Tree Regeneration Structure Following Beech Bark Disease-Motivated Harvests: Factors Associated with Patterns and Management Implications

Abstract

In the northern Appalachian region of North America, mortality of mature American beech (Fagus grandifolia Ehrh.) via the introduced beech bark disease (BBD) can result in dense thickets of beech saplings that inhibit the regeneration of other species. It is unknown if similar structures characterize more recently infested managed forests in the Great Lakes region. If these dense beech sapling layers do exist, management would be aided by knowing which site/regional factors they are associated with and by identifying particular sapling structures that may threaten the sustainability of these forests under current management paradigms. To examine these patterns, we used a natural experiment with sample plots in 69 unevenly aged, selection silviculture-managed, maple (Acer spp.)-dominated northern hardwood stands. Our stands were dispersed across northern Michigan, USA and had undergone BBD-motivated partial harvests favoring beech removal (mean = 5.5 years before measurement). In each stand, we quantified tree regeneration structure in relation to winter deer use (fecal pellet count density), site quality (habitat type), geographic region (Eastern Upper Peninsula and Northern Lower Peninsula), and multiple measures of overstory stand density. We also examined the density effects of taller regeneration strata on subordinate strata. Across sites, the small sapling recruit class (i.e., >137 cm tall and <5 cm diameter at 137 cm tall) was dominated by beech and was often dense (44% of subplots > 2000 stems ha−1 and 16% of subplots > 5000 ha−1) but never exceeded the > 10,000 stems ha−1 reported in the northern Appalachian region. Beech sapling density was higher in the Northern Lower Peninsula, on lower quality sites, at lower postharvest overstory densities, and on sites with higher densities of preharvest overstory beech. In contrast to the beech-dominated small sapling recruit class, seedlings (i.e., <25 cm tall) were generally more species diverse than sapling strata and were dominated by maple species. Although generally dense, seedling density was negatively related to small sapling recruit density, suggesting that saplings may suppress the seedling stratum. The general pattern for the small sapling recruit layer of browsing-insensitive beech (and ironwood, Ostrya virginiana Mill. K. Koch) dominance and low representation of browsing-sensitive species (e.g., Acer spp.) circumstantially supports the notion that regeneration structure is heavily influenced by deer. However, current deer use was generally low in our stands, and relationships with tree regeneration structure were weak. Instead, regeneration structure is likely shaped by a combination of factors operating at long time scales (i.e., legacies of deer browsing pressure, selection silviculture (given beech and ironwood are shade tolerant), overstory composition, and site quality) and by those effects that are more proximal, such as postharvest overstory density. Minimum stocking criteria for species considered desirable for management (e.g., sugar maple and Acer saccharum Marshall) suggest many stands are inadequately stocked in the sapling recruit classes. Although future regeneration dynamics are unclear, current patterns suggest that many stands with high beech/ironwood small sapling recruit densities may require management intervention to overcome insufficient recruitment of species targeted for management.

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

“…Rather, our sapling compositions, from a much broader region, are characterized by roughly even representation of sugar maple, ironwood, and American beech (Figure 1). This pattern is similar to that found by Angers et al (2005), near Ottawa, Canada, and Elenitsky et al (2020), in northeastern Michigan, both of which are in regions of relatively lower lake‐effect snowfall and therefore likely higher deer browsing pressure (Henry et al, 2021; Shi et al, 2006). Our results highlight the importance of considering patterns over broad geographic scales to inform landscape‐level management recommendations.…”

“…Lack of significant associations could be due to the low average richness values in our stands, as differences among stands are likely to be small and may require greater statistical power than we were able to achieve. It also may be that, while species compositional shifts among site qualities are well documented in our region (e.g., more sugar maple on higher quality sites, more red maple on lower quality sites; Burger & Kotar, 2003;Elenitsky et al, 2020), changes to diversity are negligible to modest on the range of site qualities we examined. However, regeneration diversity was higher in stands with greater canopy diversity (Table 3), offering clear support for prediction 1b.…”

“…Right‐skewed species include red maple, which has been increasing in relative density in recent decades (Fei & Steiner, 2007). Many of the species, which compete with sugar maple for sapling growing space, are negative parabolic, including American beech, which creates well‐documented dense thickets (Collin et al, 2017; Elenitsky et al, 2020; Gravel et al, 2011), and ironwood, which is a subcanopy species. Relative density analyses interpreted in the context of stand dynamics and species silvics prove to be a useful method of quantifying potential species and stand trajectories, including identifying undesirable stand trajectories for management such as high proportions of beech and ironwood.…”

“…Concerningly, NHF canopy diversity has declined since European colonization (Schulte et al, 2007) and is lower in managed stands than in unmanaged stands (Crow et al, 2002; Neuendorff et al, 2007; Powers & Nagel, 2009). Current regeneration (i.e., seedling/sapling class) in some areas has low diversity, with low density of sugar maple and some other species in the overstory, suggesting future compositional shifts in the canopy (Elenitsky et al, 2020; Hupperts et al, 2020; Leak, 2006; Matonis et al, 2011; Neuendorff et al, 2007; Powers & Nagel, 2009). These changes could inhibit long‐term forest capacity to provide multiple ecosystem goods and services, including timber, carbon sequestration, and wildlife habitat (Walters et al, 2022).…”

“…Interacting with site quality is a dramatic gradient of annual snowfall (e.g., differences of a few meters for sites <100 km apart) driven by the Great Lakes (i.e., lake‐effect), which further influences forest and tree species distributions, particularly sugar maple given its association with high snowfall areas (Henne et al, 2007). Over the narrower range of higher site qualities supporting NHFs, tree canopy and regeneration composition vary (Burger & Kotar, 2003; Elenitsky et al, 2020). Compositional data supporting habitat classification development for the region (i.e., frequency of occurrence of saplings, Burger & Kotar, 2003) suggest that lower site quality stands supporting NHFs tend to have more species; however, the influence of site on species composition may be overridden by other factors, such as deer browsing or management (Bannon et al, 2015; Matonis et al, 2011).…”