rainforest

Typical tropical rainforest vegetation. The picture shows giant ferns in the Braulio Carrillo national park, Costa Rica.

Cross section of a South American rainforest, showing the different horizontal layers of vegetation that make up the rainforest, and the animals and plants found in each layer. To a certain extent the separate layers provide unique microhabitats, with animals and plants found only in one stratum and being adapted to life there, so for example the tapir occupies only the forest floor and some butterfly species always fly at specific heights. Other animals and plants move across strata, for example many of the monkeys and birds.

Native to the rainforests of west Kenya and Uganda, the African tulip tree (Spathodea campanulata) will grow up to a height of 24 m/80 ft in any tropical region. It is also known as the fountain tree, the fire tree, or the flame-of-the-forest. These names are references to its brilliant scarlet or orange-red flowers, which resemble tulips in shape.

Rainforest clearance and banana trees, Dominica. The destruction of rainforest for timber and fuel, its clearance for agricultural land, and the effects of extractive industries, cause soil erosion, drought, and the destruction of wildlife and their habitats.

Rainforest clearance and banana trees. During the period 1990-95, the process of deforestation in South America alone resulted in the loss of 24 million ha/59 million acres of tropical rainforest.

The different layers of vegetation in a typical rainforest. The dense canopy of leaves shades the lower layers from the sun. Lianas are woody vines that grow around, and hang from, the trees. Ferns and mosses grow at ground level where the conditions are dark, damp, and humid.

Dense forest usually found on or near the Equator where the climate is hot and wet. Moist air brought by the converging trade winds rises because of the heat and produces heavy rainfall. More than half the tropical rainforests are in Central and South America, primarily the lower Amazon and the coasts of Ecuador and Columbia. The rest are in Southeast Asia (Malaysia, Indonesia, and New Guinea) and in West Africa and the Congo.

Tropical rainforests once covered 14% of the Earth's land surface, but are now being destroyed at an increasing rate as their valuable timber is harvested and the land cleared for agriculture, causing problems of deforestation. Although by 1991 over 50% of the world's rainforests had been removed, they still comprise about 50% of all growing wood on the planet, and harbour at least 40% of the Earth's species (plants and animals).

The vegetation in tropical rainforests typically includes an area of dense forest called selva; a canopy formed by high branches of tall trees providing shade for lower layers; an intermediate layer of shorter trees and tree roots; lianas; and a ground cover of mosses and ferns. The lack of seasonal rhythm causes adjacent plants to flower and shed leaves simultaneously. Chemical weathering and leaching take place in the iron-rich soil due to the high temperatures and humidity.

Rainforests comprise some of the most complex and diverse ecosystems on the planet, deriving their energy from the Sun and photosynthesis. In a hectare (10,000 sq m/107,640 sq ft) of rainforest there are an estimated 200-300 tree species compared with 20-30 species in a hectare of temperate forest. The trees are the main producers. Herbivores such as insects, caterpillars, and monkeys feed on the plants and trees and in turn are eaten by the carnivores, such as ocelots and puma. Fungi and bacteria, the primary decomposers, break down the dead material from the plants, herbivores, and carnivores with the help of heat and humidity. This decomposed material provides the nutrients for the plants and trees.

The rainforest ecosystem helps to regulate global weather patterns - especially by taking up CO₂ (carbon dioxide) from the atmosphere - and stabilizes the soil. Rainforests provide most of the oxygen needed for plant and animal respiration. When deforestation occurs, the microclimate of the mature forest disappears; soil erosion and flooding become major problems since rainforests protect the deep tropical soils. Once an area is cleared it is very difficult for shrubs and bushes to re-establish because soils are poor in nutrients. This causes problems for plans to convert rainforests into agricultural land - after two or three years the crops fail and the land is left bare. Clearing of the rainforests may lead to global warming of the atmosphere, and contribute to the greenhouse effect.

Tropical rainforests are characterized by a great diversity of species (biodiversity), usually of tall broad-leafed evergreen trees, with many climbing vines and ferns, some of which are a main source of raw materials for medicines. A tropical forest, if properly preserved, can yield medicinal plants, oils (from cedar, juniper, cinnamon, sandalwood), spices, gums, resins (used in inks, lacquers, linoleum), tanning and dyeing materials, forage for animals, beverages, poisons, green manure, rubber, and animal products (feathers, hides, honey). Other types of rainforest include montane, upper montane or cloud, mangrove, and subtropical.

Traditional ways of life in tropical rainforests are disappearing. The practice of shifting cultivation (slash and burn), in which small plots of forest are cultivated and abandoned after two or three harvests, is being replaced by cultivation on such a large scale that the rainforests cannot regenerate. As a result hunting and gathering as a way of life is also becoming less viable. In the last 30 years, Central America has lost almost two-thirds of its rainforests to cattle ranching.