Deer impacts on forest ecosystems: a North American perspective

White-tailed deer have increased in abundance and expanded their geographic range in North America over the past century, and now exist at higher dens ities than they have in the past several hundred years. This is having nume rous impacts on the forest ecosystems they inhabit. Regional recruitment fa ilure of eastern hemlock (Tsuga canadensis) and northern white cedar (Thuja occidentalis) trees can be explained in part by deer browsing. Deer also h ave significant negative effects on understorey plants, including wild lily -of-the-valley (Maianthemum canadense) and white-flowered trillium (Trilliu m grandiflorum). Long-term studies of primary, old-growth forest stands rev eal a 48-81 per cent herb and shrub species loss accompanying increases in deer density. Graminoids, ferns and club mosses were more likely to persist in these stands than plants in all other taxonomic groups. Deer also exhib it indirect effects on forest communities by reducing host plant densities or altering forest structure. Because of their numerous direct and indirect effects on other species, and because of the magnitude of these effects, w hite-tailed deer act as a keystone herbivore. Natural regulation and maximu m sustained yield management approaches have failed to alleviate deer impac ts on forest ecosystems, but an ecosystem-based management approach offers promise.