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Dryad

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

Data files

May 25, 2023 version files 72.76 KB

Abstract

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.