DOI: 10.5061/dryad.612jm647z
title: Data for: Tree species size class patterns portend compositional shifts and low resilience in managed northern hardwood forests

description: 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 &lt;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
 &gt; 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.
description: 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 m<sup>2</sup> (50 m<sup>2</sup>
 total per stand). Within a 2 m radius<sup> </sup>circular plot
 (314 m<sup>2</sup> total per stand) we tallied stems 50 to 137
 cm tall and tallied and measured DBH for stems &gt; 137 cm tall and
 &lt; 5 cm DBH. Within a 6 m radius circular plot (2,827
 m<sup>2</sup> total per stand), we tallied and measured DBH
 for all stems &gt; 5 cm DBH. From these data we established the
 following size classes: seedlings, 0 – 137 cm tall; saplings, &gt; 137
 cm tall and &lt; 10 cm DBH; canopy trees, ≥ 10 cm DBH.. For all size
 classes we calculated density as stem counts ha<sup>-1</sup>.
 In addition, for canopy trees, we calculated density as the
 cross-sectional area of boles at breast height summed across stems (basal
 area, BA, m<sup>2</sup> ha<sup>-1</sup>). 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. <em>A Guide to Forest Communities and
 Habitat Types of Michigan</em>. 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.” <em>Forest Ecology and Management</em> 479 (June
 2020): 118541. https://doi.org/10.1016/j.foreco.2020.118541.
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