Equatorial Belt

a geographic belt of the earth located on both sides of the equator between 5°–8° N lat. and 4°–11° S lat., between the subequatorial belts. Some scientists include the equatorial belt in the tropical belt (seeTROPICAL BELT). The equatorial belt is characterized by the prevalence of equatorial air masses, weakly expressed sectors on the continents, and the presence of gilea-type forests on the plains of the latitudinal zone and zones of gilea and paramo vegetation in the mountains (seeGILEA).

The climate is caused by the large influx of solar energy. The radiation balance on land is 70–80 kcal/cm² per year, sometimes reaching 90 kcal/cm². The average monthly temperatures in the lowlands are 24°–28°C; the annual variation in the average monthly temperature is 2°–3°C, the lowest on the earth. The equatorial belt is characterized by the presence of the equatorial low, which is linked to the intertropical zones of convergence of the tradewinds, in which air masses rise, moisture condenses, and abundant precipitation falls (1,500–3,000 mm a year, up to 10,000 mm on the windward slopes of mountains). Precipitation almost always exceeds evaporation, and moisturization is excessive. The rivers have abundant water, with relatively small fluctuations in the flow rate, except those rivers whose basins lie in other geographic belts. The equatorial parts of the continents typically have no seasonal rhythms, but all natural processes are highly active. Thick weathering crusts are characteristic. Acidic red-yellow ferralitic (lateritic) soils with total humification of dead plant matter and rapid mineralization of organomineral complexes predominate. These soils are poor in bases and humus (2–3 percent) and rich in the hydroxides of iron and aluminum. The vital activities of microorganisms and small animals in the soil and on the soil surface are extremely vigorous. When the forests are cleared and the ground plowed, given the high temperatures and drainage conditions, the soils of the equatorial belt lose their fertility quickly.

The humid equatorial evergreen forests (gilea, the Brazilian selva) are distinguished by the different times of periodic functions of individual plants in the absence of seasonal aspects within the community as a whole, the dense tree growth, the unusual wealth and age of the species composition, the abundance of lianas and epiphytes, and the high productivity of the aboveground biomass (40–50 tons/ha, up to 100). Mangrove thickets are common along the sea and ocean. Economically useful plants abound, such as rubber plants (including Hevea), the cacao tree, the breadfruit tree, the cotton tree (Ceiba), various palms, and trees with valuable wood. The animals that inhabit the equatorial forests are adapted to life in the trees, for example, monkeys, lemurs, sloths, and cats. Common ground animals are the tapirs, rhinoceroses, peccaries, and hippopotamuses. Birds, reptiles, and insects are numerous. Two subzones are identified in the gilea zone: the continually humid equatorial forests and the equatorial forests with a short dry season (two to three months). The latter occur in the parts of the belt farthest from the equator and in the eastern sectors, which come under the influence of the continental tradewinds. A mixture of deciduous species (losing their leaves in the dry season) occurs in the forests, as well as patches of savanna, formed primarily as a result of human economic activities. Some of the land, mostly in the more accessible marginal parts of the equatorial forest, is planted with subsistence and plantation crops. The chief occupations of the population in the continually humid equatorial forest subzone are logging, fishing, hunting, and primitive subsistence farming.

The uniformity of annual temperatures and precipitation is maintained in the mountains of the equatorial belt. Owing to the drop in temperature with elevation, the change in the nature of precipitation, the greater thinness of the air, and the increase in insolation, three zones of humid mountain equatorial forest are identified below 3,000–3,500 m—tierra caliente, tierra templada, and tierra fria. Above this elevation is a belt of high-mountain equatorial meadows called paramos, which at 4,300–4,800 m gives way to the nival zone.

In the ocean regions, the equatorial belt is characterized by an even higher radiation balance (up to 120 kcal/cm² per year), a thicker cloud cover, abundant rainfall, and mild winds and calms. The surface waters are slightly less saline (about 34 parts per thousand) than the average ocean waters and are rich in plankton. In connection with the characteristics of circulation of the atmosphere and water, the equatorial belt is clearly delineated in the western sectors of the Pacific and Atlantic oceans; in the eastern sectors it is clearly delineated only north of the equator, and in the Indian Ocean, only in the central and eastern sectors.