Thick forests generally found in wet areas of the world are called rain forests. Although most people are familiar with the lush, hot, tropical types of rain forests filled with trees that stay green year-round, there are actually several different kinds of rain forests. Temperate rain forests can be found in cooler parts of the world, such as in the northwestern part of the United States and in southern Australia. Monsoon rain forests have a dry season and trees that shed their leaves each year. Montane rain forests grow in mountainous regions. The rest of this article will focus on tropical rain forests because of their diversity and their importance to the ecology of the entire planet.

Where rain forests are found

Structure of rain forests

The next section of the rain forest contains small trees, shrubs, and plants. Many of these are saplings (young trees). Although their straight stems reach up toward the light, these smaller trees generally do not receive enough sunlight to grow into mature trees.

It is usually dark on the floor of a rain forest because the canopy blocks so much of the sunlight. For this reason, only plants that can tolerate shade grow there. So little sunlight reaches the ground that the forest floor may be only lightly covered by ground vegetation. There may be open spaces between the tree trunks. If one of the trees that creates the canopy dies or falls, however, a gap may open in the canopy, allowing sunlight to reach farther down into the forest. In such cases the ground vegetation may become thick and dense.

The constant rain washes away many of the nutrients in the soil. To make up for that loss, bacteria, fungi, and insects on the forest floor help to break down dead plants and animals. This process creates a thin, rich top layer of soil that provides nutrients to the roots of rain forest plants and trees. Most of the trees in the rain forest therefore have shallow root systems.

Plants

Tropical rain forests are known for the diversity of their plants and animals. The forests of the Amazon alone are believed to have tens of thousands of species of plants, many of them still undiscovered. The trees found in tropical rain forests stay green all year, though they do shed their leaves occasionally. Palms are among the most common trees in tropical rain forests.

Plants depend on sunlight for much of their energy. (They convert the sunlight into energy through a process known as photosynthesis.) Because of the rain forest canopy, the plants of the forest have to compete with each other to get enough light. As a result, many of the rain forest plants use other plants to reach the sunlight at the top of the canopy. For example, lianas, which are woody climbing plants, attach to the stems of other plants with anchorlike hooks. In this way they are able to climb from the ground to the canopy and receive the necessary sunlight. Epiphytes, or air plants, are also abundant in the rain forest. These plants are not attached to the ground. They live on other plants and get water and minerals from rain and also from debris that collects on the supporting plants. Mosses, ferns, and orchids can often be found attached to the stems and leaves of larger plants.

Animals

Different areas of rain forest are home to different animals, but each area has thousands of species. Many of the plant-eating animals of the rain forest live in the canopy. Monkeys, flying squirrels, and sharp-clawed woodpeckers are among the animals that inhabit the treetops. At the lower levels of the forest are animals that run, flutter, hop, and climb in the undergrowth. On the rain forest floor are such animals as rhinoceroses, chimpanzees, gorillas, elephants, deer, leopards, and bears.

Many animals in the rain forest have unusual characteristics. For example, sloths hang upside down, resting for hours at a time. The bright colors of the tiny poison dart frog warn other animals that it is poisonous and should not be eaten. Other common animals throughout the forests include ants, beetles, snakes, and bats. There are also many brightly colored birds such as toucans, parrots, and macaws.

The importance of rain forests

A tropical rain forest is a delicate network of relationships between plants and animals. Many plants, for instance, rely on animals to spread their pollen from flower to flower. At the same time, animals may depend on plants for their food and shelter. Scientists believe that more than half of the world's plant and animal species live in tropical rain forests. In addition, millions of people live in the forests. For them the forests are sources of food, shelter, and other materials.

Even people who live far away from tropical rain forests are affected by the forests. Many rain forest plants are used as medicines to help treat diseases such as cancer. Scientists have not yet identified many of the plants in the rain forests. They believe there are many more plants there that will help treat or even cure serious diseases. In addition, products such as fruits, nuts, rubber, rattan, and wood come from rain forests.

Many of the countries in which the rain forests are located are poor. For this reason, some countries are selling the resources of rain forests at a rapid pace to make much-needed money. This often means that entire sections of the forest are destroyed to harvest only one type of tree. Rain forests are also cleared away in order to use the land for other purposes, such as cattle grazing and farming. The rain forests that are destroyed for these reasons are rarely replaced. The loss of rain forests means the loss of unique plant and animal species. Over time, some of these plants and animals may become extinct if their rain forest habitat is destroyed.

The destruction of rain forests also affects the environment of the rest of the world. Through the process of photosynthesis, plants produce oxygen and absorb carbon from the atmosphere. Because the number of plants in the rain forests is so huge, the forests produce much of the world's oxygen, which all animals need in order to live. When forests are burned, massive amounts of carbon, in the form of carbon dioxide, are released into the atmosphere. This contributes to a problem known as global warming.

One of the most important roles of the tropical rain forests is to control the water supply of the areas in which they are found. They do this by absorbing the constant rain and then releasing it slowly back into the atmosphere. Some of the water is released steadily into area rivers. Many people rely on the rivers for their water supply and to irrigate their crops. Some of the water is released back into the air through evaporation. This keeps the air moist and leads to more rain. This cycle ensures a constant supply of water for the area. When forests are destroyed, however, the cycle is disrupted and rainfall becomes irregular. At first heavy rains cause flooding and landslides since there are no plants to absorb all the water. The absence of plants also means that the water from these rains washes away quickly. Since the water is not stored in plants, it cannot be returned to the atmosphere. Eventually, the rains fall less frequently, and the region may experience long periods of drought.