Tree growth, foliar chemistry, and nitrogen cycling across a nitrogen deposition gradient in southern Appalachian deciduous forests

The declining health of high-elevation red spruce (Picea rubens Sarg.) and Fraser fir (Abies fraseri (Pursh) Poir.) in the southern Appalachian region has long been linked to nitrogen (N) deposition. Recently, N deposition has also been proposed as a source of negative health impacts in lower elevation deciduous forests. In 1998 we established 46 plots on six sites in North Carolina and Virginia dominated by American beech (Fagus grandifolia Ehrh.), sugar maple (Acer saccharum Marsh.), and yellow birch (Betula alleghaniensis Britt). We evaluated several response variables across an N deposition gradient, including annual basal area growth; foliage percent N, Al, P, K, Mg, and Ca; and forest floor percent N, Mg, and C, pH, and potential net nitrification and N mineralization rates. We found a significant linear relationship between N deposition and basal area growth in sugar maple, but not in American beech or yellow birch. In addition, we found a significant relationship between N deposition and foliar chemistry (foliar %N in all species, foliar Mg/N and %K in sugar maple, and %P in sugar maple and yellow birch). Foliar %N of the three studied species was high relative to values reported in other studies in the United States and Canada. Several forest floor response variables (%N, C/N, pH, Mg/N, and potential net nitrification and N mineralization rates and nitrification/mineralization fractions) were also correlated with N deposition. The correlations between the above response variables and N deposition are consistent with the influence of chronic N deposition on forested ecosystems measured in other regions and suggest that chronic N deposition may be influencing forest structure and chemistry within the southern region.