Rainforest

Rainforests are forests with a thick, closed, and continuous tree canopy, plants that need lots of moisture, epiphytes and lianas, and no wildfires. Rainforests are usually grouped as tropical rainforests or temperate rainforests, though other types have also been described.

Rainforests grow in many parts of the world. Photos show them in Chiapas, Mexico, and in the Olympic rainforest on the Olympic Peninsula in Washington state. Others grow at Mount Palung National Park in Borneo, at Khao Sok, at Masoala National Park in Madagascar, in Palawan, and at Kinabalu Park.

Scientists estimate that 40% to 75% of all biotic species are indigenous to rainforests. Tropical rainforests may hold many millions of plant, insect, and microorganism species that have not yet been discovered.

Tropical rainforests have been called the "jewels of the Earth" and the "world's largest pharmacy," because more than one quarter of natural medicines have been found there.

Rainforests are disappearing quickly, along with many of the species that live only in them. The causes include deforestation, the habitat loss that follows, and pollution of the atmosphere. If you are looking for a different meaning of the word, see Rainforest (disambiguation).

Definition

Rainforests have a closed, continuous tree canopy, high humidity, and plants that depend on moisture. They also have a damp layer of fallen leaves, plants called epiphytes and lianas, and no wildfires. Most rainforests are either tropical or temperate, but other types have also been described, including subtropical rainforest, littoral rainforest, cloud forest, vine thicket, and dry rainforest.

Main article: Tropical rainforest

Tropical rainforests have a warm, wet climate, usually with no long dry season. They are found near the equator, and they grow in places such as Southeast Asia, Sub-Saharan Africa, South America, Central America, Australia, and some Pacific Islands. Temperate rainforests grow in cooler regions and are much rarer. They occur in places such as North America, Europe, East Asia, South America, Australia, and New Zealand. Dry rainforests have a more open canopy and get less rainfall than other rainforests.

Main article: Temperate rainforest

Layers

Main article: Stratification (vegetation)

A tropical rainforest usually has several layers, each with plants and animals adapted to that area. The emergent layer has a small number of very large trees called emergents. They rise above the general canopy to 45–55 m, and a few species grow to 70–80 m. These trees must withstand hot temperatures and strong winds above the canopy. Eagles, butterflies, bats and certain monkeys live in this layer.

Main article: Canopy (biology)

The canopy layer contains most of the largest trees, typically 30 to 45 metres tall. It is the densest area of biodiversity in the rainforest, with leaves formed by neighbouring treetops. By some estimates, the canopy is home to 50 percent of all plant species. Epiphytic plants attach to trunks and branches and collect water and minerals from rain and debris. About a quarter of all insect species are believed to live there. Scientists only began to explore this habitat in the 1980s, when they developed ways to reach it, such as firing ropes into the trees using crossbows.

Main article: Understory

The understory lies between the canopy and the forest floor. It is home to birds, snakes, lizards and predators such as jaguars, boa constrictors and leopards. Only about 5% of the sunlight that reaches the canopy gets through to this layer. Many seedlings that will grow to canopy level live here. This layer can also be called a shrub layer.

Main article: Forest floor

The forest floor, the bottom-most layer, receives only 2% of the sunlight, so only plants adapted to low light can grow there. Away from riverbanks, swamps and clearings, the floor is mostly clear of plants. Warm, humid conditions cause decaying plant and animal matter to break down quickly, and many kinds of fungi help this decay.

Flora and fauna

It is not known how many species live in the world's tropical rainforests, but a very large share of vascular plants are native to them. Rainforests are home to many kinds of fauna, including mammals, reptiles, amphibians, birds and invertebrates. Mammals may include primates, felids and other families. Reptiles include snakes, turtles, chameleons and other families, while birds include families such as Vangidae and Cuculidae. Dozens of families of invertebrates live in rainforests. Fungi are also very common, because they can feed on the remains of dead plants and animals. The great variety of rainforest species is largely due to the many refuges found there. These are places where plants are hard for many plant-eating animals to reach, or where animals can hide from predators. Having many refuges also allows a much higher total biomass than would otherwise be possible.

Some kinds of animals are becoming less common in rainforests. For example, reptiles that feed on amphibians and reptiles are showing this trend. This trend needs to be watched closely. The seasons in rainforests affect when amphibians breed, and this can directly affect the reptiles that feed on them. This is especially true for species with specialized diets, since these are less likely to use other food sources.

Soils

Even though a tropical rainforest has a lot of vegetation, its soil quality is often quite poor. Rapid bacterial decay stops humus from building up. The iron and aluminium oxides that gather through laterization give oxisols a bright red colour and can form mineral deposits, such as bauxite. Most trees have roots near the surface because there are few nutrients deeper down, so their minerals mostly come from the top layer of rotting leaves and animals. Soils on younger ground, especially of volcanic origin, can be quite fertile.

If rainforest trees are cleared, rain can run off the bare soil and start erosion. Over time, streams and rivers form and flooding becomes possible. The soil is poor for several reasons. It is very acidic, so plant roots cannot take in many nutrients. The clay in the soil also holds nutrients poorly, so even added nutrients wash away. Heavy rain washes nutrients out of the soil faster than in other climates.

Effect on global climate

A natural rainforest gives off and takes in huge amounts of carbon dioxide. Over the whole planet, these amounts stay roughly balanced over long periods. So an undisturbed rainforest would have only a small overall effect on the carbon dioxide in the air, although it may affect the climate in other ways, such as helping to form clouds by recycling water vapour.

No rainforest today is fully undisturbed. Deforestation, which is caused by people, plays a major role in making rainforests release carbon dioxide. Other things that kill trees, whether caused by people or by nature, also add to this, such as burning and drought. Some climate models predict that a large part of the Amazon rainforest could be lost around 2050, because of drought, forest dieback, and the carbon dioxide released afterward.

Human uses

Further information: Tropical rainforest § Human dimensions

Tropical rainforests provide timber and animal products such as meat and hides. People also visit them as tourism destinations, and they provide ecosystem services. Many foods first came from tropical forests and are now mostly grown on plantations in areas that used to be primary forest. Plant-based medicines are also commonly used to treat fevers, fungal infections, burns, stomach problems, pain, breathing problems, and wounds. However, rainforests are usually not used sustainably by people from outside the region. Instead, they are being exploited or cleared for agricultural purposes.

In 2007, FUNAI reported that it had confirmed 67 different uncontacted tribes in Brazil, up from 40 in 2005, so Brazil has overtaken the island of New Guinea as the country with the most. Indonesia's Papua region on New Guinea is home to an estimated 44 uncontacted tribal groups, and these tribes are in danger because of deforestation. The Central African rainforest is home to the Mbuti pygmies, hunter-gatherers who are short in height. Colin Turnbull studied them for his 1962 book The Forest People. Pygmies in Southeast Asia are among others called "Negrito". Sarawak, a Malaysian state on Borneo, has many tribes, including the Kayan, Kenyah, Kejaman, Kelabit, Punan Bah, Tanjong, Sekapan, and Lahanan. Together they are called Dayaks or Orangulu, meaning "people of the interior." About half of Sarawak's 1.5 million people are Dayaks, who are believed to have come originally from the South-East Asian mainland.

Deforestation

Further information: Deforestation in Southeast Asia, Deforestation in Madagascar, and Deforestation of the Amazon Rainforest

Tropical and temperate rainforests were heavily cut down throughout the 20th century, both legally and illegally, for their valuable hardwoods and to clear land for farming using methods such as slash-and-burn and clearcutting. The area covered by rainforests around the world is shrinking. Biologists estimate that many species are dying out, possibly more than 50,000 a year. E. O. Wilson of Harvard University says that at this rate, a quarter or more of all species on Earth could disappear within 50 years, because their habitats are being destroyed. Growing urban areas are another cause. Littoral rainforest along coastal parts of eastern Australia is now rare because of ribbon development, which has been built to meet the demand for seachange lifestyles.

Forests are being destroyed quickly. Almost 90% of the rainforest in West Africa has been lost. Since humans arrived, Madagascar has lost two thirds of its original rainforest. At current rates, the tropical rainforests of Indonesia could be logged out in 10 years and those of Papua New Guinea in 13 to 16 years. According to Rainforest Rescue, a major reason for the rising deforestation rate, especially in Indonesia, is the growth of oil palm plantations to meet demand for cheap vegetable fats and biofuels. Palm oil is already grown on nine million hectares in Indonesia, and together with Malaysia, the island nation produces about 85 percent of the world's palm oil. Several countries, notably Brazil, have declared their deforestation a national emergency. Some reports are more hopeful. A 2009 article in the New York Times stated that for every acre of rainforest cut down each year, more than 50 acres of new forest are growing in the tropics, including degraded forest on former farmland.