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.
Bibliographical noteFunding Information:
Research was funded through United States Department of Agriculture (USDA) Forest Service, Fernow Experimental Forest, Timber and Watershed Laboratory, Parsons, W.V., under USDA Forest Service Cooperative Grants 23-165, 23-590, and 23-842. Additional funding for this research was provided by USDA National Research Initiative Competitive Grants (Grant NRICGP #2006-35101-17097) and by the Long Term Research in Environmental Biology (LTREB) program at the National Science Foundation (Grant No. DEB-0417678 and DEB-1019522). We acknowledge the excellent field assistance of Staci Joy, Brad Yurish, Mark Fisher, Dave Willson, and the late Bill Grafton, to whose memory this study is dedicated.
© 2016 Gilliam et al.
- Eastern deciduous forest
- Forest ecosystems
- Forest strata
- Herbaceous layer
- Nitrogen saturation