Rainforest

Rainforests are forests with a thick, closed, and continuous tree canopy. Their plants need lots of moisture. They have epiphytes and lianas, and wildfires do not happen there. 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 think that 40% to 75% of all biotic species live only in rainforests. Tropical rainforests may hold many millions of plant, insect, and microorganism species that nobody has found yet.

Tropical rainforests have been called the "jewels of the Earth" and the "world's largest pharmacy." This is 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. Some causes are 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 are forests with thick, closed treetops, lots of moisture in the air, and plants that need water to grow. The ground is often damp and covered in fallen leaves. Epiphytes, which are plants that grow on other plants, and lianas, which are long woody vines, are common there. Rainforests almost never have wildfires. Most rainforests are either tropical or temperate. Some other kinds have also been described, including subtropical rainforest, littoral rainforest, cloud forest, vine thicket, and dry rainforest.

Main article: Tropical rainforest

Tropical rainforests are warm and wet, and they usually do not have a long dry season. They grow near the equator, in places such as Southeast Asia, Sub-Saharan Africa, South America, Central America, Australia, and some Pacific Islands. Temperate rainforests grow in cooler places and are much rarer. They can be found in areas such as North America, Europe, East Asia, South America, Australia, and New Zealand. Dry rainforests have a more open canopy, with more space between the treetops, and they get less rainfall than other rainforests.

Main article: Temperate rainforest

Layers

Main article: Stratification (vegetation)

A tropical rainforest usually has several layers. Each layer has plants and animals that are suited to life there. The emergent layer has a small number of very large trees called emergents. They rise above the rest of the canopy, to about 45–55 m, and a few kinds grow to 70–80 m. These trees face 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, usually 30 to 45 metres tall. It is the busiest area for biodiversity in the rainforest, with leaves formed by the treetops that grow close together. Some estimates say the canopy is home to half of all plant species. Epiphytic plants grow on trunks and branches and collect water and minerals from rain and debris. Scientists believe about a quarter of all insect species live here. People only began to study this layer in the 1980s, when they found ways to reach it, such as shooting ropes into the trees with 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 layer, gets only 2% of the sunlight. Only plants adapted to low light can grow there. Away from riverbanks, swamps and clearings, the ground is mostly bare of plants. The warm, wet conditions help decaying plant and animal matter break down quickly, and many kinds of fungi help this decay.

Flora and fauna

Nobody knows exactly how many species live in the world's tropical rainforests. Still, a very large share of vascular plants grow only in these forests.

Rainforests are home to many kinds of fauna. These include mammals, reptiles, amphibians, birds and invertebrates. Mammals may include primates, felids and other families. Reptiles include snakes, turtles, chameleons and other families. Birds include families such as Vangidae and Cuculidae. Dozens of families of invertebrates live in rainforests too. Fungi are also very common, because they can feed on the remains of dead plants and animals.

Many different species can live in rainforests because there are many refuges. A refuge is a place where plants are hard for plant-eating animals to reach, or where animals can hide from predators. Because of these safe places, rainforests can support a much larger 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, and scientists need to watch it closely. The seasons in rainforests affect when amphibians breed, and this can directly affect the reptiles that eat them. This is especially true for species with specialized diets, since they are less likely to switch to other food sources.

Soils

Even though a tropical rainforest has a lot of vegetation, its soil quality is often poor. Rapid bacterial decay stops humus from building up. Iron and aluminium oxides, which gather through laterization, give oxisols a bright red colour. They can also form mineral deposits, such as bauxite. Most trees have roots near the surface because there are few nutrients deeper down. 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 wash away the bare soil and cause 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 also washes nutrients out of the soil faster than in other climates.

Effect on global climate

A natural rainforest gives off and takes in large amounts of carbon dioxide. Over the whole planet, these amounts stay roughly balanced over long periods. So an undisturbed rainforest has only a small overall effect on the carbon dioxide in the air. It can still 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, such as fires and drought, also add to this. Some scientists 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

People use tropical rainforests in many ways. They provide timber and animal products such as meat and hides. Visitors come to see them as tourism destinations, and they offer ecosystem services. Many foods first came from tropical forests, but today most are grown on plantations in places that were once primary forest. People also use plant medicines to treat fevers, infections, burns, stomach problems, pain, breathing problems, and wounds. However, people from outside the region often do not use rainforests in a sustainable way. Instead, the forests are being exploited or cleared for farming.

In 2007, FUNAI reported that it had confirmed 67 different uncontacted tribes in Brazil, up from 40 in 2005. This means Brazil now has more of these groups than the island of New Guinea. On the Indonesian side of New Guinea, the Papua region is home to an estimated 44 uncontacted tribal groups. These tribes are at risk because the forests are being cut down. The Central African rainforest is home to the Mbuti pygmies, who are short in height and live by hunting and gathering food. Colin Turnbull wrote about them in his 1962 book The Forest People. In Southeast Asia, pygmy groups are also called "Negrito". The Malaysian state of Sarawak, on Borneo, has many tribes. These include the Kayan, Kenyah, Kejaman, Kelabit, Punan Bah, Tanjong, Sekapan, and Lahanan. Together, they are called Dayaks or Orangulu, which means "people of the interior." About half of Sarawak's 1.5 million people are Dayaks, who are believed to have first come from the South-East Asian mainland.

Deforestation

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

Many tropical and temperate rainforests were cut down during the 20th century. Some of this was legal and some was not. The wood was valuable hardwoods, and the land was cleared for farming. People used methods such as slash-and-burn and clearcutting. Rainforests now cover less land around the world. Biologists believe that many species are dying out. E. O. Wilson of Harvard University warns that many more species could disappear in the coming decades, because their homes are being destroyed. Growing urban areas are another cause. Some littoral rainforest along coastal parts of eastern Australia is now rare because of ribbon development. This housing was built for people who wanted a seachange lifestyle.

Forests are being lost quickly in many places. Much of the rainforest in West Africa has been lost. Madagascar has also lost a large part of its original rainforest since people arrived. If logging continues at its current pace, the tropical rainforests of Indonesia and Papua New Guinea could be cut down within the next couple of decades. According to Rainforest Rescue, one major reason for the rising deforestation rate, especially in Indonesia, is the growth of oil palm plantations. These plantations supply cheap vegetable fats and biofuels. Much of the world's palm oil comes from Indonesia and Malaysia. Several countries, including Brazil, have declared their deforestation a national emergency. Some reports are more hopeful. A 2009 article in the New York Times said that new forests are growing in the tropics, including on degraded forest land that was once farmland. These new forests are growing faster than the old forests are being cut down each acre by acre.