leytonjfreid's Journal

Sometimes Quercus hybrids can be quite difficult to determine, but doing a detailed analysis can prove the parentage of an individual or reveal new information. Thankfully the only tools you'll need for this are a measurement device using millimeters and a macro lens (although a microscope makes this much easier). I recommend checking what measurements are actually useful for what you're IDing, as most of the time you won't need most of these factors. I recommend comparing this information to the pages on Flora of North America or similar regional databases.

Anyways, here's my list:

General:
Deciduous or evergreen
Number of trunks
Height
Trunk diameter (in inches) at 4.5'. This is called the Diameter Breast Height or DBH, not often used

Bark:
Color
Texture

Twigs:
Color
Diameter (mm)
Pubescence

Buds:
Color
Shape
Diameter (mm)
Pubescence
Length

Leaves:
Petiole color
Petiole length (mm)
Leaf shape
Leaf length and width (mm)
Base shape
Margin shape
Amount of lobes
Sinus depths (mm)
Secondary veins per side (often different from the number of lobes)
Apex shape
Lobe apex shape
Abaxial color
Abaxial pubescence (hair density, type, and number of rays)
Adaxial color
Adaxial pubescence (hair density, type, and number of rays)
Thickness

Acorns:
Sessile/pedunculate
Number of acorns per group
Cup shape
Cup depth (mm)
Cup width (mm)
Cup pubescence
How much of the nut the cap encases
Scale color
Scale shape
Scale pattern
Nut wall thickness
Nut shape
Nut length and diameter (mm)
Nut pubescence
Nut color
Apex shape
Scar diameter

I hope you found this useful and if you know of any other qualities useful for ID please comment them.

General:

1. An Updated Infrageneric Classification of the North American Oaks (Quercus Subgenus Quercus) 2021

Sect. Lobatae:

1. The evolution and diversification of the red oaks of the California Floristic Province (Quercus section Lobatae, series Agrifoliae) 2017
2. HYBRIDIZATION BETWEEN QUERCUS AGRIFOLIA AND Q. WISLIZENII (FAGACEAE) 1974
3. SELECTION AND DISPERSAL IN A MULTISPECIES OAK HYBRID ZONE 2003
4. Identifying Evergreen Hybrids in the California Red Oaks 2020

Sect. Quercus:

1. Phylogeny and Introgression of California Scrub White Oaks (Quercus section Quercus) 2016

A great paper on the taxonomy of sect. Lobatae and sect. Quercus is "An Updated Infrageneric Classification of the North American Oaks (Quercus Subgenus Quercus): Review of the Contribution of Phylogenomic Data to Biogeography and Species Diversity". It provides good descriptions and evidence for multiple clades that help simplify the taxonomy of Quercus species. Here I'll outline the basic taxonomic structure the paper provides

Sect. Lobatae

Subsect. Agrifoliae: Q. agrifolia, Q. kelloggii, Q. parvula, Q. wislizeni
"Subevergreen complex" (Q. agrifolia complex): Q. agrifolia, Q. parvula, Q. wislizeni

Subsect. Palustres: Q. palustris, Q. texana

Subsect. Coccineae: Q. acerifolia, Q. buckleyi, Q. coccinea, Q. ellipsoidalis, Q. rubra, Q. shumardii, Q.
velutina
Q. shumardii complex: Q. acerifolia, Q. buckleyi, Q. shumardii, Q. rubra
Q. coccinea complex*: Q. velutina, Q. coccinea, Q. ellipsoidalis

Subsect. Phellos: Q. arkansana, Q. elliottii, Q. falcata, Q. georgiana, Q. hemispherica, Q. ilicifolia, Q.
imbricaria, Q. incana Q. inopina, Q. laevis, Q. laurifolia, Q. marilandica, Q. myrtifolia, Q. nigra, Q.
pagoda, Q. phellos
Q. incana complex*: Q. hemisphaerica, Q. incana, Q. myrtifolia, Q. inopina
Q. ilicifolia complex*: Q. ilicifolia Q. georgiana Q. imbricaria
Q. phellos complex*: Q. pumila, Q. pagoda, Q. phellos, Q. laurifolia, Q. nigra, Q. arkansana
Q. marilandica complex*: Q. laevis, Q. marilandica, Q. falcata

Sect. Quercus

Subsect. Dumosae: Q. berberidifolia, Q. cornelius-mulleri, Q. douglasii, Q. dumosa, Q. durata, Q.
gambelii, Q. garryana, Q. john-tuckeri, Q. lobata, Q. pacifica
Grade (not monophyletic): Q. gambelii, Q. garryana, Q. lobata
Q. berberidifolia—Q. durata complex (Q. berberidifolia complex for INat): Q. berberidifolia, Q. durata
Q. dumosa complex*: Q. pacifica, Q. dumosa, Q. cornelius-mulleri, Q. john-tuckeri, Q. douglasii
Subclade: Q. cornelius-mulleri, Q. john-tuckeri, Q. douglasii (would not be included in INat taxonomy)

Subsect. Prinoideae: Q. bicolor, Q. lyrata, Q. macrocarpa, Q. muehlenbergii, Q. prinoides
Q. prinoides complex*: Q. muehlenbergii, Q. prinoides
Q. lyrata complex*: Q. bicolor, Q. lyrata, Q. macrocarpa

Subsect. Albae: Q. alba, Q. michauxii, Q. montana

Subsect. Stellatae: Q. austrina, Q. boyntonii, Q. chapmanii, Q. havardii, Q. margettae, Q. oglethorpensis,
Q. similis, Q. sinuata, Q. stellata
Q. austrina complex* (which species was named first is unclear) : Q. austrina, Q. margarettae
Q. chapmanii complex*: Q. chapmanii, Q. havardii
Q. stellata complex*: Q. stellata, Q. similis, Q. boyntonii

Subsect. Polymorphae: Q. hinckleyi, Q. laceyi, Q. mohriana, Q. polymorpha, Q. pungens, Q. vaseyana
Q. vaseyana complex*: Q. hinckleyi, Q. pungens, Q. vaseyana

*Not specifically named in the paper but given a name based off of the most senior specific epithet in the group

Manos PS, Hipp AL. An Updated Infrageneric Classification of the North American Oaks (Quercus Subgenus Quercus): Review of the Contribution of Phylogenomic Data to Biogeography and Species Diversity. Forests. 2021; 12(6):786. https://doi.org/10.3390/f12060786

While looking through papers for my work on clarifying the taxonomy of California's live oaks in sect. Lobatae, I came across the paper "The evolution and diversification of the red oaks of the California Floristic Province (Quercus section Lobatae, series Agrifoliae)". This paper provided some great incite into the taxonomy along with providing taxa that have basis for being added or changed.

1. Series Agrifoliae & a Quercus agrifolia complex
   "We find that the Agrifoliae are monophyletic and sister to all other red oak species. Within the Agrifoliae, all species are supported, with Quercus kelloggii sister to a clade of subevergreen taxa: (Quercus agrifolia – (Q. parvula + Q. wislizeni))."
   This paper provides definite support for series Agrifoliae (Which would be renamed to subsection Agrifoliae under INat's taxonomy) and proves monophyly of a Q. agrifolia complex. Subsection Agrifoliae would contain Q. kelloggii, Q. x morehus, Q. x ganderi, Q. agrifolia, Q. wislizeni, Q. parvula, and Q. agrifolia x wislizeni. If a Q. agrifolia complex were erected it would contain the latter four taxa as well as any other hybrids between them added to INat in the future. The complex would be very beneficial as telling between the three species and their hybrids can be very tricky, especially in observations with limited data. Even subsection Agrifoliae is useful as telling the difference between Q. x morehus and Q. x ganderi can be quite difficult at times.

2. Q. parvula var. tamalpaisensis being raised to Q. x tamalpaisensis
   " Notably, Q. parvula var. tamalpaisensis appears to be part of a hybrid swarm between Q. parvula and Q. wislizeni."
   The paper shows evidence for Q. parvula var. tamalpaisensis being raised to a nothotaxon. While no name was assigned in the paper, current naming convention would make it Q. x tamalpaisensis.

Hopefully this leads to an agreement for subsection Agrifoliae and a Q. agrifolia complex to be added to simplify taxonomy and clean up data. I don't have enough experience with Q. parvula to give an opinion on that subject, but I thought it interesting to bring up.
Note: the paper does not mention a Q. agrifolia complex, but the data provides support for it

Hauser, D.A., Keuter, A., McVay, J.D., Hipp, A.L. and Manos, P.S. (2017), The evolution and diversification of the red oaks of the California Floristic Province (Quercus section Lobatae, series Agrifoliae). American Journal of Botany, 104: 1581-1595. https://doi.org/10.3732/ajb.1700291

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)".

1. 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."

2. 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.

3. 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