A tree is a woody plant with a single, strong trunk and many branches that lives year after year. A large group of trees covering not less than 25 percent of the area where the tops of the trees, called crowns, interlock to form an enclosure, or canopy, at maturity make up what is called a forest. This chapter is about rain forests. Sometimes the term rain forest is used to refer to any forest in tropical or semitropical regions. (Tropical regions are those around the equator.) In this chapter, however, rain forest refers only to evergreen tropical forests that do not undergo seasonal changes. Rain forests also occur in a few regions with temperate (moderate) climates. Those are discussed in the chapter titled "Coniferous Forest." Tropical forests that undergo seasonal changes are discussed in the chapter titled "Deciduous Forest."
Tropical rain forests, also called jungles, are located for the most part in a belt around the equator. Plenty of rain and warm temperatures year-round support constant plant growth and great diversity of species. It is estimated that between 20 and 100 different kinds of trees can be found in each acre (hectare) of rain forest. Because there are no seasons, trees that would normally lose their leaves during the autumn season in cooler climates retain them for several years, becoming evergreens. While rain forests cover only about 7 percent of the Earth's surface, they contain at least half of all plant and animal species. The total number of plant and animal species living in rain forests has been estimated to be as many as 20,000,000.
Tropical evergreen rain forests occur in four main regions:
The first forests evolved during Earth's prehistoric past. Since then, all forests have developed in essentially the same way, by means of a process called succession.
The first forests evolved from ferns and other prehistoric plants, which, over time having adapted to the surrounding environment, grew more treelike. Trees that preferred a warm, humid, tropical climate developed first followed by those that adapted to drier, cooler weather.
About 1,000,000 years ago, during the great Ice Ages, glaciers (huge, slow-moving rivers of ice) covered much of the planet and destroyed many of the world's forests. By the time the glaciers finally retreated, about 10,000 years ago, they had scoured the land of plants and soil, leaving only bare rock. Birch trees were among the first to return to the regions once covered by ice. In fact, the years between 8000 and 3000
Trees compete with one another for sunlight, water, and nutrients, and a forest is constantly changing. The process by which one type of plant or tree is gradually replaced by others is called succession. Succession took place following the last Ice Age, just as it takes place today when the land is stripped of vegetation from other causes such as forest fires. Succession produces different types of forests in different regions, but the process is essentially the same everywhere. During succession, different species of trees become dominant as time progresses and the environment changes.
Primary succession usually begins on bare soil or sand where no plants grew before. When the right amount of sunlight, moisture, and air temperature are present, seeds begin to germinate (grow). These first plants are usually made up of the grasses and forbs (a nonwoody broad-leaved plant) type. They continue to grow and eventually form meadows. Over time, and as conditions change, other plants begin to grow, such as shrubs and trees. These plants become dominant and replace or take over where the grasses and forbs originally grew.
As primary succession continues, "pioneer" trees, such as macaranga, balsa, pine, and palm, begin to thrive. They are all tall, sun-loving trees, and they quickly take over the meadow. However, they change the environment by making shade. Now trees with broader leaves, such as violets and cacaos, that prefer some protection from the Sun, can take root. If conditions are right, a mixed forest of sun-loving and shade-loving trees may continue for many years, which is usually true in a rain forest with its many tree species. Eventually, however, more changes may occur.
Seedlings from pioneer trees do not usually grow well in shade. Therefore, new pioneer trees cannot grow, and may not be replaced. Eventually the mature trees die out from old age, disease, and other causes. This allows seedlings from trees that prefer heavy shade, such as beeches, to move in and eventually take over the forest. These trees produce such deep shade that only those species that can live in shade will succeed there. The result is a climax forest—one in which only one species of tree becomes dominant. If one tree dies, another of the same species grows to take its place. In this way, a climax forest can endure for thousands of years.
Few climax forests actually exist, however, because other changes often occur that interfere with a forest's stability. Fires, floods, high winds, and people can all destroy a single tree to acres of trees. Volcanoes can smother them with ash or molten lava or knock them over with explosive force. Then the process of succession must start over. Climax forests are even rarer in tropical regions than temperate regions, because one species of tree is seldom dominate.
When the land has been stripped of trees, it will eventually be covered with them again if left alone. This is called secondary succession and takes place more quickly than primary succession. Seeds from other forests in neighboring regions are blown by the wind or carried by animals to the site. Soon, the seeds take root and seedlings sprout, and the process begins again.
Source: "Rain Forest." U·X·L Encyclopedia of Biomes, Vol. 2. U·X·L, Detroit: 2000.