The history of oak woodlands in California, part I: the paleoecologic record

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. In this paper, I review the long-term history of California oaks, beginning with the Tertiary fossil record from 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, many species went locally extinct and oaks retreated west of the Sierra Nevada. During the ice ages (~ the last 2 million years), oaks nearly disappeared as cool and wet climate favored expansion of coniferous forests and oak woodlands persisted in refugia. After the last glacial maximum, oaks expanded rapidly to become the dominant trees in the Coast Ranges, Sierra Nevada foothills, and Peninsular Ranges. Within the last 10,000 years, climate change has continued to alter oak woodland distribution patterns. During this period, human impacts on oak woodlands have also caused significant changes. The human impacts on oaks, associated with Native Californians, and the Spanish, Mexican, and American populations that displaced them, will be reviewed in Part II.