DISTRIBUTION PATTERNS TROPICAL RAINFOREST

The Tropical forests exist with some essential facts of geography and climatic systems. By definition, ‘the tropics’ lie between 23o N and 23o S latitude, the area within which the sun lies directly overhead at some point in its seasonal progression.

The flux of solar energy within this region is high, due to the fact that incoming sunlight is projected at a 90o angle to the surface of the earth. This high solar energy flux results in a high rate of water evaporation over the tropical oceans and of evapotranspiration over tropical land surfaces.



The result is a rising column of warm, moist air at tropical latitudes. As this air rises it cools as a result of the gradient in air pressure through the atmosphere (adiabatic cooling). Water condenses out from this air mass, generating rainfall. The air mass ultimately dries out, and is carried poleward from the tropics as a part of wind circulation patterns known as Hadley cells.

At subtropical latitudes near 23–30o N and S, the now-dry air mass descends, creating a region of high pressure that corresponds closely to the world distribution of deserts. As a consequence of these climatic circulation patterns, the earth’s equatorial zone is warm and wet, corresponding to the broad band of tropical forests found along the earth’s equatorial axis.

WHAT IS THE TROPICS | Tropics Are The Regions Of The Earth

Tropics are the regions of the earth that lie within about 1,600 miles (2,570 kilometers) north and 1,600 miles south of the equator. Two imaginary lines, the Tropic of Cancer and the Tropic of Capricorn, form the boundaries of the tropics. The Tropic of Cancer is 23° 27' north of the equator, and the Tropic of Capricorn is 23° 27' south of the equator. These lines mark the northernmost and southernmost places on the earth where the sun ever shines directly overhead.

Most places in the tropics have warm to hot temperatures the year around. Tropical places near sea level are hot because every day the sun's rays shine almost straight down at noon. Such direct rays produce higher temperatures than do slanted rays.

The temperature does not change much in the tropics because the amount of daylight differs little from season to season. At the equator, the sun shines about 12 hours a day. At the edges of the tropics, daylight varies from about 101/2 hours a day in winter to about 131/2 hours a day in summer. Places at the edges of the tropics have cool periods in winter. Tropical places that are located at high altitudes are cool because the temperature drops about 31/2 °F per 1,000 feet (2 °C per 300 meters) of elevation.

Many tropical areas have definite rainy and dry seasons. Most places near the equator get much rain during all seasons and are covered by tropical rain forests. Farther to the north and south, one or two short dry seasons occur yearly. Such areas have forests of trees that lose their leaves during these dry seasons. Areas even farther from the equator have one long dry season each year. These areas are covered by savannas (grasslands with scattered trees and shrubs).

CLAY IS A SUBSTANCE PRESENT IN MOST KINDS OF SOIL

Clay is a substance present in most kinds of soil. Geologists define clay as extremely small particles of soil that measure less than 4 microns, or 0.000157 inch, in diameter. The word clay also refers to earthy material composed of certain kinds of silicate minerals that have been broken down by weathering.

Clay consists mainly of tiny, sheetlike particles of alumina and silica bound together by water. Various other materials in clay may give it different colors. For example, iron oxide may color clay red. Clays that contain various amounts of carbon compounds may be different shades of gray.

The clay in soil has a vital role in farming. For example, it absorbs ammonia and other gases needed for plant growth. Clay also helps soil retain minerals necessary for plant growth. Without clay, soil would not keep its fertility from year to year. However, too much clay makes soil stiff and heavy and prevents the movement of air and water through soil.

There are two general types of clay, based on how the substance reacts when mixed with water. Expandable clay swells when water is added to it. Expandable clay can absorb so much water that the clay itself becomes a liquid. Nonexpandable clay becomes soft but not liquid when mixed with water.

The petroleum industry uses expandable clays called bentonites to make drilling mud. The petroleum industry also uses another kind of expandable clay as a chemical agent in the process of oil refining.

Ceramics industries use nonexpandable clay in making bricks, pottery, tile, and many other products. For example, pottery makers mold moist clay into almost any shape and bake it in hot ovens called kilns. Heat removes the water from the clay, which becomes permanently hard and cannot be softened by adding water to it. The whitest kind of clay, kaolin or china clay, is used in making porcelain. The paper industry also uses kaolin, which serves as a filler that adds whiteness and strength to paper. In addition, kaolin gives some kinds of paper a smooth, shiny surface. Fire clay contains a large percentage of silica and can stand high temperatures. It is used in making firebrick and furnace linings.

SOIL CONSERVATION

The soils of farmlands, grazing lands, and forestlands provide many products and recreational areas. Soil conservationists work to ensure the wise use of these soils.

Wise use of farmlands involves maintaining a high level of nutrients and organic matter in cultivated soils. Farmers add organic matter to the soil by plowing under certain green plants. They also add fertilizers and rotate crops to replace nutrients that leaching and growing plants remove. In addition, farmers plow and plant their fields in ways that control erosion.

Grazing lands that have been overgrazed also suffer from erosion. Overgrazing decreases the amounts of plant life and organic matter in the soil, and the soil erodes easily. Ranchers conserve grazing lands by limiting the time that their herds graze in one area.

Forestlands also must be protected from erosion. In some cases, foresters leave unusable branches and other parts of trees on the forest floor to add organic matter to the soil. They also develop large groups of trees whose roots protect the soil by holding it in place against wind and water erosion.

CLASSIFIED SOILS IN THE TROPICS

Classified Soils in the Tropics According to the USDA Soil Taxonomy, Oxisols are the most abundant soils in the humid and perhumid tropics covering about 35 percent of the land area. Ultisols are the second most abundant, covering an estimated 28 percent of the region. About half of the Ultisols and 60 percent of the Oxisols are located in humid and perhumid tropical Africa and Asia. In tropical Africa, they are abundant in the eastern Congo basin bordering the lake region; in the forested zones of Sierra Leone; in Ivory Coast; in parts of Liberia; and in the forested coastal strip from Ivory Coast to Cameroon.


The Alfisols, which have high to moderate fertility, cover a smaller area of the humid tropics. In west Africa they are found in Ivory Coast, Ghana, Togo, Benin, Nigeria and Cameroon. They are, however, the most abundant soils in Africa's subhumid and semi-arid zones, covering about one third of these regions. The Alfisols are widely distributed in the subhumid and semi-arid tropical regions of Africa, including large areas in western, eastern, central, and southeastern Africa.

Table. Geographical distribution of soils in the humid and semi-arid tropics
(millions of hectares).

 

Soil order	Tropical Africa	Tropical Asia	Tropical America	Total	Percent
Humid Tropics1)
Oxisols	179	14	332	525	35
Ultisols	69	131	213	413	28
Alfisols	21	15	18	54	4
Others	176	219	103	498	33
Total	445	379	666	1490	100
Semi-arid Tropics2)
Alfisols	466	121	107	694	33
Ultisols	24	20	8	52	1
Others	972	178	198	1348	66
Total	1462	319	313	2094	100

¹⁾ Data from NAP (1982).
²⁾ Data adapted from Kampen and Burford (1980). Part of the subhumid tropics is included.