Tree
A tree is a large, woody perennial plant with one main trunk and many branches. Trees are the most visible component of forests, a crucial component of the world's ecology.
Closely related plants may grow into large trees, or into much smaller plants. For example, the western yew (Taxus brevifolia) grows into a tree taller than 60 feet (20 m) in the Pacific rainforests of western North America, while the closely related Canada yew ( T. canadensis) of the northeastern forests is a shrub only 3-4 ft. tall (1 m). Many kinds of plants are capable of reaching tree size. This includes plants that do not develop true woody (or xylem) tissues, such as tree-ferns and palms, as well as many species of coniferous and angiosperm trees.
Foresters and ecologists define trees strictly on the basis of their size. In some forests, for example, a tree might be defined as being taller than 10 feet (3 m) and having a diameter at breast height (DBH) greater than 4 inches (10 cm). Any ecosystem that is dominated by tree-sized plants is known as a forest.
Trees began their evolution more than 400 million years ago. The oldest known organisms are certain individuals of bristlecone pine (Pinus aristata ) growing in the Rocky Mountains, which can exceed 4,500 years of age. Some trees can be extraordinary in their age or size. The tallest organisms are redwood trees (Sequoia sempervirens) of coastal California, one of which is more than 360 feet tall (110 m). The widest tree is the "General Sherman Bigtree," a giant sequoia ( Sequoia gigantea) growing in central California which has a circumference of 115 ft (35 m). Trees are also the largest plant organisms (some stands of trembling aspen (Populus tremuloides) have developed from root-sprouts, so that "individual" trees, growing over large areas actually represent the same genetic organism, which can cover as many as 100 acres (40 ha) and contain tens of thousands of mature stems.
Many smaller plants, animals, and microorganisms rely on trees and/or forests for their necessary habitat. Trees are also responsible for immense amounts of photosynthesis, and are the dominant energy base of the food webs of all forests. Trees also store huge quantities of carbon in their biomass, and in this way reduce the amount of carbon dioxide present in the atmosphere (this is important in moderating the intensity of Earth's greenhouse effect). Intact forests also cleanse the air, water, and soil of other kinds of pollutants.
Trees also provide humans with important goods and services. These may be as simple as shade and landscape use, or as immense as the 10 million acres (25 million ha/yr) of forest that are harvested and cleared globally each year for timber, fuel, or to develop new agricultural land. In fact, ecologists have estimated that humans are now using about 28% of the global production of forested biomass. In many regions and on a global basis, humans are using trees faster than they are able to grow. Deforestation is now a common problem and overall, Earth's forest cover is now only about one-half of what it was several thousand years ago.
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