California has more than twenty-five native species, natural hybrids, and varieties of oaks (Quercus species). The form of these oaks ranges from large trees, up to about 25 m tall, to shrubs no taller than about 1.5 m. California's native oaks include representatives of three oak subgroups or subgenera. Hybridization only occurs between oaks in the same subgroup. In addition, some insects, pathogens, and other agents may selectively colonize or damage oaks in certain subgroups.