Shades of Green: Earth's Forests -- Threats to Forests

By now, it is a well-known fact that forests everywhere are facing a range of threats. The forests that remain today cover a fraction of the area that was forested even a few hundred years ago. And the speed of destruction is only increasing. Everything from the direct and obvious effects of over-cutting to the more subtle effects of climate change are threatening to destroy the last of the remaining natural forests.

Threatened forests are more than just threatened trees. When the trees disappear, so does everything that depends on them, from fungi and micro-organisms to tigers and bald eagles. Each species has its own particular requirements for habitat, and therefore preserving only certain forest ecosystems, which in many cases aren't prime sources of timber, does not protect the habitat of all forest species.

click on a sub-topic to find out more
Threats to Forests Subcategories

When a forest is cleared and new trees are planted, the diversity of animals is not brought back. A plantation of one or a few kinds of trees does not support as many life forms as a natural forest. These plantations will not produce as much high-quality timber. The trees in natural forests have been growing for hundreds and even thousands of years. Planted trees are cut as soon as eighty years after they are planted.

Planting trees and logging, then planting and logging again in continuing cycles can degrade soil and water. Water often washes soil away on cleared slopes, making the area unsuitable for new trees, and destroying riparian zones below. Plantations are also much more susceptible to pests and diseases. Pest controls and fertilizers are often used on plantations, which may have other unfavorable effects on soil and water.

All over the world, even trees that are not being cut directly are dying from other causes, including diseases, pollution and soil degradation. Photo by Maya Walters.

Much of the human-caused forest destruction stems from overpopulation. In many places, there are too many people trying to make a living from too few forest resources. Trees are cleared not only for wood and other products, but so that the land they once grew on can be turned into pastures for cattle and agricultural fields to feed growing numbers of people. As cities expand, forests are cleared to make room for housing developments, shopping malls, golf courses, and other structures that require large amounts of land.

As populations rise, cities expand into new areas which were once covered in trees. Photo by Maya Walters.

Other threats to the health of forests are more indirect. In certain areas, including much of Europe and the eastern United States and Canada, forests are declining because of air pollution. This pollution is from the fossil fuels burned in vehicles -- cars, trucks, buses -- and from industry. Carbon dioxide (C0₂), a gas which contributes to "global warming", is a well known emission from these sources, but vehicles and industry also produce sulphur dioxide and nitrous oxide gases, which create acid rain. Acid deposits can kill trees directly by leaching nutrients from them, resulting in the death of leaves and needles. The most damage, however, is caused when acid gets into the soil and releases poisonous heavy metals, which are naturally present but usually inaccessible. At the same time as the acid rain releases these poisons, it also dissolves and washes away vital nutrients in the soil.

World vegetation maps show the theoretical locations of forests: the areas which have suitable climate for forests to grow. The differences between these maps and the actual forest coverage are therefore the result of human activities or natural disasters, such as fires (which also are often caused by people). But since we can't tell for certain what the actual extent of forest coverage used to be, it is hard to get an accurate measurement of the extent of forest destruction.

We cut down the forests, but even in cities we still try to keep trees for decoration, often in unexpected places! Photo by Maya Walters.

We do know that humans have been clearing the forest for a long time. Evidence of small-scale clearing from as long as 7000 years ago has been found in Central America, for example. Recent estimates have suggested that as much as one half of the tropical forests that existed at that time have since been destroyed.

There is data which indicates that half of the forests existing in the 1950's have since been destroyed. Over half of the total forest destruction in Brazil up to 1978 actually occurred between the years 1975 and 1978.

People have been destroying forests for hundreds of years, but the rate of destruction has been increasing so rapidly that some forests won't last much longer. Some areas, such as the American Southeast, that were cut a century ago are now recovering gradually. However, much of the choice timber is now gone and these "new" Southeast forests, which are only beginning to re-grow, are once again being cut. More remote areas are also being logged heavily, often to supply the growing market for wood-chip products. Coastal temperate forests in Chile, which are home to over 700 species of plants are being cut for this reason. Remote forests in Russia are also being cut, mostly for raw logs. About one fifth of the world's forests are found in Russia. In some cases, these logs are not allowed to be shipped to North America because of fears that species of insect pests might also be unknowingly imported.

When attacked by pests or diseases, often it is only one species of tree in a forest that is severely affected, and others remain healthy. Photo by Maya Walters.

Trees are, of course, also threatened by more "natural" causes, such as pests and diseases. However, pests and diseases have been around for as long as the forests themselves, and the forests didn't begin to seriously decline until threatened by humans. In large numbers, insects can kill trees, often by eating their leaves. Diseases can wipe out entire populations of tree species. For example, in eastern forests, all the large American chestnut trees were killed back to the roots. However, the blight that killed them did not naturally occur in North America; it was brought, accidentally, by humans.

Species that are brought from their natural habitat and "introduced" into a new area by humans can be amazingly destructive. Sometimes these introduced species are types of grasses or shrubs that were transported accidentally as seeds, and upon arriving in a new habitat, manage to replace the native species. Other times, insects are introduced accidentally. With no natural enemies, they quickly multiply to almost epidemic populations, feeding heavily on the leaves of trees and other plants. When mammals are introduced, the native animal populations can suffer greatly. This has been repeated many times on small islands where humans intentionally brought familiar food and companion animals when first moving to the islands. Many native island animals evolved without any serious predators, and were quickly driven to extinction by the introduced livestock. Introduced animals such as goats also cause extensive damage to the land itself, by feeding on all the grasses and small plants, leaving the ground bare and very susceptible to erosion.

sub-topics
[deforestation] [climate change] [loss of biodiversity] [pollution] [erosion] [fire]

view the regular version of the threats to forests article for faster load times

return to the list of condensed articles