High-elevation forests in the southern Appalachians have some of the highest levels of acid deposition in the United States. High-elevation streams there are particularly susceptible to acidification, which can negatively impact aquatic biota. The decline of some high-elevation forests in the southern Appalachians has also been partly attributed to acidic deposition. Acid deposition can also contribute to nitrogen saturation, which can have long-term consequences on forest productivity.

When raindrops fall through air that contains gaseous oxides of sulphur and nitrogen, these gases can dissolve in the droplets and form dilute acids, or "acid rain." There are other forms of acid deposition besides rainfall. Total loading of pollutants responsible for acid deposition (hydrogen ions or pH, sulfates, nitrates, and ammonium) is a combination of wet, dry, and occult deposition. Wet deposition includes rain, snow, sleet, and hail. Dry deposition is acidic particulate matter and gases deposited to surfaces. Occult deposition includes fog and cloudwater, but technical problems make measurement of these sources of deposition difficult (SAMAB 1996).

Wet sulfate deposition in the southern
Appalachians (1983-1990)

The National Acid Precipitation Assessment Program (NAPAP), a 10-year effort mandated by the U.S. Congress, examined the effects of acidic deposition on a variety of aquatic resources, forests, and human health. This work revealed that some of the highest deposition loadings of sulfur, nitrogen, and acidity in the United States have been measured at high elevations in the Southeast. The annual average pH of wet precipitation in 1993 for this region was second only to areas of the northeastern and north central United States. The loading of sulfate and nitrate in wet deposition over the period of 1983-1990 is highest in upland areas, including many parks and wilderness areas.

Although it is difficult to quantify the contributions of dry deposition and cloudwater deposition to total loading in the mountainous areas, it is reasonable to expect that the loading estimates could be doubled in these sensitive areas. Unfortunately, portions of streams at high elevations are probably least able to neutralize or "buffer" incoming acidity, especially during storm-generated episodes. In some of these sensitive streams, aquatic biota (fish and invertebrates) are being affected by both chronic and episodic acidification (SAMAB 1996).

Decreases in acid deposition are expected as the Clean Air Act Amendments of 1990 are fully implemented. Sulfate concentrations in precipitation seem to be decreasing in the southern Appalachians. Therefore, it is unlikely that sulfur deposition will cause additional streams to become chronically acidified in the region. However, vehicle emissions are a second major source of nitrogen compounds, and the importance of that source is expected to grow as populations increase (SAMAB 1996).

While decreased sulfur deposition could result in decreased cation leaching from forest soils, the concurrent decrease in particulate emissions has resulted in decreased atmospheric inputs of Ca and Mg to ecosystems. Consequently, acidity of rainfall has not improved. This decrease in the concentrations of buffering chemicals that can offset the acid effects of sulfates and nitrates may contribute to slower recovery from acidification (SAMAB 1996).

There are two types of stream acidification, chronic and episodic. Chronic acidification of streams is caused by added sulfate from wet and dry deposition. Deposition of sulfate and acids to sensitive watersheds results in: (1) soil acidification, (2) leaching of base cations from soils, and (3) surface water acidification. In some watershed soils, sulfate in rain is absorbed by the soils until the soils are saturated. Then the sulfate begins to leach out into the stream waters, resulting in "delayed" acidification of streams (Church and others 1989, Church and others 1992). Even if sulfate in deposition is significantly reduced, stream recovery from acidification may not be immediate. An example of chronic acidification of a low acid neutralizing capactiy (ANC) stream is Deep Run in Shenandoah National Park, where the sulfate concentrations in the stream increased about 2 micro-equivalents per liter per year (µeq/l/yr) for the 1980-1987 period, while the pH declined from 5.6 to 5.3 and the stream lost about 0.75 µeq/l/yr of ANC (Cosby and others 1991, SAMAB 1996).

Episodic acidification is the temporary acidification of streams due to large rain events (Wigington and others 1991). One of the early instances of fish kills resulting from episodic acidification comes from study of the Raven Fork Creek drainage in the Great Smoky Mountains National Park and on the Cherokee Indian Reservation. Here, base flow pHs of about 6.0 dropped to pHs in the range of 4.3 to 4.7 during stormflow. These decreases were accompanied by increases of both nitrate and sulfate. In streams monitored in the Appalachian Highlands, nitrogen saturation is thought to be responsible for episodic acidification; nitrate is observed at high concentrations during hydrologic episodes and during baseflow periods (SAMAB 1996).

Stream sensitivity in the southern Appalachians

A simple measure of the sensitivity of stream water to chemical change is acid-neutralizing capacity (ANC), or the ability of the stream water to buffer incoming acids. In stream reaches that have become acidic, the ANC is less than or equal to zero. When acid deposition falls on stream watersheds located on bedrock that is resistant to weathering, the result can be a decrease in the ANC in the stream water, along with a decrease in pH. Depending on the chemistry of the deposition as well as the chemistry of watershed soils, there may also be increases in sulfate, nitrate, and aluminum (leached by acids from soils and sediments) (SAMAB 1996).

An EPA study concluded that the following percentages of combined lengths of streams were acidic: 0.8 percent in the Ridge and Valley Province, 0.5 percent in the southern Appalachians, and none in the southern Blue Ridge. These acidic streams were generally on forested watersheds covering less than 30 square kilometers (11.6 square miles), in the upland areas of the Southern Appalachian Assessment region (Herlihy and others 1991). Percentages of highly sensitive reaches of streams (a spring baseflow ANC of less than 50 µeq/l) were: 6.5 percent in the Ridge and Valley Province, 3.5 percent in the southern Appalachians, and 7.8 percent in the southern Blue Ridge. Local geology is the primary factor controlling the sensitivity of streams to acid inputs. Stream reaches most likely to be acidic or to have low ANC values are in upland areas (Kaufmann and others 1988). Within the Mid-Atlantic region, 70 percent of the acidic streams had aluminum in excess of 100 micrograms per liter, a concentration often associated with biological effects (Kaufmann and others 1991, SAMAB 1996).

Biological effects of stream acidification

The southern Appalachians are popular for fishing but acid deposition may continue to reduce the number of streams suitable for sensitive fish species. Intensive site studies indicate that both aquatic insects and fish species common to streams are sensitive to changes in pH, calcium, and aluminum concentrations in stream waters. Of the 344 native brook trout streams in the mountains of western Virginia, 49 percent had a ANC less than 50 µeq/l, and 10 percent were acidic. Sulfate was the major anion in those streams, and sulfate was retained in soils in all watersheds (Webb and others 1989). Studies of diversity of aquatic insect species indicate a loss of sensitive species (such as mayfly larvae) from streams that have experienced either chronic or episodic acidification. Both chronic and episodic exposures to acidity in streams in Shenandoah National Park have resulted in lethal and sublethal effects on fish, particularly brook trout and blacknose dace (Bulger and others 1994). In St. Marys River, in the George Washington and Jefferson National Forests in Virginia, declines in fish populations and changes in benthic fauna have been reported in association with an historical change in pH from 6.8 to 5.2. Comparison of a 1988 biological survey with results obtained in the 1930s indicated declines in most kinds of benthic invertebrate and acid-sensitive fish that Mohn and others (1988) suggested were the result of acidification. At Fridley Run, also on the George Washington and Jefferson National Forests, liming has increased stream pH from 4.7 to 6.4 and reduced aluminum concentrations to the point that brook trout can now reproduce in the treated stream reach (Hudy and others 1995). However, these site remediation treatments are expensive and difficult to maintain. Occasional or chronic acidification of streams by sulfates and nitrates can lead to elevated levels of dissolved aluminum, which can reduce survival and diversity of macroinvertebrate and fish populations in sensitive streams (SAMAB 1996).

The EPA (1995) concluded that the regions of the United States most at risk from continued acid deposition are in the eastern part of the country. Systems being lakes and streams, stretching from the Adirondacks in New York to the southern Blue Ridge in Georgia. The current estimate in the mid-Appalachian region is that about 30 percent of stream reaches are likely to become acidic during the worst rainfall episodes; this estimate is about seven times the number of stream reaches that are now chronically acidic (SAMAB 1996).

In contrast to the damage acid rain causes to streams and lakes, it has proven to be much more difficult to demonstrate negative effects on forests. One factor that complicates the situation is that most forests are highly chemically buffered by the presence of organic matter. A second complicating factor is that many forests are chronically short of nitrogen, a major component of acid rain. In some cases, acid rain has acted as a forest fertilizer rather than as a damaging agent, at least initially. This does not mean that the acid rain is not having a progressive, chronic negative effect on forests. In some forests, nitrogen deposition can result in nitrogen saturation. Nevertheless, even where forests exhibit clear symptoms of distress in areas subjected to acid rain, it has been a very challenging task to link acid rain to any forest decline (Kimmins 1997).

The National Acid Precipitation Assessment Program (NAPAP), a 10-year study mandated by the U.S. Congress, examined the effects of acidic deposition on a variety of resources and human health. It was concluded that "With the possible and notable exception of high-elevation red spruce in the northern Appalachians, acidic deposition has not been shown to be a significant factor contributing to forest health problems in North America" (NAPAP 1991, Adams 1999). Decline of spruce-fir forests in the Appalachians observed since the 1960s has been attributed, at least in part, to acidic deposition. Acidic deposition has been reported to decrease the availability of calcium and increase the availability of aluminum and induce decline of red spruce at high elevation sites above 1800 m. However, a foliar and soil nutrient study by Bryant and others (1997) did not provide any evidence that acidic deposition was contributing to forest decline of spruce-fir forests.

Potential mechanisms of forest damage from acid deposition

Several theories about how acid rain may damage forests have been advanced. For example, it has been suggested that exposure to acid deposition may directly damage the delicate inner tissues of foliage, and through calcium and magnesium leaching out of the leaves. However, while foliar leaching may be increased by acid rain, research results do not support the idea that this is the major mechanism of acid rain damage (Kimmins 1997).

Acid deposition can affect soil processes in several ways. When rates of sulphur and nitrogen deposition by acid rain exceed uptake of these nutrients by trees, soil acidification may occur. This can lead to increased concentrations of aluminum in solution in the mineral soil water, a condition that is toxic to plants and can kill fine roots. Death of fine roots reduces nutrient and water uptake by plants, leading to nutritional problems and an increased susceptibility to drought, diseases, and insects (Kimmins 1997).

Soil acidification can also lead to the leaching of positively charged nutrient ions, such as calcium and magnesium, out of the soil. This can result in decreased availability of these elements to plants. Alternatively, the presence of excessive amounts of aluminum or ammonium nitrogen in the soil solution may restrict the uptake of magnesium even where adequate supplies are present in the soil. These positively charged ions compete with the positively charged magnesium at the root surface, preventing or restricting its uptake. Changes in the chemistry of poorly buffered upper mineral soil layers caused by acid rain can cause fine feeding roots to be restricted to the relatively well-buffered forest floor. This can increase trees susceptibility to drought where the forest floor dries out in the summer, or it may render fine roots more susceptible to winter frost damage (Kimmins 1997).

Adams (1999) suggests that altered nutrient cycles caused by acid deposition can be compounded by increased harvesting intensities and short rotations, actions which also result in the increased removal of calcium and magnesium in aboveground biomass, and altered nutrient cycling. By changing base cation availability of the soil, harvesting intensity and acid deposition may threaten long-term term productivity of forests in the Appalachians (Adams 1999).

Nitrogen saturation occurs when supplies of ammonium and nitrate are in excess of the total combined plant and microbial demand. The termis applied toecosystems where the biota are unable to utilize all of the N that is added to the system, either through N fixation, atmospheric N inputs or other sources (Aber and others 1989, Stoddard 1994). Often nitrogen saturationleads tochronic acidification of streams (Adams 1999) .

Nitrogen leakage into streams may be caused by several factors including the maturation of forests, effects of insect infestation, and excess nitrogen supply in deposition. Fixed nitrogen is an important nutrient for plant growth, but as forests mature, a balance is reached between plant use and recycling back into the system by decaying plant materials. Insect defoliation, for example, by gypsy moths or balsam wooly adelgids, also causes rapid recycling of nitrogen (Webb and others 1995, Nodvin and others 1995, SAMAB 1996).

A nitrogen saturated system may show one or more of the following symptoms (Aber and others 1989, Stoddard 1994):

• elevated concentrations of nitrate, aluminum and hydrogen in streams,
• frost damage or other disruptions of physiological function,
• increased emissions of trace gases such as nitrous oxide, and
• increased cation leaching from soils.

High-elevation spruce-fir forests of the Northeast (McNulty 1993) and hardwood forests of the central Appalachian region (Adams and others 1997) are believed to be N saturated as a result of high N deposition (Adams 1999).

Nitrogen saturation can result in significant decreases in soil fertility and nutrient deficiencies. As nitrate and sulfate anions move through the soil, cations (most often Ca and Mg) are removed from exchange sites to maintain charge neutrality of the soil solution. Adams and others (1997) reported that at the Fernow Experimental Forest, 5 years of experimental additions of ammonium sulfate at twice the ambient input levels (additions of 40 kg S ha ^-1 year ^-1 and 35 kg N ha ^-1 year ^-1 ) resulted in increased leaching of nitrate from a 25-year-old mixed hardwood forest, along with significant increases in leaching of Ca and Mg. By the fifth year of treatments, mean annual export of Ca was 2-3 times greater than inputs. Results from other watershed manipulation studies suggest that bed weathering did not increase (Norton and others 1997). Ca deficiencies have been identified in high-elevation red spruce (McLaughlin and others 1991) and may also be linked to sugar maple decline (Long and others 1997) on nonglaciated soils. As acidic loading continues, base saturation of the soil will decrease until base cations no longer are being desorbed from the soil, and aluminum concentrations in soil solution and stream water will increase. This effect is of concern because elevated soil solution aluminum concentrations have been found to interfere with root growth in red oak seedlings (Joslin and Wolfe 1989), and with Ca uptake in spruce (Shortle and Smith 1988). The acidification associated with N saturation also affects the cycling of nutrients by changing rates of litter decomposition (Adams and Angradi 1996), nutrient uptake and concentration in aboveground biomass (Adams and others 1995), soil base saturation, and N mineralization and nitrification (Aber and others 1989, Adams 1999).

In Great Smoky Mountains National Park, many streams have higher concentrations of baseflow nitrate than sulfate. In fact, streams in Great Smoky Mountains National Park have the highest recorded nitrate concentrations of any streams draining undisturbed watersheds in the United States. Silsbee and Larson (1982) report nitrate concentrations in Great Smoky Mountains National Park streams ranging from 0.2 to 90 µeq/l, often higher concentrations than are found in deposition. This finding suggests that watersheds in this region are net sources of nitrogen to streams. Old-growth forests, such as those in Great Smoky Mountains National Park, may no longer be acting as nitrate sinks, and nitrate may be leaching out of these old-growth watersheds (SAMAB 1996).