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A rainforest, or a wet forest, is a forested biome with high annual rainfall. Tropical rainforests arise due to the Intertropical Convergence Zone, but temperate rain forests also exist. In addition to prodigious rainfall, many rainforests are characterized by a high number of resident species and tremendous biodiversity of their flora and fauna.

The largest tropical rainforests exist in the Amazon basin (the Amazon Rainforest), in Nicaragua (Los Guatuzos, Bosawás and Indio-Maiz), the southern Yucatán Peninsula-El Peten-Belize contiguous area of Central America (including the Calakmul Biosphere Reserve), in much of equatorial Africa from Cameroon to the Democratic Republic of Congo, in much of southeastern Asia from Myanmar to Indonesia and Papua New Guinea, eastern Queensland, Australia and in some parts of the United States. Outside of the tropics, temperate rainforests can be found in British Columbia, southeastern Alaska, western Oregon and Washington, the western Caucasus (Ajaria region of Georgia), parts of the western Balkans, New Zealand, Tasmania, and parts of eastern Australia.

While it is commonly believed that rainforests provide much of the oxygen for the planet, and are the "lungs of the world", most rainforests do not in fact provide oxygen for the rest of the world. Through the decomposition of dead plant matter, rainforests consume as much oxygen as they produce, except in certain conditions (primarily swamp forests) where the dead plant matter does not decay, but is preserved underground instead (ultimately to form new coal deposits over enough time). Still, rainforests act as major consumers of atmospheric carbon and may play a large role in cooling air that passes through them. As such, many scientists feel that the rainforests are of vital importance within the global climate system.

Characteristics

Rainforests are characterized by high rainfall, with definitions setting minimum normal annual rainfall between 2000 mm (about 78 inches or 2 meters) and 1700 mm (about 67 inches). The soil can be poor because high rainfall tends to leach out soluble nutrients.

Rain forests are home to two-thirds of all the living animal and plant species on the planet. It has been estimated that many hundreds of millions of new species of plants, insects, and microorganisms are still undiscovered and as yet unnamed by science. Tropical rain forests are called the "jewel of the earth", the "Earth's lungs", and the "world's largest pharmacy" because of the large amount of natural medicines discovered there.

Despite the growth of flora in a rainforest, the actual quality of the soil is quite poor. Oxisols, infertile, and deeply weathered, have developed on the ancient Gondwanan shields. Rapid bacterial decay prevents the accumulation of humus. The concentration of iron and aluminium oxides by the laterization process gives the oxisols a bright red color and sometimes produces minable deposits (e.g. bauxite). On younger substrates, especially of volcanic origin, tropical soils may be quite fertile.

The undergrowth in a rainforest is restricted in many areas by the lack of sunlight at ground level. This makes it possible for people and other animals to walk through the forest. If the leaf canopy is destroyed or thinned for any reason, the ground beneath is soon colonised by a dense tangled growth of vines, shrubs and small trees called jungle.

Trees

There are several common characteristics of tropical trees. Tropical species frequently possess one or more of the following attributes not commonly seen in trees of higher latitudes.

Many species have broad, woody flanges (buttresses) at the base of the trunk. Originally believed to help support the tree, now it is believed that the buttresses channel stem flow and its dissolved nutrients to the roots. Large leaves are common among trees of the C layer. Young individuals of trees destined for the B and A layers may also have large leaves. When they reach the canopy new leaves will be smaller. The large leaf surface helps intercept light in the sun-dappled lower strata of the forest. Drip tips facilitate drainage of precipitation off the leaf to promote transpiration. They occur in the lower layers and among the saplings of species of the emergent layer (A layer).

Trees are often well connected in the canopy layer especially by the growth of woody climbers or lianas, plants with epiphytic adaptations, allowing them to grow on top of existing trees in the competition for sunlight.

Other characteristics that are more frequent in tropical tree species than in temperate forests include:

• Exceptionally thin bark, often only 1-2 mm thick. It is usually very smooth, although sometimes covered with spines or thorns.
• Cauliflory, the development of flowers (and hence fruits) directly from the trunk, rather than at the tips of branches.
• Large fleshy fruits attract birds, mammals, and even fish as dispersal agents.

Rainforest layers

The rainforest is divided into four different parts, each with different plants and animals, adapted for life in that particular area.

Emergent layer

This layer contains the emergents, a small number of very large trees which grow above the general canopy, reaching heights of 45-55 m, a few species rarely to 60 m or 70 m tall. The trees are often evergreens, but some are deciduous in dry seasons. They need to be able to withstand the harsh temperatures and high winds. Eagles, butterflies, bats and certain monkeys inhabit this layer.

Canopy layer

The canopy layer contains the majority of the larger trees, typically 30-45 m tall. The densest areas of biodiversity are found in the forest canopy, a more or less continuous cover of foliage formed by adjacent treetops.

The canopy, by some estimates, is home to 40% of all plant species, suggesting that perhaps half of all life on Earth could be found there. The fauna is similar to that found in the emergent layer, but more diverse. A quarter of all insect species are believed to exist in the rainforest canopy.

Scientists have long suspected the richness of the canopy as a habitat, but have only recently developed practical methods of exploring it. As long ago as 1917, U.S. naturalist William Beebe declared that "another continent of life remains to be discovered, not upon the Earth, but one to two hundred feet above it, extending over thousands of square miles".

True exploration of this habitat only began in the 1980s, when scientists developed methods to reach the canopy, such as firing ropes into the trees using crossbows. Exploration of the canopy is still in its infancy, but other methods include the use of balloons and airships to float above the highest branches and the building of cranes and walkways planted on the forest floor.

Understory layer

There is a space between the canopy and the forest floor, which is known as the understorey (or understory). This is home to a number of birds, snakes, and lizards. The leaves are much larger at this level. Insect life is also abundant.

Forest floor

This region receives only 2% of the rainforest's sunlight. Thus, only specially adapted plants can grow in this region. Away from river banks, swamps and clearings where dense undergrowth is found, the forest floor is relatively clear of vegetation, as little sunlight penetrates to ground level. It also contains decaying plant and animal matter, which disappears quickly due to the warm, humid conditions promoting rapid decay.

The rainforest as a source of drugs

Tropical rain forests are called the 'world's largest pharmacy' because of the large amount of natural medicines discovered there. Nearly half of the medicines that we use come from the rainforests. For example, rain forests are responsible for containing the "basic ingredients of birth control hormones, stimulants, and tranquilizing drugs" (Banks 36). Curare (a paralyzing drug) and quinine (a malaria cure) are also found there. Scientists believe that the cures for many more diseases will be discovered there in the future. Currently, 121 prescription drugs currently sold worldwide come from plant-derived sources. And while 25% of Western pharmaceuticals are derived from rainforest ingredients, less than 1% of these tropical trees and plants have been tested by scientists.

Degradation of the rainforests

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Tropical and temperate rain forests have been subjected to heavy logging and agricultural clearance throughout the 20th century, and the area covered by rainforests around the world is rapidly shrinking. It is estimated that the rainforest was reduced by about 58,000 km² annually in the 1990s. Rainforests used to cover 14% of the Earth's surface. This percentage is now down to 6% and it is estimated by some that the remaining natural rainforests could disappear within 40 years (mid-21st century). Biologists have estimated that large numbers of species are being driven to extinction, possibly more than 50,000 a year, due to the removal of habitat with destruction of the rain forests. Protection and regeneration of the rainforests is a key goal of many environmental charities and organisations, including EcologyFund and the Nature Conservancy.

See also

• Cloud forest
• Illegal logging
• Jungle
• Tropical and subtropical moist broadleaf forests

Bibliography

• Richards, P. W. (1996). The tropical rain forest. 2nd ed. Cambridge University Press ISBN 0521421942
• Whitmore, T. C. (1998) An introduction to tropical rain forests. 2nd ed. Oxford University Press. ISBN 0198501471

External links

• Forest Conservation Portal
• Rainforest Live - Facts & Figures
• Save The Rainforest
• Tropical Rainforests
• Rainforest facts and information from The Nature Conservancy
• Rainforest Alliance
• Virtual Rainforest Trip
• CFACT.org (a free-market group that disputes some claims)
• Rainforest people - Pygmies Culture of the African rainforest people, with photos of nature and animals
• Amazon Conservation Team An organization working in partnership with indigenous people in conserving biodiversity, health, and culture in tropical America.
• Save the Amazon Rainforest
• Los Guatuzos, Nicaragua
• Indio-Maiz, Nicaragua
• British Columbia's Rainforests Essays by Elmer G. Wiens.

• Old growth forests
• Rainforests
• Terrestrial biomes