Tropical rain forests grow near the equator. More than half are found in Latin America; one-third of all tropical rain forests occur in Brazil alone.
Other large tropical rain forests are found in West Africa, in Southeast Asia, and on various Pacific islands. Together, these rain forests contain
more than half of all the plants and animals in the worldsome 30 million species.
The term jungle is usually applied to the dense, scrubby vegetation that develops following the destruction of a true tropical rain forest.
A Layered Ecosystem
The dominant plants in the tropical rain forest are broadleaf evergreen trees. Unlike evergreens that grow in colder parts of the United States, those
of the tropical rain forest have no protection against cold weather or droughts. These trees grow to heights of 90 feet (27 meters), forming a
canopya tight, umbrella-like covering over the forest. The canopy is exposed to the Sun, but shades the plants that grow beneath it. Most
of the organisms in this biome make their home in the canopy, one of the four so-called "layers of life" in the tropical rain forest.
The branches of the trees that form the canopy are usually covered with vines and epiphytes, plants that grow on other plants. The vines, or
lianas, are rooted in the soil, but use the surrounding trees for support, achieving heights of up to 200 feet (60 meters). As a result, the
leaves of these vines form part of the canopy, and thus receive direct sunlight.
Rain-forest epiphytes include succulents, orchids, mosses, ferns, and bromeliads (which are related to pineapples). Epiphytes use the trees only for
support, not for food; some can nonetheless harm trees. Epiphytes commonly called stranglers initially live on a tree, but eventually grow on their own.
For instance, the strangling fig (or ficus) first takes root in a fork where a branch separates from the trunk of a tree. As the fig matures, it sends
out long roots that find their way to the surface and begin growing in the soil. Other roots snake down the trunk of the tree, interweaving into a tight
mesh. Eventually the roots of the fig completely cover the tree. Over time, this network of fig roots prevents the tree inside from growing, thus killing it.
Another stratum of plant life in the tropical rain forest is called the emergent layer. This layer is made up of trees that reach above the canopy,
attaining heights of 100 to 165 feet (30 to 50 meters); a few trees grow as high as 200 feet (60 meters). These lofty trees receive more sunlight than
even those in the canopy, but they are also exposed to higher temperatures, less humidity, and more wind.
The layer directly below the canopy, the understory, receives up to 15 percent less light than the canopy, so anything that grows there must be
able to thrive in shade. Understory flora includes young trees and some herbaceous plants (that is, any plant that has a fleshy green stem as opposed to
a woody stem). Many popular houseplants originated in the understory.
Beneath the understory is the floor of the forest. Since little sunlight filters down to the floor, few plants are able to grow there. Instead, the floor
is covered with a thin layer of plant matter (leaves, seeds, fruits, and more) that falls from the upper layers. Given the perennially warm and moist
conditions in the rain forest, this floor material decomposes quicklyeven before the organic material is washed away. The nutrients of the decomposed
matter are soon taken up again by living plants and animals. This means that most of the nutrients in a tropical rain forest are located in living
plants and animals. In fact, despite the great variety of plants and animals that live in them, tropical rain forests actually contain less organic matter
than do temperate forests.
Although tropical rain forests are rich in plant life, the soil in such areas is surprisingly poora phenomenon largely the result of the heavy
rains that wash the nutrients out of the soil on almost a daily basis. Some studies have found that the diversity of life in the tropical rain forest
is related to the changing and unstable nature of the forests over geologic time.