Rain Forest.

Ci-dessous un extrait traitant le sujet : Rain Forest.. Ce document contient 4718 mots. Pour le télécharger en entier, envoyez-nous un de vos documents grâce à notre système d’échange gratuit de ressources numériques ou achetez-le pour la modique somme d’un euro symbolique. Cette aide totalement rédigée en format pdf sera utile aux lycéens ou étudiants ayant un devoir à réaliser ou une leçon à approfondir en : Echange

---

Rain Forest.
I

INTRODUCTION

Rain Forest, woodland characterized by lush vegetation and comparatively high temperature and rainfall throughout the year. Rain forests are the world's most
biologically diverse ecosystems. Although they account for less than 7 percent of the land surface on Earth, they contain more than 50 percent--some scientists
estimate as high as 90 percent--of its plant and animal species. One hectare (about 2.5 acres) of tropical rain forest may contain more than 600 species of trees. By
comparison, the forests of the United States and Canada combined contain only around 700 tree species. Even more impressive are the number and diversity of animal
species that call rain forests their home. One study found more species of ants living on a single rain forest stump than exist in all of the British Isles.
Rain forests also play a critical role in global climate regulation by absorbing carbon dioxide, a greenhouse gas believed to be principally responsible for global warming.
Plants naturally absorb carbon dioxide and give off oxygen gas in the process of photosynthesis, and tropical rain forests absorb more carbon dioxide than any other
terrestrial ecosystem on earth. Global emissions of carbon dioxide have increased nearly 30 percent in the last century. There is general agreement among the scientific
community that by absorbing some of the gas, rain forests play a vital role in lessening its impacts.
To be classified as a rain forest, a forest must have a closed canopy, in which the treetops, or crowns, touch each other, creating a shaded forest interior. In addition,
temperature and rainfall must be high and relatively even throughout the year. Forests that meet these criteria are found flanking the equator in South and Central
America, Asia, Africa, and Australia. In South America, a vast, forested area of the Amazon River basin in Brazil and neighboring countries is by far the largest rain
forest in the world. It encompasses more than 3.5 million sq km (about 1.4 million sq mi)--about half of the total global rain forest cover. The larger of two large rain
forests in Asia is centered along the Malay Archipelago, including the islands of Borneo and Sumatra, the Malay Peninsula, and the Republic of the Philippines. The other
main rain forest in Asia is found primarily on the island of New Guinea and in northern Australia. In Africa, most of the rain forest is concentrated along the Atlantic coast
and the Congo River Basin.
In regions of the northern and southern hemispheres, small areas of temperate rain forest are found along the coasts where rainfall and humidity are high and winters
are mild. Such temperate rain forests are found in the Pacific Northwest of the United States and southwestern Canada, for example. They are dominated by only a few
tree species, however, making them differ dramatically from highly diverse tropical rain forests. This article focuses on tropical rain forests.

II

RAIN FOREST CHARACTERISTICS

Tropical rain forests belong to a broader forest category called tropical moist forests, of which there are many different types. Forest scientists distinguish rain forests
from other types by considering factors such as temperature, rainfall, length of dry season, and altitude.

A

Temperature

Rain forests are typically hot and steamy--the average annual temperature is 25° C (77° F). Temperature near the equator varies little over the course of a year, so
rain forest temperatures are about the same year round--the average minimum monthly temperature in a rain forest is a balmy 18° C (64° F). Rain forests are not
found where the temperature drops close to 0° C (32° F) because their resident plants and animals are not well adapted to withstand frost. Temperature in a rain forest
depends not only on distance from the equator but also on altitude. As elevation increases, nighttime temperatures go down significantly. This daily temperature
variation affects forest ecology, and true rain forests typically are not found above 1,000 m (3,280 ft).

B

Rainfall

Rain forests can average as little as 1.8 m (6 ft), or as much as 9.0 m (30 ft), of rainfall a year. What distinguishes a true rain forest is the distribution of precipitation
throughout the year--there are no dry seasons. Every month, typically more than 100 mm (4 in) of rain falls. If a rain forest does have dry periods, they are usually
short and unpredictable.
In many climates, rainfall evaporation is carried away to fall as rain in distant places, but in the rain forests, nearly 50 percent of the precipitation comes from local
evaporation. The warm, humid air surrounding a rain forest forms a microenvironment that permits little water to escape. Much of the rain that falls on the rain forest is
intercepted by the trees in the canopy. Some of it rolls off the leaves and down the trunks to the forest below, but a high percentage evaporates and hangs as tiny
droplets of water in the humid atmosphere. Gentle and continuous winds lift the tiny droplets higher in the atmosphere, where they cool and form clouds. When enough
of these drops enter the atmosphere and cool, they condense and fall as rain, beginning the cycle again.

C

Soil

Despite their incredible lushness and high diversity, one of the peculiarities of rain forests is that the soil is poor in nutrients that can be absorbed by plant roots. The
nutritious minerals have been washed out of the soils by heavy rainfall and high temperatures over thousands of years. To compensate for the nutrient-poor rain forest
soils, most tropical trees absorb the nutrients they can find and hold them in their living tissue. In contrast, the rich soils of temperate forests are better able to retain
nutrients, enabling a temperate forest tree to absorb small amounts of minerals as the tree needs them. When tropical trees die, nutrients are released into the soils by
decomposition. Rather than remaining in the soil reservoir as they would in a temperate forest, in the rain forest, the nutrients are rapidly absorbed again by other
living organisms.

III

FOREST STRUCTURE

Rain forest structure is distinct from most other forest types because of its many layers of vegetation, referred to as strata. The three strata of a rain forest are the
understory, the midstory, and the overstory. The lowest stratum is the understory, composed of palms, herbaceous plants (such as wild ginger), and tree seedlings and
saplings. Just 2 percent of the sunlight penetrates the many layers of leaves and branches above, so understory plant species have developed special traits to cope with
low light levels. Many have deep red coloring on the underside of their leaves to capture some of the scarce light that does manage to reach the forest understory. This
red coloring enables understory plants to absorb light of different wavelengths than the lush, green-foliaged canopy plants do. Above the forest floor but below the
canopy are one or more midstory strata, made up of woody plants, such as large shrubs and midsized trees.
The overstory is the canopy, in which the tree crowns form a continuous layer that captures the bulk of the rainwater and sunlight hitting the forest. The height of the
canopy varies from region to region and forest to forest, ranging from 20 to 50 m (65 to 165 ft). The lush, green canopy is teeming with life, and forest researchers
have developed ingenious methods for accessing this mysterious ecosystem. Researchers use hot air balloons, cables, catwalks, towers, sophisticated tree-climbing
gear, and even robots to study the millions of plants and animals that make their home high up in the forest canopy. Canopy researchers also use huge cranes that are

dropped into the heart of the forest by helicopters. Suspended from the crane's long, movable arm is a large gondola that functions as a mobile treetop laboratory.
Moving from tree to tree, forest researchers collect specimens, conduct experiments, and observe life in the canopy frontier.
The highest stratum of the rain forest is made up of the emergent trees, those individuals that stick up above the forest canopy. Emergents, which do not form a
continuous layer, are usually the giants of the forest, reaching heights of 35 to 70 m (115 to 230 ft) or more, and trunk girths of over 2 m (6.6 ft) in diameter. Less
than one percent of the trees in the forest reside in the canopy and emergent layers. However, these trees tend to be so large that they collectively account for the
vast majority of the woody mass, or biomass, of the forest.
The nicely ordered strata of the rain forest, including the continuous layer of the canopy, are regularly disturbed by naturally occurring events, such as falling trees.
Trees in a rain forest canopy are often interconnected by vines, and a falling tree may pull as well as push other trees down with it, producing a domino effect of falling
trees. The resulting opening in the forest canopy enables light to pour onto the forest floor. New plants and animals then move into the area and begin to grow.
Other natural disturbances create even larger openings in the forest canopies. For example, along the hurricane belt in the Caribbean and the typhoon belt along the
western Pacific, some forests are substantially altered when high winds and storms blow down hundreds of trees every few decades. On a smaller scale, large mammals,
such as elephants, regularly raze rain forest vegetation in the Congo River Basin in Africa. Scientists have found that these natural disturbances and the subsequent
forest regeneration are a vital process that leads to healthy and diverse forests.

IV

PLANT AND ANIMAL LIFE

Rain forest ecosystems contain more plant and animal species than virtually any other habitat in the world. Although their range has contracted and expanded with
climate changes over the last several million years, in general, rain forests are some of the oldest ecosystems on Earth. As a result of this continuity, rain forests boast
millions of different species, many of which are endemic, or unique, to rain forest habitats.

A

Plants

Although they contain numerous species, rain forests are remarkably uniform in their general appearance. Most trees have tall, slender trunks that do not branch until
near the crown. Many, such as kapok trees, are supported by thick buttresses that can stretch out 10 m (33 ft) or more. These buttresses provide needed support for
rain forest trees, top heavy because nutrient-poor rain forest soils lead to fragile, shallow root systems. Rain forest tree bark tends to be thin and smooth. Notable
exceptions are palms, which are common in some rain forests and virtually absent in other types of forests.
Rain forest plants have many unique physical characteristics that exploit the particular habitat, or niche, that a species occupies. Understory and midstory plants, such
as relatives of the banana tree, tend to have particularly large leaves to capture as much light as possible--what little light that is not intercepted by the canopy above.
These large leaves do not dry out as they would high in the canopy, where the intense sunlight creates a drier environment. These traits can change, however, when a
plant's environment changes. Canopy trees may change their shape over the course of their life, depending on the environment around them. Leaves often get smaller
as trees grow larger. In some cases, leaves of juvenile plants may be almost 10 times larger than adults of the same canopy tree species.
On the nutrient-poor forest floor, many rain forest plants, such as human-sized members of the palm family called Astrocaryum sciophilum, collect falling debris from
other plants in their cup-shaped leaves to create their own little compost pile. Rain forests also feature insectivorous plants, which derive some of their nutrients by
trapping animals, particularly insects, in their leaves. Among the insectivorous plants are the pitcher plants native to tropical Asia. Insects land upon the pitcher plant's
tubular-shaped leaves, then slide into a cavity, or pitcher, at the plant's center that is full of digestive juices. Here, the insect quickly dissolves and the nutrients it
contains become available for the plant's use. Rain forests are also home to the largest flower in the world, the giant rafflesia flower, commonly known as the corpse lily.
Weighing up to 7 kg (15 lb) with petals spanning almost 1 m (1 yd), the giant rafflesia flower is best known for its putrid smell. The plant emits the stench of rotting
meat to attract certain flies, who serve as its pollinators.
Rain forests are often teeming with climbing plants, such as rattan palms. These thick, woody climbers--25 cm (10 in) in diameter--are frequently found connecting the
trees, ascending up into the treetops and looping back down. By climbing trees, these lianas, or vines, expose their leaves and flowers to sunlight, birds, and insects
without expending the energy needed to build their own supporting tissues. Epiphytes, including mosses, bromeliads, and orchids, grow on tree trunks or nestled in the
crook of a tree. Lacking permanent roots in the soil, epiphytes must obtain their nutrients from other living plants or by trapping water and organic matter as they fall
to the forest floor. Bromeliads can store up to 38 liters (10 gallons) of water in the reservoirs formed by their overlapping leaves. Most live harmoniously with their host
trees, though some are less benign. Strangler figs, which begin their lives as epiphytes, germinate in the canopy trees and send roots down to the soil. As they grow,
these parasites enwrap their hosts until they literally strangle them to death, at which point the figs become free-standing trees.

B

Animals

Almost 90 percent of the rain forest animal species are insects, and of these, most are beetles. A single rain forest tree can host more than 150 species of beetles.
Living high in the forest canopy, most of these beetles and other insect species have eluded scientists until recently, when technology has improved access to the upper
stratum. To this day, scientists are unsure how many animal species exist in the world, largely because they have identified just a small fraction of the millions--some
estimate as many as 30 million--of insects that live in the rain forest.
Among the most fascinating rain forest insects yet encountered are leafcutter ants, remarkable because they actually cultivate their own food. These ants cut the leaves
of particular plants and carry them back to their underground nests, where they fertilize them with saliva. This careful tending causes growth of a particular fungus,
which the ants harvest and rely on as their sole source of food.
In 15 sq km (6 sq mi) of rain forest, as many as 100 different mammal species may be found. These animals occupy every available niche, from burrows in the forest
floor to the branches of emergent trees. Most rain forest mammals are nocturnal (active during the night) or crepuscular (active during the twilight of dusk or dawn),
and they spend the heat of the day sleeping. In fact, almost half the mammals of the rain forest are bats, flying mammals known for their nighttime activity. Some rain
forest mammals, including gorillas, elephants, tapirs, agoutis, and wild pigs, are ground-dwellers, but most, like their insect counterparts, live high in the treetops in the
forest canopy. Canopy-dwellers have evolved an array of fascinating traits to survive in the branches of trees. For instance, some Asian rain forests are especially
known for animals with the ability to glide. Borneo alone has more than 30 species of mammals, reptiles, and amphibians that can glide from one tree to another. Most
rain forest monkeys in Central and South America use their specialized tails, which are capable of grasping branches, like a fifth limb while climbing, feeding, and even
playing high above the forest floor.
The three-toed sloth spends most of its life defenseless, hanging upside down from tree branches. To avoid predators, its movements are so slow that they are virtually
undetectable to would-be predators, even vigilant jaguars. The sloth has also developed a relationship with a rain forest plant that makes it even more elusive to its
predators: Although it has brown fur, the sloth blends in with the surrounding green canopy because a particular species of green alga lives in its fur.

C

Plant-Animal Interactions

Like the green alga and three-toed sloth, many plants and animals of the rain forest depend on each other--often to a higher degree than in other ecosystems. For
example, 90 percent of the trees depend on animals to disperse their seeds. By comparison, in other types of forests, often 50 percent or more of the trees rely on
wind to disperse their seeds. These plant and animal relationships are often mutualistic--that is, both the animal and plant benefit from the relationship. Some animals
protect a plant species against plant-eating enemies, while the host plants provide lodging. For instance, many tropical plants, such as the snakewood tree, have hollow
structures in their stems or twigs that stinging or biting ants use as homes. In exchange for a place to live, the ants protect the plants by marching out to fight wouldbe predators--climbing vines as well as hungry animals--as soon as they detect their presence.
In some cases, the plant and animal species are so dependent on each other that they cannot live independently. For example, every species of fig tree is dependent on
one or more species of fig wasps; conversely, every species of fig wasp is dependent on one or more species of fig trees. Without the wasps performing the annual task
of pollination, the fig trees would be unable to reproduce and would eventually face extinction, and without a nursery for their eggs and larvae, the fig wasp would face
a similar fate.

V

ECONOMIC VALUE

Rain forest regions are incredibly rich in natural resources and commodities--particularly timber, minerals, and petroleum--that fetch high prices in international
markets. Many trees that grow in rain forests are highly prized for the durability and beauty of their wood. Teak, rosewood, and mahogany are rain forest hardwoods
used to make furniture and cabinetry all over the world. Teak, which resists corrosion from weather, is also highly valued in shipbuilding. Crude oil and natural gas
deposits in Southeast Asia, Central Africa, and South America attract multinational corporations. For many farmers and ranchers, rain forests offer wide expanses of
grazing land or land that can be converted for agricultural purposes. Rain forests are also home to a number of exotic animals, such as parrots and monkeys, which
bring high prices when captured and sold as pets.
The high species diversity makes rain forests highly prized for new medicinal drugs or agricultural crops. Drugs that originated in the rain forest range from early forms
of the birth control pill, first manufactured from wild yams growing in Central America, to highly specialized medicines derived from the Madagascar rosy periwinkle used
to treat a rare form of leukemia. And the drugs already in use represent just a fraction of the potential medicines the rain forests may hold. Of the thousands of plants
identified as carrying potential anticancer compounds, more than half are native to rain forests. Multinational research projects investigating rain forest plants as
treatments for a multitude of diseases--cancer and acquired immunodeficiency syndrome (AIDS) chief among them--are under way in hundreds of laboratories across
the globe.
Perhaps the greatest asset of tropical forests, however, comes from maintaining Earth's life-support systems. These critical environmental services include cycling
essential nutrients such as nitrogen and phosphorus, absorbing carbon dioxide from the atmosphere, regulating temperature and precipitation, protecting watersheds
from soil erosion, and harboring pollinators for agricultural crops. Economists have estimated that the economic value of these services outweighs other possible uses of
the land. For example, economists estimate that environmental services provided by one hectare (2.5 acres) of Costa Rican rain forest are worth between $1,300 and
$2,700 each year--substantially higher than the profits that most farmers would make by converting the land to cattle pastures, agricultural fields, or tree plantations.

VI

INDIGENOUS PEOPLES

Most of the world's tropical rain forests are inhabited, and have been for thousands of years, by indigenous peoples who depend on the forests for their livelihoods.
Many indigenous peoples live deep within the rain forest in areas that, to this day, are accessible only by river. Anthropologists believe that as many as 1,000 different
cultures of indigenous peoples may be living in rain forests worldwide. Among the many small groups are the Yanomamo, the Ashaninka, and the Kayapó of South
America; the Baka Pygmies of Cameroon; and the Penan and Bentian Dayak of Borneo. Although each indigenous group has a different culture and customs, they all
share a dependence on the rain forest habitat in which they live.
Indigenous tribes often possess a great wealth of knowledge about the rain forests, including the medicinal uses of different plant species, the habits of breeding birds,
and rainfall patterns. This knowledge has been passed down verbally from generation to generation. Many indigenous peoples collect fruits, nuts, firewood, construction
materials, and game meat from the rain forests. Most also depend on small-scale agriculture for food and medicinal plants.
Using a practice known as shifting cultivation, most indigenous peoples living in the rain forest clear small plots to plant gardens for food and medicine. Sometimes they
clear the land by burning the forest. Called slash-and-burn agriculture, this method makes the clearing process easier and releases nutrients into the soil quickly.
Because rain forest soils are nutrient-poor, garden production decreases significantly after a couple of years, at which point the garden is abandoned, and a new plot is
cleared and planted. Indigenous peoples have relied on these agricultural methods for thousands of years. In the past, the abandoned plots were allowed to regenerate
for many years before they were cleared and farmed again. These traditional shifting cultivation practices did not significantly damage the rain forest because the rain
forests were so vast and populations of indigenous peoples relatively small.
In the last half of the 20th century, indigenous tribes became vastly outnumbered by colonists migrating to the region. Attracted by seemingly unoccupied land, smallscale farmers and cattle ranchers threaten the survival of indigenous peoples and their rain forest habitat. Logging, mining, and oil and gas extraction have also
drastically reduced the size of rain forests around the globe, and as the forests shrink, indigenous peoples are forced to compete for the limited land that remains. In
this competitive environment, even the once-sustainable agricultural practices of indigenous peoples can cause significant damage to the fragile rain forest ecosystem.

VII

DEFORESTATION

Despite their uniqueness and extraordinary value, tropical rain forests are being destroyed and badly degraded at an unsustainable rate. Some scientists estimate that
in the early 1990s tropical forests were being destroyed at a rate of approximately 28 hectares (70 acres) a minute, or about 14 million hectares (35 million acres) each
year--an area about the size of the state of Wisconsin. This figure marked a decrease since the 1980s, when approximately 16 million hectares (40 million acres) were
destroyed each year, largely due to a reported decline of deforestation in the Amazon River basin in the early 1990s. However, satellite images indicate that rates may
have rebounded in the late 1990s as burning in the Amazon increased again. Over the past three decades alone, about 5 million sq km (about 2 million sq mi)--or 20
percent of the world's tropical forests--have been cleared. During this time, deforestation in tropical Asia reached almost 30 percent. High rates of deforestation are
inevitably followed by alarming rates of plant and animal extinction because many rain forest species cannot survive outside their pristine rain forest habitat. Some
scientists estimate that dozens of rain forest species are becoming extinct every day.
Causes of deforestation vary from location to location, but certain patterns tend to be consistent across all forests. Logging companies in search of valuable rain forest
hardwoods, or, less often, oil companies in search of petroleum, are often the first to enter a remote area of rain forest. Some logged forests, if left alone, can
regenerate in a few decades. But typically, logged forests are not left alone--the roads built by logging companies often provide access for landless farmers to enter a
new area, as well as a means to transport agricultural crops to market. For every 1 kilometer of new road built through a forested area, 4 to 24 sq km (1.5 to 9.3 sq

mi) are deforested and colonized.
Once the loggers abandon the land, a typical cycle of destruction ensues. When the landless farmers arrive, they clear the land for planting. Poor rain forest soils
produce a low crop yield, especially after a couple of years. At that point, the farmers often sell their lands to cattle ranchers or large plantation owners. After nutrients
have been exhausted and soils compacted by cattle, lands are then abandoned and often laid to waste. Rain forest does not readily regenerate on these lands without
human intervention. Meanwhile, the colonist farmers and cattle ranchers move to a new piece of land made accessible by logging roads, where the cycle of deforestation
begins again.

VIII

RAIN FOREST CONSERVATION

Since the 1970s, an increasing number of national and international organizations have been established to promote rain forest conservation. In recent years, two
principal approaches have been used to conserve tropical rain forests: strict protection and sustainable development. The first, outright protection through the
development of national parks and preserves, has been an essential element in biodiversity conservation. In this method of conservation, entire tracts of the rain forest
are set aside, and uses are carefully regulated. Protected areas are especially important for preserving the most biologically distinctive rain forests--that is, those
forests with exceptionally high species diversity with many species endemic to that forest. At the end of the 20th century, the number of national parks in tropical
countries increased significantly, and today approximately 5 percent of all forests are classified under some protected status. International organizations such as the
World Bank and the World Wildlife Fund have launched a movement for all countries in the developing world to set aside 10 percent of their forests in protected areas.
Yet protecting these designated areas is costly, and even impossible in some areas. Communities living near the rain forests may rely on the rain forest for food and
firewood. When restricted from using rain forest land, these communities find it more difficult to meet their subsistence needs. To lessen these adverse effects,
community-based programs have been developed that provide sustainable economic alternatives to destructive harvesting and land use. One alternative for some
forests is sustainable rain forest logging, in which the trees logged are carefully selected to ensure a minimal impact on the forest ecosystem.
In 2006 an effort was made to combine these two approaches of outright protection and sustainable development. The governor of the state of Pará in northern Brazil
set aside a vast area the size of England in the Amazon rain forest. The total area encompassed 150,000 sq km (58,000 sq mi). Of this amount, nearly one-third was
designated as completely protected. Access was even off limits to the general public. Only scientific researchers and indigenous people could access this area. The
remaining two-thirds was earmarked for sustainable development, which meant limited logging under a strict management plan and a natural resources management
plan for local communities living in the area. Moreover, the protected area was formed adjacent to existing nature reserves along Brazil's border with the neighboring
countries of Guyana, Suriname, and French Guiana, so that a vast protected corridor was created.
Other alternatives to rain forest destruction include the harvesting and selling of sustainable rain forest products, such as vegetable ivory seeds from palms, known as
tagua nuts, and brazil nuts. Still others involve exploring medicinal plants and drug development as ways to strengthen and diversify the economies of countries with
large tracts of rain forest. More recently, farms dedicated to raising beautiful rain forest butterflies have become increasingly popular. Many rain forest communities host
ecotourists--vacationers who focus on nature study and outdoor activities that have minimal ecological impact--as a way to attract tourism to their region while still
preserving their fragile rain forest homeland.

Contributed By:
Elizabeth Losos
Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.