Black oak / live oak hybrids (updated 2/21/2021 and 11/3/2021)

The captions above identify the tree I saw on today's hike was probably Quercus × morehus (Oracle oak) but that Q. × chasei (Chase oak) remains a possible alternative. The first is a hybrid of Q. kelloggii (black oak) and Q. wislizeni (interior live oak). The second, following Jepson, is a hybrid of Q. kelloggii (black oak) and Q. agrifolia var. agrifolia (coast live oak). The first name is well-established. The second is less so and my use of it here needs discussion before commenting on the identification itself.

Jepson recognizes neither as taxa but recognizes both Q. x morehus and Q. x chasei as being the "accepted name for [a] taxonomically recognized and/or fertile hybrid". Both appear in the notes for the parent species entries and in the Jepson Online Interchange database. Calflora omits Chase oak. CalPhotos has both.

There's one other player here. Many sources use Quercus × ganderi (Gander oak) for hybrids between Q. kelloggii and Q. agrifolia of all varieties. However, the original 1946 article on Q. ganderi by C. B. Wolf clearly focuses only on hybrids of Q. kelloggii and Q. agrifolia var. oxyadenia (southern coast live oak). The latter is found only in the Transverse and Peninsular Ranges of southern California. Its hybrids would not be a player Bay Area. Wolf uses the common name "Gander Oak". (Frank Gander, a botanist from San Diego, was the original finder.) Jepson limits Q. × ganderii to Q. agrifolia var. oxyadenia hybrids. Jepson does not introduce the common name "Gander Oak".

Usage in various other sources is, well, a mess. Calflora, as noted, omits Q. × chasei but does include Q. × ganderi. Some of the Calflora observation entries listed under Q. × ganderi are identified in the associated text as Q. chasei, suggesting a merge of what were in the past separate items. CalPhotos has entries for both Chase and Gander oak but pictures for the latter appear to include hybrids from both Q. agrifolia varieties. Ertter and Naumovich take an intermediate position, using use Q. chasei (no "×") in their listing but citing Q. ganderi as a synonym in the associated notes. INaturalist mixes names in yet a different way. "Chase Oak (Quercus × chasei)" is shown as an inactive taxon while "Chase Oak (Quercus × ganderi)" is the active one.

So what names seem best to apply to the Q. kelloggii × Q. agrifolia hybrids? If variety is collapsed, Q. ganderi has priority: it was coined before Q. chasei. H. E. McMinn, et al., described the Chase oak in 1949. (McMinn found the first such tree on the ranch owned by Mr. and Mrs. Chase, near Gilroy.) If variety is not collapsed, using Q. chasei for hybrids with variety agrifolia and Q. ganderi for those with variety oxyadenia remains appropriate. Jepson follows the latter in the notes and interchange. I, by policy, follow Jepson. There are no rules for common names, but it seems reasonable to keep the common and Latin names aligned: Chase oak for Q. chasei and Gander oak for Q. ganderi.

One more complication. All the above applies only to F1 hybrids. But both oracle and chase oaks produce acorns that, at least in theory, can grow into F2 hybrid trees. I've read these second-generation acorns often produce weak or short-lived plants, but not always. And that, in turn, opens up at least the possibility of a three-way hybrid tree with parentage from Q. kelloggi, Q. wislizeni, and Q. agrifolia. Such a hybrid would defy these naming conventions and would probably mix characteristics of all. I don't know that anyone has claimed to have found an example of that specific three-way hybrid, but there is a report of a possible Q. kelloggi / Q. agrifolia / Q. parvula var. parvula F2 hybrid from the Channel Islands. For now, it's probably OK to discount these wide hybrids since F2's are apparently even less common than the already rare F1's, but it's worth remembering the possibility is out there.


So what did I see today? The semi-evergreen leaves of an intermediate form make it clear today's tree is hybrid of Q. kelloggii and either Q. wislizeni or Q. agrifolia var. agrifolia. As noted, let's assume it's one of the F1 hybrids or a "simple" F2 that just re-mixes the same parent species. The second picture shows it's a small tree, maybe 30' high, which fits either, although the Chase oak can grow much larger. The four INaturalist entries for this tree agree this is an Oracle oak. Oracle oaks are considerably more common than Chase oaks. So all the indirect methods point to the Oracle Oak.
Unfortunately, there are issues with the few direct identification tools I have. Ertter and Naumovich note chasei differs from morehus "in more ovoid lvs w/ < 10 side veins", a description that does not always match what I see in the CalPhotos pictures. In any case, the leaves here really aren't ovate (i.e., widest below the middle), which I think is what's meant. ("Ovoid" according to Jepson e-flora glossary means the same sort of thing but applies to three-dimensional objects like fruits.) At least some of the leaves have 10 or more side veins, although the count is mixed. So at least the words, if not the pictures, tend toward morehus. McMinn suggests chasei has more leathery leaves, which these do not appear to have. However, again, leatheriness is not all that pronounced on the Chase oak Calflora pictures. I'd like to think oracle oaks would have flatter leaves, like the interior oak, and chase oaks would have more convex ones, like the coast live oak, but the CalPhotos pictures don't always bear that out. McMinn notes the chase oak leaves can be flat. So that's not a useful characteristic today. Both Etter/Naumovich and McMinn call out one more potentially definitive identifier for chasei: the presence of axillary hairs on the leaf bottoms ("hairy armpits"). I see none here on the few turned up leaves in the last picture -- an argument for Oracle oak -- but there are only a few such leaves in view, those hairs don't form on every leaf, and they may decay as the leaf ages. I didn't know enough in the field to make a close check.

Reviewing all, the evidence does tend toward the Oracle oak but not unequivocally. A revisit to this easily-found tree for a second look at the leaves, perhaps in spring when new ones are out, might reduce the uncertainty. A look at other black oak - live oak hybrid trees may also be informative.
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