Rain is supposed to be healthy for plants and trees. It provides them with water, which is essential for their growth. So why is some rain killing and damaging the forests? Because the raindrops contain sulfuric, nitric and other acids. These acids originate in sources such as gasoline and diesel used in cars, fossil-fuel-fired electric power plants, industrial boilers, and residential furnaces. When high concentrations of acidic compounds from such sources mix with atmospheric moisture, they create acids which are deposited in precipitation. This process creates acid rain.

Above: Traffic is one of the leading causes of air pollution. Below: Large cities build up concentrations of air pollution, and surrounding areas experience poorer air quality, a fact which affects both people and forests. Photo credit Corel Photo Clipart CD.

Acid rain harms forests in several ways. One detrimental effect of the rain is that the leaves are damaged. For example, the wax on the leaves is worn away, making them less resistant to more rain and insects. The leaves also lose nutrients, leading to the eventual demise of the tree. Furthermore, the fallen leaves do not decompose as quickly because acid rain causes many organisms living in the soil to have diminished respiration rates. This in turn increases the ammonia in the soil because the nutrients that are usually released in decomposition are not being made.

Emissions from industry, plus those from vehicles, are the largest sources of air pollution. Photo credit Corel Photo Clipart CD.

This decrease of nutrients prevents the forest from growing. Acid rain also stifles new growth and kills many of the seedlings. Another effect from acid rain is that the tree's growth is decreased because the microorganisms that live near it die. Acid rain also causes physical damage to the roots, decreasing the rate of growth and eventually leading to the death of the tree. Acid causes the pH level in the soil to fall and when it reaches 4.0, aluminum and other metals are leached out of the soil. The aluminum is filtered through the roots, damaging them, and causing the tree's death.

Pollution affects different forests in different ways. Some are so severely damaged that many or all of the trees begin to die. Some of these forests that have suffered major declines due to pollution are described here. find out more...

The United States and 24 other nations have agreed to keep the nitrogen oxide emission levels at 1987 rates as a part of the Long-Range Transboundary Air Pollution Agreement. The Clear Air Act was intended to cut in half sulfur dioxide emissions from power plants by the year 2000.

Alternative forms of transportation can be used by more people at once, and therefore create far less air pollution than cars. Photo credit Corel Photo Clipart CD. In cars, catalytic converters are used to lessen the emissions of harmful exhaust. Pollutants from factories and such are caught in filters or changed into less dangerous liquid or solid forms.

Other sources of pollution kill the animals in the forest, and when even one important predator or prey species disappears, the entire ecosystem can disintegrate. Almost 40% of the endangered freshwater fishes in North America are at risk because of water pollution. Much of that water pollution has come from pulp mills, where wood chips are manufactured into pulp and paper products. The toxic organochlorine compounds ruin fish habitat. Often, pollutants collect in increasing quantities over time, and have long-lasting effects that at first go unnoticed. By the time chemicals are determined to be the cause of plant and animal deaths, it is often too late to prevent more damage. Once an area is contaminated with pollutants, it is very difficult to clean it up.

Pesticides are often sprayed on newly cleared and replanted areas, to keep down the growth of brush and help keep insects from eating new trees. However, no one knows what the long-term effects will be of spraying these pesticides on forest soils.	Beneficial insects and even essential micro-organisms within the soil are undoubtedly also harmed. However, very little is known about the actual effects on these unseen creatures, and there is not enough evidence to convince many people that the pesticides may seriously damage forest ecosystems.
When chemical pesticides kill too many insect predators, the insects they feed on suddenly experience a population explosion. In these unnaturally large numbers, any species can become a pest; the pesticides have effectively created a new pest where there were previously only harmless insects.	More pesticides are then brought in to control these new pests. The large amounts of pesticides now in the environment mean that some species are beginning to become resistant to them, and still more pesticides are needed to combat these creatures.

In a short-term economic view, pesticides still save money and for this reason have more benefits than drawbacks. However, the drawbacks include such a serious potential impact on all forest life, and on top of this are often a danger to human health, that we may not continue to depend on them as much as we do now.