Overabundant deer and invasive plants drive widespread regeneration debt in eastern United States national parks

Abstract

Advanced regeneration, in the form of tree seedlings and saplings, is critical for ensuring the long-term viability and resilience of forest ecosystems in the eastern United States. Lack of regeneration and/or compositional mismatch between regeneration and canopy layers, called regeneration debt, can lead to shifts in forest composition, structure, and, in extreme cases, forest loss. In this study, we examined status and trends in regeneration across 39 national parks from Virginia to Maine, spanning 12 years to apply the regeneration debt concept. We further refined the concept by adding new metrics and classifying results into easily interpreted categories adapted from the literature: imminent failure, probable failure, insecure, and secure. We then used model selection to determine the potential drivers most influencing patterns of regeneration debt. Status and trends indicated widespread regeneration debt in eastern national parks, with 27 of 39 parks classified as imminent or probable failure.Deer browse impact was consistently the strongest predictor of regeneration abundance. The most pervasive component of regeneration debt observed across parks was a sapling bottleneck, characterized by critically low sapling density of native canopy species and significant declines in native canopy sapling basal area or density for most parks. Regeneration mismatches also threaten forest resilience in many parks, where native canopy seedlings and saplings were outnumbered by native subcanopy species, particularly species that are less palatable deer browse. The devastating impact of emerald ash borer eliminating ash as a native canopy tree also drove regeneration mismatches in many parks that contain abundant ash regeneration, demonstrating the vulnerability of forests that lack diverse understories to invasive pests and pathogens. These findings underscore the critical importance of an integrated forest management approach that promotes an abundant and diverse regeneration layer. In most cases, this can only be achieved through long-term (i.e., multidecadal) management of white-tailed deer and

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

“…This is particularly important for developing meaningful assessments of regeneration success following harvest. Several factors, including shrub competition (Kern et al, 2013; Walters et al, 2016), deer browsing (Bradshaw & Waller, 2016; Côté et al, 2004; Henry et al, 2021; Miller et al, 2023; this paper), and light resource limitation (i.e., canopy BA, Henry et al, 2021; Walters et al, 2022; this paper), may assert strong and variable bottlenecks on seedling to sapling transitions such that tree seedlings are poor predictors of regeneration success (Harris et al, 2022; Walters, Farinosi, & Willis, 2020; Walters, Roloff, et al, 2020). Managers should focus on saplings (taller than 1.5 m and <10 cm dbh) for assessment of regeneration success following harvest or other management activity.…”

Long-term interactive impacts of the invasive shrubLonicera maackiiand white-tailed deer (Odocoileus virginianus) on a deciduous forest understory

“…In many of these same areas, invasive plants are impacting native plants (Bialic-Murphy et al 2020; Boyce 2009; Waller et al, 2016). Both deer and invasive plants are implicated in regeneration failure (Miller and McGill 2019; Miller et al 2023)—insufficient densities of juvenile trees to replace canopy trees.…”

Individual and combined effects of non‐native earthworms and native white‐tailed deer on understorey plant survival, growth and reproduction

“…By selectively removing more competitive plants and reducing shading, large herbivores may ameliorate the effects of warming and nutrient deposition, although these effects have mostly been investigated in grasslands (Aguilera & Gibbs, 2023; Borer et al., 2014; Post et al., 2022, 2023). However, benefits of low browse pressure cannot be extrapolated to higher densities of domestic or wild large herbivores (Eldridge et al., 2016; Miller et al., 2023; Scasta et al., 2020; van Klink et al., 2014; Wardle & Bardgett, 2004). In the past, ungulate communities in northeastern forests included native woodland bison, woodland caribou, moose and elk, but we know little about their densities or impacts on plant communities and all of these species went regionally extinct due to human hunting pressure.…”