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 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.
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.
The rainforest is divided into four different parts, each with different plants and animals, adapted for life in that particular area.
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.
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.
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.
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, 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.
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.
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.