Red spruce ecosystem level changes following 14 years of chronic N fertilization

In the early 1980s, nitrogen (N) deposition was first postulated as a cause of N saturation and spruce mortality across the northeastern US. In 1988, a series of high elevation spruce-fir forest N addition plots were established on Mt. Ascutney (southeastern) Vermont to test this hypothesis. The paired plots each received, in addition to ambient N deposition, 15.7 kg N ha−1 year−1 (low N addition), 31.4 kg N ha−1 year−1 (high N addition) or no N addition (control) from 1988 to 2002. Over the years, potential and annual in situ forest floor net N mineralization and net nitrification, foliar and forest floor elemental concentrations, and basal area growth by species were measured on each plot. Live basal area decreased by 18% on the low N addition plots, and by 40% on the high N addition plots between 1988 and 2002, while the control plots had a 9% increase in basal area over the same time period. Initially, none of the plots had measurable rates of in situ nitrification, but by 2002, 21% of the mineralized N was being annually nitrified on the high N addition plots, compared to no net nitrification on the control plots. We also observed a significant increase in forest floor N concentration on the high N plots from 1988 to 2000. Reductions in live basal area, and increased net nitrification suggest that we induced late stage N saturation on the high N addition plots. The low N addition plots exhibited symptoms of mid-stage N saturation, with a smaller reduction in live basal area and net N mineralization, and a smaller increase in net nitrification compared to the high N addition plot values. Other correlations between forest floor and vegetation composition and function, and N saturation will be discussed in the paper.