Complex Effects of Ungulate Browsers on Tree Recruitment and Herbaceous Layers in New England Temperate Forests

Browsing by ungulates is a leading biotic disturbance in northern forest ecosystems and an important determinant of habitat, biodiversity, and ecosystem services. A large body of work has revealed that white-tailed deer (Odocoileus virginianus) at high densities alter forest understories in strong and predictable ways; however, less is known about how lower densities of deer and the combined effects of multiple herbivores influence forest understory vegetation, particularly in stands following canopy disturbance. Using fenced exclosures, remote cameras and other field observations, I explored the foraging response and browsing effects of low densities of deer and moose (Alces alces) on tree recruitment and herbaceous layers (low shrubs, herbs, and small trees) in stands disturbed by logging and simulated Hemlock Woolly Adelgid (HWA) attack over 4-7 years in Massachusetts, USA. I also examined the effects of 15 years of deer exclusion on an intact hardwood forest in southwestern Connecticut exposed to decades of high densities of deer. In Massachusetts, large variations in tree densities developed over time in different stand disturbance types (simulated HWA attack, logging), altering the foraging response of herbivores and mitigating browsing effects. Still, moose + deer browsing delayed tree recruitment by about three years in logged stands, whereas deer alone had relatively minor effects. Delayed tree recruitment by browsers corresponded with reduced abundance of forest indicator herbs and shrubs and greater abundance of open/disturbance indicator plant species in plots browsed by deer + moose. Richness of native herbs and low woody plants also increased with the addition of browsers. Among major tree taxa, pin cherry (Prunus pensylvanica) and oak (Quercus spp.) recruitment declined with browsing. In the Connecticut forest, sedge and exotic species abundance and richness generally declined with deer exclusion, whereas forb abundance increased. The direction in which native species richness was altered by deer exclusion depended on the plant functional group (i.e., shrub richness increased, but herb richness declined). My results revealed complex effects of herbivory over time on forest understories, highlighting the importance of examining ungulate-forest interactions across a range of ungulate densities and forest conditions and utilizing ≥3 browser treatments whenever feasible