Savanna is vegetation consisting of widely scattered trees rising from a more or less continuous lower layer dominated typically by grasses. Although the term savanna most commonly refers to a vegetation in tropical and subtropical latitudes, structurally similar vegetation can be found in the sub arctic lands of the northern hemisphere the taiga. As discussed here, savanna is that of low latitudes, included in Class C in the world vegetation map as a relative of the tropical savanna woodland, monsoon forest, and thornbush and tropical scrub.
Like its relatives, the tropical savanna is a response to wet-dry tropical climate regime in which the severe drought period is one of relatively cooler temperature but which experiences great heat just preceding the onset of the rains.
As with the savanna woodland, the tropical savanna supports trees and shrubs which are xerophytic or deciduous. In some localities the trees are palms, giving a palm savanna. Grasses in the tropical savanna are characteristically tall, with stiff coarse blades, commonly higher than the height of a man, and even up to 12 ft (4 m) high. In the dry season these grasses form a yellowish straw mat that is highly flammable and subject to periodic burning. It is widely held that, to a greater or lesser degree, periodic burning of the savanna grasses is responsible for the maintenance of the grassland against the invasions of forest.
Fire does not kill the underground parts of grass plants, but limits tree growth to a few individuals of fire-resistant species. The browsing of animals, which kills many young trees, is also a factor in maintaining grassland at the expense of forest.
The African savanna is perhaps the most celebrated of the tropical grasslands, spanning the continent from west to east in two great belts, centered about on the 10th parallels of latitude north and south, and connected in east Africa by a broad north-south belt. In equatorial latitudes the African savanna replaces the rainforest because of the aridity associated with highlands in Sudan, Kenya, and Tanzania. Characteristics are flat-topped acacia tress (Acacia), and the grotesque baobab (Adansonia digitata), which has a large barrel-shaped water-storing trunk. Elephant grass (Pennisetum pureum), forming almost impenetrable thickets may grow to heights of 16 ft (5 in).
Savanna in South America is exemplified in the campo-cerrado of the interior Brazilian Highlands. Here the trees are largely evergreen, deep-rooted and capable of tapping lower levels of soils moisture not available to the grasses during the dry season.
Other important savanna areas occur in northern Australia, India, and southeast Asia. The mesquite savanna of Texas provides an American example. Underlain by a limestone formation, the extremely low, flat, plain of the Everglades is flooded by a shallow layer of runoff from summer rains and becomes a swamp. In winter the area becomes extremely dry. Coarse saw grass (Cladium effusum) covers much of the plain. Scattered trees are represented by palms and, on higher ground by pines.
The pedogenic process most closely associated with tropical savanna is laterization promoted by the high temperatures associated with the rainy season. However, laterization gives way to calcification as the savanna is traced toward higher latitudes where thronbush and ultimately, steppe grasslands are encountered.
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