Rain Forest

I

Introduction

Rain Forest, woodland characterized by lush vegetation and comparatively high temperature and rainfall throughout the year. Rain forests are the world’s most biologically diverse ecosystems. Although they account for less than 7 percent of the land surface on Earth, they contain more than 50 percent—some scientists estimate as high as 90 percent—of its plant and animal species. One hectare (about 2.5 acres) of tropical rain forest may contain more than 600 species of trees. By comparison, the forests of the United States and Canada combined contain only around 700 tree species. Even more impressive are the number and diversity of animal species that call rain forests their home. One study found more species of ants living on a single rain forest stump than exist in all of the British Isles.

Rain forests also play a critical role in global climate regulation by absorbing carbon dioxide, a greenhouse gas believed to be principally responsible for global warming. Plants naturally absorb carbon dioxide and give off oxygen gas in the process of photosynthesis, and tropical rain forests absorb more carbon dioxide than any other terrestrial ecosystem on earth. Global emissions of carbon dioxide have increased nearly 30 percent in the last century. There is general agreement among the scientific community that by absorbing some of the gas, rain forests play a vital role in lessening its impacts.

To be classified as a rain forest, a forest must have a closed canopy, in which the treetops, or crowns, touch each other, creating a shaded forest interior. In addition, temperature and rainfall must be high and relatively even throughout the year. Forests that meet these criteria are found flanking the equator in South and Central America, Asia, Africa, and Australia. In South America, a vast, forested area of the Amazon River basin in Brazil and neighboring countries is by far the largest rain forest in the world. It encompasses more than 3.5 million sq km (about 1.4 million sq mi)—about half of the total global rain forest cover. The larger of two large rain forests in Asia is centered along the Malay Archipelago, including the islands of Borneo and Sumatra, the Malay Peninsula, and the Republic of the Philippines. The other main rain forest in Asia is found primarily on the island of New Guinea and in northern Australia. In Africa, most of the rain forest is concentrated along the Atlantic coast and the Congo River Basin.

In regions of the northern and southern hemispheres, small areas of temperate rain forest are found along the coasts where rainfall and humidity are high and winters are mild. Such temperate rain forests are found in the Pacific Northwest of the United States and southwestern Canada, for example. They are dominated by only a few tree species, however, making them differ dramatically from highly diverse tropical rain forests. This article focuses on tropical rain forests.

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II

Rain Forest Characteristics

Tropical rain forests belong to a broader forest category called tropical moist forests, of which there are many different types. Forest scientists distinguish rain forests from other types by considering factors such as temperature, rainfall, length of dry season, and altitude.

A

Temperature

Rain forests are typically hot and steamy—the average annual temperature is 25° C (77° F). Temperature near the equator varies little over the course of a year, so rain forest temperatures are about the same year round—the average minimum monthly temperature in a rain forest is a balmy 18° C (64° F). Rain forests are not found where the temperature drops close to 0° C (32° F) because their resident plants and animals are not well adapted to withstand frost. Temperature in a rain forest depends not only on distance from the equator but also on altitude. As elevation increases, nighttime temperatures go down significantly. This daily temperature variation affects forest ecology, and true rain forests typically are not found above 1,000 m (3,280 ft).

B

Rainfall

Rain forests can average as little as 1.8 m (6 ft), or as much as 9.0 m (30 ft), of rainfall a year. What distinguishes a true rain forest is the distribution of precipitation throughout the year—there are no dry seasons. Every month, typically more than 100 mm (4 in) of rain falls. If a rain forest does have dry periods, they are usually short and unpredictable.

In many climates, rainfall evaporation is carried away to fall as rain in distant places, but in the rain forests, nearly 50 percent of the precipitation comes from local evaporation. The warm, humid air surrounding a rain forest forms a microenvironment that permits little water to escape. Much of the rain that falls on the rain forest is intercepted by the trees in the canopy. Some of it rolls off the leaves and down the trunks to the forest below, but a high percentage evaporates and hangs as tiny droplets of water in the humid atmosphere. Gentle and continuous winds lift the tiny droplets higher in the atmosphere, where they cool and form clouds. When enough of these drops enter the atmosphere and cool, they condense and fall as rain, beginning the cycle again.

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