Plant Species of San Jacinto Mountain:
Possible hybrids of Quercus agrifolia and Q. wislizeni on the PCT North of SR74
Possible hybrids of Quercus agrifolia and Q. wislizeni on the PCT North of SR74
This is a quickly-written first draft article, which I might significantly revise in the future, as more work is done. The purpose of this draft is to provide this information for iNat posts of these oaks, so it doesn't have to be repeated in each obs.
On 11 October 2025, Scott White posted an iNat observation of a plant that appeared to be Quercus wislizeni, but which had cupped leaves, that he had photographed on 10 October 2025. iNat user @scion882 immediately commented that this plant was a hybrid between Q. wislizeni and Q. agrifolia. I then analyzed the other oak observations in the vicinity of this plant, and found observations that included:
- cupped leaves with no hairs underneath, lighter color on top
- cupped leaves, darker color on top
- flattish leaves with hairs underneath
- normal-looking Q. wislizeni with flat leaves and no hairs.
The oaks in all of these categories had leaves with the typical yellow-green lower leaf of Q. wislizeni, whenever the bottoms of the leaves were photographed. Links to those different observations were given in my comment on Scott's post.
Scott then added another comment that "This stand could indeed be Q.w. introgressing with Q.a.", and "my guess is that, if this stand represents introgression, it's not an F1 hybrid, but instead represents 1 or more generations of backcrossing to Q.w.".
Stimulated by this post and conversation, I found a paper by Brophy and Parnell, 1974, discussing hybridization between these two species. They found that a number of characteristics were useful in discriminating the two species.
Guided by their results, on 12 October 2025 I studied and photographed most of the oaks that appeared to be Q. wislizeni in the vicinity of Scott's oak, and sampled a few more in the first 3.6 miles of the trail. Each specimen was posted at iNat.
I made measurements of their characteristics from my photographs, with the exception of the estimate that the "hybrid" had about 50% of its leaves cupped. Both Don Rideout and I looked at the lower leaves for it, and Don used his 60x zoom camera to examine the uppermost leaves, too. The estimate that all the other oaks had fewer than 10% cupped leaves was from field observations as well as from my photographs.
My initial analysis supports Scott's guess, which greatly surprised me since I had thought all of these oaks were normal Q. wislizeni. In the future, I will gather more data on nearby oaks to understand these oaks better.
It is interesting that this area has abundant (hybrid) Q. wislizeni plants, but very few Q. agrifolia plants. Normally, in such a situation, I would expect that the Q. agrifolia plants would be the ones forming hybrids, due to the abundance of pollen from Q. wislizeni. But maybe the hybrids work better with pollen from Q. agrifolia.
Some of the measured characteristics are given in Table 1, which also links to the iNat observation of each specimen. The oak called "hybrid" is the one Scott posted, even though it appears all these oaks are hybrids as well. The other oaks are numbered in the order encountered on the trail, with the "hybrid" being oak #2. Oak #6 is the only Q. agrifolia on the trail, which was initially determined in 2008 when it was alive, but found to be long dead on this trip.
Additional measurements are:
- All of the oaks had fewer than 10% cupped leaves except for the "hybrid", which had ~50% cupped leaves.
- None of the oaks had any axillary tufts of hairs on the underneath of the leaf.
- Several of the oaks had small acorns on some twigs, and fully-developed acorns on the same or other twigs, implying that their acorns take two years to develop, typical of Q. wislizeni.
In addition, Don and I, along with our hiking companion Beth Cobb, found that the leaves of these oaks were exceptionally prickly. I don't recall ever being stabbed so much by leaves of Q. wislizeni before.
Notes on some of the measurements:
- For the leaf width and length, I measured only a single leaf for each plant.
- For the lateral vein angle, I measured only the most prominent veins in the middle of one leaf, and I measured the angle at the base of the lateral veins, where they meet the midvein. The lateral veins typically sweep upward (but infrequently downward) as they progress to the edge of the leaf, so you will measure a higher (lower) value if you use the end of the vein.
- For the leaf colors and hairs, I looked at all the leaves in all my photographs of each plant.
Table 1. Some Measured Characteristics
| Specimen Name and iNat obs link | Width / length | lateral vein angle min | lateral vein angle max | average lateral vein angle | upper leaf color | lower leaf color | upper leaf hairy | lower leaf hairy |
|---|---|---|---|---|---|---|---|---|
| Wisliz 1 | 0.72 | 37 | 50 | 43.5 | Bright / yellow / green | Yellow-green | no to yes | no |
| "hybrid" | 0.50 | 37 | 48 | 42.5 | dark green | Yellow-green | no to yes | no to yes |
| Wisliz 3 | 0.44 | 35 | 40 | 37.5 | Bright / yellow / green to dark green | Yellow-green to green | no | no |
| Wisliz 4 | 0.45 | 40 | 45 | 42.5 | Bright / yellow / green to dark green | Yellow-green | no | no |
| Wisliz 5 | 0.59 | 25 | 35 | 30 | Light green to dark green | Yellow-green | no | no |
| Wisliz 7 | 0.56 | 20 | 25 | 22.5 | Light green to dark green | Yellow-green | no to yes | no |
| Wisliz 8 | 0.63 | 20 | 30 | 25 | gray to dark green | Yellow-green | yes | yes |
| Wisliz 9 | 0.60 | 5 | 35 | 20 | Light green to dark green | Yellow-green to green | no | no to yes |
Brophy and Parnell plotted the leaf width to length ratio vs. the average angle of the lateral veins, which showed a good separation of the species for pure stands of each species. They found that populations with suspected hybrids did not fall within the pure species delinations.
I find very similar results for the "Q. wislizeni" oaks on the PCT north of SR74. See Fig. 1.
Fig. 1. The leaf width to length ratio plotted vs. the average angle of the lateral veins for specimens in Table 1. The "hybrid" is plotted separately (the one with 50% cupped leaves), as are specimens with no hairs on their leaves, and specimens with hairs. The areas where Brophy and Parnell had delineated the values for the pure species are shown.
Note the very interesting results! Only three of the oaks fall within the Q. wislizeni area of the plot, and one of those is the putative "hybrid" with cupped leaves! Those three are in the edge of that area, very close to the Q. agrifolia side, which also indicates that these are probably not pure Q. wislizeni specimens.
Three out of four of the plants that have hairs on their leaves and yellow-green undersides, plot in the Q. agrifolia side of the diagram, with the fourth falling right at the dividing line between the two species.
This plot is very similar to Figs. 3 and 4 in Brophy and Parnell, of populations that were intermediate to the two species, with no individual exactly intermediate between the two species, and with the hybrid population showing a tendency for most of the intermediates to be in the direction of Q. agrifolia for these two characteristics.
Brophy and Parnell used seven characteristics to score their populations for the extent of their hybridization. That further analysis supported their conclusions that there were few F1 hybrids in their sample, and most hybrids had their parental characteristics combined more or less at random.
Having parental characteristics combined more or less at random also appears to be the case here, except that these plants, with just one exception, all have the typical yellow-green lower leaf color of Q. wislizeni. Since that is the primary characteristic used by botanists to determine a plant as Q. wislizeni, it is not surprising that all these plants had been called Q. wislizeni in the past.
Further work needed:
- measure populations of the pure species in the San Jacinto Mountain Area, to see if they fall into the same areas as the northern California populations analyzed by Brophy and Parnell.
- measure the other characteristics found to be useful in delineating the hybrids by Brophy and Parnell to see if they confirm the hybrid hypothesis for these plants.