The Oak (Quercus) Biodiversity of California and Adjacent Regions

Twenty species of oak (Quercus) are known from California. The white oak group is the most diverse, and includes a complex of scrub oak species that are often encountered in chaparral, mixed forest and desert margin habitats. The Protobalanus group (e.g., Quercus chrysolepis) is a unique and distinctive clade of western North American species that appears to be most closely related to the white oak group. Within the white oaks and red oaks (black oaks), the majority of California species do not have obvious, close relationships to species outside of the region. In addition, these species either have lobed leaves (e.g., Q. kelloggii, Q. lobata, Q. garryana) or have leaves that appear to be derived from ancestors with lobed leaves by reduction (e.g., Q. dumosa, Q. berberidifolia, Q. john-tuckeri, Q. cornelius-mulleri, Q. agrifolia). Lobed leaves, such as those found in numerous oaks of the eastern U.S., are characteristic of species from temperate or cold climates, but not any of the more tropical species, thus suggesting a temperate ancestry for the bulk of California oak species in the white oak and red oak groups. In contrast, the Protobalanus group, which is truly evergreen and has entire or merely dentate leaves, probably has a more tropical origin.