Rainforest
A closed and continuous tree canopy defines the rainforest, separating it from other forest types. High humidity and moisture-dependent vegetation are essential features of these ecosystems. The presence of epiphytes and lianas adds to the structural complexity found within these areas. A moist layer of leaf litter covers the ground beneath the trees. Wildfires are notably absent from these environments, allowing for a unique ecological balance. These physical criteria distinguish rainforests from dry forests or savannas. Other vegetation associations include subtropical rainforests and cloud forests. Littoral rainforests grow along coastal areas while vine thickets form dense undergrowth. Even dry rainforests exist with more open canopies in areas of lower rainfall.
Tropical rainforests typically lie within 10 degrees north and south of the equator. Mean monthly temperatures exceed 18 degrees Celsius during all months of the year. Average annual rainfall is no less than 2000 millimeters and can exceed 10000 millimeters. Many tropical forests are associated with the monsoon trough known as the Intertropical Convergence Zone. Southeast Asia hosts vast forests from Myanmar to Papua New Guinea. Sub-Saharan Africa contains the Congo Rainforest stretching from Cameroon to the Congo. South America holds the Amazon rainforest which spans multiple countries. Central America includes Bosawás and the southern Yucatán Peninsula. Australia and Pacific Islands like Hawaii also support these ecosystems. Temperate rainforests occur in fewer regions around the world. They appear in North America including Alaska and British Columbia. Europe features parts of the British Isles and southern Norway. East Asia has much of Japan and Korea covered by these forests. Southern Chile and New Zealand host temperate zones as well.
The emergent layer contains very large trees called emergents reaching heights of 45 to 55 meters. Some species grow to 70 to 80 meters tall above the general canopy. Eagles, butterflies, bats and certain monkeys inhabit this uppermost layer. The canopy layer typically reaches 30 to 45 meters in height. This dense cover forms a more or less continuous foliage layer. Estimates suggest the canopy is home to 50 percent of all plant species. Epiphytic plants attach to trunks and branches obtaining water from rain. A quarter of all insect species are believed to exist in the forest canopy. Naturalist William Beebe declared in 1917 that another continent of life remains to be discovered. True exploration of this habitat only began in the 1980s when scientists developed methods to reach it. The understory lies between the canopy and the forest floor receiving only 5% of sunlight. Jaguars, boa constrictors and leopards hunt within this dimly lit region. The forest floor receives just 2% of sunlight allowing only low-light adapted plants to grow.
Estimates vary from 40% to 75% of all biotic species being indigenous to the rainforests. Many millions of species of plants insects and microorganisms remain undiscovered in tropical rainforests. Rainforests support a very broad array of fauna including mammals reptiles amphibians birds and invertebrates. Mammals may include primates felids and other families while reptiles include snakes turtles chameleons. Dozens of families of invertebrates are found in these dense forests. Fungi feed on decomposing remains of plants and animals throughout the ecosystem. The great diversity is largely the result of diverse physical refuges where plants are inaccessible to herbivores. Having numerous refuges results in much higher total biomass than would otherwise be possible. Some species show declining populations such as reptiles that feed on amphibians. Seasonality affects reproductive patterns of amphibians which directly impacts snake species with specialized feeding. These trends require close monitoring by scientists studying the region.
Despite lush vegetation soil quality in tropical rainforests is often quite poor. Rapid bacterial decay prevents the accumulation of humus in the ground. Iron and aluminium oxides concentrate through laterization giving oxisols a bright red colour. Most trees have roots near the surface because insufficient nutrients exist below. Trees obtain minerals from the top layer of decomposing leaves and animals. Soil acidity creates little difference between root pH and soil pH hindering nutrient absorption. Clay particles present in tropical rainforest soil trap nutrients poorly allowing them to wash away. High volume of rain washes nutrients out of the soil more quickly than other climates. If trees are cleared rain accumulates creating run-off beginning soil erosion. Streams form eventually leading to flooding possibilities. On younger substrates especially volcanic origin soils may be quite fertile.
On the 18th of January 2007 FUNAI confirmed the presence of 67 different uncontacted tribes in Brazil. This number rose from 40 in 2005 making Brazil home to the largest number of uncontacted tribes. The province of Irian Jaya or West Papua hosts an estimated 44 uncontacted tribal groups. Central African rainforest is home to the Mbuti pygmies one of hunter-gatherer peoples living there. They average below one and a half meters height according to studies by Colin Turnbull. The Forest People study was published in 1962 documenting their lives. Tribes in Southeast Asia are referred to as Negito among others. Many tribes live in the Malaysian state of Sarawak on Borneo island. The Kayan Kenyah Kejaman Kelabit Punan Bah Tanjong Sekapan and Lahanan are some examples. Collectively they are called Dayaks meaning people of the interior. About half of Sarawak's 1.5 million people are Dayaks believed to come originally from South-East Asian mainland.
Tropical and temperate rainforests have been subjected to heavy legal and illegal logging throughout the 20th century. Large numbers of species are being driven to extinction possibly more than 50,000 a year. E.O. Wilson of Harvard University stated that a quarter or more of all species could be exterminated within 50 years. Almost 90% of West Africa's rainforest has been destroyed since human arrival. Madagascar has lost two thirds of its original rainforest over time. At present rates tropical rainforests in Indonesia would be logged out in 10 years. Papua New Guinea faces similar threats with forests gone in 13 to 16 years. Brazil declared deforestation a national emergency after Amazon destruction jumped by 69% in 2008. Expanding urban areas destroy littoral rainforest along eastern Australia coastlines. Oil palm plantations expand to meet demand for cheap vegetable fats and biofuels. Palm oil is cultivated on nine million hectares in Indonesia producing about 85 percent of world supply. A 2009 New York Times article noted that for every acre cut down more than 50 acres grow back as secondary forest.
Common questions
What defines a rainforest?
A closed and continuous tree canopy defines the rainforest, separating it from other forest types. High humidity and moisture-dependent vegetation are essential features of these ecosystems.
Where are tropical rainforests located geographically?
Tropical rainforests typically lie within 10 degrees north and south of the equator. Southeast Asia hosts vast forests from Myanmar to Papua New Guinea while South America holds the Amazon rainforest which spans multiple countries.
How tall do trees grow in the emergent layer of a rainforest?
The emergent layer contains very large trees called emergents reaching heights of 45 to 55 meters. Some species grow to 70 to 80 meters tall above the general canopy.
When did scientists begin true exploration of the rainforest canopy?
True exploration of this habitat only began in the 1980s when scientists developed methods to reach it. Naturalist William Beebe declared in 1917 that another continent of life remains to be discovered.
Which tribes live in the Central African rainforest?
Central African rainforest is home to the Mbuti pygmies one of hunter-gatherer peoples living there. They average below one and a half meters height according to studies by Colin Turnbull.
What percentage of West Africa's rainforest has been destroyed since human arrival?
Almost 90% of West Africa's rainforest has been destroyed since human arrival. Madagascar has lost two thirds of its original rainforest over time.