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Rainforest

2007 Schools Wikipedia Selection. Related subjects: Environment

   Rainforests are forests characterized by high rainfall, with
   definitions setting minimum normal annual rainfall between 1750 mm and
   2000 mm.
   The Daintree Rainforest in Queensland, Australia. Enlarge
   The Daintree Rainforest in Queensland, Australia.

   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,
   Scotland and Norway, the western Caucasus ( Ajaria region of Georgia),
   parts of the western Balkans, New Zealand, Tasmania, and parts of
   eastern Australia.

   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 amount
   of natural medicines discovered there. Rainforests are also often
   described as the "Earth's lungs"; however, this appellation has no
   scientific basis as rainforests produce little or no net oxygen .

   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 colour 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.

   In contradiction to popular belief rainforests are not major consumers
   of carbon dioxide and like all mature forests are approximately carbon
   neutral . Recent evidence suggests that the majority of rainforests are
   in fact net carbon emmiters. However rainforests do play a major role
   in the global carbon cycle as stable carbon pools and clearance of
   rainforest leads to increased levels of atmospheric carbon dioxide.
   Rainforests may also play a role in cooling air that passes through
   them. As such rainforests are of vital importance within the global
   climate system.

Trees

   Rainforest on Fatu-Hiva, Marquesas Islands
   Enlarge
   Rainforest on Fatu-Hiva, Marquesas Islands

   There are several common characteristics of rainforest trees.
   Rainforest species frequently possess one or more of the following
   attributes not commonly seen in trees of drier climates.

   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 and shrubs of the
   understorey and forest floor layers. Young individuals of trees
   destined for the canopy and emergent 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 and are made possible because the lower layers are largely
   protected from winds which damage large leaves in the canopy. Canopy
   leaves are usually smaller than found in understorey plants or are
   divided to reduce wind damage. The leaves of rainforest understorey
   trees also often have drip tips which facilitate drainage of
   precipitation off the leaf to promote transpiration and inhibit the
   growth of microbes and bryphytes which would damage or smother the
   leaf.

   Trees are often well connected in the canopy layer especially by the
   growth of woody climbers known as lianas, or by 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 rainforest tree species
   than in drier 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,
   although on occasion a few species will grow to 60 m or 70 m tall. They
   need to be able to withstand the hot temperatures and dry 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 is called dendronautics: see Dendronautics.

Understorey 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, as well as predators such as jaguars, boa
   constrictors, ocelots, and leopards. Armadillos also live here. 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
   reaches the understorey. This layer can also be called a shrub 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.

Fauna

   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. Birds include such families as vangidae and Cuculidae.
   Dozens of families of invertebrates are found in rainforests. More than
   half of the world's species of plants and animals are found in the
   rainforest. 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 rain forests are also the source of many medicinal drugs, with
   over half the medications originating from the rainforest. 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. 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
   environmental groups 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 rainforests . Protection and regeneration of the rainforests is
   a key goal of many environmental charities and organizations. Every
   minute about 40 hectares (100 acres) of the world's tropical rainforest
   is destroyed. At this rate, it is possible that all the rainforests in
   the world will have been destroyed by the year 2025. (It is doubtful
   that this rate will be sustained as the relative cost of logging rises
   with dwindling resources.) About half the worlds species of animals and
   plants depend upon these forests for their survival.

   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.

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