ABSTRACT Increases in nitrogen (N) emissions during the 20th century resulted in the atmospheric deposition of N leading to nitrogen saturation in several terrestrial ecosystems. The Clean Air Act of 1970 has proven to be among the more effective pieces of environmental legislation, decreasing the level of N deposition throughout the United States. Recent studies suggest a hysteretic model may be an appropriate way to describe such changes, predicting a time lag of varying duration in response to decreased N. To assess predictions from the hysteretic model, we used research results from the Fernow Experimental Forest (FEF; Tucker County, West Virginia) which has been the site of a 35‐year whole‐watershed N manipulation experiment. A published study based on the first 25 years of N fertilization found substantial change in forest herbaceous communities in response to annual additions of 35 kg N/ha, driven largely by the response of nitrophilic Rubus . The decade of herb layer sampling at FEF since those 25‐year results comprises 5 years of continued additions of N, followed by 5 years of no additions. Results did not support the predictions of a significant time lag in the response of the herbaceous layer to decreased N inputs. After the first 5 years of recovery, cover/biomass decreased by ~30%, species diversity (H′) increased significantly, as did species richness (S). Species evenness (J), which had decreased significantly during the 25‐year period of N additions, did not change with decreased N. Much of this was driven by changes in Rubus cover, which decreased by > 40% in the 5 years of no added N. These data demonstrate the sensitive nature of forest herb communities to changes in inputs of N, and that the success of the Clean Air Act can extend to the recovery of forest biodiversity from reductions in N inputs.
Gilliam et al. (Wed,) studied this question.