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