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définition - Rainforest

rainforest (n.)

1.a forest with heavy annual rainfall

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définition (complément)

voir la définition de Wikipedia

synonymes - Rainforest

rainforest (n.)

rain forest

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dictionnaire analogique

Wikipedia

Rainforest

                   
  The Daintree Rainforest near Cairns, in Queensland, Australia.

Rainforests are forests characterized by high rainfall, with definitions based on a minimum normal annual rainfall of 1750-2000 mm (68-78 inches). The monsoon trough, alternatively known as the intertropical convergence zone, plays a significant role in creating the climatic conditions necessary for the Earth's tropical rainforests.

Around 40% to 75% of all biotic species are indigenous to the rainforests.[1] It has been estimated that there may be many millions of species of plants, insects and microorganisms still undiscovered in tropical rainforests. Tropical rainforests have been called the "jewels of the Earth" and the "world's largest pharmacy", because over one quarter of natural medicines have been discovered there.[2] Rainforests are also responsible for 28% of the world's oxygen turnover, sometimes misnamed oxygen production,[3] processing it through photosynthesis from carbon dioxide and consuming it through respiration.

The undergrowth in a rainforest is restricted in many areas by the poor penetration of sunlight to ground level. This makes it easy to walk through undisturbed, mature rainforest. If the leaf canopy is destroyed or thinned, the ground beneath is soon colonized by a dense, tangled growth of vines, shrubs and small trees, called a jungle. There are two types of rainforest, tropical rainforest and temperate rainforest.

Contents

Tropical

  General distribution of tropical rainforest

Tropical rainforests are characterized in two words: warm and wet. Mean monthly temperatures exceed 18 °C (64 °F) during all months of the year.[4] Average annual rainfall is no less than 168 cm (66 in) and can exceed 1,000 cm (390 in) although it typically lies between 175 cm (69 in) and 200 cm (79 in).[5].

Many of the world's rainforests are associated with the location of the monsoon trough, also known as the intertropical convergence zone.[6] Tropical rainforests are rainforests in the tropics, found in the equatorial zone (between the Tropic of Cancer and Tropic of Capricorn). Tropical rainforest is present in Southeast Asia (from Myanmar (Burma) to Philippines, Malaysia, Indonesia, Papua New Guinea and northeastern Australia), Sri Lanka, sub-Saharan Africa from Cameroon to the Congo (Congo Rainforest), South America (e.g. the Amazon Rainforest), Central America (e.g. Bosawás, southern Yucatán Peninsula-El Peten-Belize-Calakmul), and on many of the Pacific Islands (such as Hawaiʻi). Tropical rainforests have been called the "Earth's lungs", although it is now known that rainforests contribute little net oxygen addition to the atmosphere through photosynthesis.[7][8]

Temperate

  General distribution of temperate rainforest.

Temperate forests cover a large part of the globe, but temperate rainforests only occur in few regions around the world. Temperate rainforests are rainforests in temperate regions. They occur in North America (in the Pacific Northwest, the British Columbia Coast and in the inland rainforest of the Rocky Mountain Trench east of Prince George), in Europe (parts of the British Isles such as the coastal areas of Ireland and Scotland, southern Norway, parts of the western Balkans along the Adriatic coast, as well as in the North West of Spain and coastal areas of the eastern Black Sea, including Georgia and coastal Turkey), in East Asia (in southern China, Taiwan, much of Japan and Korea, and on Sakhalin Island and the adjacent Russian Far East coast), in South America (southern Chile) and also in Australia and New Zealand.

Layers

A tropical rainforest is typically divided into four main layers, each with different plants and animals adapted for life in that particular area: the emergent, canopy, understorey and forest floor layers.

Emergent layer

The emergent layer contains a small number of very large trees called emergents, which grow above the general canopy, reaching heights of 45–55 m, although on occasion a few species will grow to 70–80 m tall.[9][10] They need to be able to withstand the hot temperatures and strong winds that occur above the canopy in some areas. 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 50 percent of all plant species, suggesting that perhaps half of all life on Earth could be found there. Epiphytic plants attach to trunks and branches, and obtain water and minerals from rain and debris that collects on the supporting plants. 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, 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.[11]

Understorey layer

The understorey layer lies between the canopy and the forest floor. The understorey (or understory) 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 understorey. Only about 5% of the sunlight shining on the rainforest canopy reaches the understorey. This layer can be called a shrub layer, although the shrub layer may also be considered a separate layer.

Forest floor

  Rainforest in the Blue Mountains, Australia

The forest floor, the bottom-most layer, receives only 2% of the sunlight. Only plants adapted to low light can grow in this region. Away from riverbanks, swamps and clearings, where dense undergrowth is found, the forest floor is relatively clear of vegetation because of the low sunlight penetration. It also contains decaying plant and animal matter, which disappears quickly, because the warm, humid conditions promote rapid decay. Many forms of fungi growing here help decay the animal and plant waste.

Flora and fauna

  West Usambara Two-Horned Chameleon (Bradypodion fischeri) in the Usambara Mountains, Tanzania.

More than half of the world's species of plants and animals are found in the rainforest.[12] 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. Fungi are also very common in rainforest areas as they can feed on the decomposing remains of plants and animals. Many rainforest species are rapidly disappearing due to deforestation, habitat loss and pollution of the atmosphere.[13]

Soils

Despite the growth of vegetation in a tropical rainforest, soil quality 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 colour and sometimes produces mineral deposits such as bauxite. Most trees have roots near the surface, because there are insufficient nutrients below the surface; most of the trees' minerals come from the top layer of decomposing leaves and animals. On younger substrates, especially of volcanic origin, tropical soils may be quite fertile. If rainforest trees are cleared, rain can accumulate on the exposed soil surfaces, creating run-off and beginning a process of soil erosion. Eventually streams and rivers form and flooding becomes possible.

Effect on global climate

A natural rainforest emits and absorbs vast quantities of carbon dioxide. On a global scale, long-term fluxes are approximately in balance, so that an undisturbed rainforest would have a small net impact on atmospheric carbon dioxide levels,[14] though they may have other climatic effects (on cloud formation, for example, by recycling water vapour). No rainforest today can be considered to be undisturbed.[15] Human induced deforestation plays a significant role in causing rainforests to release carbon dioxide,[16] as do other factors, whether human-induced or natural, which result in tree death, such as burning and drought.[17] Some climate models operating with interactive vegetation predict a large loss of Amazonian rainforest around 2050 due to drought, forest dieback and the subsequent release more carbon dioxide.[18] Five million years from now, the Amazon rainforest may long since have dried and transformed itself into savannah, killing itself in the progress (changes such as this may happen even if all human deforestation activity ceases overnight).[19] The descendants of our known animals may adapt to the dry savannah of the former Amazonian rainforest and thrive in the new, warmer temperatures.[19]

Human uses

  Amazon River rain forest in Peru

Tropical rainforests 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. Many foods originally came from tropical forests, and are still mostly grown on plantations in regions that were formerly primary forest.[20] Also, plant derived medicines are commonly used for fever, fungal infections, burns, gastrointestinal problems, pain, respiratory problems, and wound treatment.[21]

Native peoples

On January 18, 2007, FUNAI reported also that it had confirmed the presence of 67 different uncontacted tribes in Brazil, up from 40 in 2005. With this addition, Brazil has now overtaken the island of New Guinea as the country having the largest number of uncontacted tribes.[22] The province of Irian Jaya or West Papua in the island of New Guinea is home to an estimated 44 uncontacted tribal groups.[23] The tribes are in danger because of the deforestation, especially in Brazil.

Central African rainforest is home of the Mbuti pygmies, one of the hunter-gatherer peoples living in equatorial rainforests characterised by their short height (below one and a half metres, or 59 inches, on average). They were the subject of a study by Colin Turnbull, The Forest People, in 1962.[24] Pygmies who live in Southeast Asia are, amongst others, referred to as “Negrito”.

Deforestation

  Jungle burned for agriculture in southern Mexico.

Tropical and temperate rainforests have been subjected to heavy logging and agricultural clearance throughout the 20th century and the area covered by rainforests around the world is shrinking.[25] Biologists have estimated that large numbers of species are being driven to extinction (possibly more than 50,000 a year; at that rate, says E. O. Wilson of Harvard University, a quarter or more of all species on Earth could be exterminated within 50 years)[26] due to the removal of habitat with destruction of the rainforests.

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.[27]

The forests are being destroyed at a rapid pace.[28][29][30] Almost 90% of West Africa's rainforest has been destroyed.[31] Since the arrival of humans 2000 years ago, Madagascar has lost two thirds of its original rainforest.[32] At present rates, tropical rainforests in Indonesia would be logged out in 10 years and Papua New Guinea in 13 to 16 years.[33]

Several countries,[34] notably Brazil, have declared their deforestation a national emergency.[35] Amazon deforestation jumped by 69% in 2008 compared to 2007's twelve months, according to official government data.[36] Deforestation could wipe out or severely damage nearly 60% of the Amazon Rainforest by 2030, says a new report from WWF.[37]

However, a January 30, 2009 New York Times article stated, "By one estimate, for every acre of rain forest cut down each year, more than 50 acres of new forest are growing in the tropics..." The new forest includes secondary forest on former farmland and so-called degraded forest.[38]

See also

References

  1. ^ "Rainforests.net - Variables and Math". http://www.rainforests.net/variables.htm. Retrieved 2009-01-04. 
  2. ^ Rainforests at Animal Center
  3. ^ Killer Inhabitants of the Rainforests
  4. ^ Susan Woodward. Tropical broadleaf Evergreen Forest: The rainforest. Retrieved on 2008-03-14.
  5. ^ Newman, Arnold. The Tropical Rainforest : A World Survey of Our Most Valuable Endangered Habitat : With a Blueprint for Its Survival. New York: Checkmark, 2002. Print.
  6. ^ Hobgood (2008). Global Pattern of Surface Pressure and Wind. Ohio State University. Retrieved on 2009-03-08.
  7. ^ Broeker, Wallace S. (2006). "Breathing easy: Et tu, O2." Columbia University Columbia.edu
  8. ^ Moran, E.F., "Deforestation and Land Use in the Brazilian Amazon," Human Ecology, Vol 21, No. 1, 1993"
  9. ^ Bourgeron, Patrick S.. "Spatial Aspects of Vegetation Structure". In Frank B. Golley. Tropical Rain Forest Ecosystems. Structure and Function. Ecosystems of the World (14A ed.). Elsevier Scientific. pp. 29–47. ISBN 0-444-41986-1. 
  10. ^ "Sabah". Eastern Native Tree Society. http://www.nativetreesociety.org/worldtrees/sea_ei/malaysia/sabah2005.htm. Retrieved 2007-11-14. 
  11. ^ Dendronautics - Introduction
  12. ^ Rainforest Facts
  13. ^ Impact of Deforestation—Extinction
  14. ^ Grida.no
  15. ^ 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
  16. ^ Malhi, Y and Grace, J. 2000 " Tropical forests and atmospheric carbon dioxide”, Tree 15
  17. ^ "Drought may turn forests into carbon producers". The Age (Melbourne). 2004-03-06. http://www.theage.com.au/articles/2004/03/06/1078464675256.html?from=storyrhs. 
  18. ^ Cox, P. M.; Betts, R. A.; Collins, M.; Harris, P. P.; Huntingford, C.; Jones, C. D. (2004). "Amazonian forest dieback under climate-carbon cycle projections for the 21st century". Theoretical and Applied Climatology 78. Bibcode 2004ThApC..78..137C. DOI:10.1007/s00704-004-0049-4. http://web.archive.org/web/20070109023123/http://www.metoffice.gov.uk/research/hadleycentre/pubs/HCTN/HCTN_42.pdf.  edit
  19. ^ a b The Future is Wild television program
  20. ^ Myers, N. (1985). The primary source. W. W. Norton & Company, New York, pp. 189-193.
  21. ^ Final Paper: The Medicinal Value of the Rainforest May 15, 2003. Amanda Haidet May 2003
  22. ^ Brazil sees traces of more isolated Amazon tribes
  23. ^ BBC: First contact with isolated tribes?
  24. ^ The Tribal Peoples, ThinkQuest
  25. ^ Entire rainforests set to disappear in next decade, The Independent
  26. ^ Talks Seek to Prevent Huge Loss of Species, New York Times
  27. ^ Littoral Rainforest-Why is it threatened?
  28. ^ Thomas Marent: Out of the woods, The Independent
  29. ^ Brazil: Amazon Forest Destruction Rate Has Tripled, FoxNews.com, September 29, 2008
  30. ^ Papua New Guinea's rainforests disappearing faster than thought
  31. ^ Rainforests & Agriculture
  32. ^ Science: Satellite monitors Madagascar's shrinking rainforest, 19 May 1990, New Scientist
  33. ^ China is black hole of Asia's deforestation, AsiaNews.it, 24 March 2008
  34. ^ Amazon deforestation rises sharply in 2007, Usatoday.com, January 24, 2008
  35. ^ Vidal, John (20 May 2005). "Rainforest loss shocks Brazil". guardian.co.uk (London). http://www.guardian.co.uk/news/2005/may/20/brazil.environment. Retrieved 7 July 2010. 
  36. ^ Brazil: Amazon deforestation worsens, Msnbc.com, August 30, 2008
  37. ^ Benjamin, Alison (6 December 2007). "More than half of Amazon will be lost by 2030, report warns". guardian.co.uk (London). http://www.guardian.co.uk/environment/2007/dec/06/conservation.endangeredhabitats. Retrieved 7 July 2010. 
  38. ^ New Jungles Prompt a Debate on Rain Forests, The New York Times, January 30, 2009

Further reading

  Part of the Illawarra Brush, in New South Wales, Australia.


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