Federal agencies of several nations have or are currently developing guidelines for critical forest soil acid loads. These guidelines are used to establish regulations designed to maintain atmospheric acid inputs below levels shown to damage forests and streams. Traditionally, when the critical soil acid load exceeds the amount of acid that the ecosystem can absorb, it is believed to potentially impair forest health. The excess over the critical soil acid load is termed the exceedance, and the larger the exceedance, the greater the risk of ecosystem damage. This definition of critical soil acid load applies to exposure of the soil to a single, long-term pollutant (i.e., acidic deposition). However, ecosystems can be simultaneously under multiple ecosystem stresses and a single critical soil acid load level may not accurately reflect ecosystem health risk when subjected to multiple, episodic environmental stress. For example, the Appalachian Mountains of western North Carolina receive some of the highest rates of acidic deposition in the eastern United States, but these levels are considered to be below the critical acid load (CAL) that would cause forest damage. However, the area experienced a moderate three-year drought from 1999 to 2002, and in 2001 red spruce (Picea rubens Sarg.) trees in the area began to die in large numbers. The initial survey indicated that the affected trees were killed by the southern pine beetle (Dendroctonus frontalis Zimm.). This insect is not normally successful at colonizing these tree species because the trees produce large amounts of oleoresin that exclude the boring beetles. Subsequent investigations revealed that long-term acid deposition may have altered red spruce forest structure and function. There is some evidence that elevated acid deposition (particularly nitrogen) reduced tree water uptake potential, oleoresin production, and caused the trees to become more susceptible to insect colonization during the drought period. While the ecosystem was not in exceedance of the CAL, long-term nitrogen deposition pre-disposed the forest to other ecological stress. In combination, insects, drought, and nitrogen ultimately combined to cause the observed forest mortality. If any one of these factors were not present, the trees would likely not have died. This paper presents a conceptual framework of the ecosystem consequences of these interactions as well as limited plot level data to support this concept. Future assessments of the use of CAL studies need to account for multiple stress impacts to better understand ecosystem response.

We evaluated foliar and forest floor chemistry across a gradient of N deposition in the Northeast at 11 red spruce (Picea rubens Sarg.) sites in 1987/1988 and foliar and forest floor chemistry and basal area growth at six paired spruce and deciduous sites in 1999. The six red spruce plots were a subset of the original 1987/1988 spruce sites. In 1999, we observed a significant correlation between mean growing season temperature and red spruce basal area growth. Red spruce and deciduous foliar %N correlated significantly with N deposition. Although N deposition has not changed significantly from 1987/1988 to 1999, net nitrification potential decreased significantly at Whiteface. This decrease in net potential nitrification is not consistent with the N saturation hypothesis and suggests that non-N deposition controls, such as climatic factors and immobilization of down dead wood, might have limited N cycling. Data from the 1999 remeasurement of the red spruce forests suggest that N deposition, to some extent, is continuing to influence red spruce across the northeastern US as illustrated by a significant correlation between N deposition and red spruce foliar %N. Our data also suggest that the decrease in forest floor %N and net nitrification potential across sites from 1987 to 1999 may be due to factors other than N deposition, such as climatic factors and N immobilization in fine woody material (<5 cm diameter).

The dieback of Picea rubens Sarg. (red spruce) in the Appalachian Mountains of New England has been correlated with emissions transported from the Great Lakes region, including acids, metals, and oxidants. In 1994–1995, metal stress in red spruce foliage evidenced by phytochelatin concentrations increased with red spruce damage index in spruce-fir dominated stands in high elevation forests. In this study, we revisited those same forests after two decades to examine metal stress impacts on high-elevation forests following reductions in atmospheric pollutant loading. We measured metal concentrations in soils, lichens, and foliage, and concentrations of phytochelatin and its precursors in foliage of red spruce trees at 1000 m along a west–east transect from New York to New Hampshire, and along an 800–1000 m elevational transect on Whiteface Mountain, NY. Path analysis showed that foliar Hg had a direct positive effect on foliar phytochelatins, metal-binding peptides produced by the metal stress response in plants. Essential metals Cu and Zn decreased the concentration of Hg in foliage. However, we could not determine the relative importance of atmospheric vs soil pathways for metal exposure. While metal stress was still occurring on Whiteface Mountain in 2013, the overall visual health of red spruce trees across the region was significantly improved compared to 1993–1995. Thus, although metal stress is still measurable in red spruce, the physiological impact may be lessened by decreases in the deposition of metals and acids, thus providing evidence of positive forest health outcomes from improvements in regional air quality in the Northeastern US.