Effects of atmospheric nitrogen deposition on the herbaceous layer of a central Appalachian hardwood forest

Abstract Gilliam, F. S. , A. W. Hockenberry (Department of Biological Sciences, Marshall University, Huntington, WV, 25755-2510), and M. B. Adams (U.S.D.A. Forest Service, Timber and Watershed Laboratory, Parsons, WV, 26287-0404). Effects of atmospheric nitrogen deposition on the herbaceous layer of a central Appalachian hardwood forest. J. Torrey Bot. Soc. 133(2): 240–254. 2006. —Additions of nitrogen (N) have been shown to alter species diversity of plant communities, with most experimental studies having been carried out in communities dominated by herbaceous species. We examined seasonal and inter-annual patterns of change in the herbaceous layer of two watersheds of a central Appalachian hardwood forest that differed in experimental N treatment. This study was carried out at the Fernow Experimental Forest, West Virginia, using two adjacent watersheds: WS4 (mature, second-growth hardwood stand, untreated reference), and WS3 (∼25-yr old, treated with 35 kg N ha−1 yr−1 as (NH4)2SO4 yr−1). Seven circular 0.04-ha sample plots were established in each watershed to represent its full range of elevation and slope aspect. The herbaceous layer was sampled by identifying and visually estimating cover (%) of all vascular plants ≤ 1 m in height within 10 1-m2 circular sub-plots in each sample plot, for a total of 140 1-m2 sub-plots. Sampling was carried out in mid-July of 1991 and repeated at approximately the same time in 1992. In 1994, these same plots were sampled each month from May to October. Seasonal patterns of herb layer dynamics were assessed for the complete 1994 data set, whereas inter-annual variability was based on plot data from 1991, 1992, and the July sample of 1994. There were no significant differences between watersheds for any sample year for any of the other herb layer characteristics measured, including herb layer cover, species richness, evenness, and diversity. Cover on WS4 decreased significantly from 1991 to 1992, followed by no change to 1994. By contrast, herb layer cover did not vary significantly across years on WS3. Cover of the herbaceous layer of both watersheds increased from early in the growing season to the middle of the growing season, decreasing thereafter, with no significant differences between WS3 and WS4 for any of the monthly cover means in 1994. Similar seasonal patterns found for herb layer cover—and lack of significant differences between watersheds—were also evident for species diversity and richness. By contrast, there was little seasonal change in herb layer species evenness, which was nearly identical between watersheds for all months except October. Seasonal patterns for individual species/species groups were closely similar between watersheds, especially for Viola rotundifolia and Viola spp. Species richness and species diversity were linearly related to herb layer cover for both WS3 and WS4, suggesting that spatial and temporal increases in cover were more related to recruitment of herb layer species than to growth of existing species. Results of this study indicate that there have been negligible responses of the herb layer to 6 yr of N additions to WS3.