Rainforests areforests characterized by a closed and continuous treecanopy, moisture-dependent vegetation, the presence ofepiphytes andlianas and the absence of wildfire. Rainforests can be generally classified astropical rainforests ortemperate rainforests, but other types have been described.
Estimates vary from 40% to 75% of allbioticspecies beingindigenous to the rainforests.[1] There may be many millions of species of plants, insects andmicroorganisms still undiscovered in tropical rainforests. Tropical rainforests have been called the "jewels of the Earth" and the "world's largest pharmacy", because over one quarter of naturalmedicines have been discovered there.[2]
Rainforests are characterized by a closed and continuous tree canopy, high humidity, the presence of moisture-dependent vegetation, a moist layer of leaf litter, the presence of epiphytes and lianas and the absence of wildfire. The largest areas of rainforest are tropical or temperate rainforests, but other vegetation associations includingsubtropical rainforest,littoral rainforest,cloud forest, vine thicket and even dry rainforest have been described.[4][5][6][7][8]
Tropical rainforests are characterized by a warm and wet climate with no substantial dry season: typically found within 10 degrees north and south of theequator. Mean monthly temperatures exceed 18 °C (64 °F) during all months of the year.[9] Average annualrainfall is no less than 168 cm (66 in) and can exceed 1,000 cm (390 in) although it typically lies between 175 cm (69 in) and 200 cm (79 in).[10]
Dry rainforests have a more open canopy layer than other rainforests,[17] and are found in areas of lower rainfall (630–1,100 mm (25–43 in)). They generally have two layers of trees.[18]
A tropical rainforest typically has a number of layers, each with different plants and animals adapted for life in that particular area. Examples include the emergent,canopy,understory andforest floor layers.[19][20]
Emergent layer
Theemergent layer contains a small number of very largetrees calledemergents, which grow above the generalcanopy, reaching heights of 45–55 m, although on occasion a few species will grow to 70–80 m tall.[21][22] They need to be able to withstand the hot temperatures and strong winds that occur above the canopy in some areas.Eagles,butterflies,bats and certainmonkeys inhabit this layer.
Thecanopy layer contains the majority of the largest trees, typically 30 metres (98 ft) to 45 metres (148 ft) tall. The densest areas ofbiodiversity 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.Epiphytic plants attach totrunks andbranches, and obtain water and minerals from rain and debris that collects on the supporting plants. 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,naturalistWilliam 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." A 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 usingcrossbows.Exploration of the canopy is still in its infancy, but other methods include the use ofballoons andairships 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 calleddendronautics.[23]
Theunderstory orunderstorey layer lies between the canopy and the forest floor. It is home to a number ofbirds,snakes andlizards, as well aspredators such asjaguars,boa constrictors andleopards. The leaves are much larger at this level and insect life is abundant. Manyseedlings that will grow to the canopy level are present in the understory. Only about 5% of the sunlight shining on the rainforest canopy reaches the understory. This layer can be called ashrub layer, although the shrub layer may also be considered a separate layer.[citation needed]
Theforest floor, the bottom-most layer, receives only 2% of the sunlight. Only plantsadapted to low light can grow in this region. Away fromriverbanks,swamps and clearings, where dense undergrowth is found, the forest floor is relatively clear of vegetation because of the low sunlight penetration. It also containsdecaying plant and animal matter, which disappears quickly, because the warm, humid conditions promote rapid decay. Many forms offungi growing here 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 rainforests.[24] Rainforests support a very broad array offauna, includingmammals,reptiles,amphibians,birds andinvertebrates. Mammals may includeprimates,felids and other families. Reptiles includesnakes,turtles,chameleons and other families; while birds include such families asvangidae andCuculidae. 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 plants and animals.
The great diversity in rainforest species is in large part the result of diverse and numerous physicalrefuges,[25] i.e. places in which plants are inaccessible to many herbivores, or in which animals can hide from predators. Having numerous refuges available also results in much higher totalbiomass than would otherwise be possible.[26][27]
Some species of fauna show a trend towards declining populations in rainforests, for example, reptiles that feed on amphibians and reptiles. This trend requires close monitoring.[28] The seasonality of rainforests affects the reproductive patterns of amphibians, and this in turn can directly affect the species of reptiles that feed on these groups,[29] particularly species with specialized feeding, since these are less likely to use alternative resources.[30]
Despite the growth ofvegetation in a tropical rainforest,soil quality is often quite poor. Rapidbacterial decay prevents the accumulation ofhumus. The concentration ofiron andaluminiumoxides by thelaterization process gives theoxisols a bright red colour and sometimes producesmineraldeposits such asbauxite. Most trees have roots near the surface because there are insufficient nutrients below the surface; most of the trees' minerals come from the top layer of decomposing leaves and animals. On younger substrates, especially ofvolcanic origin, tropical soils may be quite fertile. If rainforest trees are cleared, rain can accumulate on the exposed soil surfaces, creating run-off, and beginning a process of soilerosion. Eventually, streams and rivers form and flooding becomes possible. There are several reasons for the poor soil quality. First is that the soil is highly acidic. The roots of plants rely on an acidity difference between the roots and the soil in order to absorb nutrients. When the soil is acidic, there is little difference, and therefore little absorption of nutrients from the soil. Second, the type of clay particles present in tropical rainforest soil has a poor ability to trap nutrients and stop them from washing away. Even if humans artificially add nutrients to the soil, the nutrients mostly wash away and are not absorbed by the plants. Finally, these soils are poor due to the high volume of rain in tropical rainforests washes nutrients out of the soil more quickly than in other climates.[31]
Effect on global climate
A natural rainforest emits and absorbs vast quantities ofcarbon 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,[32] though they may have other climatic effects (oncloud formation, for example, by recyclingwater vapour). No rainforest today can be considered to be undisturbed.[33] Human-induceddeforestation plays a significant role in causing rainforests to release carbon dioxide,[34][35][36] as do other factors, whether human-induced or natural, which result in tree death, such as burning and drought.[37] Some climate models operating with interactive vegetation predict a large loss of Amazonian rainforest around 2050 due todrought,forest dieback and the subsequent release of more carbon dioxide.[38]
Tropical rainforests providetimber as well as animal products such as meat and hides. Rainforests also have value astourism destinations and for theecosystem services provided. Many foods originally came from tropical forests, and are still mostly grown onplantations in regions that were formerly primary forest.[39] Also, plant-derived medicines are commonly used for fever, fungal infections, burns, gastrointestinal problems, pain, respiratory problems, and wound treatment.[40] At the same time, rainforests are usually not usedsustainably by non-native peoples but are beingexploited or removed foragricultural purposes.
Native people
On 18 January 2007,FUNAI reported also that it had confirmed the presence of 67 differentuncontacted tribes in Brazil, up from 40 in 2005. With this addition,Brazil has now overtaken the island ofNew Guinea as the country having the largest number of uncontacted tribes.[41] The province of Irian Jaya orWest Papua in the island of New Guinea is home to an estimated 44 uncontacted tribal groups.[42] The tribes are in danger because of the deforestation, especially in Brazil.
Central African rainforest is home of theMbutipygmies, one of the hunter-gatherer peoples living in equatorial rainforests characterised by their short height (below one and a half metres, or 59inches, on average). They were the subject of a study byColin Turnbull,The Forest People, in 1962.[43] Pygmies who live in Southeast Asia are, amongst others, referred to as "Negrito".There are many tribes in the rainforests of the Malaysian state of Sarawak. Sarawak is part of Borneo, the third largest island in the world. Some of the other tribes in Sarawak are: theKayan,Kenyah,Kejaman,Kelabit,Punan Bah, Tanjong, Sekapan, and the Lahanan. Collectively, they are referred to as Dayaks or Orangulu which means "people of the interior".[44]
About half of Sarawak's 1.5 million people are Dayaks. Most Dayaks, it is believed by anthropologists, came originally from the South-East Asian mainland. Their mythologies support this.
Satellite photograph of thehaze aboveBorneo andSumatra, 24September 2015
Tropical andtemperate rainforests have been subjected to heavylegal andillegal logging for their valuablehardwoods andagricultural clearance (slash-and-burn,clearcutting) throughout the 20th century and the area covered by rainforests around the world is shrinking.[45] Biologists have estimated that large numbers of species are being driven toextinction (possibly more than 50,000 a year; at that rate, saysE. O. Wilson ofHarvard University, a quarter or more of all species on Earth could be exterminated within 50 years)[46] due to the removal of habitat with destruction of the rainforests.
Forests are being destroyed at a rapid pace.[48][49][50] Almost 90% ofWest Africa's rainforest has been destroyed.[51] Since the arrival of humans,Madagascar has lost two thirds of its original rainforest.[52] At present rates, tropical rainforests inIndonesia would be logged out in 10 years andPapua New Guinea in 13 to 16 years.[53] According toRainforest Rescue, an important reason for the increasingdeforestation rate, especially in Indonesia, is the expansion ofoil palm plantations to meet growing demand for cheap vegetable fats andbiofuels. In Indonesia,palm oil is already cultivated on nine million hectares and, together withMalaysia, the island nation produces about 85 percent of the world's palm oil.[54][unreliable source?]
Several countries,[55] notablyBrazil, have declared their deforestation a national emergency.[56] Amazon deforestation jumped by 69% in 2008 compared to 2007's twelvemonths, according to official government data.[57]
However, a 30 January 2009New York Times article stated, "By one estimate, for everyacre of rainforest cut down each year, more than 50 acres of new forest are growing in the tropics." The new forest includes secondary forest on former farmland and so-calleddegraded forest.[58]
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^Barquero-González, J.P., Stice, T.L., Gómez, G., &Monge-Nájera, J. (2020). Are tropical reptiles really declining? A six-year survey of snakes in a tropical coastal rainforest: role of prey and environment.Revista de Biología Tropical, 68(1), 336–343.
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^Cramer, Wolfgang; Bondeau, Alberte; Schaphoff, Sibyll; Lucht, Wolfgang; Smith, Ben; Sitch, Stephen (2005). "Twenty-first century atmospheric change and deforestation: Potential impacts on tropical forests".Tropical Forests and Global Atmospheric Change. Oxford University Press. pp. 17–30.doi:10.1093/acprof:oso/9780198567066.003.0002.ISBN978-0-19-856706-6.
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