Savanna Zones

natural zones of low latitudes, primarily in the subequatorial belts of the northern and southern hemispheres; areas of savanna are also encountered in tropical and subtropical zones. Savanna zones have a seasonal humid climate with a marked change from dry to rainy periods, which causes the seasonal rhythm of all natural processes. There is a predominance of ferralite soils and herbaceous vegetation (with some individual trees, groups of trees, or sparse forests).

Savanna zones are most common in Africa, where they cover approximately 40 percent of the continent and are found primarily in the Sudan, in Somalia on the plateaus of East Africa, in the southern Congo River basin, and in the northern Kalahari. They also occur in South America (in the Orinoco and Mamoré river valleys, on the Caribbean coastal lowlands, in the central and northeastern sections of the Brazilian Highlands), Central America, Asia (on the Deccan Plateau and the Indo-Gangetic Plain, in the inner regions of the Malay Peninsula), and northern and eastern Australia.

Savanna zones, as a whole, are characterized by trade winds and monsoons, with a predominance of dry tropical air in the winter and humid equatorial air in the summer. The duration of the rainy season decreases, from eight or nine months to two or three months, as the distance from the equatorial belt increases. The annual total of precipitation decreases similarly, from 2,000 mm to 250 mm. Seasonal fluctuations in temperature are relatively small (from 15° to 32°C), but daily fluctuations are great (to 25°C).

The alternation of dry and wet seasons determines the basic features of the natural environment. During the rainy season, the flow rates of rivers increase 15 to 20 times. The mechanical load (sediment discharge) increases sharply, erosion on slopes and flat surfaces is intensive, and the soils undergo elutriation. Plants vegetate rapidly; trees and shrubs usually bloom at the end of the dry season or the beginning of the rainy season and bear fruit during the rainy period. Lowlands without drainage frequently become swampy during the rainy season.

In the dry season, the water level of the rivers drops sharply, small streams dry up, and the level of subsurface waters decreases. Wind erosion and, in the driest regions, deflation are intensive. A rise in soil dissolution is observed, the vegetation of plants slows or ceases, and many trees lose their foliage. Some animals travel large distances in search of water and food; others hibernate.

The relief in savanna zones is often represented by smooth surfaces, above which rise monadnocks protected by dense fer-ralite crusts. Soils are usually red, reddish brown, and red-chestnut with a high content of ferric oxide, manganese, and aluminum. The drought-resistant vegetation is represented by various types of savannas, savanna forests, sparse xerophytic forests, and shrubs. Narrow strips of forest are common along the rivers. The abundance of plant food (the aboveground green mass reaches 30 tons per hectare) favors the existence of a rich and varied animal life, including numerous large ungulates, carnivores, rodents, cursorial birds, reptiles, and insects.

The vertical zonality in the mountains of the savanna zones is usually represented by sparse forests and savannas, above which lie dry and mesophytic deciduous and evergreen forests. Higher up, there are shrubs and mountain steppes. Depending on the length of the rainy season and the total amount of precipitation, humid high-grass savanna, savanna forest, typical savanna, desert savanna, and sparse forest subzones are distinguished in the savanna zones of the subequatorial belts.

Land forms in the savanna zones have been highly altered by man. Grain and tuber crops, cotton, jute, peanuts, sisal, sugarcane, and other plants are cultivated, and livestock is raised, primarily in the more arid regions.