maple regeneration counts for three key size classes as a function of plot- and stand-level predictors, including deer use, forest structure, and site quality. Among our 141 stands, sugar maple seedlings (<50 cm tall) were consistently abundant, averaging 69,000 stems ha−1 and present in 76% of plots per stand, on average (25 plots per stand, each 2 m2). In contrast, small (50–137 cm tall) and large (>137 cm tall and < 5 cm DBH) sugar maple sapling densities were much lower, averaging 2,300 and 1,100 stems ha−1, respectively, and occurring, on average, in 31% and 32% of plots per stand (25 plots per stand, each 12.6 m2). Under a wide range of potential sugar maple stocking criteria, most stands are understocked in saplings; e.g., only 29% of stands had > 2,500 small sugar maple sapling stems ha−1 and 35% of stands had > 1,000 large sapling stems ha−1. Based on our models, sapling densities negatively associated with deer use and were more abundant on medium than high quality sites. Across all size classes, negative associations with subcanopy trees and/or shrub densities suggest light limitation, whereas positive associations with sugar maple canopy trees > 25 cm DBH suggest persistent seed limitations. Overall, our study supports need for alternative forest and/or deer management strategies over much of the range of northern hardwood forests in Michigan to promote higher densities of large sugar maple regeneration for canopy recruitment. Medium quality sites with abundant large sugar maple canopy trees and low deer browsing pressure (for example, the deep snow region in the north western Upper Peninsula) are the exception; high sugar maple sapling densities suggest forests in this region are thriving under single-tree selection management.