Acid deposition forests

Receptors. The receptor can be a person, animal, plant, material, or ecosystem. The criteria and hazardous air pollutants were so designated because, at sufficient concentrations, they can cause adverse health effects to human receptors. Some of the criteria pollutants also cause damage to plant receptors. An Air QuaUty Criteria Document (12) exists for each criteria pollutant and these documents summarize the most current Hterature concerning the effects of criteria pollutants on human health, animals, vegetation, and materials. The receptors which have generated much concern regarding acid deposition are certain aquatic and forest ecosystems, and there is also some concern that acid deposition adversely affects some materials. 

Since SO2 and NO2 are criteria pollutants, their emissions are regulated. In addition, for the purposes of abating acid deposition in the United States, the 1990 Clean Air Act Amendments require that nationwide SO2 and NO emissions be reduced by approximately 10 million and 2 million t/yr, respectively, by the year 2000. Reasons for these reductions are based on concerns which include acidification of lakes and streams, acidification of poorly buffered soils, and acid damage to materials. An additional major concern is that acid deposition is contributing to the die-back of forests at high elevations in the eastern United States and in Europe. 

The impact of acid deposition on forests depends on the quantity of acidic components received by the forest system, the species present, and the soil composition. Numerous studies have shown that widespread areas in the eastern portion of North America and parts of Europe are being... 

When a forest system is subjected to acid deposition, the foliar canopy can initially provide some neutralizing capacity. If the quantity of acid components is too high, this limited neutralizing capacity is overcome. As the acid components reach the forest floor, the soil composition determines their impact. The soil composition may have sufficient buffering capacity to neutralize the acid components. However, alteration of soil pH can result in mobilization or leaching of important minerals in the soil. In some instances, trace metals such as Ca or Mg may be removed from the soil, altering the A1 tolerance for trees. 

This interaction between airborne acid components and the tree-soil system may alter the ability of the trees to tolerate other environmental stressors such as drought, insects, and other air pollutants like ozone. In Germany, considerable attention is focused the role of ozone and acid deposition as a cause of forest damage. Forest damage is a complex problem involving the interaction of acid deposition, other air pollutants, forestry practices, and naturally occurring soil conditions. 

Consider a lake with a smaU watershed in a forest ecosystem. The forest and vegetation can be considered as an acid concentrator. SO2, NO2, and acid aerosol are deposited on vegetation surfaces during dry periods and rainfalls they are washed to the soil floor by low-pH rainwater. Much of the acidity is neutralized by dissolving and mobilizing minerals in the soil. Aluminum, calcium, magnesium, sodium, and potassium are leached from the soil into surface waters. The ability of soils to tolerate acidic deposition is very dependent on the alkalinity of the soil. The soil structure in the... 

A second area of concern is reduced tree growth in forests. As acidic deposition moves through forest soil, the leaching process removes nutrients. If the soil base is thin or contains barely adequate amounts of nutrients to support a particular mix of species, the continued loss of a portion of the soil minerals may cause a reduction in future tree growth rates or a change in the types of trees able to survive in a given location. 

Acid deposition occurs when sulfur dioxide and nitrogen oxide emissions are transformed in the atmosphere and return to the earth in rain, fog or snow. Approximately 20 million tons of SOj are emitted annually in the United States, mostly from the burning of fossil fuels by electric utilities. Acid rain damages lakes, harms forests and buildings, contributes to reduced visibility, and is suspected of damaging health. 

The extent and severity of the damage attributable to acid depositions are difficult to estimate, since impacts vary according to soil type, plant species, atmospheric conditions, insect populations, and other factors that are not well understood. Nitrates in precipitation may actually increase forest growth in areas with nitrogen-deficient soils. 

The documentation of regional level terrestrial consequences of acid deposition is complicated. For example, forested ecosystems m eastern North America can he influenced by other factors such as high atmospheric ozone concentrations, drought, insect outbreaks and disease, sometimes from non-native sources. However there is a general consensus on some impacts of acidic depositon on both soils and forests m sensitive regions. 

Mechanisms of Forest Response to Acidic Deposition. Lucier, A. A. Ed., Springer-Verlag NY, 1990. 

Cronan, C. S. (1984). Biogeochemical responses of forest canopies to acid precipitation. In "Direct and Indirect Effects of Acidic Deposition on Vegetation" (R. A. Linthurst, ed.), pp. 65-79. Butterworth, Boston, MA. 

Lin, T. C. (1998). Acid deposition and forest ecosystem sensitivity in Taiwan. In V. N. Bashkin and S-U. Park (Eds.) Acid Deposition and Ecosystem Sensitivity in East Asia, NovaScience Publishers, USA, pp. 379-412. 

The effect of acid deposition on forests remains a topic of some dispute among experts. While there is little doubt that sulfur dioxide and other forms of acid deposition do cause damage to trees, a number of other factors may also be responsible for the widespread die-off of trees observed in forests in Europe and North America over the past 50 years. For example, other elements of polluted air, such as ozone or heavy metals, may also contribute to at least some extent to these disasters. 

Saxena, V. K., and N.-H. Lin, Cloud Chemistiy Measurements and Estimates of Acidic Deposition on an Above Cloudbase Coniferous Forest, Atmos. Environ., 24A, 329-352 (1990). 

Binkley, D. Driscoll, C. T. Allen, H. L. Schoeneberger, P. McAvov, D. Acidic Deposition and Forest Soils Context and Case Studies of the Southeastern United States Ecological Studies Springer-Verlag New York, 1989 Vol. 72, 149 pp. 

For Further Reading W. Jackson, Altars of Unhewn Stone Science and the Earth San Francisco, North Point Press (1987). J. Raloff, Pollution helps weeds take over prairies, Science News 150, 356 (1996). S. E. Schwartz, Acid deposition unraveling a regional phenomenon, Science, 243, 753 (1989). W. H. Smith, Air pollution and forest damage, Chemical and Engineering News, November 11,1991, p. 30. 

A second area of concern is that of reduced tree growth in forests. The leaching of nutrients from the soil by acid deposition may cause a reduction in future growth rates or changes in the type of trees to those able to survive in the altered environment. In addition to the change in soil composition, there are the direct effects on the trees from sulfur and nitrogen oxides as well as ozone. 

Rustad, L. E., I. J. Fernandez, R. D. Fuller, M. B. David, S. C. Nodvin, and W. A. Halterman. 1993. Soil solution response to acidic deposition in a northern hardwood forest. Agriculture Ecosystems and Environment 47 117-134. 

Cronan CS, Schofield CL. 1990. Relationships between aqueous aluminum and acidic deposition in forested watersheds of North America and Northern Europe. Environ Sci Technol 24 1100-1105. 

Effects of Acidic Deposition on Forest Ecosystems in the Northeastern United States An Evaluation of Current Evidence, R.L. Burgess, ed. ESF-84-016, State University of New York, College of Environmental Science and Forestry, Syracuse, NY, January 1984. 

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