Stasis in forest regeneration following deer exclusion and understory gap creation: A 10‐year experiment

Abstract

Chronically elevated ungulate browse pressure in temperate forests worldwide often generates ecological legacies characterized by low plant diversity and contributes to the formation of dense, nearly monodominant, and highly recalcitrant layers of understory vegetation. Once established, these recalcitrant layers combined with continued browsing may jointly constrain tree establishment and diversity so completely that understory recovery may be unattainable without mitigating browse pressure, the recalcitrant layer, or both. Here, we investigate the independent and synergistic effects of both white‐tailed deer (Odocoileus virginianus) browsing and hay‐scented fern (Dennstaedtia punctilobula) competition on tree regeneration in a 10‐year experiment. Specifically, we examine how tree seedling establishment, growth, and composition are filtered by fern cover versus fern removal (gaps), browser presence versus absence (exclosures), and their combined effects during 10 years at three hardwood forest sites in Pennsylvania, USA. Fern gaps enhanced establishment for multiple tree species, increasing seedling density and diversity, particularly in the first 3 years post‐treatment, and enhancing richness (≤1 species) over the course of the experiment. Excluding deer for a decade increased the height growth of other regeneration and altered species composition, but had no effect on diversity, richness, and density. Notably, we observed higher Prunus serotina seedling densities outside exclosures, possibly due to greater secondary dispersal. We argue that browsing legacies in second growth forests established at the turn of the last century created two conditions inimical to diverse forest regeneration: an overstory dominated by two species, P. serotina and Acer rubrum (86% of basal area), and a dense recalcitrant understory layer dominated by a native fern. The first condition limits propagule supply, the second strongly filters seedling establishment, and both create impoverished forest understories composed of few individuals and species. In undisturbed forest understories, the inertia toward impoverishment was sustained across the decade even where browsing was eliminated. Consequently, stand replacing disturbances (whether natural or anthropogenic) that disrupt the understory layer and reinitiate succession may be necessary to propel forests out of their current stasis and down a pathway leading to greater diversity.

Chronic browsing by an introduced mammalian herbivore in a tropical island alters species composition and functional traits of forest understory plant communities

“…Our data demonstrate a significant decrease in species richness and a slight decrease in plant abundance in understory communities along a gradient of chital habitat use (Figure 3). These results align with previously reported effects of introduced chital on understory plant communities in the Andaman Islands (Ali, 2004) and with studies from other ecosystems with introduced herbivores (Camargo- Sanabria et al, 2014;Martin et al, 2011;Reed et al, 2022;Royo & Carson, 2022;Simberloff et al, 2013;Stephan et al, 2017). Although it is possible that both vegetation and chital habitat use are simultaneously responding to underlying environmental factors, similarity in rainfall, topography and bedrock among our sites leads us to expect low environmentally driven covariation.…”

“…Persistent effects of ungulate browsers on communities within browse height can indirectly affect vegetation in adult size classes by affecting recruitment probabilities and composition of recruits (Bellingham & Allen 2003, Coomes et al, 2003; Reed et al, 2022; Royo & Carson, 2022). Decreased abundances and altered composition in smaller stems over long timescales can reduce overall recruitment and alter adult tree composition (Ramirez, 2021).…”

Legacy of thinning on woody species composition and structure in southern Appalachian Mountain hardwood forests: restoration implications

“…Modeling characteristics of the woody understory in deciduous broadleaved forests have proven difficult given myriad factors that influence seedling establishment and recruitment, including seed production and dispersal coupled with seedbed conditions (Willis et al 2016), seed predation (Royo & Carson 2008), herbivory (Boerner & Brinkman 1996), climate (Fisichelli et al 2014), topographic and edaphic conditions (Frey et al 2007), and competition (Knight et al 2008). Although herbivory has long-lasting effects on composition and structure of developing vegetation (Royo & Carson 2022), Odocoileus virginianus density across our study area averages <10 individuals/km 2 (North Carolina Wildlife Resources Commission 2020), suggesting that at least currently, browse pressure has is negligible. Model performance varied from a low of r 2 = 0.05 to r 2 = 0.40, suggesting for many of the understory metrics we examined, the limited number of legacy effects and environmental variables we examined were not the primary drivers of the woody understory structure, diversity, and function.…”

A 17-Year Study of the Response of Populations to Different Patterns in Antlerless Proportion of Imposed Culls: Antlerless Culling Reduces Overabundant Deer Population

“…In addition, long-term overbrowsing by herbivores has led to almost complete population increase failure of tree species and inhibited the natural regeneration of trees [ 2 , 5 , 6 ]. In these areas, even if the herbivore population declines, it is possible that ground vegetation will not increase for a long time [ 7 , 8 , 9 ]. Therefore, immediate reduction in the herbivore population to an appropriate density is required to conserve natural vegetation, and herbivore population management has become an important issue in reducing damage in large areas of Europe, North America, and Asia [ 2 , 10 , 11 , 12 ].…”