Native Americans entered California 13-15,000 years ago. Although population densities were never high, these peoples nevertheless had significant impacts on their environment. Most notably, they used fire to maintain meadows, clear underbrush, encourage the growth of certain plants, and ensure easy access to a major food source, the acorns of oak trees.  Rapid population growth in California began with the discovery of gold in 1848, and continues today with associated agricultural expansion, land clearance, and urbanization.

Invasive plant species from the Mediterranean Basin have dramatically altered many plant communities.  European grasses, in particular, have supplanted native evergreen bunchgrass communities. Decades of fire suppression has increased undergrowth density in many areas, particularly woodlands. Controlled burns and vegetation management plans are now being implemented in many areas to help reduce fuel loads and encourage native plant communities.