Tropical rainforest is a forest of tall trees in a region of year-round warmth and plentiful rainfall. Almost all such forests lie near the equator. They occupy large regions in Africa, Asia, and Central and South America, and on Pacific islands. The largest tropical rainforest is the Amazon rainforest, also called the selva. It covers about a third of South America. Tropical rainforests stay green throughout the year.
A tropical rainforest has more kinds of trees than any other area in the world. Scientists have counted 179 species in one 1-hectare area in South America. Most northern temperate forests have fewer than seven species. About half of the world's species of plants and animals also live in tropical rainforests. More species of amphibians, birds, insects, mammals, and reptiles live in tropical rainforests than anywhere else.
The tallest trees of a rainforest may grow as tall as 60 metres. The crowns (tops) of other trees form a covering of leaves about 30 to 45 metres above the ground. This covering is called the upper canopy. The crowns of smaller trees form one or two lower canopies. All the canopies shade the forest floor so that it receives less than 1 per cent as much sunlight as does the upper canopy.
Most areas of the forest floor receive so little light that few bushes or herbs can grow there. As a result, a person can easily walk through most parts of a tropical rainforest. Areas of dense growth called jungles occur within a tropical rainforest in areas where much sunlight reaches the ground. Most jungles grow near broad rivers or in former clearings.
The temperature in a rainforest rarely rises above about 35 ºC or drops below about 20 ºC. In many cases, the average temperature of the hottest month is only 1º to 3 ºC higher than the average temperature of the coldest month.
At least 200 centimetres of rain falls yearly in a tropical rainforest. Thundershowers may occur on more than 200 days a year. The air beneath the lower canopy is almost always humid. The trees themselves give off water through the pores of their leaves. This process, called transpiration, may account for as much as half of the rain in the Amazon rainforest.
All tropical rainforests resemble one another. But each of the three largest ones--the American, the African, and the Asian--has a different group of animal and plant species. For instance, each rainforest has many species of monkeys, all of which differ from the species of the other two rainforests. In addition, different areas of the same rainforest may have different species. For example, many kinds of trees that grow in the mountains of the Amazon rainforest do not grow in the lowlands of that forest.
A tropical rainforest is always green. Most trees in the forest lose old leaves and grow new ones throughout the year. But certain species of trees may lose all of their leaves for a short time during the year. Different kinds of trees bear flowers and fruit at different times of the year. Thus, some kind of tree is in bloom or in fruit at any time of the year. Some short trees bear fruit on the trunk or on large, low branches. Some tall trees bear large fruit on long, drooping, ropelike stalks.
Tropical rainforest trees include some species of great beauty and others that provide fruit, timber, and other useful products. Cassias, dhaks, shellseeds, and tabebuias bear bright-coloured flowers. But most rainforest trees have smaller, less noticeable flowers, and the canopy always appears mostly green. Brazil nuts, cashews, durians, mangosteens, sapodillas, and many kinds of figs and palms yield fruit. Valuable timber comes from balsas, brazilwoods, lauans, logwoods, mahoganies, and rosewoods. Kapoks bear fruit that contains a fluffy fibre used to stuff life jackets and upholstery. Cinchonas provide the drug quinine. Curare, another important drug, comes from various woody vines that grow in a tropical rainforest.
In a tropical rainforest, many plants grow on tree branches, where they receive more sunlight than they would on the ground. Such plants, called epiphytes or air plants, include ferns, mosses, orchids, and bromeliads. Climbing plants called lianas twine around tree trunks and branches. Some lianas form loops and knots as they grow toward the sunlight.
Several kinds of strangler trees grow in rainforests. These trees start life as air plants. But unlike other species of air plants, they develop roots that reach down to the ground. The roots surround the tree on which the strangler lives. In time, the strangler may kill the other tree by depriving it of food, light, and water.
In a tropical rainforest, most plant nutrients (the chemicals necessary for growth) are locked up in the living vegetation. Small amounts of nutrients are stored in a thin layer of soil near the surface, where decaying vegetation mixes with the soil. The roots of most rainforest trees remain close to the supply of nutrients near the surface. In some species, the roots form large growths called buttresses that extend between the roots and the trunk. The buttresses may help keep the trees upright.
A tropical rainforest has no dominant species of trees. Most kinds of trees are widely scattered throughout the forest and depend on animals for pollination. By contrast, in nontropical forests, certain tree species dominate and pollination occurs chiefly by wind.
A great variety of animals live in a tropical rainforest. Many of these animals spend their lives in the trees and never descend to the ground. The fruit and nuts of the upper and lower canopy furnish food for bats, gibbons, monkeys, squirrels, parrots, and toucans. Sloths and some monkeys feed on the leaves. Hummingbirds and sunbirds sip nectar from flowers. Frogs, lizards, and snakes also dwell among the branches. Large birds and large snakes prey on the smaller animals.
Many canopy animals are especially suited to treetop life. Flying lemurs and flying squirrels glide from tree to tree. Galagos and marmosets jump from branch to branch. Several kinds of anteaters, monkeys, opossums, and porcupines sometimes hang by their tail.
Antelope, deer, pigs, tapirs, and many kinds of rodents roam the forest floor. They feed on roots, seeds, and leaves, and also on fruit that drops to the ground. Chimpanzees, coatis, and several members of the cat family live on the floor and in the trees. Ants may be found at all levels in a rainforest. Bees, butterflies, mosquitoes, moths, termites, and spiders are also abundant.
Over the years, few people have dwelt in tropical rainforests. Most such people clear small areas and plant crops there. They chop down the trees, burn them, and plant seeds among the ashes. But after a few years, the thin layer of soil no longer provides good harvests. The farmers then move elsewhere and begin the process all over again. Such farming, called slash-and-burn cultivation, can support only a small population.
A few groups of rainforest people practise no agriculture. For example, the Pygmies of the Central African rainforest live by hunting wild animals, gathering wild plants, and trading with agricultural tribes.
Today, the rapid growth of the world population and the increasing demands for natural resources threaten many tropical rainforests. People have destroyed large areas of rainforests by clearing land for farms and cities. Huge mining, ranching, and timber projects also have caused much damage. Scientists estimate that from 5.5 million to 22 million hectares of tropical rainforests are destroyed yearly. They fear that further forest destruction will lead to the elimination of the local peoples, and hundreds of thousands of species of plants and animals.