The political boundaries of the state of California cover an area of 411 x 103 km2, 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 MTE area, excludes these desert regions and adds northwestern Baja California and southern Oregon to the floristic province. Under this definition, the California floristic province covers 324 x 103 km2. 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 the topographic diversity within the floristic region is very high. Thus, this region covers the Coast Ranges extending 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 2700 m, while Mt. Whitney in the Sierra Nevada is the highest point in the continental United States at 4400 m elevation. The Transverse Ranges in Southern California have a number of peaks reaching above 3000 m. The dynamic geologic history of uplift, faulting, and tectonics has produced complex mosaics of soil structure and parent material, and produced sharp climate shifts over the Quaternary with 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 400–800mm annual rainfall. Rainfall is strongly centered on the winter months, and 6 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 1500m in northern California and 1800m 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 of California support mixes of conifer and hardwood forests, extending into massive coast redwood forests along the northwestern coast. Mean annual rainfall reaches its highest levels above 2500 mm in this region.