Forests

Scientists have classified up to 26 types of forests worldwide. Forest biomes are divided into three main types: tropical, temperate, and boreal forests, or taiga.

Tropical Forests

Tropical forests occur near the equator, in the region between 23.5°N and 23.5°S, known as the tropics. This region includes parts of Asia, Africa, South America, and Central America. Stable temperature and abundant rainfall make tropical forests the most productive biome type. Tropical forests have only two seasons—one wet and one dry. Tropical rain forests are characterized by long wet seasons and tall trees and plants that grow year-round. Tropical dry forests have long dry seasons during which trees lose their leaves.

Competition for light is intense in a tropical rain forest. Most of the plants are trees, and some have evolved to grow as tall as 50 to 60 m (164 to 197 ft). The treetops form a continuous layer, called the canopy, which shades the forest floor. Though you may think of the tropical rain forest as an impenetrable jungle, much of the forest floor is relatively free of vegetation because so little sunlight reaches the ground. The very dense growth known as jungle is found along riverbanks and in disrupted areas where sunlight can reach the forest floor. Because of the intense competition for sunlight, many small plants live on the branches of tall trees. These plants are called epiphytes (EP-uh-FlETS) and include mosses, orchids and bromeliads, such as the one shown in Figure 21-3. Epiphytes use other organisms as support, but they are not parasitic because they make their own food.

Tropical forests have the highest species richness of all the biomes. One hectare of tropical rain forest (about the size of two football fields) may contain as many as 300 species of trees. An area of temperate deciduous forest of the same size, by contrast, would probably contain fewer than 12 species of trees. Animal life is also very diverse in the tropical rain forest. Rain-forest vertebrates include many kinds of monkeys, snakes, lizards, and colorful birds, such as parrots. Insect species are particularly diverse in tropical rain forests. There may be more than 1 million species of tree-dwelling beetles in the tropical rain forest biome alone. Overall, tropical rain forests probably contain about one-half of the world's species.

Temperate Forests

Temperate forests occur in eastern North America, western and central Europe, and northeastern Asia. This biome is characterized by distinct seasons and a moderate climate. Temperate forests can be characterized by the type of tree that is most common, such as coniferous trees, which bear seeds in cones and tend to be evergreen, or deciduous trees, which shed their leaves each year.

figure 21-3

The tall trees in a rain forest are home to many epiphytic species, such as this bromeliad. The red structure in the middle of the bromeliad shown here is a fruiting structure similar to a pineapple.

figure 21-3

The tall trees in a rain forest are home to many epiphytic species, such as this bromeliad. The red structure in the middle of the bromeliad shown here is a fruiting structure similar to a pineapple.

Word Roots and Origins epiphyte from the Greek word epi, meaning "on" or "upon" and phyton, meaning "a plant"

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figure 21-4

A temperate deciduous forest is characterized by deciduous trees. Being deciduous is an adaptation that conserves the tree's energy and avoids freezing during winter.

figure 21-4

A temperate deciduous forest is characterized by deciduous trees. Being deciduous is an adaptation that conserves the tree's energy and avoids freezing during winter.

Word Roots and Origins taiga from the Russian word taiga, meaning "forest"

figure 21-5

Organisms of the taiga are adapted for dry, cold conditions and a reduced availability of food in the winter season. The conifers have needle-shaped leaves, which are adapted to conserve water.

Temperate Deciduous Forests

Trees that lose all of their leaves in the fall and regrow them each spring characterize temperate deciduous forests. Deciduous forests stretch across eastern North America, much of Europe, and parts of Asia and the Southern Hemisphere. These regions have pronounced seasons, with precipitation unevenly distributed throughout the year. Compared with taiga, discussed below, temperate deciduous forests have warmer winters and longer summers and receive more precipitation. Deciduous trees have broad, thin leaves with a large surface area that permits maximum light absorption. Familiar deciduous trees include the birch, beech, maple, oak, hickory, sycamore, elm, ash, willow, and cottonwood. Bears, wolves, white-tailed deer, foxes, raccoons, and squirrels are typical mammals of the temperate deciduous forests. Large areas of temperate deciduous forest in the United States, Europe, and Asia have been cut for timber or cleared to make way for farms, towns, and cities. Figure 21-4 shows a stand of trees in a temperate deciduous forest.

Taiga

South of the tundra and north of the temperate regions is the taiga (TIE-guh), a forested biome dominated by coniferous trees, such as pines, firs, and spruces. Taiga, also called boreal forest, stretches across large areas of northern Europe, Asia, and North America between 50°N and 60°N. During the long winter, snow covers and insulates the ground, protecting tree roots against freezing.

Plants living in the taiga are adapted for long, cold winters; short summers; and nutrient-poor soil. On a coniferous tree, the waxy, needle-shaped leaves remain on the tree all winter long. The shape of the needle is a leaf adaptation that reduces water loss, because the small holes through which the leaves exchange air are partially enclosed in the needle. Typical mammals of this biome include moose, bears, wolves, lynxes, and hares. Animals that are adapted to survive the winter may stay in the taiga year-round, but others migrate to warmer climates in the fall and return in the spring. Many species hibernate six to eight months of the year. Figure 21-5 shows a representative area of taiga.

figure 21-5

Organisms of the taiga are adapted for dry, cold conditions and a reduced availability of food in the winter season. The conifers have needle-shaped leaves, which are adapted to conserve water.

Grasslands are, as the name suggests, dominated by a variety of grasses. Grasslands are known by different names in different parts of the world: prairies in North America, steppes in Asia, pampas in South America, and veldts in southern Africa.

Temperate Grasslands

Temperate grasslands usually form in the interior of continents, at about the same latitude as temperate deciduous forests. However, rainfall patterns make these areas too dry to support trees. This biome once covered large areas of North America, Asia, Europe, Australia, and South America.

Temperate grasslands have rich, fertile soil. In areas that have remained relatively undisturbed by humans, grasslands support large herds of grazing mammals, such as the North American bison shown in Figure 21-6a. Grass can survive repeated grazing by animals and occasional fires that sweep across the area because the actively growing part of the plant is at or below the ground rather than at the tip of the stem. Because grasslands have such rich soil, much of the world's temperate grassland has been transformed into farmland for growing crops such as wheat and corn. Only fragments of undisturbed prairie remain in the midwestern United States.

Savanna

Savannas (suh-VAN-uhz) are tropical or temperate grasslands that have scattered deciduous trees and shrubs. The savannas of Africa are the best known, but this biome also occurs in South America and Australia. Savannas receive more rainfall than deserts do but less rainfall than tropical or temperate forests do. Alternating wet and dry seasons characterize savannas. Like temperate grasslands, savannas support large numbers of herbivores, such as zebras, wildebeests, giraffes, and gazelles, as Figure 21-6b shows. Large carnivores, such as lions, leopards, and cheetahs, feed on these herbivores.

Because most of the rain falls during the wet season, the plants and animals of the savanna must be able to deal with prolonged periods of drought. Some trees of the savanna shed their leaves during the dry season to conserve water, and the above-ground parts of grasses often die during the dry season and regenerate after a period of rain.

Chaparral

Chaparral (SHAP-uh-RAL) is a biome that is dominated by dense, spiny shrubs and has scattered clumps of coniferous trees, as Figure 21-6c shows. Chaparral is characterized by mild, rainy winters and hot, dry summers with periodic fires. This biome is located in the middle latitudes, about 30° north and 30° south of the equator. Chaparral is found primarily in coastal regions—for example, around the Mediterranean Sea and in southern California.

figure 21-6

Temperate grasslands once covered a large portion of the United States and supported huge herds of herbivores, such as bison (a). An area rich in wildlife, the savanna biome supports great herds of large herbivores (b). The chaparral biome supports grasses, shrubs, and clumps of small trees (c).

figure 21-6

Temperate grasslands once covered a large portion of the United States and supported huge herds of herbivores, such as bison (a). An area rich in wildlife, the savanna biome supports great herds of large herbivores (b). The chaparral biome supports grasses, shrubs, and clumps of small trees (c).

(a) Temperate grassland
(c) Chaparral

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