Decadal-scale comparisons of vegetation and soil properties in forests of the Adirondack Mountains, NY

In 2004, trees were remeasured in permanent vegetation plots that had been established in spruce-fir, northern hardwood, and pine-dominated stands in 1984 in the Adirondack Park, NY. Over the 20-yr interval, live basal area (BA) in red spruce/balsam fir-dominated plots decreased by 15% and live red spruce BA decreased by 42% (P \u3c 0.05). Excluding American beech, live BA in the northern hardwoods decreased significantly over the 20-yr interval. Further, using the same growth measurements and the ambient N deposition gradient in the Adirondack Mountains, the data indicated that N deposition had either a neutral or positive effect on basal area growth, with the positive effects especially apparent within the smaller size classes of several species. The nature of change in vegetation measured in these plots suggested that other co-varying factors (e.g., temperature, tropospheric ozone, soil acidification) may be partially counteracting the species-dependent fertilization effect of N deposition and that northern hardwood plots were not aggrading over the 20-yr interval. In 2005/6, soils were also sampled and analyzed for C, N, soil organic matter (SOM), and HCl-extractable Ca. Those results were compared to data collected in 1932 and 1984 to determine spatial patterns and temporal trends in soil chemistry. As measured in 2005/6, average C (24 kg m-2) and N (1.1 kg m-2) contents of the combined organic and mineral horizons were high for northeastern U.S. forests. Contrary to expectations, atmospheric inputs of N deposited over the past 50 years only accounted for a small portion of soil N, and the differences in N among the plots along the N deposition gradient. There was a significant increase in SUM content between 1932 and 2005/6, which amounted to an increase in soil C content of about 0.5 Mg C ha-1yr-1 at 16 sites that had a history of agriculture and/or fire, indicating that at least some Adirondack forest soils were accumulating C during the 20th century. Further, between 1932 and 2005/6, median HCl-extractable Ca amounts decreased significantly (-79%) in organic horizons and in the whole-profile (organic + mineral horizons; -64%), and the overall rate of whole-profile Ca depletion was 13.6 kg ha-1yr-1 for the pooled data over the 74-yr interval