|California scrub oak in the Santa Monica Mountains National Recreation Area. Photo © 2010 Michael O'Brien.|
California scrub oak (Quercus berberidifolia) Liebm.
coastal sage scrub oak (Quercus dumosa) Nutt. (Fagaceae) [52,59,79,94,207]
These species are in the white oak subgenus (Lepidobalanus) [214,217].
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) [171,213]. Hybridization and introgression have led to genetic "blurring" of these species [214,217], 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 [52,79,207].
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. However, the history of taxonomic confusion means that for oaks in the Q. dumosa complex, species designations used in this review cannot be guaranteed.
To help the reader follow information about the 2 species covered in this review, the following conventions are followed:
1. "California scrub oak" refers to entities that lie within the geographical range of Q. berberidifolia as defined by The Jepson Herbarium , with the exception of entities on desert ecotones (where the reports may refer to other species) or in coastal areas where the distributions of California scrub oak and coastal sage scrub oak overlap.
2. "Coastal sage scrub oak" refers to Q. dumosa according to its current classification. Thus, this name coastal sage scrub oak is used only when a) a report clearly identified this species.
3. "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. Although there is confusion over taxonomy, managers may be heartened by the fact that the shrubby oaks have very similar responses to fire .
Hybrids: 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 , 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) [79,214,217]. Putative California scrub oak × Tucker oak hybrids can be found in the South Coast Ranges above Ventura and on the north slope of the Tehachapi Mountains . Named California scrub oak hybrids are:
Quercus × howellii Tucker (Q. berberidifolia × Q. garryana), Howell's oak . These hybrids are noted in Sonoma  and Marin  counties.
Quercus × macdonaldii Green & Kellogg (Q. berberidifolia × Q. lobata), MacDonald oak ; noted in Santa Barbara County and on the Channel Islands 
Coastal sage scrub oak rarely comes in contact with other white oaks due to its restricted range. Some high-elevation California scrub oaks near coastal sage scrub oak populations show signs of introgression with coastal sage scrub oak . Named coastal sage scrub oak hybrids are:
Quercus × grandidentata Ewan (Q. dumosa × Q. engelmannii) [89,218], largeleaf oak
Quercus × kinselae (C. .H. Muller) Nixon & C. H. Muller (Q. dumosa × Q. lobata), Kinsel's oak
Quercus × townei Palmer (Q. dumosa × Q. engelmannii), Town's oak . Town's oak is also known by the synonym Quercus dumosa Nuttall var. kinselae C. H. Muller .
Complete FEIS review for Quercus berberidifolia, Q. dumosa
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