The political boundaries of the state of California cover an area of 424,000 square kilometers but the area includes more than the core area of Mediterranean-type climate. These political boundaries include winter rainfall portions of the Sonoran and Mojave Deserts, as well as areas of cold desert habitats east of the Sierra Nevada. The California Floristic Province, generally defined as the core Mediterranean-type ecosystem area, omits these desert regions but adds northwestern Baja California and southern Oregon. Under this definition, the California Floristic Province covers 324,000 square kilometers. Because of the differences between the political and floristic province boundaries of California, some caution must be used in assessing figures on California biodiversity in the literature.
The geomorphic structure of California is complex and topographic diversity within the floristic region is very high. The region encompasses the Coast Ranges, which extend north and south along the state; the broad Central Valley; the Sierra Nevada range; and the Transverse Ranges of Southern California. The Coast Ranges reach elevations as high as 2,700 m, while Mount Whitney in the Sierra Nevada is the highest point in the continental United States at 4,400 m. The Transverse Ranges of Southern California include a number of peaks above 3000 m. A dynamic geologic history of uplift, faulting, and tectonics has produced complex mosaics of soil structure and parent material. Sharp climate shifts over the Quaternary has brought associated glaciation in the Sierra Nevada.
The foothill regions throughout most of California are typically dominated by mosaics of chaparral shrublands and both evergreen and deciduous woodlands with oak species as the typical dominants. These areas commonly receive 400–800 mm annual rainfall. Rainfall is strongly centered on the winter months, and six months without rain is common. Drier areas along the coast and inland at the transition to desert environments support coastal sage scrub dominated by drought deciduous shrubs and a few species of deeply rooted evergreen sclerophylls.
Mountain areas above 1,500 m in northern California and 1,800 m in southern California show a transition to montane conifer forests, subalpine forests, and alpine communities with increasing elevation. Higher rainfall areas along the central and northern coast support mixes of conifer and hardwood forests. These grade into massive coast redwood forests along the northwestern coast. Mean annual rainfall reaches its highest levels above 2500 mm in this region.