Forests in which the dominant trees shed all their leaves during certain seasons of the year, and then grow new ones, are classed as deciduous forests. In an evergreen forest, the dominant trees grow new leaves before shedding the old ones. Thus, they remain green throughout the year.
In some other systems, forests are classified according to the usable qualities of the trees. A forest of broadleaf trees may be classed as a hardwood forest because most broadleaf trees have hard wood, which makes fine furniture. A forest of needleleaf trees may be classed as a softwood forest because most needleleaf trees have softer wood than broadleaf trees have.
Many scientists classify forests according to various ecological systems. Under such systems, forests with similar climate, soil, and amounts of moisture are grouped into formations. Climate, soil, and moisture determine the kinds of trees found in a forest formation. One common ecological system groups the world's forests into six major formations. They are (1) tropical rain forests, (2) tropical seasonal forests, (3) temperate deciduous forests, (4) temperate evergreen forests, (5) boreal forests, and (6) savannas.
Tropical rain forests grow near the equator, where the climate is warm and wet the year around. The largest of these forests grow in the Amazon River Basin of South America, the Congo River Basin of Africa, and throughout much of Southeast Asia.
Of the six forest formations, tropical rain forests have the greatest variety of trees. As many as 100 species-none of which is dominant-may grow in 1 square mile (2.6 square kilometers) of land. Nearly all the trees of tropical rain forests are broadleaf evergreens, though some palm trees and tree ferns can also be found. In most of the forests, the trees form three canopies. The upper canopy may reach more than 165 feet (50 meters) high. A few exceptionally tall trees, called emergents, tower above the upper canopy. The understory trees form the two lower canopies.
The shrub and herb layers are sparse because little sunlight penetrates the dense canopies. However, many climbing plants and epiphytes crowd the branches of the canopies, where the sunlight is fullest.
Most of the animals of the tropical rain forests also live in the canopies, where they can find plentiful food. These animals include such flying or climbing creatures as bats, birds, insects, lizards, mice, monkeys, opossums, sloths, and snakes.
Tropical seasonal forests grow in certain regions of the tropics and subtropics. These regions have a definite wet and dry season each year or a somewhat cooler climate than that of the tropical rain forest. Such conditions occur in Central America, central South America, southern Africa, India, eastern China, and northern Australia and on many islands in the Pacific Ocean.
Tropical seasonal forests have a great variety of tree species, though not nearly as many as the rain forests. They also have fewer climbing plants and epiphytes. Unlike the trees of the rain forest, many tropical seasonal species are deciduous. The deciduous trees are found especially in regions with distinct wet and dry seasons. The trees shed their leaves in the dry season.
Tropical seasonal forests have a canopy about 100 feet (30 meters) high. One understory grows beneath the canopy. Bamboos and palms may form a dense shrub layer, and a thick herb layer blankets the ground. The animal life resembles that of the rain forest.
Temperate deciduous forests grow in eastern North America, western Europe, and eastern Asia. These regions have a temperate climate, with warm summers and cold winters.
The canopy of temperate deciduous forests is about 100 feet (30 meters) high. Two or more kinds of trees dominate the canopy, and another 15 to 25 kinds may be present. Most of the trees in these forests are broadleaf and deciduous. They shed their leaves in fall. The understory, shrub, and herb layers may be dense. The herb layer has two growing periods each year. Plants of the first growth appear in early spring, before the trees have developed new leaves. These plants die by summer and are replaced by plants that grow in the shade of the leafy canopy.
Large animals of the temperate deciduous forests include bears, deer, and, rarely, wolves. These forests are also the home of hundreds of smaller mammals and birds. Many of the birds migrate south in fall, and some of the mammals hibernate during the winter.
Some temperate areas support mixed deciduous and evergreen forests. In the Great Lakes region of North America, for example, the cold winters promote the growth of heavily mixed forests of deciduous and evergreen trees. Forests of evergreen pine and deciduous oak and hickory grow on the dry coastal plains of the Southeastern United States.
Temperate evergreen forests. In some temperate regions, the environment favors the growth of evergreen forests. Such forests grow along coastal areas that have mild winters with heavy rainfall. These areas include the northwest coast of North America, the south coast of Chile, the west coast of New Zealand, and the southeast coast of Australia. Temperate evergreen forests also cover the lower mountain slopes in Asia, Europe, and western North America. In these regions, the cool climate favors the growth of evergreen trees.
The strata and the plant and animal life vary greatly from one temperate evergreen forest to another. For example, the mountainous evergreen forests of Asia, Europe, and North America are made up of conifers. The coastal forests of Australia and New Zealand, on the other hand, consist of broadleaf evergreen trees.
Boreal forests are found in regions that have an extremely cold winter and a short growing season. The word boreal means northern. Vast boreal forests stretch across northern Europe, Asia, and North America. Similar forests also cover the higher mountain slopes on these continents.
Boreal forests, which are also called taiga, have the simplest structure of all forest formations. They have only one uneven layer of trees, which reaches up to about 75 feet (23 meters) high. In most of the boreal forests, the dominant trees are needleleaf evergreens-either spruce and fir or spruce and pine. The shrub layer is spotty. However, mosses and lichens form a thick layer on the forest floor and also grow on the tree trunks and branches. There are few herbs.
Many small mammals, such as beavers, mice, porcupines, and snowshoe hares, live in the boreal forests. Larger mammals include bears, caribou, foxes, moose, and wolves. Birds of the boreal forests include ducks, loons, owls, warblers, and woodpeckers.
Savannas are areas of widely spaced trees. In some savannas, the trees grow in clumps. In others, individual trees grow throughout the area, forming an uneven, widely open canopy. In either case, most of the ground is covered by shrubs and herbs, especially grasses. As a result, some biologists classify savannas as grasslands. Savannas are found in regions where low rainfall, poor soil, frequent fires, or other environmental features limit tree growth.
The largest savannas are tropical savannas. They grow throughout much of Central America, Brazil, Africa, India, Southeast Asia, and Australia. Animals of the tropical savannas include giraffes, lions, tigers, and zebras.
Temperate savannas, also called woodlands, grow in the United States, Canada, Mexico, and Cuba. They have such animals as bears, deer, elk, and pumas.
Source : World Book 2005