Rainforest - Because of the abundant rainfall and prevailingly warm temperatures, the equatorial region is characterized by growth of rainforest, or selva, a vegetation type unexcelled for luxuriance of tree growth and numbers of species. Broadleaf trees rise to heights of 100 to 150 ft (30-45 m), forming a dense leaf canopy through which little sunlight can reach the ground. Giant lianas (woody vines) hang from the trees. The forest is evergreen, although individual species have a rhythm of leaf-shedding.
Rainforest is the home of the small forest animals, of which the monkeys are perhaps the best representatives, taking advantage of the continuous forest canopy for living and traveling.
With a large water surplus and prevailingly high temperatures, chemical processes are active on the rocks and soils in the wet equatorial regions. Leaching out of all soluble constituents of the deeply decayed rock results in a distinctive type of soil, termed a latosol. Reddish or yellowish, and often containing irregular nodules of reddish iron hydroxides, this soil is especially rich in hydroxides of iron, manganese, and aluminum. These have been left behind in the soil after the soluble minerals (including silica) have been carried down through the soil and into the streams and rivers. Large concentrations of the iron, manganese,, or aluminum minerals occurring as lenses or layers in the soil are termed laterite. These minerals may be extracted as ores of commercial value. Bauxite is the foremost ore of aluminum in use today.
Vegetation and climate work hand in hand to make lateritic soils and ore bodies. At the prevailingly warm temperatures, bacteria in the upper soil layer are unusually vigorous and consume virtually all-dead vegetation. Thus humus, the black, partially decomposed plant matter present in most soils of middle-latitude and arctic climates, is almost entirely absent on well-drained sites. Rotting and decay of bedrock may be very deep in wet equatorial regions of low topographic relief. It is reported that even to depths as great as 300 ft (90 m) the rock has been found to be soft and crumbly from chemical action.
Stream flow tends to be fairly constant and extremely copious because a large water surplus exists throughout much of the year and provides ample runoff. Flood plains have meanders and. many swampy sloughs where the river channels have shifted their courses. Although water is abundant, river systems such as the Amazon carry relatively little material in chemical solution.
Hilly or mountainous belts have very steep slopes, on which flows, slides, and avalanches of soil and rock frequently occur, stripping away all forest and soil down to the bedrock.
Of economic importance are several forest products. Rain-forest lumber, such as mahogany ebony, or balsawood, is a valuable tropical product. Quinine, cocaine, and other drugs come from the bark and leaves of tropical plants; cocoa comes from the seed kernel of the cacao plant. Rubber, made from the sap of the rubber tree, is now largely an economic product of Malaya, Sumatra, and Ceylon, although the tree comes from South America, where it was first exploited.
Although most of the wet equatorial climate lies in a belt 100 of latitude on either side of the equator, the Malabar coast of India and the coasts of Burma and Thailand, located between 10° and 25° N lat., have a warm, wet climate with large annual total of rainfall, which also supports rainforest.
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