Plants of Southern California: Quercus agrifolia, Q. wislizeni, and hybrids between them

Introduction

Quercus agrifolia, coast live oak (henceforth "clo"), and Q. wislizeni, interior live oak (henceforth "wisliz"), are closely-related oaks that can occasionally form hybrids. I have recently come across several specimens that appear to be Q. wislizeni in most respects, but have leaves cupped like those of Q. agrifolia, and with hairs on their leaves, which wisliz is not supposed to have. The question is: how do I definitively determine that these are hybrids, and not just variants within wisliz?

I was very pleased to find a paper on hybrids between these two species by Brophy and Parnell from 1974 which gave seven characteristics that reliably distinguished the two species. They used five of those seven characteristics to score individual specimens from areas that seemed to contain possible hybrids, and found that these populations indeed had intermediate scores.

However, when I tried to separate our species in southern California using two of those characteristics, the {leaf width / leaf length} and the {average angle of lateral vein to midvein}, there wasn't good separation between our species; see Quercus agrifolia, Q. wislizeni, and hybrids between them using the Brophy and Parnell Analysis.

This page looks at the other five characteristics given by Brophy and Parnell to distinguish the species, with the goal of using them to try to identify hybrids.

Characteristics that separate Q. agrifolia and Q. wislizeni

Table 1 summarizes some characteristics that separate Q. agrifolia and Q. wislizeni. Those characteristics are individually discussed below.

Table 1. Some characteristics that separate Q. agrifolia and Q. wislizeni

Characteristic	Q. agrifolia	Q. wislizeni
Total number of leaf side veins on both sides of main vein	6 to 12	14 to 20
Leaf convexity	edges gen cupped down	edges gen flat
Color of underside of leaf	dull green (yellow-green)	shiny yellow-green
Axillary tufts of hairs in underside of leaves	present	absent
Relative size of vein islets in underside of leaves	small	large
Acorn development	Mature in year 1	Mature in year 2
Habit	tree	gen shrub

Number of Secondary Leaf Veins

Counting the number of secondary leaf veins is not unambiguous. Which side of the leaf do you use to count the veins? If you use the underneath of the leaf, do you count only the more-prominent raised side veins? Or do you include less-prominent side veins? What about the very minor side veins that don't go all the way to the edge of the leaf?

Despite that ambiguity, after you count the veins on a number of different leaves, you'll develop some consistency in how you count veins. And fortunately there is usually a very clear difference in the number of veins for the two species.

I count the total number of side veins on each side of the main vein for a leaf. That helps a bit with ambiguous vein counting, too.

Brophy and Parnell only counted the number of veins on one side of the leaf, and reported clo as having 4 to 6 side veins, and wisliz having 7 to 10 side veins. The total vein count would then be 8 to 12 for clo, and 14 to 20 for wisliz.

My counts for southern California so far, which have small sample sizes, are 6 to 10 for clo from the Hemet area (13 specimens), and 14 to 20 for wisliz (19 specimens). This is a bit larger than reported by Brophy and Parnell for northern California plants of clo, and exactly the same as reported by them for wisliz from northern California.

Leaf convexity

The leaves of clo are almost always cup-shaped. However, in heavy shade the leaves can be planar. The leaves of wisliz are almost always not cupped, but some specimens are reported to have cupped leaves.

Color of underside of leaf

The quickest discriminant in the field for wisliz is the shiny yellow-green color of the bottom of its leaf, unobscured by any hairs.

Rest of this page not yet written

Go to:

• Native and Introduced Plants of Southern California