Tropical rainforest

Amazon river rainforest in Brazil

Tropical rainforests are rainforests which are generally found near the equator. They are common in Asia, Africa, South America, Central America, and on many of the Pacific Ocean islands.

Within the WWF's biome classification scheme, tropical rainforests are considered a type of tropical wet forest (or tropical or subtropical moist broadleaf forest), and may be called lowland equatorial evergreen rain forests.

Characteristics

Rainforests are characterized by high rainfall, with rainfall of 100-600cm a year. For this reason, the soil can be poor because high rainfall tends to leach out soluble nutrients. Oxisols, infertile, and deeply weathered, have developed on the ancient Gondwanan shields. 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 (e.g. bauxite). On younger substrates, especially of volcanic origin, tropical soils may be quite fertile, as are the soils of many seasonally flooded forests, which are annually replenished with fertile silt.

Rainforests are home to two-thirds of all the living animal and plant species on the planet. It has been estimated that many hundreds of millions of new species of plants, insects, and microorganisms are still undiscovered and as yet unnamed by science. Tropical rain forests are called the "jewel of the earth", and the "world's largest pharmacy" because of the large amount of natural medicines discovered there. Tropical rain forests are also often called the "Earth's lungs", however there is no scientific basis for such a claim as tropical rainforests are known to be essentially oxygen neutral, with little or no net oxygen production.

Tall, broad-leaved evergreen trees are the dominant plants, forming a leafy canopy over the forest floor. Taller trees, called emergents, may rise above the canopy. The upper portion of the canopy often supports a rich flora of epiphytes, including orchids, bromeliads, mosses, and lichens, who live attached to the branches of trees. The undergrowth or understory in a rainforest is often restricted by the lack of sunlight at ground level, and generally consists of shade-tolerant shrubs, herbs, ferns, small trees, and large woody vines which climb into the trees to capture sunlight. The relatively sparse understory vegetation makes it possible for people and other animals to walk through the forest. In deciduous and semi-deciduous forests, or forests where the canopy is disturbed for some reason, the ground beneath is soon colonised by a dense tangled growth of vines, shrubs and small trees called jungle. yitytyt utit oowowippppppppppppp

Temperature ranges from 5 degrees C to 50 degrees C

Locations

Distribution map of tropical rainforests around the world

About half of the world's tropical rainforests are in the Amazon Basin (the Amazon Rainforest), primarily in Brazil.Other areas of rainforest include Nicaragua (Los Guatuzos, Bosawás and Indio-Maiz), the southern Yucatán Peninsula-El Peten-Belize contiguous area of Central America (including the Calakmul Biosphere Reserve), in much of equatorial Africa from Cameroon to the Democratic Republic of Congo, in much of southeastern Asia from India to Indonesia and Papua New Guinea and northern and eastern Australia.

Trees

Rainforest on Fatu-Hiva, Marquesas Islands

There are several common characteristics of tropical rainforest trees. Tropical rainforest species frequently possess one or more of the following attributes not commonly seen in trees of higher latitudes or trees in drier conditions on the same latitude.

Many tree species have broad, woody flanges (buttresses) at the base of the trunk. Originally believed to help support the tree, now it is believed that the buttresses channel stem flow and it's dissolved nutrients to the roots.

Large leaves are common among trees and shrubs of the understory and forest floor layers. Young individuals of trees destined for the canopy and emergent layers may also have large leaves. When they reach the canopy new leaves will be smaller. The large leaf surface helps intercept light in the sun-dappled lower strata of the forest and are made possible because the lower layers are largely protected from winds which damage large leaves in the canopy. Canopy leaves are usually smaller than found in understory plants or are divided to reduce wind damage. The leaves of rainforest understorey trees also often have drip tips which facilitate drainage of precipitation off the leaf to promote transpiration and inhibit the growth of microbes and bryphytes which would damage or smother the leaf .

Trees are often well connected in the canopy layer especially by the growth of woody climbers known as lianas or by plants with epiphytic adaptations, allowing them to grow on top of existing trees in the competition for sunlight.

Other characteristics that are more frequent in tropical rainforest tree species than in temperate forests or drier tropical regions include:

• Exceptionally thin bark, often only 1-2 mm thick. It is usually very smooth, although sometimes covered with spines or thorns.
• Cauliflory, the development of flowers (and hence fruits) directly from the trunk, rather than at the tips of branches.
• Large fleshy fruits attract birds, mammals, and even fish as dispersal agents.

Layers

Main article: Rainforest § Rainforest layers

The rainforest is divided into five different parts, each with different plants and animals, adapted for life in that particular area. These are: the emergent layer, canopy layer, understory layer, shrub layer, and forest floor. Only the emergent layer is unique to tropical rainforests, while the others are also found in temperate rainforests.

The emergent layer contains a small number of very large trees which grow above the general canopy, although on occasion a few species will grow to 60 m or 70 m tall. They need to be able to withstand the hot temperatures and dry winds. Eagles, butterflies, bats and certain monkeys inhabit this layer.

Human uses

Habitation

Most tropical rainforests have historically not supported dense human populations. Food resources within the forest are extremely dispersed due to the high biological diversity and what food does exist is largely restricted to the canopy and requires considerable energy to obtain. Rainforest soils are often thin and leached of many minerals, and the heavy rainfall can quickly leach nutrients from rainforest plots cleared for cultivation. Nonetheless humans have and do exploit rainforests for food and shelter in several parts of the world. Many agriculturalists, particularly within the Amazon and New Guinean rainforests, obtain their food primarily from farm plots cleared from the forest and hunt and forage within the forest to suplement this. Other peoples described as rainforest dwellers are hunter-gatherers who subsist in large part by trading high value forest products such as hides feathers and honey with agricultural people living outside the forest. Other groups of hunter-gatherers have exploited rainforest on a seasonal basis but dwell primarily in adjacent savanna and open forest environments where food is much more abundant.

Cultivated foods and spices

Coffee, chocolate, bananas, mangoes, papayas, avocados and sugar cane all originally came from tropical rainforests, and are still mostly grown on plantations in regions that were formerly primary forest. In the mid-80s and 90s, 40 million tons of bananas were consumed worldwide each year, along with 13 million tons of mangoes. Central American coffee exports were worth US$3 billion in 1970. Much of the genetic variation used in evading the damage caused by new pests is still derived from resistant wild stock. Tropical forests have supplied 250 cultivated kinds of [fruit], compared to only 20 for temperate forests. Forests in New Guinea alone contain 251 tree species with edible fruits, of which only 43 had been established as cultivated crops by 1985.

Tropical rain forests are called the 'world's largest "pharmacy"' because of the large amount of natural "medicines" discovered there. Nearly half of the "medicines" that we use were discovered in rainforests. For example, rainforests contain the "basic ingredients of birth control hormones, cocaine, stimulants, and tranquilizing drugs" (Banks 36). Curare (a paralyzing drug) and quinine (a malaria cure) are also found there. Scientists believe that the cures for many more diseases will be discovered there in the future. Currently, 121 prescription drugs currently sold worldwide come from plant-derived sources. And while 25% of Western pharmaceuticals are derived from rainforest ingredients, less than 1% of these tropical trees and plants have been tested by scientists.

Tourism

Currently one the largest economic values of tropical rainforests comes in the form of tourism. People travel both nationally and internationally to experience rainforests firsthand. The economic benefits of tourism are the most promising way in which rainforests may be preserved.

Animal products

Rainforests provide numerous animal products including honey, game meat and associated trophies such as hides and ivory.

Ecosystem services

In addition to extractive human uses rainforests also have non-extractive uses that are frequently summarised as ecosystem services. Rainforests play an important role in maintaining biological diversity, modulating precipitation infiltration and flooding, increasing scientific knowledge and in the spiritual wellbeing of humans. Such ecosystem services are of use to humans without the need for any modification or management of the forest itself.

See also

• Rainforest
• Tropical and subtropical moist broadleaf forests
• Temperate rain forest
• List of tropical and subtropical moist broadleaf forests ecoregions