In Africa, the thorntree-tall grass savanna rings the savanna woodland, with acacia the dominant tree species. The definition of a desert is a region with less than 25 cm of rainfall per year, and where potential evapotranspiration exceeds precipitation totals, i.e. , potential for outgoing moisture exceeds incoming moisture. In North America, it's found in the Great Basin and Colorado Plateau of the western US, but has expanded into steppe grasslands due to overgrazing by livestock. In the dry tropical areas, the thorntree desert consists of trees and shrubs adapted to a climate with a long dry season. Such plants are called xerophytes .
Trees here are largely deciduous, shedding their leaves in the dry season.
Xerophytic plants are usually widely dispersed over the ground. Most desert animals are nocturnal, active only during the night. Some animals remain dormant until moisture returns, e.g. the spadefoot toad. In the American Southwest, the plants are quite large giving the appearance of a woodland, consisting of saguaro cactus, barrel cactus, prickly pears, and creosote bush. The dry desert formation class generally receives less than 15 cm of rainfall per year.