Diversification Oaks For A Climate Changed Central Coastal California
The recommendations and considerations presented below are informed by 40 years of experience in growing a large variety of oaks from around the world at Stanford University and other locales scattered through coastal California, while also peering into the future through the lens of the severe climate change scenarios which are now playing out worldwide. While many professionals involved in horticulture in California downplay potentially negative climate change consequences for native plants, this belief is problematic. The history of the last 30 years of climate change research has consistently shown actual conditions experienced to have been at the extreme end of the prediction spectrum. There is no reason to expect this pattern to change, but there is reason to fear that we will be facing climate zone migration on literally a continental scale over the next century, the lifespan of reasonably well sited and planted oak tree.
As these extreme, pseudo-apocalyptic climate change scenarios become increasingly likely, and while any predictive scientific analysis of native plants within the extreme scenario remains exceedingly rare, we present here the application of severe climate change predictions to the oak trees so critical to the historical ecology and iconography of coastal California. As an example, there is no scientific certainty that coastal California will maintain a Mediterranean climate. Such climates are rare, and monsoonal patterns in the American Southwest are strengthening in recent years. If it begins to rain even quasi-consistently in the summer in central coastal California, the effect on native oaks will be spectacularly negative. Local extirpation of California native becomes likely, as we are already seeing in some areas in recent decades. Of course the arid opposite is eminently possible, with central coastal California experiencing even greater aridity. After watching the the effects of the drought of the early 1990's at Stanford, it became obvious that even profoundly drought adapted oaks can succumb to sufficient aridity.
Within currently accepted science, central coastal California could get wetter in the summer, drier in the winter, or any combination, along with seemingly inevitable warmer temperatures. And of course these extremes could switch back and forth, never settling into the type of consistent patterns we have grown accustomed to in recent centuries. These are times of profound uncertainty regarding our climate future, and any logical response to this unfolding crisis will be built out of hedged bets. Such a strategy can be seen in the enormous tree plantings at Apple Park, where more than 60 oak taxa were utilized, to account for a wide variety of potential climate scenarios. In times of uncertainty, wise investors choose a hedged strategy.
There is one element of climate change which does seem certain: temperature increase. Though the actual magnitude of the increase is unknowable, it's nearly certain to be warmer, especially in winter. This increase will inevitably favor evergreen tree species, as they have to ability to take advantage of increased photosynthetic potential of a warmer winter. Please recall deciduousness evolved because leaves containing water explode when exposed to sub-freezing temperatures, a simple fact that water expands when it freezes. If air temperatures no longer freeze, deciduousness is maladaptive, and evergreen species become dominant. We watched this over the years at Stanford, planting both Coast Live Oak and Valley Oak at the same planting site. Twenty to thirty years later, it's clear that Coast Live Oak will literally swallow, by overtopping, any nearby Valley Oak, unless heavy pruning is utilized. The Coast Live Oak is able to capture more energy, plain and simple.
Looking at any coastal California mountain range, this stark pattern plays out on an epic scale. If a total tree population comparison were made between deciduous and evergreen tree species in these mountains - comparing total population counts - deciduous trees would represent only a fraction of a single percent of the total native forest tree population. It really doesn't freeze much up there, and the evergreens rule. Evergreens rule in southern California also, which has a climate that certainly mimics the application of many climate models to the central coastal California area.
The upshot of these observations and predictions is the recommendation that appropriate areas of central coastal California should look to diversify into a wide range of primarily evergreen oak species to ensure the iconic consistency over the coming century of extreme climate change.
In this writing, we look at potential diversification oak species through the lens of existing species, creating a list of trees which maintain the aesthetic, ecological and habitat functions of the current natives, but vastly increase the climate resiliency of the target areas. We begin our considerations with the historically common oak species, and related hybrids, native in low elevation locations between San Francisco and Santa Barbara. We will be looking exclusively at tree form oaks, all of which have demonstrated substantial drought tolerance, and are known to perform well in.
Coast Live Oak (Quercus agrifolia) is the vastly dominant oak type in low elevation locations of coastal California. Valley Oak (Quercus lobata) maintains a presence throughout these regions, becoming dominant in the wettest valley locations, and continuing into the foothills were moisture conditions are favorable. Blue Oak (Quercus douglasii) is rare in valley locations, and fairly common in the foothills. Some of the most iconic oaks in the foothill area are a well-know hybrid of the Valley and Blue Oaks, the Jolon Oak (Quercus x jolonensis). Finally, we consider California Black Oak (Quercus kelloggii), which is a fairly rare component of the foothills of coastal California with little or no ability to withstand valley or flatland locations. A few highly rare hybrids of Coast Live Oak and Black Oak, Gander's Oak (Quercus x ganderi), can be found in these areas with sufficient observation.
Coast Live Oak
The iconic character of coastal California is based around evergreen oak trees. If Coast Live Oak is at risk, what other oaks can be brought in to augment or replace it? We begin this discussion with other California natives.
Relatives and Analogues
From southern California, we have Engelman Oak (Quercus engelmannii), which grows from Pasadena south to San Diego, and inland to Ramona. Engelman oaks planted at Stanford in recent testing have performed well. There are multiple young examples in East Palo Alto, and Engelman is clearly performing well at Apple Park nearby in Cupertino, as Engelman oak is used as a street tree, and performing extremely well. Engelman oak is closely related to many of the oaks of the southwestern United States, which are also proving to perform well in coastal California. Engelman oak typically has bluish-colored foliage, mimicking the extirpating Blue oak. Engelman oak is semi-evergreen, typically losing about half it's foliar canopy by the time new growth resumes in the spring. Engelman oak has a loose, open canopy, and has proven to be popular with those who have seen it. The City of Pasadena plants Engelman oak extensively as a street tree. Engelman oak has proven to have much greater tolerance of summer rainfall or irrigation, which makes perfect ecological sense given it's origin in a monsoonal climate with consistent summer rainfall.
Also from southern California is the incredibly rare Island Oak (Quercus tomentella), native to the Channel Islands off Santa Barbara and Los Angeles. While few of these trees have been planted in the Bay Area, those planted are thus far proving adaptable. Mature examples can be found in Berkeley, Davis and Sacramento. Young specimens are growing along the 101 in East Palo Alto. Island Oaks are also clearly visible in the perimeter plantings at Apple Park in Cupertino, and, again appear to be growing well. Hints of powdery mildew have been seen, but a greater issue is that some of the seedlings grown from mature specimens revert to a shrubby growth habit. For both of these reasons, any Island Oaks planted at Stanford should have been grown in an area where powdery mildew is common, and grown for a sufficient length of time so that the genetic growth habit becomes evident, and powdery mildew exposure in guaranteed. Any plants with poor growth habit or powdery mildew susceptibility should be discarded and not planted.
The other evergreen oak trees native to California grow in mountainous inland areas. These areas have very different soil and very different climates than can be found around Stanford. These mountain species have proven ill-adapted to growing in the flatlands of the Bay Area, rarely surviving long before expiring from catastrophic leaf disease or drowning with poor soil drainage by comparison to natively evolved soils, which are highly rock based containing little or no clay, and as such allow lightning fast soil drainage. The clay soils around the Bay are impossible for trees with such adaptation. Virtually all of the so-called California scrub oaks occur in upper-elevation chaparral, again with ferociously fast soil drainage, combined with high levels of susceptibility to powdery mildew.
Staying within the evergreen/semi-evergreen paradigm, the next closest set of oaks to consider are found in the American Southwest. One is the beautiful and ornament Silverleaf Oak (Quercus hypoleucoides), while the other is the most common oak in Arizona, Emory Oak (Quercus emoryii). Silverleaf Oak has proven to grow in Portland, Oregon, and can be found over a very large range of elevation change in the mountains of the southwest. Young specimens have grown well in southern California tree nurseries, and are now performing surprisingly well at Apple Park in Cupertino. There are no known examples of Emory oak in California, but it appears that Emory oak is fairly closely related to the native California Interior Live Oak (Quercus wislizenii), which has proven a poor performer in coastal California. Emory oak is certainly worthy of trial at Stanford.
Most of the other evergreen/semi-evergreen oak species of the southwestern US are related to California native Engelman oak, with some scientific authors going so far as to claim that Engelman oak (Q. engelmannii) is actually the same species as the Mexican Blue Oak (Q. oblongifolia). While there is clearly an element of accuracy in these claims of conspecificity, actual observation of both types in the wild suggests a more subtle relationship. It is indeed possible to find example of Engelman oak in habitat that share a strongly bluish ovalized leaf form with the Mexican Blue oak, they are rare, and the truth of Engelman oak foliage types is remarkably variable, and congruence with all the related southwestern US oak types can be found. Observing Engelman oak in habitat after viewing the Arizona White Oak (Q. arizonica), Gray Oak (Q. grisea) and Mexican Blue Oak in habitat, one is struck that Engelman oak populations look like all the Southwestern oak species got marooned together, and have been merrily hybridizing with one another for eons, blurring all the morphological edges. All three of these Southwestern types are performing well in the initial stages of the Apple Park landscape.
Moving to the edges of the Southwest, and moving south into northern Mexico, we encounter Escarpment Oak (Quercus fusiformis) occupying large areas that experience considerable drought, but also intermittent heavy summer rains. Escarpment oak is closely related to Southern Live Oak (Quercus virginiana), and looks quite similar. Escarpment Oak generally has slightly larger leaves, much larger acorns, and a much more upright growth habit than the widespreading Southern Live Oak. Hybrids between the two are exceedingly common, and virtually impossible to fully sort the genetics. In Texas where all the forms are common, people simply call these trees Live Oaks, with live, just as in California, referring to a tree that holds green leaves in the winter. Escarpment Oak performance at Apple Park has been outstanding.
Long ago, Stanford learned that lawns kills the native Coast Live Oak, which evolved with the warmest 6 to 8 months of the year being without rainfall. Add summer irrigation to the root zone of a Coast Live Oak, and watch the Oak Root Fungus which lives dormantly in many California soils awaken, and begin eating oak roots, eventually killing the tree. The story has been repeated many thousands of times over in coastal California. Several decades ago, Stanford began planting Southern Live Oak in lawn situations, with good results. This approach is common in southern California, where Southern Live Oak is much more commonly planted. Southern Live Oak is also a heavily used urban and plaza tree, through most of the municipalities of the southern third of the United States, from Raleigh to Miami to New Orleans to Austin to Tuscon to Phoenix to Los Angeles. Apple Park utilizes Southern Live Oak in such locations.
In northwestern Mexico can be found one of the highest concentrations of oak diversity anywhere in the world. Most of the Southwestern species already mentioned grow well south into Mexico, and there they grow alongside compatible evergreen oaks. Because of the range overlap with oak taxa already proven to perform well in California, it is reasonable to assume these additional species are likely to perform well. There are already Mexican oaks growing around California, with more being planted all the time. But these are oak types native to the wetter eastern side of Mexico, on the Atlantic side. There are multiple oaks from these areas represented at the Shields Oak Grove at the U. of California, Davis, and they have suffered with drought in the years that the Grove was unirrigated, but now are performing decently with summer irrigation.
Two of these wetter-zone Mexican oaks are represented in the test plantings at Stanford, Monterrey Oak (Quercus polymorpha) and Canby Oak (Quercus canbyi) are both performing decently in multiple locations at Stanford, both in the Arboretum area and also near the Foothills. Monterrey Oak appears to be performing better in the Arboretum location, with it's deeper soils. Canby Oak survives in both locations, but clearly would perform better with additional irrigation. Neither of these trees are recommended for further planting at this time, but the specimens already in place should be observed. If rainfall patterns do change over coming decades, both of these species should fare quite well.
The much more appropriate arid adapted oak taxa can be found in northwestern Mexico, in areas that have become particularly dangerous in recent decades due to criminal activity. Potential species of interest include: Q. invaginata, Q. durifolia, Q. sideroxyla, Q. mcvaughii, Q. scytophylla, Q. albocincta, Q. deserticola, Q. coccolobifolia, Q. coffeicolor, Q. viminea
The relative and analogue species listed above can be considered a Stage 1 climate change recommendation, which gives a San Francisco Bay landscape a measure of adaptability to increased heat and summertime rainfall, but all staying within the relatedness of being on the same continent.
For greater tolerance to aridity, it is necessary to turn to the Mediterranean area itself. Here you will find trees like the Cork Oak (Quercus suber), the Levantine Live Oak (Q. calliprinos), and the ultra rare sweet acorn Quercus rotundifolia. The Holly Oak or Holm Oak (Quercus ilex) has been used extensively, and it's shortcoming have also become obvious. The strains of Holly oak brought to California are typically heavy acorn production trees, apparently from the dehasa agricultural system. Aphid, and the attendant honeydew drip have largely doomed Holly oak from typical usage, particularly in urban areas. Holly oak has shown a moderate level of invasiveness, though it creates far fewer seedlings than native Coast Live Oaks planted in the same areas. Unfortunately, those acorns drop during the Thanksgiving to Christmas Holiday season, creating a real hazard when planted in commercial shopping areas. The sudden and highly controversial removal of more than 50 mature Holly oaks in Palo Alto in 2009 highlights these issues.
Cork Oak (Quercus suber) have also been planted in California for more than a century, though never in the same large numbers as Holly Oak. Cork Oak, like many oaks, doesn't fall neatly into the evergreen/deciduous paradigm. They are semi-evergreen, and lose most of their leaves in the early spring, only to push new growth within a month or so. But during the period of relative leaflessness, many people question whether the tree has died. This aesthetic challenge has limited the acceptance of Cork Oak. Cork Oak is extremely drought tolerance - arguably even more drought tolerant than the native Coast Live Oak.
Unfortunately, even this low level of Cork Oak acceptance has been notably diminished in recent decades with the advent of container nursery production in California. This system makes no allowances for tap rooted trees, so the tap root, and the rest of the root system, wind up circled and kinked and bound, and this stunts the growth, while predisposing the trees to root diseases of various kinds. Looking around California at the many Cork Oaks planted, a strong pattern emerges. The oldest Cork Oaks are almost shockingly large, with trees that have trunks 5 and 6 feet in diameter in places like Davis. Then, looking at newer plantings, one sees that most of the trees are stunted, many have the telltale swelling and darkening of the lower trunk that indicates severe root disease, and only a small percentage of the trees planted manage to thrive.
Cork Oak can now be grown in air-pruning containers, which preserves the integrity of the root system, and also allows for rapid, defect-free growth. Cork Oak is mercifully free of both aphids and powdery mildew everywhere observed in California. Cork Oak requires good soil drainage, much like the soil needed to support native Valley Oak, although Cork Oak grows well in places where Valley Oak is severely damaged by powdery mildew. Finally, like Valley Oak, the growth habit of Cork Oak is highly variable, ranging from trees which have nearly Weeping Willow levels of pendulousness, to trees with a highly upright-growing branching structure. This variability is likely another reason for the limited acceptance of Cork Oak, as pendulous individuals come with a high maintenance price tag if planted in urban areas. Selection for growth habit is not within the current scope of the California nursery industry. Cork Oak is certainly worth further planting in appropriate locations in California, if carefully grown and selected individuals can be made available.
Levantine Live Oak (Quercus calliprinos) has been test planted at Stanford since 2003. Acorns were placed in protective cages around the Arboretum, whee young Coast Live Oaks had died. There are nearly a dozen of these replants around the Arboretum, mostly near the Vernal Pool in the Arboretum, and, with zero supplemental watering, these trees have established. Others were planted along Serra Street in later plantings. They are slow growing but dense, and bear some considerable resemblance to native Coast Live Oak. This similarity is an example of convergent evolution, as very similar temperature, humidity and day length. The acorns, however, are vastly larger than Coast Live Oak acorns, and much larger than the leaves, giving a very unusual look. Some of the trees along Serra Street have produced acorns in quantity. Levantine Live Oak has very spiny leaves, and can be painful to prune. They are not ornamental trees for California, but do have potential utility in mass plantings on difficult sites with low rainfall and good soil drainage. They can also be used as livestock barrier trees. Levantine Live Oak is the most-mentioned tree in the Bible, with examples known to live in excess of one thousand years.
Related to the Levantine live oak is the previously-mentioned Quercus rotundifolia. This species and related hybrids can be found in arid parts of the Mediterranean. Limited observation of this tree at Apple Park confirms the good performance of Q. rotundifolia in coastal central California.
Alongside Coast Live Oak, Valley Oak is the other truly iconic tree of Stanford. Valley Oaks are deciduous, with a very different leaf shape than the vastly more common Coast Live Oak. The range of land over which Valley Oak grows in California is vastly greater than the range occupied by Coast Live Oak, which demonstrates that Valley Oak is a more adaptable tree than Coast Live Oak, within limits. Those limits are defined by aridity and heat, and increased heat is the greatest certainty of this period of extreme climate change. Stanford is almost certain to continue to warm, particularly in the winter, making Valley Oak decreasingly adapted. This vulnerability means consideration of replacement oak species to maintain aesthetic and ecological function is warranted.
Relatives and Analogues
The nearest close relative to Valley Oak can be found on the Sky Islands of Arizona, and the forests of New Mexico. In these areas one can find Gambel Oak (Quercus gambelii), which form a kind of genetic bridge between the widely distributed Bur Oak (Q. macrocarpa) and Valley Oak itself. Bur Oak is most easily identified by an enlarged central sinus in the leaves. This center gap is quite distinctive and unique, even as the size of the leaves, and the trees themselves, can vary considerably in size. Careful observation of Valley Oak leaves from many individual trees, including trees at Stanford, will inevitably yield a few example of Valley Oak leaves with an enlarged central sinus, a powerful confirmation of the shared genetics of Valley Oak and Bur Oak.
Gambel Oak occupies mountainous zones between the ranges of Valley Oak and Bur Oak, and extend south into the mountains of Mexico. Most Gambel Oaks are rather short and shrubby, often with multiple stems, and rarely growing over 25 feet tall. But in New Mexico and Arizona, and likely northern Mexico also, Gambel Oaks can be found with tall, straight single trunks, reaching heights in excess of 70 feet. Such trees in New Mexico have been identified as Gambel Oak hybridized with Bur Oak, a classification which fits a variety of morphological traits on these trees.
In Arizona, it appears that genetics from the Netleaf Oak (Quercus rugosa) entered the Gabel Oak gene pool, giving the trees thicker leaves, and also giving some examples the "bunch of grapes" acorn peduncles that is characteristic of some Netleaf Oaks. These "triple threat" Arizona Gambel Oaks have been test grown in California, and are currently growing well at Apple Park, displaying little or no powdery mildew which often disfigures Valley Oak. It is safe to assume that this form of Gambel Oak, or a very closely-related oak, is the most southerly deciduous tree in North/Central America, making it an ideal climate change Valley Oak analogue.
Continuing to the east from the Catalina Mountains of Arizona, down into the Capitan Mountains of New Mexico, and you encounter the only other deciduous oak from the White Oak subgenus in the Southwest. This is the Capitan Strain of Chinkapin Oak (Quercus muehlenbergii), a very specific and as-yet scientifically unnamed ecotype of Chinkapin oak that performs in a shockingly different manner to growing in coastal California microclimates from the typical forms of Chinkapin oak, which range over much of the United States east of the Missouri River south through central Texas.
The Capitan strain of Chinkapin oak is one of two disjunct populations of Chinkapin oak outside the main range of the species, the other population occupying the Guadalupe Mountains straddling New Mexico and Texas. Based on trial of all three of these types, the main range Chinkapin oak has large, thin leaves, and the leaves are quickly and severely disfigured by a variety of fungal diseases when grown in central coastal California. The Guadalupe strain has slightly smaller and thicker leaves, and exhibits a much reduced, but still notable, level of fungal leaf infection. Remarkably, the Capitan strain, with much smaller and much thicker leaves appears nearly immune to all leaf diseases currently present in coastal California. The Capitan strain can also produce excellent fall colors, including reds, oranges and purples.
Finally, this is a consistently upright-growing strain, with excellent branch attachments, making this a practical selection for street location, and other locations where substantial clearance is required. The Capitan strain of Chinkapin oak is highly recommended as a drought adapted, habitat positive, well structured and pleasingly colored oak for Stanford and surrounding areas.
Moving eastward, there is one more deciduous oak which is proving to grow well, and be very attractive, in coastal California. This is the exceedingly rare Lacey Oak (Quercus laceyi) from the Texas Hill Country, with related types ranging southward into northern Mexico. These are small to medium size trees which tolerate substantial drought, and can display nice yellow to orange fall color. No powdery mildew or other disfiguring leaf diseases have thus far been observed at Apple Park. The tree bears a remarkable resemblance to Blue Oak (Quercus douglasii), in leaf shape, leaf coloration, and canopy shape, with Blue oak having potential to be a much larger tree.
Near Frost Amphitheater at Stanford stands a Water Oak (Quercus nigra), likely a century or more old. The tree is semi-evergreen, losing about eighty percent of it's foliage by spring. The leaves are fairly small, thin, and notably stiff, denoting a dense waxy leaf cuticle, which helps explain the utter lack of leaf disease ever observed on the tree. Although the Water oak has grown well, it shows the weak branching structure for which this tree is known in the southern US. When Hurricane Katrina hit New Orleans, the Water oaks growing all over the city were blown apart, often splitting right down the middle. The other common oak in New Orleans, the Southern Live Oak, came through the storm virtually unscathed. It should be noted that Southern Live Oak wood was commonly used for structural timber in the 1800's, so this pattern fits with historical usage.
In Menlo Park, right across San Francisquito Creek, stands a lovely, large, old Overcup Oak (Quercus lyrata), with it's cartoonish leaf sinuses and lobes. This particular tree has grown well with substantial lawn irrigation, likely continuously for decades. The leaves show little disease. While Overcup oak itself isn't particularly well-suited to a climate changed coastal California, there is a hybrid of the Overcup oak with the Southern Live Oak, which is called Comptons Oak (Quercus x comptoniae), that is performing well in limited plantings at Apple Park and elsewhere. These hybrids are quite variable, since one parent is nearly evergreen while the other parent is an early fall deciduous species. Also, Overcup oak is tall and upright growing, and Southern Live Oak is a widespreading semi-evergreen tree. The ideal is an upright-growing, late fall to early winter color tree, with leaves that show exuberant geometry of the Overcup oak parent. These hybrids are capable of producing truly excellent fall color when carefully selected.
Northern to Central Eurasia
Valley Oak is classified within the White Oak subgenus, which naturally also contains Gambel Oak and Bur Oak. White Oaks are not confined to North America, and can also be found in Eurasia, where the so-called English Oak (Quercus robur) occupies vast tracts of land, and have been the foundation of central and northern European civilizations and dynasties. In central and northern California, all English Oaks observed have been infested with aphids, and the ground beneath saturated with honeydew, excluding it from widespread planting. Various upright-growing cultivars of English Oak are available from the Oregon nursery industry, and these invariably hold dead leaves all winter, a widely-hated trait that leaves most people thinking the tree is dead. These upright English Oak cultivars are not recommended for further planting. Instead Trojan Oak (described later) can be substituted, with vastly improved drought tolerance, pest resistance, and aesthetics.
There is a subgenus of oaks in the zone between Central Europe to the wetter zones of the Mediterranean. These trees are called Mesobalanus, and include the Hungarian or Italian Oak (Quercus frainetto), the Caucasus Oak (Quercus macranthera), and the Pyrenees Oak (Quercus pyrenaica). Italian Oak has been planted around California, and has performed well in Palo Alto for 20 years, with strong growth and very minimal powdery mildew. Graft failure has been a serious problem with the 'Forest Green' cultivar, showing the necessity of procuring this tree in own root form.
Mediterranean Into Eurasia
As we venture south in Eurasia, we quickly move out of the White Oak subgenus, and into a set of closely-related trees in the Cerris oak subgenus, known in California largely through the Cork Oak, the most evergreen member of the subgenus. Tucked into a largely unnoticed corner of the Stanford campus is the original Leland Stanford residence, situated very close to San Francisquito Creek. Here can be found some of the original Stanford plantings, including American Persimmon (Diospyros virginiana) and Kentucky Coffee Tree (Gymnocladus dioecus). But the real survivor in this area is an old Turkey Oak (Quercus cerris), which likely originated, along with the rest, in a large plant purchase made in New York in the 1870's, and transported to Stanford on two of Mr. Stanford's trains. This Turkey Oak appears to be hybridized, perhaps with the common in New York White Oak (Quercus alba). Regardless, the tree is nearly 150 years old, and in excellent condition, growing in favorable near-creek alluvial soil. All Cerris group oaks show resistance or even immunity to powdery mildew, though aphids have been seen on what appear to be Turkey oak in Berkeley. There's a old set of several dozen Turkey oaks at Lake Merritt in Oakland, though these appeared to be declining in recent years, along with most of the other trees around the Lake, possibly due to the adoption of "recycled water" for irrigation.
Turkey oak is a tree that ranges in height from 45 to 75 feet tall. There is a closely related oak the Chestnut Leaf Oak (Quercus castaneifolia) that grows in the mountains of Iran, and has potential to be one of the tallest oak types in the world. Chestnut Leaf Oak is one of the key Stanford test species, with several performing spectacularly at the Palo Road planting, and another outperforming native Valley Oak on Serra Street. The mother to these trees is located in the Shields Oak Grove in Davis, where six fifty year old specimens are planted, with the tallest growing over 100 feet in that time. Chestnut Leaf Oak is used as a primary street tree around Apple Park, along with Turkey Oak and Engelman Oak. These Chestnut Leaf Oaks show limited hybridization with Bur Oak, which is very typical of seedlings originating in the Shields Grove.
Smaller than Turkey Oak, but again very closely related and similar in many morphological traits is Trojan Oak (Quercus trojana), which has been trial planted at Stanford both in the Gerona Triangle, and also in the Palo Road plantings. Trojan oak is among the best and most consistently-performing of the new oaks being trialled at Stanford and around the Bay Area. These are relatively small oaks, with the mother tree in the Shields Grove in Davis attaining about 30 feet in 50 years, while Turkey Oak grow to 50 feet in those 50 years, and the Chestnut Leaf Oaks approach 100 feet in the same time period. No powdery mildew has been observed thus far, as the trees appear immune, and no sign of aphids have appeared in initial testing. The trees are extremely drought tolerant, and maintain a lovely, nearly fastigate growth habit. This appears to be an oak worth of much greater planting, and have great promise for allees in plazas.
Yet another deciduous member of the Cerris oak subgenus is the Oak of Mount Tabor (Quercus ithaburensis), a rare and protected tree in Israel, which also grows elsewhere in the Mediterranean, particularly Italy and Greece. A few Oak of Mt. Tabor were direct seeded into sites around the Vernal Pool, and they have survived without any supplemental irrigation, which is yet another testament to the drought adaptation to be found throughout the Cerris oak group. Oak of Mt. Tabor was subsequently planted in both the Galvez Street plantings, and also the Serra Street plantings. A particularly exceptional specimen grew on Serra Street until earlier this year, showing the potential for this species at Stanford and around the Bay Area. While the Levantine Live Oak is analogous to Coast Live Oak, Oak of Mt. Tabor is roughly an ecological equivalent to Valley Oak. Oak of Mt. Tabor is the second most mentioned tree in the Bible.
Related to the Oak of Mt. Tabor is Boissier's Oak (Quercus infectoria subsp veneris), with several healthy trees to be found in the Shields Oak Grove at the Univ. of California Davis Arboretum. Judging from one large volunteer Boissier's oak to be found thriving at the edge of the Shields Grove, this is a well adapted tree to California, which makes perfect sense given the native range of this oak.
California Black Oak
California Black Oak (Quercus kelloggii) is present only in the far Foothill lands at Stanford, but is an iconic native oak growing in the Stanford vicinity. There used to be an old Black Oak outside Frost Amphitheater at Stanford, and each summer the tree suffered from fungal leaf diseases, and was typically nearly defoliated by Fall. The key problem seems to be enhanced summer humidity levels in the vicinity of the San Francisco Bay, levels of humidity not found in areas where Black Oak is native. Natural hybrids with Coast Live Oak show greater resistance to fungal leaf infections, but are exceedingly rare, require excellent soil drainage, and still only give a measure of resistance.
Relatives and Analogues
Southwestern US To Northern Mexico
Moving south and east from Stanford, the first real analogous relative to Black Oak is the Chisos Red Oak (Quercus gravesii). This species grows from the Chisos Mountains in far southwest Texas, in select pockets up into New Mexico. This is a true semi-evergreen species, and examples of the New Mexico genetics are performing well at Apple Park.
Moving slightly north and east from Chisos Red Oak range you will find the Texas Red Oak (Quercus buckleyi) growing in the mid-section of Texas, particularly in the Hill Country and Edwards Plateau region extending into Oklahoma. There are several Texas Red Oaks growing as part of the Stock Farm Road Planting, but the soils in this area are not particularly conducive to plant growth of any kind. Texas Red Oak grows well under California nursery conditions, and examples are performing well at Apple Park, so long as they are planted in well drained soil locations. Texas Red Oak has excellent red fall color beginning around Thanksgiving, and continuing through Christmas. Texas Red Oak produces large acorns, and there are some initial signs of invasiveness in favorable microenvironments.
Continuing east and north from Texas Red Oak range, you are now in Shumard Oak (Quercus shumardii) range. This is the southern- and western-most member of the related oaks often simply called Eastern Red Oaks. This includes Pin Oak (Quercus palustris), Scarlet Oak (Quercus coccinea), and the Red Oak (Quercus rubra) itself. These other three oaks have been long-planted in California, with most Pin Oaks long dead, a few surviving Scarlet Oaks, and many surviving Red Oaks, the most wide-ranging and most widely-adapted of the group. None of these can be reasonably recommended for a climate change future, but Shumard Oak is the best adapted of this group for a classic, large-growing fall color oak. Shumard Oaks are growing along the edge of Apple Park along Tantau Avenue, and are generally performing well. Shumard Oaks can hold dead leaves in the winter, a widely-hated trait called marcessance. For this reason, if Shumard Oak is desired for planting, specimens should be selected in the winter, to ensure a clean winter leaf drop.
Reversing direction, and moving south into Mexico, brings us to the related Canby Oak (Quercus canbyi). Leaf size shrinks notably, as does acorn size. Rather than being deciduous, Canby Oaks can be consider winter deciduous trees, typically losing their leaves in January and early February, after turn a nice array of colors typically seen in the fall, accenting reds, yellows, oranges and some purple coloration. There are Canby oaks at Stanford, both in the Palo Road Plantings, and also a specimen at the Gerona Triangle Planting. Canby Oaks will survive in good soil dry sites without irrigation, but for good vigorous growth they require modest additional summer irrigation. Canby Oaks are probably best used in near coastal zones of California extending northward to British Columbia.
Here we use the term exotic to mean hybrids of non-native and often artificial origin. These can come from direct artificial pollen delivery to simply collecting and growing acorns from mixed arboretum oak plantings. Because many of the test trees are grown from acorns collected from the Shields Oak Grove at the U. of California Davis, there is a high likelihood of hybridization due to the proximity of nearly 90 different oak taxa, represented within a population of 350 total oaks from all over the world.
The example of Netleaf Oak hybrids growing at Palo Road, and another at the Gerona Road Planting, provide a telling example, with both showing combined parentage of Netleaf Oak with Bur Oak, which grow directly across a 100 foot lawn from each other in the Shields Grove. Netleaf Oak leaves are oval and entire, while Bur Oak carries the characteristic central sinus. Knowing the acorns came from the Netleaf Oak mother in the Shields Grove, and then seeing the seedlings with lobes and an enlarged central sinus, it seems a reasonable assumption that you have found what might be called a Netbur Oak.
The Netbur oak at the Gerona Triangle is the largest and fastest growing oak among all trees being trialed, including the Chestnut Leaf Oak, which can grow to 150 feet. The tree is quite upright, and will likely grow into one of the largest and most iconic trees at Stanford over coming decades. It is well sited at the confluence of several drainage channels, so part of its outstanding growth, despite no supplemental irrigation, can be attributed to this generous allotment of water during the rainy season, combine with good draining soils in this part of the Stanford Campus.
The Netbur oak at Palo Road is a lovely little pyramidal tree, well less than half the height of the Gerona Netbur. Both are semi-evergreen, with each losing half to three-quarters of it's leaves by the time new growth appears in the spring. The foliage on the Palo Netbur is almost entire free of disease, while some infection is typically present on the Gerona Netbur, only visible upon close inspection, which will become more and more difficult as the tree ages, and foliage lifts up and out of close inspection range.
A common oak hybrid in the eastern United States started appearing shortly after Europeans began bringing over English oaks (Quercus robur). English oak is highly compatible with Bur oak, and their hybrid offspring often show favorable acorn production, vigorous growth, and disease-free leaves. Several of these so-called Burenglish Oaks were created through artificial hybridization at the University of Utah in the 1960's, and several of these found their way to the Shields Oak Grove in Davis, where they have stood the test of time. Selected offspring of these original Cottam Utah hybrids are performing well at the Galvez Street Planting and the Serra Street Planting.
Experimental Oaks from Northwestern Mexico
Given that many of the oaks of the American Southwest are performing well in coastal California, and given that those oak types also occur into northwestern Mexico, it seems logical that the species which co-exist with the American Southwestern types in Mexico are also likely to perform well in coastal central California. Below are several such species, culled from botanical expeditions. As a very general approximation, the northwestern "quadrant" of Mexico is an arid climate, while the southern and eastern "quadrants" experience substantially more rainfall than central coastal California. Unfortunately, virtually all of the Mexican oaks which have been grown in California arboreta and botanical gardens come from the wetter "quadrants" leaving plants from the most applicable areas untested. Further unfortunateness can be witnessed in the reality that the areas where these potentially most applicable species grow are among the most dangerous areas in all of Mexico, making the needed collecting virtually impossible.
Oaks To Avoid
Rocky Mountain Live Oak (Quercus turbinella)
English Oak and cvs. (Q. robur et. al.)
Swamp White Oak (Q. bicolor)
Post Oak (Q. stellata)
Pin Oak (Q. palustris)
White Oak (Q. alba)