thumb|upright|[[Mojave Desert in Joshua Tree National Park]]
The ecology of California can be understood by dividing the state into a number of ecoregions, which contain distinct ecological communities of plants and animals in a contiguous region. The ecoregions of California can be grouped into four major groups: desert ecoregions (such as the Mojave Desert), Mediterranean ecoregions (such as the Central Valley), forested mountains (such as the Sierra Nevada), and coastal forests.
Different authorities define the boundaries of ecoregions somewhat differently: this article follows the definitions of the World Wide Fund for Nature (WWF) and the United States Environmental Protection Agency (EPA)
Deserts
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. California has two high deserts: the Mojave Desert and the Great Basin Desert. The Mojave Desert ecoregion is marked by the presence of Joshua trees. The dry cold Great Basin desert of California consists of the Owens Valley, and is classified into Great Basin shrub steppe by the WWF, and into the Central Basin and Range ecoregion by the EPA.
The deserts in California receive between of rain per year. Plants in these deserts are brush and scrub, adapted to the low rainfall. Common plant species include creosote bush, blackbrush, greasewood, saltbush, big sagebrush, low sagebrush, and shadscale. The EPA divides the region between the Central Valley (ecoregion 7), the Southern California chaparral (ecoregion 6), the Southern California mountains (ecoregion 8), and the Southern California coast (ecoregion 85). The montane chaparral consists of a mosaic of sage scrub, chaparral, and montane species, depending on altitude. The interior woodland ecoregion contains several endemic species, due to unique soil types such as serpentine. These plant communities often occur as a mosaic,
California's Central Valley was once a large temperate grassland containing native bunchgrasses and vernal pools. Grizzly bear, gray wolf, tule elk, and pronghorn antelope used to inhabit the grasslands. The native grasslands and pools have now been largely replaced by livestock ranches and farms. The Carrizo Plain, where the native grass is preserved, is referred to as the "Serengeti of California".
thumb|Wildflowers bloom after a wildfire in [[San Diego County]]
Forested mountains
thumb|Sierra Nevada lower montane forest in [[Yosemite Valley]]
The cooler and wetter mountains of northern California are covered by forest ecoregions. Both the WWF and the EPA divide the mountains into three ecoregions: the Sierra Nevada, the Klamath Mountains, and the Eastern Cascades Slopes and Foothills (occurring on the Modoc Plateau).
The Sierra Nevada are home to half of the vascular plant species of California, with 400 species that are endemic to the region. These biotic zones include montane forest dominated by conifers such as Jeffrey pine and Lodgepole pine, subalpine forest dominated by whitebark pine, up to alpine tundra which cannot support trees. The Sierra are also notable for giant sequoia trees: the most massive on earth.
The Klamath and Siskiyou Mountains are a notable biodiversity hotspot, containing one of the four most biodiverse temperate forests in the world.
The redwood forests thrive in a thin belt up to wide next to the coast, where the trees are kept moist by winter rains and summer fog.