Site hosted by Build your free website today!

Effects of Acid Rain on Forests

Over the years, scientists, foresters, and others have watched some forests grow more slowly without knowing why. The trees in these forests do not grow as quickly as usual. Leaves and needles turn brown and fall off when they should be green and healthy.

Researchers suspect that acid rain may cause the slower growth of these forests. But acid rain is not the only cause of such conditions. Other air pollutants, insects, diseases and drought are some other causes that harm plants. Also, some areas that receive acid rain show a lot of damage, while other areas that receive about the same amount of acid rain do not appear to be harmed at all. However, after many years of collecting information on the chemistry and biology of forests, researchers are beginning to understand how acid rain works on the forest soil, trees, and other plants.

Acid Rain on the Forest Floor

A spring shower in the forest washes leaves and falls through the trees to the forest floor below. Some of the water soaks into the soil. Some trickles over the ground and runs into a stream, river or lake. That soil may neutralize some or all of the acidity of the acid rainwater. This ability of the soil to resist some pH change is called buffering capacity. A buffer resists changes in pH. Without buffering capacity, soil pH would change rapidly. Midwestern states like Nebraska and Indiana have soils that are well buffered. Places in the mountainous northeast, like New York's Adirondack Mountains, have soils that are less able to buffer acids. Since there are many natural sources of acid in forest soils, soils in these areas are more susceptible to effects from acid rain.

How Acid Rain Harms Trees

Acid rain does not usually kill trees directly. Instead, it is more likely to weaken the trees by damaging their leaves, limiting the nutrients available to them, or poisoning them with toxic substances slowly released from the soil.

Scientists believe that acidic water dissolves the nutrients and helpful minerals in the soil and then washes them away before the trees and other plants can use them to grow. At the same time, the acid rain causes the release of toxic substances such as aluminum into the soil. These are very harmful to trees and plants, even if contact is limited. Toxic substances also wash away in the runoff that carries the substances into streams, rivers, and lakes. Less of these toxic substances are released when the rainfall is cleaner.

Even if the soil is well buffered, there can be damage from acid rain. Forests in high mountain regions receive additional acid from the acidic clouds and fog that often surround them. These clouds and fog are often more acidic than rainfall. When leaves are frequently bathed in this acid fog, their protective waxy coating can wear away. The loss of the coating damages the leaves and creates brown spots. Leaves turn the energy in sunlight into food for growth. This process is called photosynthesis. When leaves are damaged, they cannot produce enough food energy for the tree to remain healthy.

Once trees are weak, they can be more easily attacked by diseases or insects that ultimately kill them. Weakened trees may also become injured more easily by cold weather.

Acid rain can harm other plants in the same way it harms trees. Food crops are not usually seriously affected, however, because farmers frequently add fertilizers to the soil to replace nutrients washed away. They may also add crushed limestone to the soil. Limestone is a basic material and increases the ability of the soil to act as a buffer against acidity.

EPA page: forests