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A forest is a biome that is dominated by trees. The characteristics of a forest depend largely upon climate, soil type, and range. Therefore the plant and animal life of a tropical rain forest is very different from that found in a boreal or temperate forest. Forests can have very rich and diverse ecologies. For instance, although the tropical rain forests now cover less than 10% of Earth’s surface, they still house a half to two thirds of all known plant and insect species.

The complex and rich biodiversity of forests makes them vulnerable to overexploitation. People have always taken wood from the forest, or used the land to grow food. But forest soils are often poor and, once disturbed, may not recover. Meanwhile, tree loss can destroy the habitats of animals living in the forest. Scientists are working to find ways of using forests sustainably if they are to continue to play their vital role in the planet’s ecosystem.

Historical Background and Scientific Foundations

Forests were first formed around 420 million years ago, during the geological Silurian period, when plants and animals emerged from the sea and began to occupy the land. These primeval forests were very different from those of modern times. They were dominated by giant horsetails, mosses, and tall ferns. Simple invertebrates would have been the only animal life present. Gymnosperms, or trees, evolved around the Triassic period of 245 to 208 million years ago and would have caused dramatic change in the forest ecosystem.

Flowering plants, birds, insects, and mammals evolved from the Cretaceous period of 144 to 65 million years ago and would have added to the rich diversity of forest life. All of these forests were tropical. The temperate forests of the Northern Hemisphere appeared after the ice ages.

The United Nations Food and Agriculture Organization (FAO) defines a forest as a region where trees provide more than 10% of the land cover. That is a broad description. At one extreme would be the African savannas, where trees might cover up to 20% of the land area, but there would be a lot of open ground in between. At the other extreme are the closed canopy forests, like tropical rain forests, where tree coverage is so dense that little light actually penetrates the forest floor. Today, forests cover around a third of the land surface of Earth.

There are several types of forests; their appearance and ecology depend upon climate. The moist tropical forests—often known as rain forests—are mostly found quite close to the equator, between latitudes 23.5 North and 23.5 South, and in areas where rainfall is more than 80 in (200 cm) a year. In tropical rain forests, temperatures typically range from 77 to 81°F (25 to 27°C). The interior of the forest is warm, rather than hot, because the rain tends to evaporate rapidly, which causes cooling. One of the best-known tropical rain forests is in the Amazon area of Brazil. Forests do not have a winter in the usual sense, although some of them have a dry season, varying in length. Temperatures do not vary during the day in the tropical rain forests and the length of the day is 12 hours. Then there are cool cloud forests, which are a type of rain forest occurring in mountainous regions. Here the trees and other vegetation are always wet because temperatures are relatively low and there is less evaporation.

The biodiversity of trees in a rain forest is high, with a hundred or more different species possibly occurring in a square mile (or square kilometer). The trees that do well in rain forests have big leaves, so they can trap as much sunlight as possible for photosynthesis from the


BIODIVERSITY: Literally, “life diversity”: the wide range of plants and animals that exist within any given geographical region.

BIOME: A well-defined terrestrial environment (e.g., desert, tundra, or tropical forest) and the complex of living organisms found in that region.

DECIDUOUS: Plants that shed leaves or other foliage after their growing season.

EVERGREEN: Bearing green leaves throughout the entire year.

SUSTAINABILITY: Practices that preserve the balance between human needs and the environment, as well as between current and future human requirements.

TEMPERATE: Pertaining to a region located between 30 and 60 degrees latitude in both hemispheres in which the climate undergoes seasonal change in temperature and moisture.

TROPICAL: The area between 23.5 degrees north and south of the equator. This region has small daily and seasonal changes in temperature, but great seasonal changes in precipitation.

relatively low levels penetrating the canopy. The leaves tend to be thin too, which makes for rapid evaporation. Trees are tall, with shallow roots. The dense canopy is home to tree-dwelling animals such as birds, bats, and monkeys. Ground vegetation may be sparse, as so little light penetrates the canopy. Therefore, the popular image of hacking through rain forest with a machete bears little comparison with the reality. The plants that do grow there include orchids, vines, mosses, ferns, and palms.

The soil in a rain forest tends to be old, thin, acidic, and low in nutrients. Life there is supported instead by the nutrients bound up in the organisms themselves. When they die, they decompose rapidly and are recycled. This is in contrast to the temperate forests, whose soils are rich in nutrients from decaying leaf litter, and these are absorbed by the roots of plants.

Temperate forests are found between 30 and 55 degrees of latitude in eastern North America, north east Asia, and western and central Europe. Temperate forests have a distinct winter season and well-defined seasons with 150 to 200 growing days during the year. Temperatures in temperate forests vary between -22° and 86° F (-30° and 30° C). They have between 15 and 25 in (75 to 150 cm) of rain a year, depending on the local climate. Compared to a tropical rain forest, there are far fewer tree species—maybe just three or four per square mile or kilometer. Animal life includes squirrels, birds, deer, foxes, wolves, and bears.

Temperate forests are divided, broadly, into those made up of deciduous trees and those made up of evergreens. Deciduous trees shed their leaves every fall. Typically they will first create a beautiful display of colors, as the chlorophyll in their leaves decays to reveal other pigments that are present but normally masked with its green. Broad-leaved deciduous forests occur wherever rainfall is plentiful and common tree species found include oak, beech, elm, and willow. Evergreen trees, as the name suggests, retain their leaves. Many evergreens are coniferous, or cone-bearing, but there are also broad-leaved evergreens. Coniferous forests are found in a wide range of temperature and moisture conditions, including colder and drier climates. They occur where moisture is limited and in colder climates. Their thin waxy leaves help reduce moisture loss from the tree.

A temperate forest will have a dense canopy, when trees are in leaf. In the spring, before the leaves come out, there will often be a dense undergrowth of ferns and wild flowers. These forests also support many species of songbirds and may have pools of water that provide a habitat for a range of amphibians and insects. The eastern part of the United States and the southern part of Canada were once heavily covered in deciduous forest, as was most of Western Europe. When temperate forest is cut down, it does tend to regrow, but the dominant tree species may differ from the original.

Boreal forests, sometimes also known as northern forests or taiga, are those found between 50 and 60 degrees of northern latitude. Two thirds of these forests are in Siberia and the rest occur in Scandinavia, Alaska, and Canada. They may also be found in mountainous regions at lower latitudes. Boreal forests are often composed of evergreens like spruce, cedar, pine, and fir, perhaps mixed in with some deciduous tree species including birch and maple. These forests grow slowly, because of the cold climate in these regions. They have long, cold, dry winters and a short, moist, and moderately warm summer with a growing season of around 130 days. Precipitation is mainly in the form of snow, with 8 to 20 in (40 to 100 cm) annually. Soil is thin and acidic, and the tree canopy dense, so vegetation on the floor of a boreal forest tends to be sparse. Animal life includes birds such as hawks and woodpeckers, and bears, lynx, foxes, wolves, and hares.

Forests have many functions. They represent a massive carbon sink because trees capture carbon dioxide in the atmosphere through photosynthesis. The amount of carbon locked away in the world’s forests is thought to be about 422 billion metric tons. They also play a role in precipitation by adding to the moisture in the atmosphere through transpiration, the evaporation of moisture through stomata, which are the tiny pores on the surfaces of leaves. They are also a habitat for a rich diversity of plants and animals.

Forests are also economically important, for wood and plant products. Annual global consumption of wood is four billion metric tons, which is more than steel and plastic combined. Around half of the people in less developed countries depend upon wood for fuel. Paper

is also a major product from wood and accounts for about a fifth of wood consumption. One quarter of the world’s forests are grown and managed specifically for wood production. Coniferous forests are the most important source of wood in North America with major production areas including the southern Atlantic States and the Pacific Northwest from northern California to Alaska. Meanwhile, boreal forest in Siberia, Canada, and the western United States are also significant commercial wood producers.

Impacts and Issues

Forests have long been used as a human resource. They have been cleared for agriculture and their products, particularly wood, harvested. The associated road building adds another impact to the forest. As the global population has grown, the pressure on the forest environment has increased so that, today, deforestation is a major environmental challenge. The impact is perhaps greatest on the tropical rain forests because of their poor soil. When trees are removed, this soil is rapidly eroded by rain. Moreover, as it is not particularly well suited to agriculture, farmers have tended to grow for a short period until the soil is exhausted and then move on, thereby expanding the area of deforestation.

A century ago, there were 4.8 million square mi (12.5 million square km) of tropical rain forest, which is roughly equivalent to the whole area of the United States. Although there is broad agreement that much of this is being cleared and lost forever, there is a range of opinion on what exactly is meant by deforestation. It may be seen as total conversion of an area into agricultural land, or just as a region within a forest that is cleared by logging. Satellite imagery can provide estimates of the extent of global deforestation and also detect the forest fires that destroy forested land, either deliberately or accidentally. Estimates of deforestation in tropical areas vary from 12 to 49.4 million acres (5 to 20 million hectares) per year. The figure given by the FAO of 12.3 million hectares is quite widely accepted and is equivalent to around 0.6% of the total each year.

In short, more than half of the tropical rain forests that used to exist has probably already been destroyed by human activity. Brazil has one of the world’s highest rates of deforestation, but it began with more tropical rain forest than most other countries, so impact may be less. Indonesia and Malaysia may have similarly high deforestation rates. Africa, Madagascar, Cameroon, Ghana, Haiti, and Liberia are losing their forests at an alarming rate. And in central America, nearly two thirds of the original tropical forest has been lost, mainly to cattle ranching.

Temperate forests are also at risk, with rates of deforestation thought to be highest in Siberia. Meanwhile, there is particular concern over the loss of the so-called old growth or primeval forests. These forests are of particularly great ecological value because they house so much of the world’s biodiversity and even indigenous human cultures. These forests are characterized by the feature of being so undisturbed by human activity that the trees live out their natural lifespan. India, Burma, Thailand, and Vietnam have little of their primeval forest left. Around three quarters of the remaining old growth forests lie in Russia, Canada, Brazil, Indonesia, and Papua New Guinea. Their survival owes more to their remoteness than to effective environmental protection measures.

Deforestation has many different impacts. It releases carbon into the atmosphere, which could accelerate climate change. It could also decrease the amount of rainfall by loss of transpiration from leaves. This, in turn, increases the risk of drought and can cause further destruction of forest through fires. There is also the loss of habitat associated with deforestation. Many plant and animal species living in the rain forests remain to be discovered and some are already endangered. It is difficult, therefore, to estimate the true impact of forest loss upon global biodiversity. We could be losing resources that could provide benefit to humanity. Some of the plants found in forests have proved to be a source of new drugs. For instance taxol, which is extracted from the Pacific yew, is a potent anti-cancer therapy.

Many developed countries now see the value of forest protection and are undertaking extensive reforestation schemes. Advances in agriculture mean that it is no longer necessary, in many places, to attempt to exploit the poor soil of the forests. Since people also travel more and have leisure time, they have applied pressure to conserve their forests as places of recreation and esthetic value. Therefore, in the United States and China there is now more forest than there was 200 years ago, thanks to replanting. Nearly half of those countries with the most forest area in the world have increased their forest biomass in the last two decades.

Forest management can be an opportunity to reforest an area, but only if it is done in a sustainable manner. The monoculture approach, where one species of tree is planted and cropped intensively, is not sustainable however. It provides rapid growth and easy harvest, but does not promote biodiversity, nor does it protect the soil from erosion. With this approach, all the trees in an area are cut regardless of size and promptly removed. However, alternative approaches have been developed. Leaving behind woody debris after logging helps nutrient recycling and protects the soil. It is also more ecologically sound to harvest selectively, taking only a percentage of the trees in an area and this has been shown to improve the quality and quantity of timber in the long-term.

Forest protection must not be confined only to rich countries. It needs global attention. Around 12% of world forests now have protected status, with various local schemes underway to restore the land and use it sustainably. There are also the “debt for nature”schemes, whereby conservation groups buy off an existing debt for a developing country. In return, they ask the country to put a conservation project in place. Notable examples of such schemes are underway in Bolivia, Ecuador, and Costa Rica.

There are many actions that can be taken on an individual level to protect forests. E-mail can cut down on the use of paper, and paper can be re-used and recycled. Products made out of tropical hardwoods like ebony, teak, and mahogany, should only be bought if they come from a forest that is being managed sustainably. There are many products that are being harvested sustainably by local people, such as Brazil nuts, cashews, and mushrooms. Buying these products supports their efforts.

See Also Ecosystem Diversity; Forest Resources; Habitat Loss; Human Impacts; Land Use; Logging; Rain Forest Destruction; Reforestation



Cunningham, W.P., and A. Cunningham. Environmental Science: A Global Concern. New York: McGraw-Hill International Edition, 2008.

Kaufmann, R., and C. Cleveland. Environmental Science. New York: McGraw-Hill International Edition, 2008.

Web Sites

University of California Museum of Paleontology. “The Forest Biome.” (accessed March 4, 2008).

Susan Aldridge

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