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. The desert biome is currently expanding at an accelerating rate during the last 100 years, most likely to increased temperatures and decreased moisture. For example, the grasslands bordering the southern edge of the Sahara have lost about 650,000 square miles to the expanding desert in the last 50 years. This has major repercussions on the distribution patterns of plant and animal species, especially those that have to migrate over the expanding deserts.