Plant Species of San Jacinto Mountain:
Quercus kelloggii, black oak, with axillary leaf tufts of hairs
Quercus kelloggii, black oak, with axillary leaf tufts of hairs
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| Fig. 1. Abaxial (bottom) view of two black oak leaves, photographed on 7 July 2025 in Humber Park. Left: a leaf with no axillary tufts of hairs in the veins. Two out of ten samples oaks had no tufts of hairs in the veins, and three others had only weak tufts. Right: a leaf with prominent axillary tufts of hairs in the veins. Five out of ten sampled oaks had such clear tufts of hairs. Click on the photos to go to the inat obs showing more pix of each oak. | |
Quercus kelloggii, black oak, is a familiar common oak in the mountains of southern California, easily recognized by its deciduous moderately- to deeply-lobed leaves, with bristle-tipped teeth on the lobes. It can infrequently hybridize with Quercus agrifolia, coast live oak, or with Q. wislizeni, interior live oak. It is always a surprise and a delight to come across one of those hybrids, which often look similar to black oak, but with leaves not as deeply lobed. The hybrids are usually found singly, in an area where both parents are present.
Distinguishing the two hybrids was thought to be fairly easy by some. If the hybrid had tufts of hairs in the axils of the veins on the underneath of a leaf, or was densely hairy underneath, it was thought this was a hybrid with Quercus agrifolia, since tufts of hairs in the vein axils are a familiar characteristic of Q. agrifolia var. agrifolia, and dense hairs on the underneath of a leaf is the defining characteristic of Q. agrifolia var. oxyadenia. This hybrid is called Gander oak, Q. x ganderi.
If such hairs are absent, it was thought this was a hybrid with Q. wislizeni since its leaf is glabrous. This hybrid is called oracle oak, Q. x morehus.
At higher elevations of San Jacinto Mountain, we only have the oracle oak hybrid. Its parents are abundant at the higher elevations of San Jacinto Mountain, but Q. agrifolia is almost non-existent at those elevations.
We have found 67 known locations for oracle oak so far; see Plant Species of San Jacinto Mountain: Quercus x morehus, oracle oak. Given that we have a minimum of something like ten thousand plants of each parent, the oracle oak hybrid is quite rare, making up no more than ~1% of the numbers of the parent species.
On 21 June 2025, I photographed an oracle oak that stimulated a lot of discussion when I posted it at iNaturalist, since it had axillary tufts of hairs. Given the stated difference between the hybrids, the axillary tufts of hairs strongly argued for a determination of Gander oak. But it was nearly inconceivable to me that it could be a Gander oak, given the populations of the parents at San Jacinto Mountain.
I then did an analysis of iNat obs of both the San Jacinto Mountain oracle oaks, and of some iNat obs of "Gander oak" from San Diego county, which showed pretty clearly that our San Jacinto Mountain oaks were indeed oracle oaks.
That analysis still left the question about the origin of the leaf tufts open.
My first hypothesis to account for the leaf tufts was that they were a feature of young leaves. Oaks often have quite hairy young leaves, which are lost as the leaf grows older. So the question became: which of the two parent species has leaf tufts on its young leaves?
I'd observed hairs on young leaves of one plant of Q. wislizeni before, so I went searching for young leaves of it, and examined them for hairs. I found none in an examination of ten or so plants.
I then photographed young leaves of one plant of Q. kelloggii, and found it had long hairs on the main veins, and what looked like weak tufts of hairs in the vein axils. Searching online, I found this iNat post showing five axial tufts of hairs!
A few days later, I photographed another Q. kelloggii with very clear axillary tufts of hairs.
So the tufts of hairs in the axils of the veins of the oracle oak are clearly coming from Q. kelloggii. Interestingly, the Jepson eFlora treatment for Q. kelloggii doesn't mention axillary tufts of leaves. But the Flora of North America treatment, published in 1997, by Richard J. Jensen, does, and says "surfaces abaxially glabrous with small axillary tufts of tomentum to densely pubescent". See example of a densely pubescent leaf from the San Jose, CA, area, and another from the Redding, CA, area. Another observation from the San Jose area shows dense hairs that are in the process of being deciduous.
The question then becomes: how many of our black oaks have such tufts of hairs, and how long do they persist on the leaves?
To begin to answer that question, I photographed the leaves of ten black oaks in the Humber Park area on 7 July 2025. I photographed the first ten plants I saw that had leaves I could reach. I photographed the lower surface for three leaves of each plant.
Five of the ten oaks, 50%, had clear axillary tufts of hairs, with a median number of axillary tufts of 2, 3, 4, 5, and 7. Three of the ten oaks, 30%, had only weak or questionable tufts. Two of the ten oaks, 20%, had no tufts.
Table 1 gives the results for each photographed leaf for each plant, and links to the iNat observation of each plant showing the photographs.
Table 1. # of Axillary Hair Tufts per Leaf
| # | Lifeform | # axillary tufts per leaf | iNat pix link | ||
|---|---|---|---|---|---|
| leaf 1 | leaf 2 | leaf 3 | |||
| 1 | big tree | 5 | 5 | 4 | iNat |
| 2 | young tree | 1? | 0 | 0 | iNat |
| 3 | big tree | 6 | 2 | 2 | iNat |
| 4 | cluster of many short stems | 6 | 7 | 7 | iNat |
| 5 | cluster of many short stems | 2? | 4 weak | 1 | iNat |
| 6 | young tree | 4 weak | 6 weak | 3 weak | iNat |
| 7 | big tree | 1 | 5 | 4 | iNat |
| 8 | big tree | 3 | 3 weak | 6 | iNat |
| 9 | big tree | 1? | 0 | (pix out of focus) | iNat |
| 10 | big tree | 1? | 1? | 4 | iNat |
The order of the leaves in Table 1 is time order, and is the same order in which the leaves are presented in the iNat obs. In the iNat obs, the first photograph is a crop of the first leaf zooming in on the vein axils; the second photograph is a photograph of the entire first leaf, and then the same for the second and third leaves. Those six photographs are followed by one or more photos of the upper surface of the leaf, and then a photo of the entire plant.
I thank iNat user @scion882 for alerting me to the iNat observations of Q. kelloggii with densely-pubescent leaves>.