Chaparral and Scrub Oaks

Continuing with my text-heavy posts, this one will address the chaparral of southern California, particularly San Diego County, focusing on the scrub oaks.

Chaparral covers a very large part of California, even extending up into the warmer, drier part of Oregon and south into Baja. Chaparral vegetation corresponds almost 100% to the area of Mediterranean climate, and this makes perfect sense. Plants of Mediterranean climates are adapted to seasonal drought conditions, somewhat variable and unpredictable rainfall during the wet season, summer high temperatures generally below the extremes of the deserts, and winter low temperature seldom below freezing. The primary stress on plants is absence of precipitation for up to 7 months out of the year (see Plant Life in the World's Mediterranean Climates by Peter Dallman, 1998).

In San Diego and northwestern Baja, the chaparral takes on a unique flavor. This is the most arid portion of the California Floristic Province. However, its proximity to the cool Pacific creates fog and overcast conditions (such as we are having right now) which keep temperatures down and provide a small amount of additional moisture during the summer. Geology also contributes to the uniqueness of the chaparral here. Much of the coastal plain consists of sedimentary rock (sandstone) that has been shaped by successive events of sea level rise and fall, creating marine terraces that are cut by canyons where creeks flow. 

The most rare version of chaparral that is found here is known as Southern Maritime Chaparral. While there is plenty of generic chaparral in San Diego County, certain pockets with the right geology and micro-climate support Southern Maritime Chaparral that includes some rare species, such as Arctostaphylos glandulosa ssp crassifolia (Del Mar Manzanita), Ceanothus verrucosus (Wart-stem Ceanothus), Comarostaphylis diversifolia  ssp. diversifolia (Summer Holly), and Quercus dumosa (Nuttall's Scrub Oak). A few of the more common shrubs are also typically found in Southern Maritime Chaparral. These include Adenostoma fasciculatum var. obtusifolium (San Diego Chamise), Cneoridium dumosum (Bush Rue), Dendromecon rigida (Bush Poppy), Rhamnus crocea (Redberry), and Yucca shidigera (Mojave Yucca). I have designed a large part of my garden around these plants, as well as others that form this distinctive vegetation type.

Yucca shidigera in bloom with Comarostaphylis on the left and Arctostaphylos on the right rear,
also Dudleya virens ssp hassei in my front yard in early spring

Arctostaphylos glandulosa ssp. crassifolia in my front yard. In 1992 I collected this plant as a seedling
from a site in Carlsbad that soon afterwards was cleared and graded to become the Villa Loma Apartments

In 1996 the U.S. Fish and Wildlife Service estimated that approximately 21,000 acres of Southern Maritime Chaparral had historically existed in San Diego County. Due to agriculture and urban development much of that has been lost, leaving somewhere between 1,500 and 3,700 acres at that time (Federal Register Volume 61, Number 195). Because Southern Maritime Chaparral contains no rare animals, persuading the public and the decision-makers to conserve these rare plants is a bit more difficult than it should be.

One of the unique species of this chaparral is Quercus dumosa (Nuttall's Scrub Oak). The City of Encinitas is named after scrub oaks. I assume that some early Spanish explorers or settlers observed a lot of scrub oaks in this area and called it Los Encinitos. These scrub oaks probably reminded them of similar-looking scrub oaks of Spain that are generically called Chaparro, and this term led to the modern word Chaparral. So what species of scrub oak did this early Spaniards see when they arrived in this area? Were they Quercus dumosa or something else?

Herein lies a whole raft of taxonomic problems which I am trying to make sense of but may not be able to. The various shrubs that have been called "scrub oak" at one time or another present a wide variety of leaf shapes, leaf hairs, acorns, and other features. Also, oaks have an annoying tendency to hybridize (perhaps because they are wind pollinated). A number of botanists have endeavored to clarify the distinction between these different types. John M. Tucker tried to sort this out in the 1950s. Beginning in the 1980s there was a renewed effort which is ongoing. Tom Chester gives an excellent treatment of the history of these studies at http://tchester.org/plants/analysis/quercus/scrub_oaks.html, (although he notes that his paper is incomplete). Another paper on this subject is Quercus berberidifolia, Q. dumosa by Janet L. Fryer. 2012. In: Fire Effects Information System, [Online], U.S. Department of Agriculture. Probably the most authoritative paper on the subject is by Kevin Nixon of Cornell University, The Oak (Quercus) Biodiversity of California and Adjacent Regions, USDA Forest Service Gen. Tech. Rep. PSW-GTR-184, 2002.

The gist of these papers, as I understand them, is that all the scrub oaks in southern California were formerly considered Q. dumosa, but there was always a lot of confusion because of the variability in leaf shape and size. It was finally concluded that leaf morphology could not be used diagnostically, but other features could. A process of splitting into multiple species began, so that there are now three species of scrub oak recognized in San Diego County (excluding the desert). These are Quercus berberidifolia, Quercus dumosa and Quercus x acutidens. The last one is theorized to be a hybrid between Quercus englemannii and Quercus cornelius-mulleri (Nixon and Steele, 1980).

Chester says that these species can be distinguished by the size of hairs (trichomes) on the undersides of the leaves. He says that the trichomes on dumosa are easily visible, whereas on both berberidifolia and acutidens they are minute and require greater magnification to see clearly. Nixon adds that the trichomes on dumosa are curly. Everyone agrees that genuine dumosa is restricted to Southern Maritime Chaparral in coastal San Diego County at such locations as Torrey Pines State Reserve. They also say that dumosa is distinctive in several ways, such as very slender, wide-angled stems, which helps to distinguish it from the others. 

This leaves the problem of how to distinguish berberidifolia from x acutidens. Around Encinitas I see many scrub oaks that do not seem to fit the description of dumosa, so they have to be one of these other two. Chester suggests that all the plants called berberidifolia in San Diego County are really acutidens and that acutidens is a sufficiently stable and widespread taxa to merit consideration as a species. He says that he did not see a true berberidifolia until he went to the Santa Monica Mountains. This is consistent with what some others have told me. If this is the case, then everything I am seeing locally that is not dumosa must be acutidens.

I contacted Nixon by email about this dilemma, and he graciously provided some fascinating insights. He notes that x acutidens is fairly common in San Diego County, but he does not say that it replaces berberidifolia, and he still considers it to be a hybrid. I don't know if he is aware of Chester's draft paper, but if so, he is not persuaded by it with respect to the status of acutidens. His most interesting comment is in regard to the acorns, which he has given me permission to quote here. 

"One feature of Q. engelmanni .... is the presence of 
'fused' cotlyedons.  Later in the season, if you cut the acorn of Q. 
engelmannii you will see that the cotyledons form a single mass instead 
of two.  We have used this character in the Q. acutidens populations, 
and it is variable - present to varying degrees in different 
individuals.  This alone strongly suggest hybridity, as it is never 
variable in any other species I know.  In Q. engelmanni it is very 
consistently fused, and indicates a relationship to a group of Mexican 
live oaks that also have fused cotyledons.  This includes Q. arizonica 
as the closest one geographically, but also about 30 other species 
scattered through dry habitats in Mexico. So even though there is 
obvious hybridization with scrub oaks in the 'Q. acutidens' 
populations, Q. engelmannii is clearly part of a different complex." 

So, to definitively identify Q. x acutidens one must cut open a number of acorns late in the season and observe whether some of them have fused cotyledons. 

I have two interests in regard to the scrub oaks. One is to try to understand some specimens that are growing in a chaparral area of Encinitas, the other is to understand what I have growing in my garden. It is clear to me that leaf size and shape are not diagnostic. Yet I am so fascinated by the variety of leaf shapes on these plants that I am going to do a little study of them, below. Perhaps this will just prove the point that leaf morphology cannot be used diagnostically. Perhaps it will also support the notion that what I am seeing in many cases is the hybrid x acutidens. So my sample will consist of two plants in my garden and two plants from a chaparral area in Encinitas. As background, the chaparral area in question is known as Encinitas Ranch. It is mature Southern Maritime Chaparral with a diverse mix of chamise, manzanita, mission manzanita, Yucca, toyon, mountain mahogany, and scrub oaks. It has not burned in quite a while, perhaps 80 years or more. First, an overview shot.

Encinitas Ranch

Scrub Oak #1 is about 8 ft. in height. In the photo the white stick is a 6 ft. ruler. The leaves are fairly uniform over the entire plant, about 1" to 1.5" and virtually all are toothed.

Looking at the undersides of the leaves with a hand lens I can see that there are trichomes but they are very minute and I can't see individual hairs. A microscope would be necessary to make out any detail of the trichomes. There were some partially developed acorns on the plant at this time (early July). Overall leaf shapes and acorns are shown below. I would have been inclined to call this dumosa except that the trichomes don't seem to match what Chester described.

Scrub Oak #2 is located about 50 yards away from #1 and presents a completely different appearance. It is about 6 ft. tall with a more sparse and open appearance. It is sufficiently open to be able to see the base, which was not possible with #1, and it is clear that #2 arises from multiple trunks.

Moreover, the leaves have a completely different appearance. The great majority of leaves are entire (smooth edged) and elongated as shown in this photo. They are still about the same size as the leaves of #1 (about 1.5 inches) but more slender, and this is accentuated by the fact that the edges are smooth and not toothed. The trichomes on the undersides of the leaves appear to be the same as on #1 as far as I can tell with a hand lens.

However, a couple of branches on this plant contain leaves that are toothed. These leaves are probably about 1% of the total leaves on the plant, but it is interesting to note that the plant is capable of producing toothed leaves, and does produce them sometimes. This leaf variability suggests to me that it is a hybrid, but I confess to complete ignorance in how this works. Below are photos of the toothed leaves and acorns. The acorn looks superficially very similar to #1.

Scrub Oak #3 is from my garden. It was sold to me as Quercus dumosa in the strict sense, that is of the type that would only be found at Torrey Pines and similar locations. Unfortunately I don't remember where I bought it so I can't say anything about its origin.

The stems are very slender and widely angled, which is supposed to indicate dumosa. There are trichomes on the undersides of the leaves, not a lot of them and not dense, but they are long and curly or wavy. They are much longer and more visible than on #1 or #2. This plant has never produced any acorns so I can't do any comparison that way. But if trichome length is diagnostic then #3 is dumosa, exactly as it was sold to me. So assuming I now know how to identify dumosa from this plant, let's move on to the next one.

Scrub Oak #4 is also from my garden. It was sold to me as berberidifolia and it was propagated by Native Sons Nursery, so my guess is that it is from central California stock.

The leaf size and shape are very similar to #1 and #3. However, the trichomes on the underside of the leaves are minute by comparison with #3 and very similar to #1 and #2. Is this really berberidifolia? I can only take the word of Native Sons Nursery that they collected acorns from a bona fide berberidifolia somewhere.

CONCLUSIONS
Some months back when I first asked some CNPS people for help in identifying the scrub oaks in this chaparral area of Encinitas, I was told they were all acutidens. This would imply, as Chester contends, that berberidifolia is not found in coastal San Diego County. Personally, I hope this is true as it would make the situation much simpler. All scrub oaks in Encinitas would have to be acutidens unless they have clearly visible, long, curly trichomes.

But of course nothing can be that simple. Calflora shows a large number of locations for acutidens in coastal San Diego County, as well as a large number of locations of berberidifolia, and they overlap completely. The berberidifolia locations are undoubtedly based on reports from highly qualified botanists, so they must be taken seriously. Interestingly, there are no observation points for either species inside the stand of chaparral in Encinitas that I am concerned with. Apparently it has never been surveyed. If both acutidens and berberidifolia are present locally and only the fused cotyledons of some acorns to tell us which is which, then I must conclude that the situation is fairly hopeless for the amateur botanist. I am not expecting to get resolution of this for all of southern California or even San Diego County. I would just like to understand the plants in this one stand of chaparral in Encinitas.

To resolve this I'm going to collect acorns in October and cut them open to look for ones that have fused cotyledons. Since Nixon says that fused cotyledons occur sporadically in x acutidens, I'm assuming I will need to look at quite a number of acorns and they will need to be fully developed. I am also going to try to germinate some acorns to see what kind of leaves I get from the seedlings. Then I'll post my results. Maybe nobody cares but me, but I really want to figure this out.