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Acid Deposition Discussion and Conclusions

Emissions of SO₂ and NO_x are decreasing. However, plant species structure and composition, soil chemistry, and microbial activities continue to change. Currently, the mortality and decline of Fraser fir and red spruce at high elevations in the Southern Appalachians are the only cases of significant ecosystem damage. Thus, less than 5 percent of the South is currently being negatively impacted by elevated sulfur and nitrogen deposition (Fenn and others 1998). In addition, atmospheric deposition reduces the number of microorganisms important to nutrient cycling and removes important nutrients from the soil, making spruce-fir forests more susceptible to canopy deterioration, drought, loss of foliage, insects, and diseases. Hardwood, pine, and mixed oak-pine forests are less sensitive than spruce-fir for several reasons, including biological nitrogen demand, higher soil cation exchange capacity, and faster nitrogen cycling.

Since most hardwood, pine, and mixed forests are nitrogen deficient, they may experience increased growth rates in response to continued elevated nitrogen deposition. Conversely, nitrogen deposition can significantly degrade some of these forests over time (years to decades), especially in areas where nitrogen levels may be high and the soil has reached or is approaching saturation.

Sulfate and nitrate concentrations have increased in streams throughout the South, but not to levels that are considered regionally problematic. Furthermore, sulfate and nitrate in some streams are low or near detection limits (Swank and Vose 1997).