Rainforests are forests characterized by high rainfall, with definitions setting minimum normal annual rainfall between 1750–2000 mm (68-78 inches).
Rainforests are home to two-thirds of all the living animal and plant species on Earth. It has been estimated that many hundreds of millions of species of plants, insects and microorganisms are still undiscovered. Tropical rainforests have been called the "jewels of the Earth," and the "world's largest pharmacy," because of the large number of natural medicines discovered there.
The undergrowth in a rainforest is restricted in many areas by the lack of sunlight at ground level. This makes it possible to walk through the forest. 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. The two types of rainforest are:
- Tropical rainforests are rainforests in the tropics, found near the Equator (between the Tropic of Cancer and Tropic of Capricorn) and present in southeast Asia (Myanmar to Indonesia and Papua New Guinea, northern and eastern Australia), sub-Saharan Africa from Cameroon to the Congo, (Congo Rainforest), South America (the Amazon Rainforest) Central America (Bosawás, southern Yucatán Peninsula-El Peten-Belize-Calakmul), and on many of the Pacific Islands(such as Hawaii). Tropical rainforests have been called the "Earth's lungs," although it is now known that rainforests contribute little net oxygen additions to the atmosphere through photosynthesis.
- Temperate rainforests are rainforest in temperate regions. They can be found 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), and in East Asia (in southern China, Taiwan, much of Japan and Korea, and on Sakhalin Island and the adjacent Russian Far East coast), and also Australia and New Zealand.
Despite the growth of vegetation
in a 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 color and sometimes produces minable deposits
such as bauxite
. On younger substrates, especially of volcanic
origin, tropical soils may be quite fertile.'''
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, 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. Human induced deforestation plays a significant role in causing rainforests to release carbon dioxide, as do natural processes such as drought that result in tree death. Some climate models run with interactive vegetation predict a large loss of Amazonian rainforest around 2050 due to drought, leading to forest dieback and the subsequent feedback of releasing more carbon dioxide.
The rainforest is divided into four different parts, each with different plants and animals, adapted for life in that particular area:
- The 'emergent layer' contains 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 70-80 m tall. They need to be able to withstand the hot temperatures and strong 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 50 percent 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, 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.
- The understory layer lies between the canopy and the forest floor. The understory (or understorey) 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 understory. Only about 5 percent of the sunlight shining on the rainforest reaches the understory. This layer can also be called a shrub layer, although the shrub layer may also be considered a separate layer.
- The forest floor layer receives only 2 percent of 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 due to the warm, humid conditions promoting rapid decay. Many forms of fungi grow here which help decay the animal and plant waste.
Flora and fauna
More than half of the world's species of plants and animals are found in the rainforest. 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 plant and animal life. As a result of all the decomposing matter on the forest floor, rainforests develop high levels of organic matter in the soil, making it very nutritious for plant and animal life. This humus is a major factor in allowing rainforests to be highly active ecosystems, with rainforests home to about 80% of the Earth's species. This amounts to over 20 million species of plants and animals. These species are rapidly disappearing due to deforestation, habitat loss and biochemical releases into the atmosphere.
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.. Also, plant derived medicines are commonly used for fever, fungal infections, burns, gastrointestinal problems, pain, respiratory problems, and wound treatment.
On 18 January 2007, FUNAI
reported 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. The province of Irian Jaya
or West Papua
in the island of New Guinea is home to an estimated 44 uncontacted tribal groups.
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. Biologists have estimated that large numbers of species are being driven to extinction (possibly more than 50,000 a year, at that rate, says Edward O. Wilson of Harvard University, a quarter or more of all species on earth could be exterminated within 50 years) 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.
The rainforests are being destroyed at an ever-quickening pace. Almost 90% of West Africa's rainforest has been destroyed. Since the arrival of humans 2000 years ago, Madagascar has lost more than 90% of its remaining eastern rainforests. At present rates, tropical rainforests in Indonesia would be logged out in 10 years, Papua New Guinea in 13 to 16 years.
Several countries, notably the Brazil, have declared their deforestation a national emergency. Amazon deforestation jumped by 69% in 2008 compared to 2007's twelve
months, according to official government data. Deforestation could wipe out or severely damage nearly 60% of the Amazon rainforest by 2030, says a new report from WWF.
- Richards, P. W. (1996). The tropical rain forest. 2nd ed. Cambridge University Press ISBN 0-521-42194-2
- Whitmore, T. C. (1998) An introduction to tropical rain forests. 2nd ed. Oxford University Press. ISBN 0-19-850147-1
- Butler, R. A. (2005) A Place Out of Time: Tropical Rainforests and the Perils They Face. Published online: rainforests.mongabay.com