A Western Interior Origin for the Chaparral Shrub Genus Arctostaphylos (Ericaceae) in California?

Arctostaphylos (manzanita) is a speciose genus of semi-xerophyllic shrubs that has evolved rapidly, adapting to wildfires and the widely diverse topography, microclimates, and edaphic substrates found throughout its range. Its present center of diversity is the California Floristic Province (CFP), a geographically defined area that includes most of California except for the Modoc Plateau in the north and the Mojave and Sonoran desert regions in the south. Over 50 species of Arctostaphylos occur within the CFP and they are among the dominant chaparral shrubs. Despite its immense biodiversity, the CFP is geologically young, in part, having formed as a result of global-scale cooling and drying within the last 15 Ma. Climate in western North America prior to this time was warm and wet, as shown by paleoclimate estimates using floristic and quantitative analyses of Paleogene and Neogene western North American angiosperm leaf assemblages. Here we review the fossil record of Arctostaphylos to elucidate the geographic origin and timing of the radiation of Arctostaphylos into the CFP. The first unequivocal record of the genus is A. masonii Wolfe. These well-preserved leaves, which show the diagnostic ladder-like marginal venation of Arctostaphylos, are from the middle Miocene (~14.5 - 14.1 Ma) Savage Canyon formation of Stewart Valley in west-central Nevada. The Savage Canyon formation represents a large lacustrine deposit that formed in a basin of high elevation eastward of the present-day Sierra Nevada (i.e. pre-Great Basin). In California Arctostaphylos first appears during the late Miocene, by which time it was well established within the coastal range. Therefore, interpretation of the present paleontological data suggests Arctostaphylos originated outside of California in the early Neogene pre-Great Basin region and subsequently dispersed into California where it quickly diversified being well adapted to the developing semi-arid Mediterranean-type climate that characterizes the CFP today.