alexbinck 的最愛

Our botanical hiking group was intrigued by the many conspicuously yellowed-but-persistent leaves on this "late deciduous" oak. (Perhaps "partially deciduous" would be a better phrase here...as there were also many green leaves still attached on this nearly mid-winter Feb 1st date.) There were many other deciduous oaks in the local area (presumably many were Q. lobata, and some Q. douglasii)...but they had all completely dropped their leaves by this time.

The intermediate evergreen/deciduous behavior here, together with leaf characters (i.e. lobing, somewhat shiny upper surface, paler lower surface, vestiture, etc.) suggested this was a hybrid of a deciduous and an evergreen pair of parent species in the "white oak group" (Quercus sect. Quercus). We were thinking perhaps Q. douglasii and one of the local scrub oaks (i.e. Q. berberidifolia or Q. durata). Later, @joergmlpts referred me to this nearby iNat obs...which appears as if it may actually be the same exact tree we saw. And there's also this similar-looking iNat obs 38209307 from very nearby. Both those observations were placed as Q. douglasii...but the observers indicated they suspected they were hybrids of Q. douglasii crossed with a scrub oak.

On researching things in the days after our visit, I found a likely candidate: Quercus x subconvexa. It was described by John Tucker in 1953 as a hybrid of Q. garryana (Oregon Oak) and Q. durata (Leather Oak)...both of which are vouchered from the area (indeed Tucker found individuals of each nearby). The agreement is excellent(!) between this oak and the many details of Tucker's description & extended discussion of Q. x subconvexa. Of particular note, diagnostically, are: 1) the lobing pattern of the leaves...each lobe with a short, sharp mucro at its tip; 2) the vestiture of the leaves (uniformly-densely distributed long-rayed stellate hairs abaxially vs. more isolated & widely-scattered tiny clumps of tightly-tufted short-rayed hairs adaxially); as well as the partial deciduousness and paucity/apparent lack of acorns.

And, intriguingly, this site appears to by quite near (and perhaps is! ) the type locality for Q. x subconvexa...which was described as a "north slope, elev. 1300 feet" (compare with topo map link here) and "approximately 5 miles north-northeast of Gilroy". I checked on Google Earth and the spot here fits the bill extremely well. Note also that all Tucker's many vouchers listed here are from the same locality (though the coords given there, 37.072628 -121.532169, appear a bit off...as they indicate a point in an open grassy area on a southwest-facing slope, about 750' to the south of the location of this observation). As seen in the 7th photo here, this observation is just north of two park benches and an interpretive sign along the Mummy Mountain Trail.

The strong fit between critical features of this oak with those discernible in photos accompanying 19 of Tucker's vouchers at the preceding SEINet link (see also here)...as well as the agreement with a preponderance of the many details in the nicely written paper Tucker(1953)... have me fairly-well convinced of the ID here.

---Comments on Individual Photos of the Series---

2nd Photo: Thumbnail is 16 mm wide. Together with Photos 8-10 here, one gets a sense of the relatively large size of the leaves here...presumably reflective of parentage from the relatively large-leaved species Q. garryana.

6th Photo: Tree-like habitus of Q. x subconvexa at center of photo (it looks like a 2nd smaller individual may also be present to the right).

8th Photo: Abaxial side is shown in the two duller leaves still attached to the twig (at upper right-center & at lower right ); and also for the detached leaf at lower left...all other leaves here exhibit the shinier adaxial side.

9th Photo: View of adaxial (upper) side, with mm scale.

10th Photo: View of abaxial (lower) side, with mm scale.

11th Photo: Pale abaxial side at left vs. shinier adaxial side at right.

12th & 13th photos show detail of abaxial sides. Though it's difficult to make out in the photos (but much better discerned under a well-lit stereo-view dissecting microscope), the abaxial hairs here are somewhat dense and "stellate". In particular, most these stellate hairs had 5-7 relatively-long & straight rays...each ray centrally-attached to the base of the (compound) hair and radiating outward & slightly upward. The attachment of the rays was visually very subtle to perceive under a stereo-view dissecting scope at 40X magnification and could easily be overlooked. I wasn't able to discern it with a hand lens.

Goodly trip leader Sandy led a group of us to this interesting oak, which had been anticipated to be an instance of "Oracle Oak", also known as Quercus × morehus...considered a hydrid of Q. kelloggii [Jeps; FNA] and Q. wislizeni(i) [Jeps; FNA]. This seemed a reasonable putative ID to me, as the leaves & acorns appeared intermediate between what I thought of as "typical" characters of the presumed hybrid's parents...both of which appeared to be present nearby (though what I was taking as Q. wislizeni is apparently better considered as Q. parvula var. shrevei...more on that below).

For instance, the leaves are mostly planar & elliptic (noticeably longer than wide) and completely glabrous with a dull, lighter green color below, as in Q. wislizeni (and Q. parvula). But they also have bristle-tipped teeth at the tips of their shallow and somewhat regularly-spaced lobes...which seemed to me a plausible indication of Q. kelloggii parentage. Note also that Q. kelloggii is deciduous whereas Q. wislizeni is evergreen (as is Q. parvula var. shrevei)...and, as an intermediate trait, the hybrid Q. × morehus typically exhibits appreciable yellowing & loss of leaves during winter. Sandy noted there was no clear indication of any deciduous leaves during our visit...but perhaps if we checked again in January or February this might be apparent?

Also, the acorns appear to have their cap scales extending quite far down and enveloping much of the nut...which seems an affinity with Q. kelloggii (e.g. cf. images here, here, and here). Acorns of Q. parvula var. shrevei appear here. Then again, the fraction of the acorn nut covered by cap scales presumably depends on the growth stage of the acorn, and perhaps also impacts on acorn nut growth due to insect interactions, pathogens, and other physiological stressors.

And with oaks, it seems that various leaf, acorn, and other characters can be remarkably variable within populations of a given species (or even within an individual!).

Much interesting info on Q. × morehus can be found in Tom Chester's write-up here relating to occurrences in the San Jacinto Mountains of southern CA — including an image of the brief original 1863 description by Kellogg; a discussion of the possible meaning of the epithet "morehus" as well as the common name "Oracle Oak"...and more. Another (older) discussion of "Q. Morehus" can be found on pp. 46–49 of the 1910 "The Silva of California" by W. L. Jepson. And there's a Q. × morehus Wikipedia page with a line drawing and further references. (One such detailed reference cited there, Wolf(1938), can be read by scrolling through pp. 47-51 at this link.)

Per the current Jepson eFlora treatment, the native CA species of "Red" or "Black" Oaks (= Quercus Section Lobatae) are: Q. agrifolia, Q. kelloggii, Q. wislizeni, and Q. parvula (the latter having 3 varieties — Q. parvula var. shrevei being the one occurring in the Santa Cruz Mountains; with the nominate variety occurring in the Channel Islands and areas of adjacent southern CA; and var. tamalpaisensis localized on Mt. Tamalpais in Marin Co.). [Note that the FNA treatment of Quercus synonymized Q. parvula under Q. wislizenii...which is a potential source of confusion in terms of which authority people may be following when they attach a name to a particular 'oak entity' of interest. But such issues are always present...albeit often unspoken & in the background.]

Note that the initial lead of "couplet 1" in the Jepson eFlora Key to Quercus separates out the "red/black oaks"...that is, the thin acorn cup scales here are enough to get us to the Section Lobatae. But beyond there the key becomes ambiguous for the material here.

When we visited this particular tree, I had been taking to heart the FNA synonymy of Q. parvula under Q. wislizeni...but now, after studying things further, I better appreciate my lack of a clear understanding for the subtle complexities involved in attaching an optimally informative name here! While oak taxonomy is notoriously difficult and differences of opinion exist, clearly there are experts who have long studied oaks and have great expertise in the subject and see merit in recognizing Q. parvula...and in particular, Q. parvula var. shrevei in the Santa Cruz Mountains. Sandy was fortunate to have Al Keuter comment on her post of this tree, and Al suggested that perhaps this might be better referred to as Q. × 'wootteni'...a (not yet formally described?) putative hybrid of Q. kelloggii and Q. parvula var. shrevei. The only mention I could find online for Q. × 'wootteni' was in the key to CA members of Quercus section Lobatae, series Agrifoliae, appearing in the appendix at the end of the 2017 paper:

The evolution and diversification of the red oaks of the California Floristic Province (Quercus section Lobatae, series Agrifoliae)

...of which Al is a co-author.

There are abundant individuals of both Q. kelloggii and what I believe are considered Q. parvula var. shrevei in the vicinity of the oak in this post.