Ayers, Harvard, Jenny Hager, and Charles E. Little. 1998. An Appalachian Tragedy: Air Pollution and Tree Death in the Eastern Forests of North America. San Francisco: Sierra Club Books.

Enclyclopedia of the Atmospheric Environment. Impacts of Acid Rain. Website. http:/ / aric/ eae/ Acid_Rain/ Older/ Impacts.html

Our trees are dying because of:


Acid rain is a general name for many phenomena including acid fog, acid sleet, and acid snow. Although we associate the acid threat with rainy days, acid deposition occurs all the time, even on sunny days.

Acid Deposition is the scientific term used to describe "Acid Rain". When atmospheric pollutants such as sulphur dioxide and nitrogen oxides mix with water vapour in the air, they are converted to sulphuric and nitric acids. These acids make the rain acidic, hence the term "acid rain". Rain returns the sulphur and nitrogen acids to Earth, and in high concentrations, can cause damage to natural environments including forests and freshwater lakes. This form of acid deposition is known as wet deposition.

With acid fog the trees become bathed in the sour oxides of nitrogen and sulfur. North Carolina State University scientists have found rime ice on Mt. Mitchell with a pH as low as 2.1, somewhere between battery acid and lemon juice.

A second method of acid deposition is known as dry deposition. Whilst wet deposition involves the precipitation of acids, dry deposition occurs when the acids are first transformed chemically into gases and salts, before falling under the influence of gravity back to Earth. Sulphur dioxide, for example, is deposited as a gas and as a salt.

Limited views are caused by sulfate-based aerosol particles. This is decreasing thanks to the 1990 amendments to the Clean Air Act in most places, but in the southern Appalachians sulfate is still slightly on the increase.

Nitrogen in the presence of sunlight creates smog -- it is not being controlled seriously and has increased rapidly. There have been reductions of views from 50 and 75 miles to 3 and 10 miles.

Sulphur Dioxide

Sulphur dioxide (SO2) is a colourless gas, belonging to the family of gases called sulphur oxides (SOx). It reacts on the surface of a variety of airborne solid particles, is soluble in water and can be oxidised within airborne water droplets.

Natural sources of sulphur dioxide include releases from volcanoes, oceans, biological decay and forest fires. The most important man-made sources of sulphur dioxide are fossil fuel combustion, smelting, manufacture of sulphuric acid, conversion of wood pulp to paper, incineration of refuse and production of elemental sulphur. Coal burning is the single largest man-made source of sulphur dioxide accounting for about 50% of annual global emissions, with oil burning accounting for a further 25 to 30%.

The major health concerns associated with exposure to high concentrations of sulphur dioxide include effects on breathing, respiratory illness, alterations in pulmonary defenses, and aggravation of existing cardiovascular disease. In the atmosphere, sulphur dioxide mixes with water vapour producing sulphuric acid. This acidic pollution can be transported by wind over many hundreds of miles, and deposited as acid rain.

Nitrogen Oxides

Nitrogen oxides (also known as oxides if nitrogen, and abbreviated as NOx) is a collective term used to refer to two species of oxides of nitrogen: nitric oxide (NO) and nitrogen dioxide (NO2). Nitric oxide is a colorless, flammable gas with a slight odour. Although somewhat toxic, its odour is insufficient to provide warning. Nitrogen dioxide is a reddish brown, nonflammable, gas with a detectable smell. In significant concentrations it is highly toxic, causing serious lung damage with a delayed effect. Nitrogen dioxide is a strong oxidizing agent that reacts in the air to form corrosive nitric acid, as well as toxic organic nitrates. It also plays a major role in the atmospheric reactions that produce ground-level ozone or smog.

Globally, quantities of nitrogen oxides produced naturally by bacterial and volcanic action, and lightning, outweigh man-made emissions. Man-made emissions are mainly due to fossil fuel combustion from both stationary sources, such as power generation (24%), and mobile sources, such as transport (49%). Other atmospheric contributions come from non-combustion processes, for example nitric acid manufacture, welding processes and the use of explosives.

In the atmosphere, nitrogen oxides mix with water vapour producing nitric acid. This acidic pollution can be transported by win over many hundreds of miles, and deposited as acid rain.

Nitrogen oxide and nitric oxide, also components of acid rain, can force trees to grow even though they do not have the necessary nutrients. As well, the trees are sometimes forced to grow well into late autumn when it is actually time for them to prepare for severe frosts in the winter.

Effects of Acid Rain

The rough canopies of mature evergreen forests are efficient scavengers of particulate and gaseous contaminants in polluted air. This results in a more acidic deposition under the forest canopies than in open land.

Acid rain acidifies the soils and waters where it falls, killing off plants and animals. Surface water acidification can lead to a decline in, and loss of, fish populations and other aquatic species including frogs, snails and crayfish. Acid rain affects trees, usually by weakening them through damage to their leaves. Certain types of building stone can be dissolved in acid rain.


Soil is the basis of wealth upon which all land-based life depends. Acid deposition is known to wash essential nutrients from soils, and aluminium which is normally bound in the soil may be released into ground water. Soil acidification may affect the health of trees and other vegetation.

Soils containing calcium and limestone are more able to neutralise sulphuric and nitric acid depositions than a thin layer of sand or gravel with a granite base. If the soil is rich in limestone or if the underlying bedrock is either composed of limestone or marble, then the acid rain may be neutralised. This is because limestone and marble are more alkaline (basic) and produce a higher pH when dissolved in water. The higher pH of these materials dissolved in water offsets or buffers the acidity of the rainwater producing a more neutral pH.

In regions where the soil is not rich in limestone or if the bedrock is not composed of limestone or marble, then no neutralising effect takes place, and the acid rainwater accumulates in the bodies of water in the area. This applies to much of the northeastern United States where the bedrock is typically composed of granite. Granite has no neutralising effect on acid rainwater. Therefore, over time, more and more acid precipitation accumulates in lakes and ponds. Such areas or catchments are termed acid-sensitive (poorly buffered), and can suffer serious ecological damage due to acid rain.

To grow, trees and other vegetation need healthy soil to develop in. Long-term changes in the chemistry of some sensitive soils occur as a result of acid rain. As acid rain moves through the soils, it can strip away vital plant nutrients such as calcium, potassium and magnesium through chemical reactions, thus posing a potential threat to future forest productivity. Furthermore, the number of microorganisms present in the soil also decreases as the soil becomes more acidic. This further depletes the amount of nutrients available to plant life because the micro- organisms play an important role in releasing nutrients from decaying organic material. Trees growing in acidified soil are more susceptible to viruses, fungi and insect pests. Other plant life may grow more slowly or die as a result of soil acidification.

Poisonous metals such as aluminium, cadmium and mercury are leached from soils through reacting with acids. This happens because these metals are bound to the soil under normal conditions, but the added dissolving action of acids causes rocks and small-bound soil particles to break down. In addition, the roots of plants trying to survive in acidic soil may be damaged directly by the acids present. Finally, if the plant life does not die from these effects, then it may be weakened enough so that it will be more susceptible to other harsh environmental influences like cold winters or high winds.

Sick soils are those depleted of their life-giving nutrients after decades of pollution-caused leaching. The metal aluminum is usually safely locked away in silicate compounds in the soil. The molecules of heavy metals brought in via pollution frees the molecules of the lightweight metal aluminum. The tree then takes up the poisonous aluminum.

Acid rain leaches the soil and acts directly on the leaves and needles -- acid affected coniferous trees have a slower metabolism than healthy trees. The stomata do not readily close and the fluids inside the cells of the needles freeze solid.


The acidity of water of freshwater lakes and streams is predominantly determined by the soil and rock types of an area, since 90% of the water entering these water courses has passed through the ground. Only 10% of water in lakes and streams comes directly from rainfall. Consequently, areas that are most susceptible to freshwater acidification have an acidic geology such as granite and a peat-based soil.

Acid rain can enter the watercourse either directly or more usually through the catchment. If the catchment has alkaline-rich soil then the acid rain may be neutralised, and water entering the lake is of low acidity. However, if the catchment has a thin, alkaline-poor soil then acid water is passed to the lake. Acidification of a lake occurs over time. At first the natural buffering capacity of the lake neutralises the additional acidity entering the lake but at some point, the lake buffering capacity runs out and the acidity of the water increases rapidly. In time, the lake water stabilises at a certain acidity, maintaining a small number of species of plants and animals, but usually lacking many fish.

Massive use of fertilizers combined with massive run-off from highways and other places causes an excess of nitrogen in our waters: ponds, lakes, and streams. This often results in great cycles of algal bloom and bust. The algae feed on the excessive nutrients and their numbers increase; the dead algae descend to the bottom and begin to decompose; oxygen is used up in the expanse of the decomposing bacteria; oxygen becomes scare; and other life forms start dying.

The onset of acidification brings about a clearer bluer water body due to the settling out of decaying organic matter. Whilst the total amount of living matter remains largely unchanged, the diversity of different species drops considerably. Rushes thrive in acidified freshwater. White Sphagnum moss may invade lakes and form a thick green carpet over the bottom of the lake on account of the clearer waters allowing more light to reach the moss. Soft bodied animals such as leeches, snails and crayfish are early victims, often being one of the first signs of the commencement of acidification. Few insect species are very resistant to acidification and species such as mayfly disappear even under moderate acidification. However, species such as dragonfly larvae, water beetle and bloodworms can grow abnormally large in their population size when competition is removed. Salmon, trout and roach are particularly at risk from freshwater acidification, pike and eel being relatively resistant.


Ozone is a chemically active form of oxygen that becomes a powerful bleach. It affects the lungs of humans and causes plant foliage to die. The cumulative effects of decades of ozone no longer allow photosynthetic processes to operate. Laws aimed at reducing the production of tropospheric ozone are based on human-health standards, not tree-health standards, but trees are much more susceptible than people.

Most injuries to plants occur internally after air replete with ozone (O3) enters through the stomata (leaf air pores).

Four major symptoms of ozone injury can be distinguished on leaves:

1. Pigmented Lesions (Stipples)

Punctate spots consisting of a few injured or killed palisade cells that mainly effects the upper leaf surfaces. Ash leaves develop a white stipple.

2. Bleaching

Large areas on the upper leaf surface changes color to a reddish­brown or bronze. Ozone damage appears as bleached patches between the veins. Virtually all species are affected.

3. Chlorosis

Chlorosis is the loss of chlorophyll in the leaf, usually on the upper surface. This is a symptom of chronic ozone injury. It can result in premature leaf fall, yellow spots (chloroses), or dead parts (necroses).

4. Bifacial Necrosis (both sides of leaf)

This problem occurs when the plant experiences a short term acute exposure to high ozone concentrations. The mesophyll tissue between the upper and lower epidermis is destroyed, along with injury to the small veins. If extremely high ozone exposure occurs, the larger veins can also be injured.

If the damge is extreme, the entire transport system can be affected, leading to plant growth rate reduction (and, subsequent problems, such as increased crop yield loss).

J. David McKee (Tropspheric Ozone: Human Health & Agricultural Impacts CRC Press Inc., 1994:175) reports that around 90% of the crop losses in the United States from air pollution can be ascribed to problems with ozone. The rising level of ozone concentration is also a significant factor in the forest decline in large Europe, Japan, India, Mexico, Great Britian, Canada and Scandinavia.


The North American shield has been affected by rising chlorofluorocarbons, mainly the Freon of air conditioners and aerosol cans, which break down the O3 molecule.

Stratospheric ozone forms a "shield" to protect the earth from the bombardment of damaging ultraviolet rays in the "B" range -- a shorter wavelength than the ultraviolet rays that give us suntans and make the sky blue.

Ultraviolet-B rays stream through a thinning stratospheric ozone layer. UV-B causes blindness and cancer.

You can see the bad effect on the leaves caused by UV-B burns by the blotchy and wrinkled leaves. UV-B especially affects white pine needles, the affected needles being bent and lying flat along the branch. This is due to the burning damage to the tender base needles as they emerge, causing a scarring that hardens the exposed side.


maple in mortal decline
ashes are yellow
beeches blighted
white pines are dying of ozone
butternut nearly extinct
red mulberry nearly extinct
hemlocks dying
dogwood dying