Thorn forest, dense, scrublike vegetation characteristic of dry subtropical and warm temperate areas with a seasonal rainfall averaging 250 to 500 millimetres (about 10 to 20 inches). This vegetation covers a large part of southwestern North America and southwestern Africa and smaller areas in Africa, South America, and Australia. In South America, thorn forest is sometimes called caatinga. Thorn forest grades into savanna woodland as the rainfall increases and into desert as the climate becomes drier.
A thorn forest consists primarily of small, thorny trees that shed their leaves seasonally. Cacti, thick-stemmed plants, thorny brush, and arid-adapted grasses make up the ground layer. Many annual plants grow, flower, and die during the brief rainy season.