Oaks of San Diego County II
Fred M. Roberts, CNPS-SD Rare Plant Botanist
Part 2: White Oaks, Laying Out the Groundwork
In Oaks of San Diego County, part 1, I covered oaks in general and in more detail, the red and golden oaks. The white oaks are a bit more challenging. I thought I might add more history into the mix. If you step back in time early in the last century, say 1927, the choice of white oaks in San Diego County and books to identify them were a good deal more limited than they are today. As a botany enthusiast, you might have a copy of Davidson and Moxley’s Flora of Southern California (1923) and Willis Jepson’s Manual of Flowering Plants of California (1924) on hand. Botany is a whole other experience at that time. Unless you physically visited one of California’s herbariums, you might only have books, their keys, and short description s in hand to sort out your oaks. It was all about interpretation, gestalt, and word of mouth. There was no internet, no Google to search, no California Consortium of Herbaria, no easily accessible photo libraries to access, and no iNaturalist where people more knowledgeable about oaks could weigh in on identifications. Only two species of white oak would be available to users of these books in San Diego County:
Quercus dumosa (scrub oak) with three varieties that were not universally recognized (Q.d. var. dumosa , Q.d. var. elegantula , & Q.d. var. turbinella), and
Q. engelmannii (Engelmann’s or Mesa oak)
In two of the next handy botany guides to come along, Phillip Munz’s 1935 Manual of Southern California Botany and Howard McMinn’s 1939 Illustrated Manual of California Shrubs , there is little change except that McMinn considered none of the Q. dumosa varieties worth recognition and Munz transferred Q.d. var. elegantula to Engelmann’s oak, treating it as a synonym with the name Q. McDonaldii var. elegantula . Clearly there was a bit of uncertainty as to what to do with this last oak. Regardless, the name elegantula effectively disappears from botanical guides a short time after. Munz dominated California botany in the latter half of the century with his A Flora of California (1959) and A Flora of Southern California (1974). With these books we still only have these two oaks to choose from here in San Diego County. So how did we get from there to here as our choices of San Diego white oaks:
Q. berberidifolia – CALIFORNIA SCRUB OAK
Q. cornelius - mullerii – MULLER’S OAK
Q. dumosa – NUTTALL’ S SCRUB OAK
Q. engelmanii – ENGELMAN N ’S OAK
Q. x acutidens – TORREY’S OAK
It is hard to believe that virtually all scrub oaks in San Diego within the white oak group were lumped under a single species. On the other hand, it certainly made it a lot easier to identify scrub oaks. It was recognized that scrub oak (as frequently ref erred to at the time) was quite variable and at least some of that variability was attributed to hybridization between this oak and Engelmann’s oak. Jepson said of it in 1923 “highly variable in leaf texture and outline and in acorn character, both of cup and nut”.
Let’s tackle Engelmann’s oak first, being the only tree in the group (at least most of the time). LeRoy Abrams, in his 1923 Illustrated Guide to Plants of the Pacific States , was the first major flora to use the name “Engelmann’s oak”. Otherwise, it was generally known as “mesa oak” until the mid - 1900s.
In San Diego County, Engelmann’s oak is an interior oak, found away from the immediate coast but generally avoiding the deserts, except at the base of the Banner Grade. It is found in oak woodlands, oak - grassland savannahs, and scattered within chaparral and coastal sage scrub communities. It is also found outside of San Diego County, most notably on the Santa Rosa Plateau of Riverside County, scattered about the interior valleys of Orange and Riverside Counties , and extending north to the foothills of the San Gabriel Mountains, where it was once moderately common in the vicinity of Pasadena and Monrovia.
Engelmann’s oak is generally a tree with semi - deciduous oblong leaves, at a relatively long 20 - 60 mm (0.75 - 2.5 inches). The leaf margins are mostly entire (smooth) or with a few coarse teeth, and a rounded tip. Distinctively, the leaves, at least in the right light, have a bluish - gray green cast. The acorn is relatively small, barrel shaped, an d the cup seems overly small. Like all white oaks, Engelmann’s oak has warty acorn cups. In its pure form, it is easily recognized but hybrids abound and make the line between Engelmann’s oak and the shrub species a bit fuzzy. One of the unique characters of Engelmann’s oak, which will have bearing later in our story, is that the cotyledons, the first pair of leaves sent out by seedling Eudicots, are fused. In other Southern California white oaks, the cotyledons are free. The oak had an interesting start when it was first discovered in San Diego County. Perhaps the first white oak collected in San Diego, there is a delightfully recounting of it in a short essay by James Lightner titled “Parry’s Forgotten Discoveries, 1849 - 1851” (available at http://www.sandiegoflora.com/wp-content/uploads/2014/11/Parrys-Forgotten-Discoveries-1849-51-1.pdf). In June 1850, Charles Parry, traveling by mule, obtained a collection between Ramona and Santa Ysabel. Parry sent the specimen to world renowned botanist, John Torrey at New York. Torrey determined the oak as Quercus oblongifolia, later to be known as Sonoran blue oak. Sonoran blue oak is found in southeastern Arizona and the Sierra Madre of western Mexico. Torrey believed that the oak Parry found was likely an extension of this species into the San Diego region. This same oak was found to occur in the vicinity of San Gabriel near Los Angeles in the 1860s and then again collected in San Diego County without a specific locality by Henry Nicholas Bolander and Alexander Kellogg in April 1874. By the 1890 s a dozen collections had been made near San Diego by collectors including Parry, Samuel B. Parish, George Vasey, Charles Orcutt, and Edward Lee Greene over a wide area of the backcountry.
Not everyone agreed that the blue - gray leaved southern California oaks were Q. oblongifolia. In 1889, Edward Lee Greene proposed the name Q. engelmannii for the oaks of California in honor of George Engelmann. The two oaks are very similar, the typical hiker might not be able to separate them side by side. The main difference lies in the size of the fruit and the form of the tubercles on the acorn cup with Engelmann’s oak being larger. Since that time , Engelmann’s oak has become a species of conservation concern, first being included in the 4th edition of the CNPS Inventory of Rare and Endangered Plants (1988) as a List 4 plant, and later as a California Rare Plant Rank (CRPR) of 4.3 plant. The primary threat has been from loss of habitat, especially in the foothills of the San Gabriel Mountains.
Let us turn our attention to the scrub oaks. The white scrub oaks have always been a little problematic. Most field botanists in the last century were content to put most of them in the Quercus dumosa bin, largely because the plants were not closely examined. Others, more observant, knew the situation was more complex but not certain what to do about it. On one of his many webpages, Tom Chester included this quote from Francis M. Fultz, from The Elfin Forest of California (1923):
“ Quercus dumosa ...is not only the...most widely distributed, and most plentiful wherever it is found, but is also the one which is most frequently shrub - like form in appearance. The dumosa varies a great deal in size, and also in shape of leaf and acorn – in fact, so much so that botanists have tried to make several species out of it, but not with very much success.”
I think the first species to tackle would be Nuttall’s scrub oak. This is the most distinctive of our white scrub oaks even though he name has resulted in some confusion. A few decades ago, when it emerged that the name Q. dumosa, after nearly a century of being applied to nearly all the white scrub oaks in California, should only be applied to a narrow subset of them, a good deal of confusion followed. This was further complicated by conservationists insisting the name Q. dumosa also belonged to a rare oak (Q . dumosa has a CRPR 1B status). Nuttall’s scrub oak is mostly typically found in coastal chaparrals and occasionally in coastal sage scrub or woodland, often on sandy soils within a few kilometers of the coast, often in widely disjunct populations, from Santa Barbara County south into northwestern Baja California, Mexico. In San Diego County, where the maritime influence spreads inland over a broader coastal plain, the distribution flares inland reaching as much as 7 kilometers (4.5 miles) on Otay Mesa.
Nuttall’s scrub oak is readily separated from other scrub oaks in San Diego County by generally having a rounded, sometimes low, often broader then tall, habit, with dense, often tangled reddish branches that are angled. The dark green leaves, which are generally less than 20 mm (0.75 inch) long are frequently round to oblong in outline, often wavy and with distinctly toothed margins, though these are sometimes smooth. Whereas the trichomes (hairs) on the leaf of most white oaks are minute and require a dissecting scope to see clearly, on Nuttall’s scrub oak you can see them with your eye, assuming your closeup vision is not failing as it is in my aging eyes. A simple trick, turn the oak leaf sideways and look along the central vein. If you can see hairs without a hand lens, and the leave s are dark green and somewhat rounded, you are almost certainly holding Nuttall’s scrub oak. There are also more trichomes on the upper leaf surface than we typically see on other shrub species. Nuttall’s scrub oak is the only California white oak that retains the juvenile character of scaled acorn cups more typical of red oaks. We don’t see this much in San Diego County, but it is a common feature on Nuttall’s oak in other parts of its range. This is likely because north of San Diego County, Nuttall’s scrub oak flower s earlier than other species and have less opportunity to hybridize. Most of our scrub oaks are straight forward evergreen shrubs. Nuttall’s scrub oak is not so clear cut. It appears to be semi - deciduous. In years with unfavorable conditions, cold or dry, most individuals can lose all or most of their leaves. There is an old individual on the Dana Point Headlands in Orange County. It loses all its leaves most winters and leaves behind an incredibly tangled mass of bare, angled branches, dense enough, you could just lay on top of the plant. At many sites, a dense layer of leaf litter forms below Nuttall’s scrub oak. A clever way to know you have one is to put a branch in a plant press. Many of the leaves will have dropped off the branch by the next time you open the press. Neither California nor Torrey’s scrub oak does this. Nuttall’s scrub oak is the species that got me interested in oaks. I wrote a book in 1995, An Illustrated Guide to the Oaks of the Southern Californian Floristic Province, largely to address all the questions I got regarding Nuttall’s scrub oak (I slipped a few other oaks in the book as well, it turned out that most of state’s oak s fell within its boundaries). Until I met Nuttall’s scrub oak, I pretty much called all oaks “Quercus oakus”. I didn’t really have much interest in the group. That all changed one day while attending a lecture given by Dr. Kevin Nixon and Dr. Kelly Steele as guest speakers circa 1981 at U.C. Santa Barbara where I was studying geography and botany.
Nixon and Steele had come to talk largely about Muller’s oak, which they described and published in a paper in 1981. At the end of their presentation, they mentioned that they were working on another possible species, thus far apparently endemic to Torrey Pines State Park. A photograph of the oak caught my attention. I had seen this oak but it wasn’t at Torrey Pines, it was in southern Orange County. I already knew Kelly as a TA for a botany field course and she introduced me to Kevin Nixon. They had an informal name for the oak “Moran’s oak” and they were likely to name it Q. moranii after the great botanical explorer and lover of dudleyas, Reid Moran. There was still work to be done before they published the name, including being certain that no previous botanist had already provided a name for it , and getting a better handle on its name.
Within a few months, I had found the oak at several places in Orange County. At the time I was more of a conservationist than a botanist and I learned how challenging it can be to protect an un - named species. Of the sites I was aware of in Orange County, most were at risk, and some were lost within a few years of discovery. Eventually we would find it at 8 locations in Orange County. In the early 1980s though, it was tough times for this oak in Orange County. No one was really looking at it in San Diego County. Mitch Beauchamp published his Flora of San Diego County in 1986 and while it included Muller’s oak, there was no evidence he was aware of “Moran’s oak” (Beauchamp also introduced the name Q. grandentata into the San Diego County botanical awareness as a hybrid of Engelmann’s oak and scrub oak but the type specimen was from Pasadena and it was actually a hybrid between San Gabriel Mountains leather oak and Engelmann’s oak so it was never a San Diego County form after all).
Circa 1984, a chance find in the foothills of the Santa Ynez Mountains in association with a potential land donation to The Nature Conservancy changed the trajectory of Moran’s oak. One of the highlights for the property was an interesting oak. Kevin Nixon was asked to look at it. Lo and behold, it was “Moran’s oak”.
In Richard Henry Dana’s book, Two Years Before the Mast , published in 1840, Dana describes his life as a sailor and fur trader on the merchant ships Pilgrim and Alert . In the mid - 1830s the Pilgrim made a few stops along the California coast, including at the Mission San Diego de Alcalá and Mission Santa Barbara. Dana, a former student at Harvard, describes his unlikely encounter with a former professor “strolling about San Beach, in a sailor’s pea - jacket, with a wide straw hat, and barefooted”. The professor was Thomas Nuttall, who would be returning to Boston on the same ship as Dana after collecting plants and animals up and down the California coast.
Among the boxes of specimens that must have been loaded on the Alert at San Diego were specimens of scrub oak collected at Santa Barbara. In 1842, Nuttall would name this Quercus dumosa. Few botanists ever saw the type specimen and the name would be ubiquitously applied to white scrub oaks across California, including those found in San Diego County. We’ll get back to that specimen later.
If Nuttall had collected his type specimen in San Diego, did he get Q. dumosa or did he get Moran’s oak? It took Kevin and Kelly some time to track down Nuttall’s original specimen (pre online database, it was a lot more challenging to track down specimens!). There was even a fear that the only specimen could be at Berlin and that it had been destroyed by bombing in World War II. Fortunately, this was not the case, and they tracked down the type. It was indeed the oak they had been calling Moran’s oak. The new oak, as it turned out, was the old oak. For 150 years, botanists had attributed the wrong name to most of the scrub oaks in California. The next name in the queue was Quercus berberidifolia.
California scrub oak’s name, Q. berberidifolia , was coined in 1854 by Friedrich Liebermann, a Dutch botanist mostly known for extensive collections in Mexico. The type for the specimen was obtained by Thomas Coulter but Coulter’s original field notes were lost at sea and we will never know for certain where the specimen was obtained. I will address that further in Part 3.
The exact distribution of California scrub oak in San Diego County is a little uncertain. Depending on whom you talk to it is found largely along the coast and western foothills, all the way to the desert edge, or not at all. It is common in chaparral and scattered in coastal sage scrub and oak woodlands. The oak is a shrub, sometimes arborescent. Generally, its habit is more erect with open branching rather than low, broad, and densely branched, separating it from Nuttall’s scrub oak, and the branches tend to be curved or arched vs. angled. The leaves are oblong to elliptic, or somewhat rounded, mostly less than 30 mm (1.25 inches) long, with smooth or toothed margins. The upper leaf surface is mostly glabrous (smooth), green and shiny. The lower leaf surface is paler with scattered minute hairs that require magnification to see. The trichomes generally have 6 - 8 rays. The acorn cup is tuberculated, and the nut is narrow or broad. The investigation of odd scrub oaks at Torrey Pines eventually lead to a completely new understanding of Quercus dumosa.
Before that, the oaks on the desert edge in eastern San Diego County were getting most of the attention. These oaks today are known as Muller’s oak, named after Cornelius H. Muller, who founded the UC Santa Barbara Herbarium in the 1950s and was curator there from 1956 - 1964. He was one of the world’s foremost oak experts. When I met him in 1981, it was said that he could identify any oak species in the world with a single leaf. Muller’s oak is largely a mountain oak found in interior chaparral along the desert edge of the eastern Transverse Ranges (San Bernardino and Little San Bernardino Mountains) and Peninsular Ranges of Riverside and San Diego Counties south to the Sierra Juarez Mountains of Baja California, Mexico . The overall foliage has a yellowish gray - green cast due to a very high density of minute, star - shaped hairs (trichomes). The leaves are leathery, about 25 - 30 mm (1 - 1.25 inches) long, usually longer then broad, elliptical in outline, tapered to the tip and with margins sparsely toothed. Both the upper and lower leaf surfaces are rich with trichomes. Upon the lower surface, the density is so thick the actual leaf surface is obscured. The trichome form a flattened rotate cluster, typically with about 12 rays (arms) but as many as 16. These are often fused at the base (this is a difficult character to see without an electron microscope). The acorn is not especially distinctive, though more often you see a more chocolate brown acorn than on other San Diego County oak species.
It had been recognized for some time that the scrub oaks of eastern San Diego County were a bit odd compared to those on the coastal slope. Of the oaks near Campo and Jacumba, Engelmann thought these simply unusual Q. dumosa. Our friend, Edward Greene had described a new oak from the mountains of northern Baja California in 1889 as Q. turbinella (shrub live oak). Greene thought the oaks in eastern San Diego County were aligned with his new species. In 1909, in his three - volume Flora of California , Willis Jepson more or less sided with Engelmann but introduced a compromise and called these oaks Q. dumosa var. turbinella. The relationship of Q. dumosa to Q. turbinella remained a matter of debate for some years.
Another “student of the oak ”, John M. Tucker, a professor at U.C. Davis much of his life, jumped into the ring. His PhD dissertation was the basis for his 1953 paper on interrelationships in the Quercus dumosa complex. The paper was largely devoted to circumstances north of our area, notably, the status of Q. turbinella and Q. turbinella subsp. californica (this latter now known as Q. john - tuckeri) but we get an honorable mention. In Tucker’s opinion, Engelmann was closer to the mark than Greene and the oaks in eastern San Diego County were not relatives of shrub live oak at all, rather something entirely different but still closely related to Q. dumosa. Nixon and Steele were following Tucker’s 1953 lead when they published their paper giving the Muller’s oak the Latin name we know it by today.
The final name in this puzzle is Q. x acutidens (Torrey’s oak). Most of us first heard the name with the publication of the Jepson Manual in 1993. The name (as a full species) is about as old as Q . berberidifolia , being published by John Torrey in 1859 based on a specimen from San Luis Rey near Oceanside, Fallbrook, or Bonsall. Edward Greene stepped in again and was unconvinced that Q. acutidens was sufficiently distinct from Q. dumosa (in the historic sense, this would be Q. berberidifolia ). By 1900, no author that I am aware of was recognizing Q. acutidens and what would become Torrey’s oaks. was dropped into the minor variant of California scrub oak. The name was largely forgotten for over a century.
Not to leave any oak leaves unturned, Nixon and Steele were also looking at the oaks of San Diego County in the vicinity of Ramona that did not quite fit into any clear bin. They also saw similar oaks farther north in the Peninsular Ranges of Riverside County. The oaks were likely of hybrid origin but their diversity was in some ways “consistent” over very large areas. Because the oaks typically had moderate to large leaves, often were round tipped, and apparently had partially fused cotyledons (the first leaves to sprout in Eudicots), they were fairly certain that one of the parents was Engelmann’s oak. Engelmann’s oak is the only California oak with fused cotyledons. This group of oaks also tended to have fairly high trichome counts. High trichome density is a character of Muller’s oak, which is found along the eastern edge of and somewhat overlaps the range of Torrey’s oak, so they concluded that Torrey’s oak represented a stable hybrid, a “protospecies” between Muller’s oak and Engelmann’s oak. In a 1994 paper published in Novon by Nixon and Muller titled “New Names for California Oaks ” Nixon and Muller offer some hint of how they saw Torrey’s scrub oak under their discussion of the newly described island scrub oak (Q. pacifica):
“it is not known from the mainland but bears a superficial similarity to some of the tree forms that are putative hybrids between Q. engelmannii ....and Q. cornelius - mulleri ...in San Diego County. The latter populations, which have been described as Q . x acutidens ..., differ in having much greater variability in leaf shape, thicker, more coriaceous leaves, denser abaxial [leaf underside] leaf vestiture, much smaller hairs typically having more than 10 rays, and variable levels of connation [fusing] of the cotyledons...”
Many of the oaks in the vicinity of Ramona fit this description and the area has some of the best examples of Torrey’s oak. However, the actual limits of the form and whether it is better treated as a species or a hybrid, and its relationship to California scrub oak will be the subject of Part III. We are leaving off things more or less where they stood in the mid - 1990s. A lot had changed since 1927. Rather than having just a couple names available for white oaks at the time, we now have five. For the rest of California, the changes wrought in the 1980s and 1990s to oak taxonomy have really clarified the situation . Unfortunately, this isn’t quite the case in San Diego County. But that is the topic of Part III.
~reprinted from the SD-CNPS newsletter with permission of the author
