In the broadest sense, the Q. dumosa complex includes at least 6 of California's shrubby white oaks, including California scrub oak, coastal sage scrub oak, leather oak (Q. durata), MacDonald oak (Q. macdonaldii), Tucker oak (Q. john-tuckeri), and Sonora scrub oak (Q. turbinella). Hybridization and introgression have led to genetic "blurring" of these species, and the Q. dumosa complex was realigned several times over the 20th century. As defined by systematists in the early 21st century, the name Q. dumosa refers only to coastal sage scrub oak: a species that lies within a very narrow geographical range in southern California and is primarily restricted to coastal sage scrub communities. Under this narrow taxonomic designation, the name Quercus dumosa is misapplied to many of California's shrubby oaks; most commonly, it has been misapplied to California scrub oak.

This taxonomic dilemma creates confusion as to which taxon is actually being discussed in literature written before the 21st century. Most 20th-century literature that refers to "Q. dumosa" actually applies to California scrub oak [52,79], but entities on the Mojave Desert ecotone that are identified in older literature as "Q. dumosa" may actually be Muller oak (Q. cornelius-mulleri) or Sonora scrub oak. Unlike these strictly southern California species, California scrub oak has a broader geographic distribution that extends into northern California, so except for desert ecotones and other areas of geographic overlap in the southern part of the state, the burden of reassigning species designations to entities discussed in older literature as "Q. dumosa" is somewhat lessened. ...

"Quercus dumosa (sensu lato, sl)" refers to reports in which the distinction between California scrub oak and coastal sage scrub oak was unclear. This includes reports of "Q. dumosa" from older literature in areas where the distributions of California scrub oak and coastal sage scrub oak overlap, since the species described may have been reclassified since the literature was published.

The ecology and reproductive habits of California's shrubby oaks are similar [despite the] confusion over taxonomy ...

Hybridization, hybrid swarms, and introgression are rampant among the scrubby oaks within the Q. dumosa complex. Additionally, these species may hybridize and intergrade with white oaks that grow primarily as trees. California scrub oak hybridizes with many other white oaks in California [52], including coastal sage scrub oak, leather oak, Tucker oak (Q. john-tuckeri), Sonora scrub oak, Engelmann oak (Q. engelmannii), valley oak (Q. lobata), and Oregon white oak (Q. garryana).

- Fryer, Janet L.: (2012) Quercus berberidifolia, Q. dumosa^©