Determining the Effects of Atmospheric Deposition

David L. Radloff, staff specialist, Forest Environment Research Staff, Forest Service.

What Is Atmospheric Deposition?

The quality of the air can affect trees and forests in many ways. Through the air, trees receive water, nutrients, and gases (such as carbon dioxide) that are essential for their growth. During recent years, forest managers and the public have become concerned that polluted air may be adversely affecting the health of trees and forests in large regions of the United States.

This concern is a part of the important environmental issue known as acid rain. Acid rain (or snow or fog) is literally precipitation that contains abnormally high concentrations of acids (especially sulfuric acid and nitric acid). These acids may end up in streams, lakes, soils, and plants possibly changing the ecological balance and causing detrimental changes in terrestrial or aquatic ecosystems. In addition, acids (or chemicals that can be converted to acids) may be deposited on the land in dry form, and potentially harmful gases also may be deposited. All these chemicals may influence the health of forests. Considered together, these chemicals are called atmospheric deposition.

Why Is There Concern About Forest Health?

Scientists have long known that atmospheric deposition of certain pollutants in high enough concentrations damages and kills trees. Examples are the death of trees in many square miles of forests from high sulfur dioxide concentrations near metal smelters and high fluoride concentrations near aluminum production plants. These severe effects are seen only in the areas near the sources of pollutants, and so are important locally but not regionally.

Today, there is concern that atmospheric deposition may affect forests by compromising their health over large areas far from the sources of pollution. The chemicals that make up atmospheric deposition can be transported many hundreds of miles from their sources. Along the way, they may change from one chemical form to another, and their concentrations may alter as some are deposited on the ground. Although we know that eventually some of these chemicals are deposited in forested areas, we do not know what effects this regional atmospheric deposition has on forests. One goal of forestry research is to determine these effects.

Is There Evidence of Damage to Forests?

Major concern about adverse effects of atmospheric deposition on forests was first expressed in Europe. In Sweden, France, Switzerland, the Federal Republic of Germany, and other countries, forest managers and researchers have observed damage to forest trees from discoloration of needles and leaves through loss of foliage to premature death. In the Federal Republic of Germany, a 1985 forest damage survey showed that about half the forest area has some visible Symptoms of damage; about 19 percent of the area shows moderate to severe symptoms. Although the causes of these changes in forest conditions in Europe are not known with certainty, biological research has implicated atmospheric deposition as a possible cause. Indeed, during the past several years, a consensus has developed among many European scientists that atmospheric deposition is a major contributor to forest health problems in Europe.

This European evidence has sensitized U.S. forest managers and the public to the possibility of forest damage from atmospheric deposition in this country. Several observations about forest conditions in the United States have heightened the concern.

First, in the San Bernardino Mountains east of Los Angeles, trees of several species have been damaged by ozone and other oxidants highly reactive chemicals that form when nitrogen oxides, hydrocarbons, and sunlight interact. The trees have shown symptoms of leaf tissue damage and discoloration, early leaf fall, reduced productivity (photosynthesis), and reduced growth. The most sensitive species is ponderosa pine, and the most sensitive trees may be weakened enough so they are ultimately killed by other contributing factors, such as insects. The result has been a change in the composition of this mixed forest ecosystem.

The San Bernardino situation is a rather special case of atmospheric deposition damage to forests because this forest ecosystem has been exposed to high concentrations of a known damaging gas for more than 40 years. No other forest area has had this kind of regional exposure history. Controlled research studies over many years demonstrated that specific components of atmospheric deposition (namely oxidants) caused this observed forest damage.

Visible symptoms characteristic of ozone damage have been spotted in other forested regions, including the Sierra Nevada of central California and scattered stands of eastern white pine (a sensitive species) throughout the Eastern United States. Although the symptoms appear to link these cases with ozone, the damage is less severe than in the forests near Los Angeles.

Second, spruce and fir forests growing at high-elevation sites in the East seem to be decreasing in health and vigor. At locations from the southern Appalachian Mountains to New England, trees in these forests are showing symptoms of needle discoloration and loss, and some trees are dying at unexpectedly young ages. Research has shown that these high-elevation, mountain sites receive some of the highest rates of atmospheric deposition in the Eastern United States. It is not known if this relationship between acid rain and forest damage is merely coincidental or based on cause and effect. Future research will discover the cause of spruce and fir decline in the East.

Third, Forest Service surveys and research studies have shown that some eastern forests are growing more slowly than expected. The rate of diameter growth of red spruce trees throughout much of the range of this species has decreased since the 1960's, on both high- and low-elevation sites. Loblolly pine, shortleaf pine, and slash pine in the important commercial forests of the Southeastern United States also appear to be growing less rapidly than they were several decades ago. This could be the result of changes in any of the many factors that influence forest health and growth, including weather, climate, insects, diseases, competition from other plants, forest age, forest-management practices, and atmospheric deposition. Again, research must be conducted to sort out these factors and determine the causes of the observed forest conditions.

Atmospheric deposition might affect forests by causing changes in soil chemistry. Researchers collect samples from soil columns in an underground lab in long-term studies to better understand the relationships among forests, nutrients, soils, and atmospheric deposition.

What Research Is Needed?

Several important decisions hinge on knowing whether atmospheric deposition is having a widespread, adverse effect on U.S. forests. Forest managers need to know how forests are likely to grow in the future so the forests can be managed to yield the greatest benefit to the public. Regulatory agencies, such as the Environmental protection Agency, need to know if forests are being damaged so effective clean air laws can be implemented. Congress needs to know what resources are being affected to determine if new legislation should be passed requiring stricter control of air pollution to reduce atmospheric deposition.

These decisions will influence actions that may involve millions to billions of dollars each year. Research on trees and forests will provide results that enable decisionmakers to select the correct course of action.

The goal of the research must be to answer three broad questions: (1) Do current or expected levels of atmospheric deposition decrease the health and productivity of our forests?

(2) What physiological and ecological mechanisms are influencing the effects of atmospheric deposition on forests? (3) What changes in future forest conditions can be expected under different levels of atmospheric deposition?

The research to answer these questions will involve a blend of statistical analysis of trends in forest conditions and atmospheric deposition, empirical experiments to determine simple cause/effect relationships, mechanistic experiments to understand the relationships, and modeling to predict future forest conditions.

What Research Is Being Conducted?

The Federal Government began a focused research program on the effects of atmospheric deposition in 1980 with the creation of the National Acid Precipitation Assessment Program (NAPAP). During the first several Years, research concentrated 01, atmospheric chemistry, meteor-0109Y, and effects on lakes and streams and agricultural crops. In 1984, research began in earnest on acid rain's effects on forests. This forestry research program expanded considerably in 1985 and again in 1986.