Deer Herbivory Alters Forest Response to Canopy Decline Caused by an Exotic Insect Pest

Abstract

Hemlock woolly adelgid (HWA; Adelges tsugae) infestations have resulted in the continuing decline of eastern hemlock (Tsuga canadensis) throughout much of the eastern United States. While the initial impacts of HWA infestations have been documented, our understanding of forest response to this disturbance remains incomplete. HWA infestation is not occurring in isolation but within a complex ecological context. The role of potentially important interacting factors, such as elevated levels of white-tailed deer herbivory, is poorly understood. Despite the potential for herbivory to alter forest successional trajectories following a canopy disturbance, little is known about herbivory-disturbance interactions, and herbivory is rarely considered in assessing forest response to a co-occurring disturbance. We used repeated censuses of deer exclosures and paired controls (400 paired plots) to quantify the impact of deer herbivory on tree seedling species abundance in 10 eastern hemlock ravines that span a gradient in HWA-induced canopy decline severity. Use of a maximum likelihood estimation framework and information theoretics allowed us to quantify the strength of evidence for alternative models developed to estimate the impacts of herbivory on tree seedling abundance as a function of varying herbivore density and canopy decline severity. The exclusion of deer herbivory had marked impacts on the abundance of the studied seedling species: Acer rubrum, Acer saccharum, Betula lenta, Nyssa sylvatica, Quercus montana, and Tsuga canadensis. For all six species, the relationship between seedling abundance and deer density was either exponential or saturating. Although the functional form of the response varied among seedling species, the inclusion of both deer density and canopy decline severity measures consistently resulted in models with substantially greater support. Canopy decline resulted in higher proportional herbivory impacts and altered the ranking of herbivory impacts by seedling species. Our results suggest that, by changing species' competitive abilities, white-tailed deer herbivory alters the trajectory of forest response to this exotic insect pest and has the potential to shift future overstory composition.

Simultaneous influence of canopy decline and deer herbivory on regeneration in a conifer–broadleaf forest

“…Studies that have examined simultaneous interactions involving a chronic disturbance process (e.g., browsing of regeneration or infection of canopy trees by pests and pathogens) suggest that they exert strong control on successional trajectories and forest community dynamics (Papaik et al 2005;Tremblay et al 2007;Busby et al 2008;Eschtruth and Battles 2008;Relva et al 2009). Given the ubiquitous nature of deer herbivory in forests worldwide (Gill 1992;Cote et al 2004;Tanentzap et al 2009) and the increasing likelihood of prolonged forest decline linked to climate change and exotic pests and pathogens (Lovett et al 2006;van Mantgem et al 2009;Allen et al 2010), an understanding of the interactions between herbivory and forest decline seems imperative for effective forest management and the prediction of future forest dynamics (Wisdom et al 2006).…”

Analysis of genetic diversity in flowering dogwood natural stands using microsatellites: the effects of dogwood anthracnose

“…These die-offs often result in restructuring of forest communities and disruption of ecosystem processes (Hall et al 2002;Becker et al 2008;Elliott and Swank 2008;Eschtruth and Battles 2008 (Havill et al 2006). In the United States alone, the projected economic loss due to invasion of non-native species is over $120 billion annually and about one-fourth of this loss is caused by exotic plant pathogens (Pimentel 1997;USBC 2001;Pimentel et al 2005).…”

Modelling natural disturbances in forest ecosystems: a review

“…By reducing tree height ungulate browsing resulted in trees being exposed for longer time periods to extreme conditions in the blowdown area, thus leading to increased tree mortality. In a recent statistical approach, Eschtruth and Battles (2008) modelled the effect of insect-related decline on ungulate herbivory, finding higher herbivory impacts and changes in affected species as a result of the interaction. However, to date disturbance interactions are rarely accounted for in studies of ungulate herbivory (Wisdom et al, 2006).…”