A thorn forest is a dense, scrublike vegetation characteristic of dry subtropical and warm temperate areas with a seasonal rainfall averaging 250 to 500 mm (9.8 to 19.7 in). 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, and consists primarily of small, thorny trees that shed their leaves seasonally. Trees typically do not exceed 10 metres (33 ft) in height, usually averaging between 7 and 8 metres (23 and 26 ft) tall. Thorn forest grades into savanna woodland as the rainfall increases and into desert as the climate becomes drier.