Declining biodiversity is a global challenge to sustainability and resilience of ecosystems facing multiple novel stressors, including climate change and invasive pests-pathogens. Northern hardwood forests (NHF) of the Great Lakes region have experienced declines in canopy tree diversity since European colonization, with current tree regeneration patterns in some areas suggesting perpetuation of this trend. However, regional (10⁶ ha) patterns and possible causes are underexplored. To address this information gap, we used data from 141 managed NHF stands, Michigan, USA to examine diversity indices across tree size classes (seedling, sapling, and canopy); stand-level drivers of regeneration (seedling and sapling) diversity and individual species density; and patterns among species in relative abundances across size classes. Diversity was similar across size classes (mean asymptotic species richness of 6 – 7 species and <3 effective common species); however, some species contributing to sapling diversity are unlikely to contribute to future canopy diversity (Fraxinus americana and Fagus grandifolia, insect/pathogen limited; Ostrya virginiana, small maximum size). For the 11 most common species, conspecific canopy density (+, significant for 11 seedlings, 11 species) and stand basal area (-, saplings, 3 species) were the most consistent drivers of density, with less consistent effects for deer use, site quality, and substrate. Patterns of relative density by size class among the 18 species present on > 10 % of sites were consistent with theories of establishment or recruitment limitations due to deer browsing, deep shade, or limited suitable substrate. For seven species (including e.g., Tsuga canadensis, Betula alleghaniensis, Populus grandidentata, Tilia americana), relative abundance was lowest in the seedling layer and increased in larger size classes, suggesting seedling establishment substrate and/or early shade mortality limitations. For two species (Acer saccharum, Quercus rubra.), patterns of reduced sapling relative abundance compared to seedling or canopy strata suggests a sapling recruitment bottleneck from deer browsing/shade, whereas six species (e.g., Fagus grandifolia, Ostrya virginiana, Pinus strobus) had highest relative density for saplings suggesting sapling to canopy recruitment limitations. Lastly, seedlings were relatively most abundant for Acer rubrum and Prunus spp., indicating disproportionally high seedling establishment, if not sapling or canopy recruitment, and for F. Americana, consistent with recent pest-related canopy mortality. Our results suggest sustained low diversity and shifting composition are being driven by limited local seed availability, deep shading, limited seedling establishment substrate, and abundant deer. Management aimed at overcoming these specific limitations may be necessary to promote future NHF resilience.

Our data are from 141 managed northern hardwood forest stands distributed throughout northern Michigan. Data were collected from a systematic grid of 25 survey points within a 12-ha square within each stand and collected species-level data in the following plots at each survey point. We counted all tree seedlings shorter than 50 cm and ocularly estimated percent cover of shrubs and groundcover composition (e.g., hardwood litter, bare mineral soil, downed woody material) within 2 quadrats, each 1 m² (50 m² total per stand). Within a 2 m radius circular plot (314 m² total per stand) we tallied stems 50 to 137 cm tall and tallied and measured DBH for stems > 137 cm tall and < 5 cm DBH. Within a 6 m radius circular plot (2,827 m² total per stand), we tallied and measured DBH for all stems > 5 cm DBH. From these data we established the following size classes: seedlings, 0 – 137 cm tall; saplings, > 137 cm tall and < 10 cm DBH; canopy trees, ≥ 10 cm DBH.. For all size classes we calculated density as stem counts ha^-1. In addition, for canopy trees, we calculated density as the cross-sectional area of boles at breast height summed across stems (basal area, BA, m² ha^-1). At the stand scale we assigned a dominant habitat type, an index of site quality, based on assessment of herbaceous plant assemblages and key indicator species (Burger and Kotar 2003). We assessed winter deer use at the stand scale with fecal pellet transect surveys in spring of 2017 at a subset of 50 sites, surveying 628 m of transect (6 m wide) subdivided into 103 segments at each site. We modeled estimated winter deer use (percent transect segments occupied by deer pellets) for all stands in 2017 with a spatial model in a Bayesian framework; the model also incorporated well-established climate and landscape factors known to influence local deer populations, plus deer pellet transect surveys conducted in 2019 at 139 sites following experimental harvest (Henry et al., 2021; Appendix A).

Burger, Timothy L., and John Kotar. 2003. A Guide to Forest Communities and Habitat Types of Michigan. WI, USA: University of Wisconsin-Madison.

Henry, Catherine R., Michael B. Walters, Andrew O. Finley, Gary J. Roloff, and Evan J. Farinosi. 2021. “Complex Drivers of Sugar Maple (Acer Saccharum) Regeneration Reveal Challenges to Long-Term Sustainability of Managed Northern Hardwood Forests.” Forest Ecology and Management 479 (June 2020): 118541. https://doi.org/10.1016/j.foreco.2020.118541.

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