Thornbush and tropical scrub (which may be treated as two different formation classes) consist of xerophytic trees and shrubs responding to a climate with a very long dry season and only a short, but intense, rainy season. Thornbush, also referred to as thorn forest, and thorn-woods, consists of tall, closely spaced woody shrubs commonly bearing thorns and largely deciduous. Cacti may also be present. The lower layer of herbs may consist of annuals, which largely disappear in the dry season, or of grasses. In the drier areas the lower layer may consist only of scattered grass clumps with much bare ground between. One example of the thornbush is the catinga of northeastern Brazil, and open thorn forest of the dry highlands. Another is the dorn veld of South Africa.
Tropical scrub is a dense growth of low woody shrubs which may occur in patches or clumps separated by barren ground. Scrub may develop in stony, sandy, or gravelly sites in areas of thornbush.
Thornbush and tropical scrub are found in many parts of the world where the wet-dry tropical climatic regime is transitional into the tropical desert regime. Soils show the influence of aridity and are subject to calcification, and (in poorly drained sites) of salinization. Reddish brown and reddish-chestnut soils occur in much of the area of these formation classes.
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