Quantifying the role of interacting forest disturbances: The compounded effects of an exotic insect pest and deer herbivory on forest regeneration and exotic plant invasion

Eastern hemlock forests throughout much of the northeastern Unites States are currently threatened by a suite of novel stressors including the infestation of an exotic insect pest, the hemlock woolly adelgid (HWA); elevated white-tailed deer populations; and the invasion of exotic plants. Although changes in forest ecosystem function, structure, and composition resulting from individual disturbance events are often well documented, field studies designed to quantify the impacts of interacting disturbances are rare. I used observational and manipulative field experiments, greenhouse studies, GIS models, and statistical models to assess the relative roles of these stressors and the impact of their interactions in influencing forest community dynamics in ten hemlock forests in the Delaware Water Gap National Recreation Area that span a gradient in HWA induced canopy decline severity.; These studies quantified the dramatic increase in understory light availability, vascular plant cover, and species richness caused by HWA infestation and provided insight into the spatial and temporal patterns of HWA spread and hemlock decline. Further, changes in light availability were strongly correlated with direct assessments of hemlock decline and HWA infestation severity.; Results from repeated censuses of deer exclosures found that herbivory caused marked declines in the abundance of common tree seedling species and accelerated the rate of exotic plant invasion. In addition, through use of a maximum likelihood estimation framework and information theoretics, study results provided compelling evidence of an important nonlinear interaction between deer herbivory and canopy disturbance. HWA canopy disturbance magnified the impact of herbivory on tree seedling and exotic plant abundance at a given deer density. In addition, an assessment of the relative importance of canopy disturbance, herbivory, species diversity, and propagule pressure in controlling exotic plant invasion demonstrated the dominant role of the nonlinear interaction between canopy disturbance and propagule pressure.; The consequences of the documented interactions, and their nonlinear functional forms, have profound implications for forest community dynamics. These results emphasize that, in an age where forests face a suite of novel stressors, it is critical that we improve our understanding of the relative roles of these stressors and the consequences of their interaction in determining forest response.