Effects of Acid Deposition on Forests

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 et al. (1997) did not provide any evidence that acidic deposition was contributing to forest decline of spruce-fire 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).