The rainforest is a moist, densely wooded area usually found in a warm, tropical wet climate. Annual rainfall is about 80 inches (200 centimeters) and ranges as high as 400 inches(1,000 centimeters in some tropical rainforests. The average temperature in most rainforests is 80 degrees F (27 C) . Broadleaf evergreen trees, vines, sparse undergrowth, and nutrient-poor soils are common characteristics of this kind of forest. The tallest trees tower to a height of perhaps 200 feet (60m), averaging around 120 feet (37m), and below these giants are as five tiers of straight trunked-trees that refrain from branching until their umbrella heads finally find their own available space in the strata. The upper layer, often difficult to see from the forest floor, consists of the scattered giant monoliths called emergent trees, that tower over the canopy.
As an example an outstanding specimen of tualang (Koopmpassia excelsa) has been recorded at a height of 276 feet (84m). It is not uncommon for the expansive umbrella crown of a massive emergent tree to spread over an entire acre(0.4ha). The base of the medium- and larger size trees very often send out immense flying snake , or plank butresses from a height of 20 to 30 feet (6.1-9.1m) up the trunk. These sinuous, wing like flanges of high-tensile-strength wood help stabilize the top heavy trees during the frequent gale-force winds and occasionl hurricanes and tornadoes that lash the canopy. The heads of trees are bound together by woody vines called lianas, some approaching 800 feet (244m) long, and this further serves to stabilize them in the high winds that sweep over the crowns. Some species of vine coil to the left in the Northern Hemisphere and to the right in the Southern, to make best use of the sun's rays. Below the canopy, visibility is usually good for 50 feet (15m), and often much more. Due to the dim light, which averages less than 1 percent of that falling on the canopy above, vegetation is relatively sparse at ground level.
Tropical rainforests encircle the planet, forming an uneven green belt between the Tropic of Cancer and the Tropic of Capricorn. These rainforests are home to nearly half the earth's plant and animal species. Tropical rainforests once covered more than 4 billion acres (1.6 billion hectares) of the earth. Today, nearly half the tropical rainforests are gone. They now cover only about 7 percent of the earth's land, including parts of South and Central America, Central Africa, and Southeast Asia. The largest continuous rainforest lies in South America, where about 2.7 million square miles (6.9 million square kilometers) of forest cloak the Amazon Basin. Though fewer in number than in the tropics, rainforests grow in temperate regions as well. Temperate rainforests have a more seasonal climate, with less constant temperatures and less rain, than do rainforests near the Equator. Though plant and animal life are abundant, the species are not as diverse as they are in warmer, tropical rainforests.
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