Rainforest
:''This article is about the rainforest in general. For more specific information, see
Temperate rain forest or
Tropical Rainforests.
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.
Rainforests act as major consumers of
carbon dioxide 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.
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.
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.
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. The science of accessing tropical forest canopy is called dendronautics, see
DendronauticsUnderstory 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.
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,
turtle,
chameleons and other families. Birds include such families as
vangidae and
Cuculidae. Dozens of families of invertebrates are found in rainforests.
Many foods, such as
coffee,
chocolate,
bananas,
mangoes,
papayas,
avocados and
sugar cane, originally came from tropical forests, and are still mostly grown on plantations in regions that were formerly primary forest.
[Myers, N. 1985. The primary source. W. W. Norton and Co., New York, pp. 189-193.] Tropical rain forests are also the source of many medicinal drugs, with nearly half the medicines that we use having been discovered there. Scientists believe that the cures for many more diseases will be discovered there in the future.
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.
*
Cloud forest*
Deforestation*
Illegal logging*
Jungle*
Tropical and subtropical moist broadleaf forests*
rubber tappingGeneral references
* 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
Specific references
*
Rainforest Portal*
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.
*
Dendronautics An organization involved with tropical rain forest canopy exploration.
*
United Nations Forum on Forests*
The Coalition for Rainforest Nations
