[Note: Blue oak (Quercus douglasii), gray pine (Pinus sabiniana), and buckeye (Aesculus californica) are three species that thrive in rugged local foothill woodland and chaparral habitat. Today and in the following weeks, this series will look at these species and the adaptive strategies they have evolved over time to thrive in their challenging environment.]
Darwin, the product of a strictly codified class society beset by enormous economic inequalities, identified a major factor of evolution to be competition for finite resources. Those species that clambered to the top of the evolutionary heap benefitted from genetic mutations and / or adaptations that gave them a biological edge over their competitors.
Some species cooperate by sharing resources. For example, in Tortuguero, a tiny strip of beach along the northeastern shoulder of Costa Rica, four separate species of sea turtle lay their eggs each year. They migrate to the beach at different times, ranging from early March to October, and feed on different resources. Millions of turtles, and untold numbers of their babies, have shared the same tiny piece of real estate for eons.
Quercus douglasii also goes by a number of other common names, including white oak, mountain oak, mountain white oak, and iron oak. But it acquired its most familiar and descriptive common name from the same person from whom its Latin binomial (scientific name) is derived. In 1831 David Douglas, a Scottish botanist, christened it the blue oak for the bluish cast of its deeply lobed leaves. (A digression: Douglas, for whom the Douglas-fir and hundreds of other western plants are named, lived from 1799 to 1834. He made three trips to the American Northwest between 1823 and 1831, encountering the blue oak on his last trip while traveling from the Columbia River in Oregon to San Francisco. He died under curious circumstances, apparently after falling into a bull trap while climbing Mauna Kea in Hawai'i.)
Surviving Drought and Fire:
Adaptations to survive the long, hot, dry summers and sparse winter rains of our Mediterranean climate include thick leaves with a bluish-green color. According to the US Department of Agriculture (USDA), blue-gray-green leaf color reduces heat absorption. During severely hot and dry years, blue oaks will sometimes shed their leaves and go dormant to conserve energy, allowing these tough little oaks to survive temperatures above 100° F for several weeks at a time. Like many plants growing in marginal soils, blue oaks are slow growers, usually increasing only a few inches each year.
A further strategy to survive drought and fire conditions is the blue oak's extensive root system, which allows it to grow through cracks in rocks to depths of 80 feet in order to reach ground water, helping it to survive in fire-prone and arid regions (Blue Planet Biomes).
Although blue oaks can tolerate fast-burning grass fires, they have less success in surviving hotter brush fires, according to University of California Department of Agriculture and Natural Resources (UCANR). If a tree does survive a fire event, it can reproduce both through seeds and by sprouting from burnt stumps. Blue oaks can produce sprouts after a low- to moderate-severity surface fire, and younger trees have the edge on older trees for fire survival odds. On younger trees, the light-colored bark (hence “white oak”) is thick and helps reduce fire damage, the USDA notes, whereas the bark of mature blue oaks is thin and will flake off as the trees age, making older blue oaks less insulated against fire. After a fire, blue oaks can also re-establish from acorns that have dropped from surviving parent trees and/or been dispersed by animals, among other possibilities.
Native American Uses:
All parts of the blue oak were woven deeply into the culture and survival of California's native peoples. It was one of more than a dozen oak species whose acorns contributed a major source of dietary nutrients and calories. Because of their superior flavor, blue oak acorns were among the most commonly gathered.
A Plant Guide published by the Natural Resources Conservation Service (NRCS) provides an exhaustive list of the ways California Native Americans used blue oak wood, bark, and acorns, including as “medicine, dyes, utensils, games, toys, and construction materials.” Locally, the Maidu used oak shoots to frame cradleboards and oak posts to construct shelter, and the Yana used an oak paddle in cooking. Traps for birds were baited with acorns, and split acorns became dice for gambling.
UC Master Gardeners of Butte County are part of the University of California Cooperative Extension (UCCE) system. To learn more about us and our upcoming events, and for help with gardening in our area visit our website. If you have a gardening question or problem, email the Hotline at mgbutte@ucanr.edu or leave a phone message on our Hotline at 530-552-5812. To speak to a Master Gardener about a gardening issue, or to drop by the MG office during Hotline hours, see the most current information on our Ask Us section of our website.
- Author: Brent McGhie, UC Master Gardener of Butte County
No doubt you have seen many species of oaks living in wild areas of Northern California. But native oaks can also be attractive and beneficial additions to the home landscape.
The key to living with oaks is to disturb them as little as possible. This is especially true of the root crown (the base of the trunk) and the root zone. The root zone of an oak tree covers an area one-third again larger than the outermost edge of the tree's foliage. Strive to keep this area as natural as possible.
The fungal disease “oak root rot” (Armillaria mellea) has evolved with California oaks. It is parasitic on oaks and other plant species, but if oaks are growing naturally in our moist winter/dry summer environment, this fungus is kept in check. However, if oaks are provided with summer irrigation in their root zone, Armillaria can become an aggressive, deadly pathogen. Do not water any plants under oaks during the summer: if they need watering, they do not belong there. Instead, native plants that are adapted to our Mediterranean climate can be grown under oaks. If winter rains are unusually low, a supplemental watering can be provided in the early spring. Do this by watering deeply in the outer two-thirds of the root zone.
Other threats to the root zone include soil compaction, paving, trenching, and changes in grade or drainage. If soil is mounded under an oak, the oxygen supply to the roots can be reduced, which can suffocate the tree. Oxygen exchange with the roots may also be reduced if the soil is compacted or covered with paving. Suffocation can also occur when drainage patterns are altered and the soil around an oak becomes saturated for extended periods. Excavating by grading or trenching can cut or damage roots. When enough roots are damaged, trees will die.
Learn more about the many benefits native oaks provide to our local ecosystem: Attend our workshop, “Plant an Acorn; Harvest a Community” on Tuesday, November 7th. For descriptions of this and all the other workshops in the Master Gardeners' Fall Workshop Series, visit our website. All workshops are free, but registration is required.
UC Master Gardeners of Butte County are part of the University of California Cooperative Extension (UCCE) system. To learn more about us and our upcoming events, and for help with gardening in our area visit our website. If you have a gardening question or problem, email the Hotline at mgbutte@ucanr.edu or leave a phone message on our Hotline at 530-552-5812. To speak to a Master Gardener about a gardening issue, or to drop by the MG office during Hotline hours, see the most current information on our Ask Us section of our website.
You may have noticed that blue oaks are turning brown in the foothills right now. They aren't dying – they are utilizing an adaptive strategy that they have evolved over time to thrive in their challenging environment.
Some species cooperate by sharing resources. For example, in Tortuguero, a tiny strip of beach along the northeastern shoulder of Costa Rica, four separate species of sea turtle lay their eggs each year. They migrate to the beach at different times, ranging from early March to October, and feed on different resources. Millions of turtles, and untold numbers of their babies, have shared the same tiny piece of real estate for eons.
Here in our own backyard, there are species that have evolved to exploit ecological niches that very few others claim. These are the various oaks, conifers, and woody shrubs that populate the foothill woodland and chaparral zone of the Western Sierra Nevada and Coast Range mountains. A dominant species in this environmental nook is the blue oak (Quercus douglasii). According to Andrew Conlin, Soil Scientist with the Natural Resources Conservation Service (NRCS), “the presence of blue oak woodlands indicates really rough growing conditions: shallow, tough soils” as opposed to the rich loamy soils of the valley, which are preferred by valley oaks.
Quercus douglasii also goes by a number of other common names, including white oak, mountain oak, mountain white oak, and iron oak. But it acquired its most familiar and descriptive common name from the same person from whom its Latin binomial (scientific name) is derived. In 1831 David Douglas, a Scottish botanist, christened it the blue oak for the bluish cast of its deeply lobed leaves. (A digression: Douglas, for whom the Douglas-fir and hundreds of other western plants are named, lived from 1799 to 1834. He made three trips to the American Northwest between 1823 and 1831, encountering the blue oak on his last trip while traveling from the Columbia River in Oregon to San Francisco. He died under curious circumstances, apparently after falling into a bull trap while climbing Mauna Kea in Hawai'i.)
Surviving Drought and Fire:
Adaptations to survive the long, hot, dry summers and sparse winter rains of our Mediterranean climate include thick leaves with a bluish-green color. According to the US Department of Agriculture (USDA), blue-gray-green leaf color reduces heat absorption. During severely hot and dry years, blue oaks will sometimes shed their leaves and go dormant to conserve energy, allowing these tough little oaks to survive temperatures above 100° F for several weeks at a time. Like many plants growing in marginal soils, blue oaks are slow growers, usually increasing only a few inches each year.
A further strategy to survive drought and fire conditions is the blue oak's extensive root system, which allows it to grow through cracks in rocks to depths of 80 feet in order to reach ground water, helping it to survive in fire-prone and arid regions (Blue Planet Biomes).
Although blue oaks can tolerate fast-burning grass fires, they have less success in surviving hotter brush fires, according to University of California Department of Agriculture and Natural Resources (UCANR). If a tree does survive a fire event, it can reproduce both through seeds and by sprouting from burnt stumps. Blue oaks can produce sprouts after a low- to moderate-severity surface fire, and younger trees have the edge on older trees for fire survival odds. On younger trees, the light-colored bark (hence “white oak”) is thick and helps reduce fire damage, the USDA notes, whereas the bark of mature blue oaks is thin and will flake off as the trees age, making older blue oaks less insulated against fire. After a fire, blue oaks can also re-establish from acorns that have dropped from surviving parent trees and/or been dispersed by animals, among other possibilities.
USDA research also reveals that the blue oak's post-fire recovery is likely aided by the fact that it withstands extreme drought by dropping leaves under water stress and producing a flush of new leaves when wet weather returns. In fact, in wet years, crown-scorched blue oaks may produce a flush of new leaves soon after fire.
Native American Uses:
A Plant Guide published by the Natural Resources Conservation Service (NRCS) provides an exhaustive list of the ways California Native Americans used blue oak wood, bark, and acorns, including as “medicine, dyes, utensils, games, toys, and construction materials.” Locally, the Maidu used oak shoots to frame cradleboards and oak posts to construct shelter, and the Yana used an oak paddle in cooking. Traps for birds were baited with acorns, and split acorns became dice for gambling.
Besides providing physical sustenance to native peoples, I imagine that the peaceful beauty of blue oak woodlands fed their souls. Twisted, dwarfed blue oak silhouettes are a classic component of the California landscape. These trees are prime examples of successful adaptation to truly demanding habitat and climate conditions.
Sources and further information:
Calscape: Blue Oak, Quercus douglasii (calscape.org)
Blue Planet Biomes: http://www.blueplanetbiomes.org/blue_oak.htm
UCANR: Welcome - UC Oaks | Oak Woodland Management (berkeley.edu)
NRCS: Quercus douglasii Hook (usda.gov)
UC Master Gardeners of Butte County are part of the University of California Cooperative Extension (UCCE) system. To learn more about us and our upcoming events, and for help with gardening in our area, visit our website. If you have a gardening question or problem, email the Hotline at mgbutte@ucanr.edu (preferred) or call (530) 538-7201.
- Author: Jeanette Alosi
By Brent McGhie, Butte County Master Gardener, July 17, 2015
On good sites with deeper soils and available moisture, blue oaks can't compete with other trees, such as live oaks. They are, however, superbly adapted to the shallow, rocky soils of the hot, dry foothills, where other oaks would struggle to survive. These trees can tolerate temperatures above 100° for several weeks at a time. In addition to an extensive system of lateral feeder roots, which are fairly shallow, blue oaks develop deep sinker roots, which can grow through cracks in rocks to as deep as 80 feet in pursuit of groundwater.
Normally blue oaks are winter-deciduous, dropping their leaves in the fall. However, when water becomes too scarce, even for blue oaks, they become drought-deciduous and simply drop their leaves and remain dormant until the following spring. Drought dormancy is common among desert and chaparral plants, but is a rare feature for oaks and for trees in general.
For homeowners who are lucky enough to have blue oaks growing on their land, this means “fall” may come as early as July or August in dry years. In fact, it has been noted that this year the leaves of some blue oaks in the Butte County foothills are already beginning to turn brown. These trees may look like they are dying or dead, but don't be fooled into cutting them down. They will normally produce a fall crop of acorns from stored food and then follow up with new leaves in the spring.
A tree owner may be tempted to water drought-stressed oaks, but should avoid doing so. Summer irrigation will encourage “oak root rot” and other soil pathogens and could very likely do far more harm than good. Our blue oaks have survived many droughts before this one. Keep in mind that the key to living with oaks is to disturb them as little as possible.