Forests
Forests are a crucial component of the world's ecology, providing a wealth of biological diversity. Although trees are the most visible component, within the woodland and forest biomes that cover about 30% of the world's ice-free land surface, there are other animal and plant species as well. Forest trees provide a huge number of products upon which human society depends, including fuel, fiber, paper, materials for construction and building, food, and medicines. They also play a vital role in many ecosystems, such as maintaining the structure and fertility of forest soil, and preventing soil erosion and floods. In addition, forests provide the invaluable ecological function of absorbing carbon dioxide and releasing oxygen through photosynthesis. Through this process, forests control greenhouse gas levels, which help maintain global temperatures, and produce the atmosphere upon which all living things depend.
Trees began their evolution more than 400 million years ago. Botanists believe the first tree was a plant probably less than a meter tall with tiny stems and no true leaves. By 350 million years ago, huge Devonian forests of fern-like trees had grown and were home to a great variety of insects. By the time warm-blooded mammals appeared, seed-bearing conifers had become dominant, succeeding earlier spore-bearing plants. Conifers in turn were overtaken by deciduous trees with broad leaves and flowers, which had spread worldwide by 60 million years ago.
Today, three types of forest trees occur in regions of varying distances from the equator. Evergreen broadleaf biomes or tropical rain forests flourish in the tropical parts of Africa and southeastern Central America, where the annual rainfall is never less than 130 centimeters. Throughout the year, the trees sprout new leaves and shed old ones. Tropical forests contain more species of animals than any other biome.
In moist, temperate latitudes where winters are mild, the deciduous broadleafs dominate. Temperate deciduous forests occur in much of the eastern United States, extending northward into southeast Canada, as well as in central and northern Europe, China, the eastern region of the former Soviet Union, Korea, Japan, coastal Brazil, east Africa, the eastern coast of Australia, and most of New Zealand. In these forests, where rainfall is somewhat evenly distributed throughout the year, the seasons change and leaves fall from the tree branches, creating a rich soil that supports lush forest growth. Wolves, bears, foxes, and mountain lions once inhabited these forests; today the wildlife largely comprises squirrels, raccoons, opossums, rabbits, and rodents.
Evergreen coniferous forests are found mostly in mountainous regions of the temperate zone and at higher, colder latitudes of northern Europe, Asia, and North America. Also called boreal forests or taiga, which means "swamp forest," these forests occur where the winters are long, cold, and dry and summers are short. Bogs can be found in certain parts of the taiga, along with cold lakes and streams. In the taiga's far north, the dense forest becomes scattered, eventually giving way to tundra. Moose, elk, deer, grizzly bear, black bear, wolves, lynx, and wolverines can be found in the taiga, along with rabbits, hares, porcupines, and rodents. Besides in the boreal forests, evergreen conifers also grow in such biomes as montane coniferous forests, temperate rain forests, and pine barrens.
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