We evaluated the impact of long-term over-browsing by white-tailed deer on the diversity and density of trees in a forest in the Allegheny High Plateau region of central Pennsylvania. We compared tree diversity and density inside a 60 year-old deer exclosure to an adjacent reference site. Browsing caused a 55–100% decline in density of four tree species (Prunus serotina, Acer saccharum, Betula lenta, Cornus alternifolia) and created a forest dominated (> 70% of all stems) by Acer pensylvanicum, an understory tree that is known to be highly browse-tolerant. The total density of trees that are capable of ascending into the canopy (i.e., non subcanopy tree species) declined by 85%. Browsing caused a significant decline in both mean species richness and Shannon diversity and created communities that contrasted significantly in tree species composition (ANOSIM, R  =  0.8105, P < 0.0001). Our results suggest that long-term over-browsing can create low density, depauperate communities where dominance is concentrated in only a few browse-resistant species (Acer pensylvanicum and Fagus grandifolia; 82% of all individuals vs. 37% inside the exclosure). We suggest that this may lead to novel forest dynamics in the event of a large canopy disturbance because these two species were never co-dominant in this region and the beech saplings (typically root sprouts) will likely succumb to future bouts of beech bark disease. Our results combined with those of many other studies call for the long-term reduction in the size of the deer herd throughout this region.