Rain forests are ecosystems characterized by high annual precipitation and an abundance of many large trees, generally of very old age. (An ecosystem is an ecological community, or the plants, animals, and microorganisms in a region considered together with their environment.) Rain forests can be found in both tropical and temperate regions. (Temperate regions have mild or moderate climates; tropical regions have high enough temperatures and enough rain to support plant growth year round.) Rain forests require a humid climate, with an average precipitation of at least 80 to 100 inches (200 to 250 centimeters) per year. Because of the great amount of precipitation, forest fires occur only rarely. As a result, trees in a rain forest are able to grow to a very large size and a very old age.
Tropical rain forests can be found in equatorial regions of Central and South America, west-central Africa, and Southeast Asia, including New Guinea and the northeastern coast of Australia. Tropical rain forests are the most complex of the world's ecosystems in terms of both their physical structure and the tremendous biodiversity of species they support. Because they support such a wide variety and number of plants, animals, and microorganisms, tropical rain forests represent the highest peak of ecosystem development on Earth.
Biomass: The sum total of living and once-living matter contained within a given geographic area.
Biodiversity: The wide range of organisms—plants and animals—that exist within any geographical location.
Canopy: The "covering" of a forest, consisting of the highest level of tree branches and foliage in the forest.
Ecosystem: An ecological community, including plants, animals, and microorganisms, considered together with their environment.
Old-growth forest: A mature forest, characterized by great age and many large and very old trees, with a complex physical structure.
Temperate: Mild or moderate.
Tropical: Characteristic of a region or climate that is frost free with temperatures high enough to support—with adequate precipitation—plant growth year round.
Productivity of tropical rain forests. Tropical rain forests have a very complex canopy, consisting of many layers of foliage (leaves) intertwined with each other. This canopy makes up one of the densest leafy surfaces found in any of Earth's ecosystems. The presence of so many leaves make it possible for tropical rain forests to capture solar energy and convert it to plant production with a high degree of efficiency.
In the tropical rain forests, woody tissues of trees account for about 80 percent of the biomass. (Biomass is the sum total of living and dead plants and animals.) Another 15 percent of the organic matter occurs in soil and litter (the uppermost, slightly decaying layer of organic matter on the forest floor), and about 5 percent is foliage. In contrast, a much larger fraction of the biomass in temperate forests occurs as organic matter in the soil and on forest floor. The reason for this difference is temperature. In tropical rain forests, dead biomass decays very rapidly because of warm and humid environmental conditions.
This fact explains a strange contradiction about tropical rain forests. In spite of the abundance of living and dead plants and animals they contain, they are very fragile environments. If trees are cut down, the vast majority of the forest's biomass is lost. In addition, the soil in tropical rain forests is generally not very fertile. When trees are removed, it is usually difficult to get other plants and crops to grow in the same place. The destruction of tropical rain forests in order to obtain land for agriculture, then, has had some surprising results. Very rich, productive stands of trees have been lost, but those stands have not been replaced by farms that are as rich. In fact, the land is often simply lost to any form of productive plant growing.
Biodiversity in tropical rain forests. An enormous number of species of plants, animals, and microorganisms occurs in tropical rain forests. In fact, this type of ecosystem accounts for a much larger fraction of Earth's biodiversity than any other category. Some scientists estimate that as many as 30 million to 50 million species may occur on Earth, and that about 90 percent of these species occur in tropical ecosystems, the great majority of those in rain forests.
Most of the undiscovered species are probably insects, especially beetles. However, tropical rain forests also contain immense numbers of undiscovered species of other arthropods (invertebrates with external skeletons), as well as many new plants and microorganisms. Even new species of birds and mammals are being discovered in tropical rain forests, further highlighting the fact that so much still is to be learned about that natural ecosystem.
Temperate rain forests are most common on the windward sides of coastal mountain ranges. In such areas, warm, moisture-laden winds from the ocean are forced upwards over the mountains. There they cool, form clouds, and release their moisture as large quantities of rainfall. Temperate rain forests are found primarily along the west coasts of North and South America and in New Zealand.
Many types of temperate rain forests exist. In northern California, for example, coastal rain forests are often dominated by stands of enormous redwood trees more than 1,000 years old. Old-growth rain forests elsewhere on the western coast of North America are dominated by other conifer (cone-bearing) species, especially Douglas-fir, western hemlock, sitka spruce, red cedar, and fir. Rain forests also occur in wet, frost-free parts of the Southern Hemisphere adjacent to the ocean. In parts of New Zealand, for example, the most common tree species in temperate rain forests are southern beech and southern pines.
Most species that are found in temperate rain forests also live in younger forests. However, some important exceptions exist. For example, in temperate rain forests of the Pacific coast of North America, the spotted owl, marbled murrelet, and some species of plants, mosses, and lichens appear to require the special conditions provided by old-growth forests and do not survive well in other ecosystems.