The ecology of California is amazingly diverse.

California's high mountains block most moisture from reaching the eastern parts of the state, which are home to California's desert and xeric shrub ecoregions. The low desert of southeastern California is part of the Sonoran desert ecoregion, which extends into Arizona and parts of northern Mexico. Southern California's high deserts constitute the Mojave desert ecoregion, which has affinities to the Great Basin shrub steppe , which covers California's Owens Valley and Modoc Plateau , as well as most of neighboring Nevada.

The southern and central Coast Ranges, including the San Francisco Bay Area and Los Angeles region, comprise the California chaparral and woodlands ecoregion, which extends across the Mexican border into northwestern Baja California. The California chaparral and woodlands ecoregion contains numerous plant communities, including oak savanna, oak woodland, conifer woodlands, chaparral, coastal sage scrub, and coastal grassland; these plant communities often occur as a mosaic, with patches of distinct plant communities situated in response to site conditions, including variations in sun exposure (north or south slopes), wind exposure, and soils, and variations in rainfall between wetter seaward slopes and the drier 'rain shadow' on the landward slopes.

California's Central Valley was once a large temperate grassland, the California Central Valley grasslands ecoregion, which was formerly home to great herds of grazing pronghorn and Wapiti; some writers have referred to it as "America's Serengeti". The Central Valley has been mostly converted to farms and rangeland; its once great seasonal wetlands have been drained and its perennial bunch grasses replaced by exotic annual grasses or farm fields, but patches of the native bunchgrasses still exist, as do some of the small seasonal wetlands known as vernal pools.

The mountains of northern California have a cooler and wetter climate, and are home to temperate coniferous forests, including the Sierra Nevada forests , Northern California coastal forests , Klamath-Siskiyou forests , and Central and Southern Cascades forests . The highest peaks are home to Mountain-tops contain tundra, fellfields (stony ground with patches of meadow), and krummholz (dwarf forests). These forests are home to some of the world's largest trees, the Coast redwood (Sequoia sempervirens) of the Northern California coastal forests, and the Giant Sequoia (Sequoiadendron giganteum), which lives in scattered groves in the Sierra Nevada forests.

Botanists generally consider the California chaparral and woodlands, Sierra Nevada forests, Central Valley grasslands, Northern California coastal forests, and Klamath-Siskiyou as a single California Floristic province, which does not include the deserts of eastern California, which belong to other floristic provinces. Many Bioregionalists, including poet Gary Snyder, identify the central and northern Coast Ranges, Klamath-Siskiyou, the Central Valley, and Sierra Nevada as Alta California or the Shasta Bioregion.

See also: Geography of California