Long‐term biological legacies of herbivore density in a landscape‐scale experiment: forest understoreys reflect past deer density treatments for at least 20 years

Abstract

Summary1. Ungulate browsers, when at high densities, are major drivers of vegetation change in forests world-wide. Their effects operate via a variety of generalizable mechanisms related to plant palatability and relative growth rate with respect to browsing pressure. 2. Though such impacts are obviously long-lasting when they determine composition of tree regeneration, we document in a unique long-term (30 year) experiment that biological legacies of initial deer density persist in the understorey herbaceous vegetation at least 20 years after deer densities were equalized. 3. We sampled understorey vegetation in former clear-cut areas where density of white-tailed deer (Odocoileus virginianus) was manipulated (3.9-31.2 deer km À2 ) for 10 years (1979)(1980)(1981)(1982)(1983)(1984)(1985)(1986)(1987)(1988)(1989)(1990), and stands experienced ambient deer density (ca. 10-12 deer km À2 ) for the next 20 years to determine whether initial deer density treatments still influenced understorey vegetation in 30-year-old, closed-canopy forests. 4. Stands initially (1979-1990) exposed to higher deer densities had ca. five times higher fern cover and three times the seedling and forb cover in 2010, as well as significantly lower angiosperm species density, compared to stands initially exposed to lower deer densities. 5. These results appear driven by deer avoidance of ferns, allowing them to expand at high deer density and sequester sites for decades. 6. Synthesis. Our long-term, experimental results show unequivocally that elevated deer densities cause significant, profound legacy effects on understorey vegetation persisting at least 20 years. Of relevance regionally and globally where high deer densities have created depauperate understoreys, we expect that deer density reduction alone does not guarantee understorey recovery; stands may need to be managed by removing recalcitrant understorey layers (e.g. ferns).

Wild boar rooting intensity determines shifts in understorey composition and functional traits

“…Based on our results and on previous scientific literature (Nuttle et al 2014), forest managers aiming at restoring a diverse herbaceous community should be aware that the thickets of Ruscus aculeatus probably attained such strong dominance due to intense rooting disturbance and that this dominance may last for decades. Managers should thus consider the idea of implementing Ruscus aculeatus eradication measures in combination with effective measures of wild boar population control (enhanced trapping or exclosures).…”

“…In that system deer overbrowsing resulted in the formation of a long-lasting (recalcitrant sensu Royo and Carson 2006) herb-layer dominated by unpalatable ferns, that lasted more than 20 years even after a substantial decrease in deer density (Nuttle et al 2014). The existence of this legacy effect points to the limits of a synchronic approach for the study of wild boar impact on understorey.…”

“…Wild boar activities impact crops, pastures and forests (BarriosGarcia and Ballari 2012) and cause direct economic damage by reducing agricultural incomes and transmitting diseases to humans and livestock (Barrios-Garcia and Ballari 2012). Although wild boar is a natural component of forest ecosystems, when the population is over-abundant it may have a profound impact on several ecosystem components since the effects of large herbivores are positively related to their density, often in a nonlinear way (Nuttle et al 2014). Such impact acts through several mechanisms ranging from the modification of habitat structure to the alteration of disturbance regime and food webs (Valenzuela et al 2014).…”

“…For instance, the selective predation of large-seeded species may alter the composition of woody understorey, since in temperate forest systems large-seeded species are mainly trees and shrubs (Siemann et al 2009). In the long-term, this selective predation may also favour certain tree species compared to others, and may affect the successional trajectory of the stand (Nuttle et al 2014). Mechanical damage may instead favour clonal species, since clonality may allow a rapid recolonization of rooted patches (Catorci et al 2012).…”

Scarification and gap size have interacting effects on northern temperate seedling establishment

“…Indeed, browsing from elevated deer populations has been linked to altered regeneration patterns in several recent studies (Long et al, 2007;Witt and Webster, 2010;Kain et al, 2011;Randall and Walters, 2011;Matonis et al, 2011;Nuttle et al, 2014). Not all species, however, may be equally affected by browsing, as deer have been shown to browse certain species preferentially, while avoiding others (Horsley et al, 2003;Rooney and Waller, 2003;Long et al, 2007;Kain et al, 2011;Randall and Walters, 2011;Schumacher and Carson, 2013;Nuttle et al, 2014).…”