Tree species size class patterns portend compositional shifts and low resilience in managed northern hardwood forests

Abstract

Declining temperate forest biodiversity is a global challenge to future sustainability and resilience. Canopy tree diversity has declined in northern hardwood forests (NHFs) of the Great Lakes region since European colonization, with current tree regeneration patterns suggesting this trend may continue. However, regional (10 6 ha) patterns and drivers are underexplored. 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 (+, seedlings, significant for 10 species) and stand basal area (−, saplings, 3 species) most consistently associated with density; deer use, site quality, and substrate were less predictive. Patterns of relative density by size class among 18 species present on >10% of sites were consistent with species life history strategy and/or establishment/recruitment barriers. For seven species (including Tsuga canadensis, Betula alleghaniensis), relative density was lowest in the seedling layer, suggesting seedling establishment substrate and/or early shade mortality limitations. For two species (Acer saccharum, Quercus rubra), reduced sapling relative density compared with seedling or canopy strata suggests a sapling recruitment bottleneck from deer browsing/shade, whereas six species (e.g., F. grandifolia, O. virginiana, Pinus strobus) had highest relative density for saplings, suggesting sapling to canopy recruitment limitations. Lastly, seedlings were relatively most dense 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. Collectively our analyses suggest sustained low diversity and shifting composition are driven by limited local seed availability, limited seedling establishment