Deposition of atmospheric nitrogen can have significant effects on a variety of ecosystems and through a variety of pathways. Changes in plant diversity have been documented in relatively simple ecosystems, such as grasslands or single-species tree plantations. However, documenting effects in mixed species natural forests is more difficult and requires diligence and patience. After 25 years of nitrogen fertilization in a mixed species deciduous forest at the Fernow Experimental Forest in West Virginia, Forest Service scientists documented an increase in the herbaceous layer cover in the fertilized stand, an almost twenty-fold increase in cover of Rubus spp. (blackberry) within the herbaceous layer, along with a decrease in the species diversity of the herbaceous layer species. The response of Rubus to nitrogen fertilization was dependent on canopy openness, specifically the amount of available light. Finally, the dominance of Rubus spp. in the herbaceous layer likely contributed to the observed decline in herb layer plant species diversity. Evaluation of tree species diversity over time, along with biogeochemical monitoring, are continuing in this dedicated evaluation of effects of elevated nitrogen deposition on forests.