Characteristics of Quercus wislizeni, Q. agrifolia, and hybrids
Identification of Quercus wislizeni, Q. agrifolia, and hybrids has been shown to be quite muddy, with many having disagreeing views on which traits are natural for which species. In this post I will go over some of the traits listed by the paper "HYBRIDIZATION BETWEEN QUERCUS AGRIFOLIA AND Q. WISLIZENII (FAGACEAE)".
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Ratio of leaf width to length:
"A good separation can be made between Q. agrifolia and Q. wislizenii populations by use of the ratio of leaf width to leaf length. This measure of leaf shape shows that leaves of Q. agrifolia have a strong tendency to be oval, with the width over half the length. By contrast, the leaves of Q. wislizenii have a tendency to be oblong, or two to three times longer than wide." -
Angle of lateral vein to midvein
" The angle between the lateral veins and the midvein and the number of lateral veins are distinctly different for standard Q. agrifolia and standard Q. wislizeni"
The paper used 42.1-65.7 degrees as the standard Q. wislizeni criteria and 26.5-39 as the standard Q. agrifolia criteria, with anything in between being intermediate. -
Convexity of the leaf, coloration of the abaxial side, presence of axillary hairs, and size of the vein islets
"Leaves of Q. agrifolia show a marked tendency to be convex and dull green on the abaxial surface. They have small vein islets when viewed through a hand lens or microscope, and have tufts of brownish, stellate trichomes in the axils of the major veins. On the other hand, leaves of Q. wislizenii exhibit a strong tendency to be plane, to be yellowish green on the abaxial surface, to have relatively large vein islets and to be glabrous."
The paper also states that "When all seven characters are considered simultaneously, clear differentiation is evident between Q, agrifolia and Q, wislizeni".
Hopefully this clears up a lot of the confusion that we've had over these species. I highly recommend reading this article as it is full of very interesting information.
Edit: The paper also mentions the difficulty of distinguishing between species in areas where they are both found, further providing evidence that genes flow readily between the species.
Brophy, W. B., & Parnell, D. R. (1974). HYBRIDIZATION BETWEEN QUERCUS AGRIFOLIA AND Q. WISLIZENII (FAGACEAE). Madroño, 22(6), 290–302. http://www.jstor.org/stable/41423936
Comments
This paper has an important graphic, figure 7, that maps both type species at their apex locations and prime axis and the area of the hybrid PCA space. The hybrid space is defined by the F1 hybrids and the intergraded population. The restricted area of the two type taxa is distinctive by the narrow range from their point location. Conceptually, this approach may be applied to other Quercus taxa to separate the hybrids from their parents and to different F1 individuals from the intergraded population.
Jstor links are some time restricted. An alternative link is Brophy & Parnell 1974 or
https://www.biodiversitylibrary.org/page/47865969
Interesting paper and new to me, thanks for sharing.
An important takeaway here is that these two species, even in pure assemblages, were overlapping when only considering a single characteristic. It was only by looking at multiple characteristics that they could be consistently separated.
The problem is compounded in the case of hybrids, where traits may congregate in this intermediate space. I have seen many people try to suggest hybridization based on singular intermediate traits, but my hypothesis is that in variable, closely related taxa such as within Subsection Agrifoliae, singular intermediate traits are fairly common even among uncrossed individuals. Therefore, it is only by examining multiple traits in aggregate, and finding that they consistently cluster in the intermediate space that we can confirm a hybrid individual.
I hope that made sense.
If you have not seen it, the key at the end of Hauser et al. was also very detailed and useful. It's easy to miss because it's after the citations: https://bsapubs.onlinelibrary.wiley.com/doi/epdf/10.3732/ajb.1700291
Notably, they also failed to consistently separate the three evergreen Agrifoliae species on the basis of the PCA from leaves alone. However, they were separable by leaf and nut characteristics in combination.
This looks interesting, and thank you.
A comment on PCA , it is an empirical statistical technique, as I understand it. As you mention, it does not necessarily separate the study features. The clearest separation, as I understand it, is a process of sorting the different mixes until a separation is found. This may be a result of my not understanding the mathematics.
Regardless figure 4 provides a satisfying tree with the Agrifoliae as a clade with two subclades, the Q. kelloggii and the Q. agrifolia group. Figure 7 provides a time line of decent. As I read it Q. kelloggii has a 1 MYA with the Agrifoliae a 15 MYA divergence time. This and the hybridization pattern suggest a single regional intergradation process for the larger 20 MYA clade.
That is my quick read. My take away is that "Fossils attributed to Q. kelloggii are earliest fossils for the Agrifoliae...". This supports the notion for including all of the California Red-oaks in a single taxon. Always, different and thorough reading should be entered into the discussion.
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