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Increasing rates of atmospheric deposition of nitrogen (N) present a novel threat to the biodiversity of terrestrial ecosystems. Many forests are particularly susceptible to excess N given their proximity to sources of anthropogenic N emissions. This study summarizes results of a 25‐yr treatment of an entire central Appalachian hardwood forest watershed via aerial applications of N with a focus on effects of added N on the cover, species richness, and composition of the herbaceous layer. Research was carried out on two watersheds of the Fernow Experimental Forest (FEF), West Virginia. The long‐term reference watershed at FEF (WS4) was used as a reference; WS3 was experimentally treated, receiving three aerial applications of N per year as (NH4)2SO4 totaling 35 kg N ha−1 yr−1, beginning in 1989. Cover of the herbaceous layer (vascular plants ≤1 m in height) was estimated visually in five circular 1‐m2 subplots within each of seven circular 400‐m2 sample plots spanning all aspects and elevations of each watershed. Sampling was carried out in early July of each of the following years: 1991, 1992, 1994, 2003, and 2009—2014, yielding 10 yr of data collected over a 23‐yr period. It was anticipated that the N treatment on WS3 would decrease species richness and alter herb layer composition by enhancing cover of a few nitrophilic species at the expense of numerous N‐efficient species. Following a period of minimal response from 1991 to 1994, cover of the herb layer increased substantially on N‐treated WS3, and remained high thereafter. There was also a coincidental decrease in herb layer diversity during this period, along with a sharp divergence in community composition between WS4 and WS3. Most changes appear to have arisen from unprecedented, N‐mediated increases of Rubus spp., which are normally associated with the high‐light environment of openings, rather than beneath intact forest canopies. These findings support the prediction that N‐mediated changes in the herbaceous layer of impacted forests are driven primarily by increases in nitrophilic species.