Oak woodlands are a fixture of California geography, yet as recently as 10,000 years ago oaks were only a minor element in the landscape. The first fossil evidence for California's oaks is in the early Miocene (~20 million years ago) when oaks were present across the west, intermixed with deciduous trees typical of eastern North America. As climate became drier, species dependent upon summer precipitation went locally extinct and oaks retreated west of the Sierra Nevada. During the Pleistocene (the last 2 million years) oak abundance declined during cool glacial periods and expanded during warm interglacials. After the last glacial maximum (~18,000 years ago), oaks expanded rapidly to become the dominant trees in the Coast Ranges, Sierra Nevada foothills, and Peninsular Ranges. During the Holocene (the last 10,000 years) oaks in the Sierra Nevada were most abundant during a warm dry period between 8000 and 6000 years ago. Native American use of fire to manipulate plants for food, basketry, tools, and other uses helped maintain oak woodlands and reduce expansion of conifers where these forest types overlapped. Fire suppression, initiated by the Spanish and reinforced during the American period has allowed oak woodland density to increase in some areas in the Coast Range, but has decreased oaks where pines are dominant. Extensive cutting of oaks has reduced their populations throughout much of the state.
Mensing, Scott. 2015. The paleohistory of California oaks. In: Standiford, Richard B.; Purcell, Kathryn L., tech. cords. Proceedings of the seventh California oak symposium: managing oak woodlands in a dynamic world. Gen. Tech. Rep. PSW-GTR-251. Berkeley, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station: 35-47.