Nitrogen saturation in a high elevation New England spruce-fir stand.

High rates of nitrogen deposition were first postulated as a cause of N saturation and spruce mortality during the 1980s. To test this hypothesis, N addition plots were established in 1988, in a high elevation spruce-fir forest in southeastern Vermont, an area of relatively low N deposition (5.4 kg N ha-1 year-1). Four replicated treatment plots received NH4Cl-N or NaNO3-N at rates ranging from 15.7 to 31.4 kg N ha-1 year-1. The N was applied in three equal doses each year between June and August from 1988 to 1994. Two control plots were established. Spruce, fir, and birch trees on the N addition plots receiving <20 kg N ha-1 year-1 had the highest rate of basal area growth between 1988 and 1990 and then had the highest rate of decline between 1991 and 1994. Spruce, fir, and birch trees on the N addition plots receiving > 25 kg N ha-1 year-1 showed moderate rated of decline from 1988 to 1994. Numerous birch and maple sprouts were noted on the sites with the highest rates of decline, but no spruce or fir seedlings were observed. In July 1994, net N mineralization potential was highest on control plots. A strong positive correlation existed between forest floor %N and net nitrification potential. Results suggest that N saturation has caused foliar nutrient imbalances and that the stands may be changing in species composition and structure. No long-term effects of N-form additions on N saturation and forest health were observed. Continued N additions may change the stands from a slow growing and slow N-cycling coniferous forest, to a fast N-cycling and fast growing deciduous forest.