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Chaparral is a semiarid biome that occurs on the west coast of every continent between about 30° and 40° North latitude. This smallest biome is unique for its Mediterranean climate: mild, moist winters and hot, dry summers. Mature chaparral consists almost solely of woody evergreen shrubs with small leathery leaves. The numerous species form impenetrable thickets from five to eight feet (1.5 to 2.5 m) tall. During the long dry summers the typically resinous foliage and dry woody stems become explosively combustible.

Wildfires raze large areas to ash-covered earth every few decades. Fires are not harmful to this community; they are in fact necessary for maintaining its vigor. Following fires the bare ground is briefly colonized by a large number of annual species, but the land is soon reclaimed by the shrubs which sprout from seeds or root crowns. Trees and succulents are rare life forms in chaparral because they are more vulnerable to destruction by the very hot fires.

This young biome evolved from early Tertiary tropical forest during the Pliocene and Pleistocene. The uplift of the great mountain ranges of western North America blocked the summer monsoon moisture from reaching the far west, creating a summer dry season (see chapter on “Deep History of the Sonoran Desert”).

The main area of chaparral occurs west of the coast, transverse, and peninsular ranges and is called Californian chaparral. Disjunct patches of chaparral occur inland of these ranges and are called interior chaparral. Interior chaparral differs in having only a few species; it is often comprised almost entirely of manzanita (two species of Arctostaphylos) and shrub live oak (Quercus dumosa). Interior chaparral also receives substantial summer rainfall, though the plants do not respond to it.

California chaparral borders the western edge of the Sonoran Desert in California and northern Baja California, and interior chaparral is scattered along the desert’s northeastern edge where it meets the Mogollon Rim of Arizona. Interior chaparral also occurs in isolated patches on the lower slopes of some mountain islands.