Forests help clean air by removing carbon dioxide and pollutants and releasing oxygen. These are normal physiological and biochemical processes of plant metabolism and growth. Excessive amounts of chemical pollutants in the air may interfere with these processes causing physical damage to trees and other plants.
Ozone is a gas normally found in air in small quantities. High concentrations of ozone cause chronic health problems in people and may injure some plant species. Tree species that seem to be sensitive to ozone damage include eastern white pine and black cherry. Additionally, other species of pine, ash, poplar, maple and oak may be sensitive. On hardwoods, symptoms include purple speckling on upper surfaces of leaves. On pines, symptoms include yellow mottling on needles, shorter needles, and loss of needles. While damage from high concentrations of ozone does not seem to kill trees, it is an additional stress on the health of trees. Also, blackberry and milkweed are known to be affected by changes in ozone levels.
Since 1992, ozone damage in Maryland has been monitored through the Forest Health Monitoring System. The extent to which ozone is affecting forest health and ecosystem diversity remains unclear. While ozone levels can differ within a few miles due to elevation changes and other site factors, air quality data indicate increases in ozone levels across Maryland. Scientists are concerned about connections between trends in forest health and concentrations of ozone in the atmosphere. Ozone levels may be reaching concentrations where natural adaptations may no longer be effective in buffering the gas.
Acid deposition includes rain, snow, fog, gases, aerosols, and particulates like dust that are acidic in nature. Acid precipitation and dry deposition have pH levels below 7.0. Chemicals in the air react with precipitation and deposition making them more acidic. These chemicals include sulfate, nitrate, nitrite, and ammonium ions.
Sulfate, nitrate, nitrite, and ammonium ions come from both manufactured and natural sources. Natural sources of these chemicals include fires, volcanos, and marshes. These have been reacting with precipitation and dry deposition throughout time. However, automobiles, power plants, and factories, have increased the amounts of these chemicals to the point that natural adaptations may no longer be effective in keeping the pH levels in balance.
In 1986, a survey conducted by the National Acid Precipitation Assessment Program determined that the highest percentage of streams sensitive to acidic inputs in the United States occurs in the East. Further, sensitive streams in the East are concentrated in the Coastal Plain and Appalachian Plateau of the mid-Atlantic States. In 1991, the National Atmospheric Deposition Program/National Trends Network also found that Maryland is located in or near the region of highest levels of acid precipitation in the United States. Maryland also receives higher levels of sulfate and nitrate in the form of precipitation than the rest of the country.
Results of studies indicate that approximately one-third of all headwater streams in Maryland are susceptible to impacts from acid deposition or are already acidic. Approximately 65 small streams on the Coastal Plain and the Appalachian Plateau in Maryland are monitored by the network. At each of these streams, macroinvertebrate samples are collected in spring and fall each year to determine the amount of aquatic life living in the streams. Information about macroinvertebrates contribute to the evaluation of the impact of acid deposition on these streams.
Actual measured effects of acid deposition on forests are limited. While no direct adverse effects on forests have been documented, indirect effects may result from long- term changes in pH levels in streams and lakes and soil chemistries. Research on the effects of acid deposition has demonstrated that surface waters are more vulnerable to adverse impacts than ground water.
The ability of the vegetation, soils, and bedrock within the watershed to buffer acidic inputs and lessen impacts depends on the path water takes through the watershed. Rain seeping through leaf litter to a small stream and then to a large stream has a different impact than rain flowing across a rock outcropping into a pond then following a stream to a lake.
The potential for long-term effects of acid deposition on soils is greatest on poorly buffered soils, particularly at high-elevations where soils are thin or where soils are already moderately acidic. Changes in soil chemistry may worsen for 50 to 60 years before effects on forests are observed. Similarly, it may take 40 or 50 years for watersheds to show improvements in response to less acid precipitation and deposition. Improvements in stream water chemistry are not expected until the year 2000, or later.