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3.4 Acid Deposition Results

Although sulfur is an essential nutrient for soil and plant metabolic processes, sulfur deposition can contribute to degradation of soil chemistry (Reuss and Johnson 1986). Long-term increases in soil acidity resulting from sulfur deposition are believed to affect nutrient cycling by leaching nutrients, such as calcium and magnesium (Fenn and others 1988). Research has also shown that sulfur deposition provides the stimulus to mobilize aluminum in soil solutions (Reuss and Johnson 1986). Dissolved aluminum interferes with the uptake of calcium and other root functions (Johnson and others 1991).

Currently, high-elevation spruce-fir forests are the most susceptible to the effects of sulfur deposition (McLaughlin and Percy 1999) because they lack the ability to buffer sulfur deposition and are low in base cation pools. Future rates of sulfur deposition are expected to decrease, which could lead to a reduction in the effects of sulfur deposition on base cations in high-elevation spruce-fir forests.
Recent evidence indicates that most Southern Appalachian soils supporting spruce-fir ecosystems are poorly buffered, high in aluminum, and nitrogen saturated (Johnson and others 1991). Nitrogen saturation occurs when ammonium (NH₄) and nitrate (NO₃) are present in quantities that exceed total combined plant and microbial demand. Excess levels of nitrogen have been found to affect soil and plant calcium:aluminum ratios (Johnson and others 1991), cause aluminum toxicity (Shortle and Smith 1988), and decrease calcium uptake and leaching of base cations (McLaughlin and others 1998) in these sensitive forests. A lack of calcium changes the wood structure of spruce and fir and may change the ability of branches to withstand stress (McLaughlin and others 1998). Furthermore, excess levels of nitrogen decrease the rates of some critical functions of soil microorganisms, including decay of forest floor material (Drohan and Sharpe 1997). These effects on forest soils are most dramatic in the sensitive soils under spruce-fir forests. Conversely, in an oak-pine forest in the North Carolina Piedmont, Johnson and others (1995) predict that forest floor nutrient contents will be virtually unaffected by a 50 percent reduction in sulfur deposition over the next 20 years.

Effects of acid deposition on tree growth have been associated with nutrient limitations caused by increases in soil aluminum concentrations. Studies of historical tree-ring chemistry (Bondietti and McLaughlin 1992) have shown that calcium concentrations in stemwood increased as growth increased during the late 1940s and 1950s. However, decreases in tree growth were associated with increases in aluminum:calcium ratios in the wood, suggesting that the availability of calcium was reduced at the same time aluminum concentrations increased. McLaughlin and Kohut (1992) have shown evidence for the competitive inhibition of calcium uptake by aluminum. Dendroecological and plot-based data have shown declines in radial growth of red spruce radial growth decline (LeBlanc and others 1992) and canopy crown deterioration during the mid to late 1980s in the Southern Appalachian Mountains (Peart and others 1992).

While acid deposition has affected tree growth in spruce-fir forests of the Southern Appalachians (McLaughlin and others 1998), damage to these ecosystems is not limited to acid deposition.Reams and Van Deusen (1993) reported that stand disturbances and changes in stand dynamics have resulted in radial growth declines in spruce-fir forests. In addition, the balsam woolly adelgid was introduced into North America at the beginning of the 20th century, and the exotic insect has been active in the Southern Appalachians since the late 1950s (McLaughlin and others 1998). The damage to mature Fraser fir in the Southern Appalachians by the woolly adelgid has been extensive over the past 15 years (Dull and others 1988). Although heavy infestation is unquestionable evidence that the adelgid plays a major role in killing these trees (see the HLTH-2 Chapter for more details), it is also important to consider the influence of predisposing factors, including abiotic stressors such as acid deposition, on the susceptibility of forests to pathogens (Manion 1981).

Hardwood forests in the South are considered less sensitive than spruce-fir forests to nitrogen deposition because they still have adequate stores of base cation nutrients, and the soils still maintain considerable capacity to retain the deposited nitrogen (NAPAP 1998). In most hardwood forests, virtually all nitrogen deposition is either adsorbed in the soil or used by vegetation and microorganisms. Much of this nitrogen may be removed later by forest harvesting. These systems, therefore, have not shown negative effects from increases in nitrogen deposition and may respond with increased growth. Research has shown that 22.8 pounds per acre per year of nitrogen fertilizer increased basal area growth of trees by 67 percent (McNulty and Aber 1993).

Impacts of nitrogen deposition on forest health have not been detected in the pine and oak-pine forests of the South (NAPAP 1998). However, nitrogen is a major contributor to the depletion of base cations in many buffered soils supporting southern pine and oak-pine forests. Over the course of decades, therefore, nitrogen deposition is likely to reduce pine forest productivity (NAPAP 1998). Increases in growth are expected for some nitrogen-deficient soils, while negative effects are expected to be limited to the most acidic soils.

In the future, nitrogen deposition will continue to impact the structure and function of high-elevation spruce-fir forests. In addition, some hardwood, pine, and oak-pine forests that are sensitive to nitrogen deposition could respond with reduced growth rates and accelerated tree mortality over the long term. However, research has predicted that in oak-pine forests in the North Carolina Piedmont vegetation will respond positively to a 200-percent increase in nitrogen deposition over the next 20 years. A 3- to 9-percent increase in vegetation nutrient content and a 10- to 30-percent increase in forest floor nutrient content are expected (Johnson and others 1995).

Currently, the SAMI Class I Wilderness Areas are much more sensitive to acid precipitation than any other areas surveyed by the National Stream Survey (NSS) in the Southern Appalachians (Herlihy and others 1996). The Wilderness Areas of greatest concern are Otter Creek and Dolly Sods in West Virginia. There, the percentage of acidic stream length is high, pH is low, and sulfate and inorganic aluminum concentrations are high. Additionally, stream nitrate concentrations, an indicator of acid deposition effects, have been shown to have a strong correlation with forest age.The highest concentrations occur in old-growth forests, where biological demand for nitrogen is lowest. The Wilderness Area of least concern is the Sipsey, in Alabama, because sulfate concentrations are not increasing and acid neutralizing capacity (ANC) of streams in this area is high.

ANC has been used to determine stream quality because stream acidification affects fish and other aquatic species. Research in the South has shown that the biological response of brook trout can be altered by ANC (Table 1). Furthermore, the Southern Appalachian Assessment has shown that 70 percent of sampled streams have suffered moderate to severe fish community degradation and about 50 percent of the stream miles in West Virginia and Virginia show habitat disruption (SAMAB 1996). However, streams targeted by the NSS in the Southeastern Highlands (which includes the Ozarks/Ouachita, Piedmont, Southern Appalachians, and Southern Blue Ridge and ecological subregions in the states of Arkansas, Georgia, North Carolina, and Tennessee) appear to be buffered from sulfur deposition by a substantial amount of sulfate adsorption in watershed soils (Rochelle and Church 1987). As a result, sulfate concentrations in these streams are low.