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This article is about the rainforest. For more specific information, see Temperate rain forest or Tropical rainforest. For Jenny Diski's novel, see Rainforest (novel).

Rainforests, or rain forests, are forests characterized by high rainfall, with definitions setting minimum normal annual rainfall between 1750 mm and 2000 mm (68 inches to 78 inches).

Rainforests 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. Tropical rain forests are called the "jewels of the earth", and the "world's largest pharmacy" because of the large number of natural medicines discovered there. Tropical rain forests are also often called the "Earth's lungs", however there is no scientific basis for such a claim as tropical rainforests are known to be essentially oxygen neutral, with little or no net oxygen production..

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 colonized by a dense tangled growth of vines, shrubs and small trees called jungle.

Distribution

Tropical rainforest

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, northern and eastern Australia and in the Hawaiian Islands. The majority of tropical rainforest is found within a 20 degree band around the equator.

Temperate rainforest

Outside of the tropics, temperate rainforests can be found in North America including the northwestern coast of the United States and the Pacific coast of Canada. In Europe they are found in coastal portions of Ireland, Scotland and southern Norway, parts of the western Balkans along the Adriatic coast, coastal areas of the eastern Black Sea including Georgia and coastal Turkey. In Asia portions of southern China, Taiwan, much of Japan, Korea, Sakhalin Island and the adjacent coast of Russia.

Soils

Despite the growth of vegetation in a rainforest, the actual quality of the soil is often quite poor. 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.

Effect on global climate

A natural rainforest emits and absorbs vast quantities of carbon dioxide. Over the long term these fluxes are approximately in balance, so that an undisturbed rainforest would have little net impact on atmospheric CO₂ levels, though they may have other climatic effects (on cloud formation, for example, by recycling water vapour). No rainforest in the 21st century can be considered to be undisturbed. Human induced deforestation plays a significant role in causing rainforest to release carbon dioxide, as do natural processes such as drought that result in tree death and these droughts themselves are believed to be exacerbated by human induced climate change. Some climate model runs with interactive vegetation predict a large loss of Amazonian rainforest around 2050 due to drought, leading to forest dieback and the subsequent feedback of releasing more carbon dioxide.

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 a small number of very large trees which grow above the general canopy, reaching heights of 45-55 m, although on occasion a few species will grow to 70 m or 80 m tall. They need to be able to withstand the hot temperatures and strong winds. Eagles, butterflies, bats and certain monkeys inhabit this layer.

Canopy layer

The canopy layer contains the majority of the largest 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. The science of accessing tropical forest canopy using airships, or similar aerial platforms, is called dendronautics.

Understory layer

There is a space between the canopy and the forest floor, which is known as the understory (or understorey). This is home to a number of birds, snakes, and lizards, as well as predators such as jaguars, boa constrictors, and leopards. The leaves are much larger at this level. Insect life is also abundant. Many seedlings that will grow to the canopy level are present in the understory. Only about 5% of the sunlight shining on the rainforest reaches the understory. This layer can also be called a shrub layer although some have a perception that the shrub layer is an individual layer.

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. Many forms of fungi grow here which help decay the animal and plant waste.

Fauna

More than half of the world's species of plants and animals are found in the rainforest. Rainforests support a very broad array of fauna including mammals, reptiles, birds and invertebrates. Mammals may include primates, felids and other families. Reptiles include snakes, turtles, chameleons and other families while birds include such families as vangidae and Cuculidae. Dozens of families of invertebrates are found in rainforests. This amounts to over 5 million species of plants and animals.

Human uses

Many foods originally came from tropical forests, and are still mostly grown on plantations in regions that were formerly primary forest. Tropical rainforests are also the source of medicinal drug components. Tropical rainforests also provide timber as well as animal products such as meat and hides. Rainforests also have value as tourism destinations and for the ecosystem services provided.

Deforestation

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. 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 rainforests Error in Webarchive template: Empty url.. Protection and regeneration of the rainforests is a key goal of many environmental charities and organizations. (It is doubtful that this rate will be sustained as the relative cost of logging rises with dwindling resources.)

Another factor causing the loss of rainforest is expanding urban areas. Littoral Rainforest growing along coastal areas of eastern Australia is now rare due to ribbon development to accommodate the demand for seachange lifestyles.

About half of the mature tropical rainforests, between 750 to 800 million hectares of the original 1.5 to 1.6 billion hectares that once graced the planet have already been felled. The devastation is already acute in South East Asia, the second of the world's great biodiversity hot spots. Most of what remains is in the Amazon basin, where the Amazon rainforest covered more than 600 million hectares, an area nearly two thirds the size of the United States. The forests are being destroyed at an ever-quickening pace. Unless significant measures are taken on a world-wide basis to preserve them, by 2030 there will only be 10% remaining with another 10% in a degraded condition. 80% will have been lost and with them the natural diversity they contain will become extinct.

Many tropical countries, including Indonesia, Thailand, Malaysia, Bangladesh, China, Sri Lanka, Laos, Nigeria, Liberia, Guinea, Ghana and the Cote d'lvoire have already lost large areas of their rainforest. Eighty per cent of the forests of the Philippine archipelago have already been cut down. "In 1960 Central America still had four fifths of its original forest; now it is left with only two fifths of it. Half of the Brazilian state of Rondonia's 24.3 million hectares have been destroyed or severely degraded in recent years. Several countries, notably the Philippines, Thailand and India have declared their deforestation a national emergency.

See also

• Tropical rainforest
• Temperate rain forest
• Cloud forest
• Jungle
• Endangered species
• Tropical and subtropical moist broadleaf forests
• Deforestation
• Illegal logging
• Rubber tapping
• Coalition for Rainforest Nations
• Holocene extinction event

References

General references

• Richards, P. W. (1996). The tropical rain forest. 2nd ed. Cambridge University Press ISBN 0-521-42194-2
• Whitmore, T. C. (1998) An introduction to tropical rain forests. 2nd ed. Oxford University Press. ISBN 0-19-850147-1
• Butler, R. A. (2005) A Place Out of Time: Tropical Rainforests and the Perils They Face. Published online: rainforests.mongabay.com

Specific references

1. Broeker, W.S., 2006 "Breathing easy, Et tu, O2" Columbia University http://www.columbia.edu/cu/21stC/issue-2.1/broecker.htm.
2. Moran, E.F., "Deforestation and Land Use in the Brazilian Amazon", Human Ecology, Vol 21, No. 1, 1993 “It took more than 15 years for the "lungs of the world" myth to be corrected. Rain forests contribute little net oxygen additions to the atmosphere through photosynthesis.”
3. Lewis, S.L. , Phillips, O.L., Baker, T.R., Lloyd, J. et al 2004 “Concerted changes in tropical forest structure and dynamics: evidence from 50 South American long-term plots” Phil. Trans. R. Soc. Lond. 359
4. Malhi, Y and Grace, J. 2000 " Tropical forests and atmospheric carbon dioxide”, Tree 15
5. http://www.theage.com.au/articles/2004/03/06/1078464675256.html?from=storyrhs
6. Bourgeron, Patrick S. "Spatial Aspects of Vegetation Structure". In Frank B. Golley (ed.). Tropical Rain Forest Ecosystems. Structure and Function. Ecosystems of the World (14A ed.). Elsevier Scientific. pp. 29–47. ISBN 0444419861.
7. "Sabah". Eastern Native Tree Society. Retrieved 2007-11-14.
8. http://www.dendronautics.org/
9. Myers, N. (1985). The primary source. W. W. Norton and Co., New York, pp. 189-193.

External links

• Rainforest Portal
• Rainforest Action Network
• United Nations Forum on Forests
• The Coalition for Rainforest Nations

• Rainforests