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The woodland on the property is mixed evergreen forest, composed of a variety of native California tree species, including Coast Redwood (Sequoia sempervirens), California Bay Laurel (Umbellularia californica), Pacific Madrone (Arbutus menziesii), Douglas Fir (Pseudotsuga menziesii), and various species of oak (Quercus sp.).

Our evergreen woodland areas are made up of a number of native tree species, including oaks (click any image below to enlarge)

Of these trees, the most confounding, and confusing, to identify are some of our oaks.

Some of our oak species are easier to identify than others

The challenge with identifying which oak species inhabit our woodlands is that oaks in this region are highly adaptive to the environment they are growing in, and some species, within a particular oak lineage, may hybridize with each other.  This can result in significant variability in growth habit, and leaf morphology, making species identification difficult.

Leaves of some oaks may vary considerably, on the same branch, especially at the leaf margin

It isn’t difficult just for the gardener, as there is significant discussion among ecologists, and botanists, as to the identity of some of California’s oak species. Some of the oaks growing here are easily identified, such as our Canyon Live Oaks (Quercus chrysolepis), but the identity of the other Live Oak species is proving to be more challenging.

We had presumed until now, not looking very closely at our oaks, that Coast Live Oaks (Quercus agrifolia) likely predominated here.  However, the Interior Live Oak (Quercus wizlizeni), and Coast Live Oak look very similar at first glance.  Both belong to the Red Oak subgenus, Erythrobalanus, and their leaf margins typically have bristle-like teeth, although the margins may be smooth.  However, an important distinguishing feature between the two is that the leaves of the Interior Live Oak are flat, not convex like their Coast Live Oak cousins.

Live Oaks are evergreen, and have thick, dark green leaves

Underneath the leaves, the Coast Live Oak also has fine tufts of hairs where the lateral veins meet the central midline vein, whereas the ventral leaf surface of the Interior Live Oak is glabrous (hairless).

The ventral surface of this leaf lacks the axillary hairs characteristic of Coast Live Oaks (Q. agrifolia)

The leaves from this particular tree are clearly hairless, ruling out a Coast Live Oak, so  that should narrow the species to Interior Live Oak…or does it?

To complicate matters further there has been some significant discussion in the literature in recent years regarding a mainland form of the Santa Cruz Island Oak (Quercus parvula). Quercus parvula has a limited range, found in coastal Santa Barbara county, and of course on Santa Cruz Island.  The mainland relative of the Santa Cruz Island Oak is known as the Shreve Oak (Quercus parvula var. shrevei), and has a much wider distribution along California’s coast.

The acorn cups of Interior, Shreve, and Coast live oaks have overlapping scales, resembling roof shingles

This Shreve Oak subspecies reportedly grows from San Francisco, south through the Central Coast ranges, and is especially concentrated in the Santa Cruz, and Santa Lucia mountain ranges.  However, the key to California Oaks I have in front of me suggests that the Shreve Oak should have convex leaves, more similar to the Coast Live Oak, not flat leaves like the Interior Live Oak [1].

The leaves of the trees in question are flat, not convex

As a result of that key, I thought I could rule out the Shreve Oak here, but in 2002 Dodd et al. stated that “central coast populations [of Interior Live Oak] at low elevations, typically occupying redwood forest should be treated as Shreve oak“[2].  Taxonomists were advocating the split of the Shreve Oak from populations of oaks that were once believed to be Interior Live Oaks, while admitting that differentiation between the two species based on morphological features alone may be virtually impossible.

For many species, the characteristics of the acorns and their cups may be diagnostic, but not always

A more recently published text describes Shreve Oaks as single trunked trees, with leaves similar to the Interior Live Oak, but generally longer, up to 5 inches in length[3].  Here these oaks are indeed single trunked, generally taller than they are wide, but the leaf sizes tend to be smaller, in the 2-3 inch range, certainly nowhere near 5 inches in length.

Our mystery oaks are typically taller than wide

As Live Oak leaf morphology is variable though, depending on local growing conditions, a species identification based on leaf length alone would be inadequate.

For any Live Oak species, leaf length may be affected by local growing conditions

Our attempts to conclusively identify this oak species has caused more than a few headaches, so for now it seems that we’ll simply have to accept the absolute identity of these Interior Live Oak/Shreve Oaks on the property, without a more expert analysis, will remain a mystery.

The canopy of these oaks is low to the ground, and provides welcome shade on hot summer afternoons

It really seems quite remarkable that something as large as a 50 foot oak tree could be as difficult to identify as some of the species of fungi we find growing here in the winter.

Regardless of species, all of our oaks provide important habitat for a range of plant and animal species

Whether these oaks are officially Interior Live Oaks, or Shreve Oaks, is really of little consequence to the wildlife species that depend on our oak woodland habitat.  Arboreal Salamanders use the cavities in our oaks to lay their eggs. The Coast Range Fence Lizards, Skilton Skinks, Alligator Lizards, and Ringneck snakes forage, protected, among the oak leaf litter.  Pocket Gophers, Western Gray Squirrels, and Acorn Woodpeckers rely heavily on the acorns produced in the fall as stores for winter.

The mature acorn as has already fallen from this cup

With the exception of the Coast Live Oak, acorns from oaks in the Red oak subgenus take two full years to mature. As a result, it’s not uncommon to see maturing acorns, and tiny developing acorns on the same branch of an Interior Live Oak, or Shreve Oak.

It's common to find acorns in various stages of development on Live Oaks, except for the Coast Live Oak whose acorns develop within a single season

Walking around the property the past few weeks there is no question where the acorn-laden oaks are, as our resident Acorn Woodpeckers (Melanerpes formicivorus) this fall are relishing the tremendous abundance of acorns being produced this year.

The abundance of acorns this year will be beneficial to the wildlife species that depend on them for food

Each morning we can hear the woodpeckers calling to each other as they busily forage from tree to tree, collecting the acorns to store in their granary trees.

Acorn Woodpecker (Melanerpes formicivorus) - (Source: Wikimedia Commons. Author: Marlin Harms)

These woodpeckers will rely on these acorn reserves during inclement winter weather, and during the spring nesting season.

Remnant of an old granary tree here at Curbstone Valley, a communal pantry, used by Acorn Woodpeckers. Up to 50,000 acorns may be stored in any one granary.

These oaks also support other life, whether literally, as a vining support for our native honeysuckle, or by providing the perfect soil conditions for our golden chanterelles.

California Honeysuckle growing through an oak branch

Our mystery oaks however, aren’t the only oaks in our woodlands.  We also have Scrub Oaks, and Canyon Live Oaks growing here, along with some false oak species, including the Tanbark Oak, and of course, our dreaded poison oak, which is just starting to turn a vivid shade of red for the fall.

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[1] Pavlik, Bruce M., Muick, Pamela C., Johnson, Sharon G., and Popper, Marjorie.  1991.  Oaks of California.  Cachuma Press.

[2]  Dodd, Richard S.; Kashani, Nasser; Afzal-Rafii, Zara  2002.  Population diversity and evidence of introgression among the black oaks of California.   In: Standiford, Richard B., et al, tech. editor. Proceedings of the Fifth Symposium on Oak Woodlands: Oaks in California’s Challenging Landscape. Gen. Tech. Rep. PSW-GTR-184, Albany, CA: Pacific Southwest Research Station, Forest Service, U.S. Department of Agriculture: 775-785.

[3] Costello, L.R., Hagen, B.W., and Jones, K.S.  2011.  Oaks in the Urban Landscape: Selection, Care, and Preservation.  University of California Agriculture and Natural Resources.