We’d never expected to come upon a well-researched tree article that made us laugh. This one from the HCN Newsletter cracked us up: It was riff on a widely-circulated but biased pamphlet on eucalyptus, and it was spot on. [Edited to Add from the Comments below: “To truly appreciate this parody, you might want to read the NPS publication upon which the parody is based.” It’s entitled “Eucalyptus: A Complex Challenge.” The link here goes to a PDF of that pamphlet.]
As the introduction to the article says (some emphasis added):
We dedicate the following article to those who may be unaware of the history, problems and hazards of oak trees. The style and tone parodies a widely distributed brochure about eucalyptus trees, but the information in this article is based on facts.
We republish the article with permission. [We’d also like to clarify that we don’t actually dislike oaks – not does the author. We love oaks as we love eucalyptus as we love trees.]
OAKS: A COMPLEX CHALLENGE by Lynn Hovland
Oak trees could take over the world and threaten the health of ecosystems. There are 500-600 oak species. Approximately 20 of them are said to be native Californians—but they were not always native. Evolutionary studies suggest that the oak genus Quercus first appeared in southeast Asia about 60 million years ago. Several oak species later immigrated to Europe, North Africa, North America and Central America. According to Colin Tudge in The Tree, no other forest tree is as widespread throughout the world as oaks.
One reason why these aliens have become so pervasive (dare we say “invasive”?) is that they display an uncanny ability to survive. They resprout aggressively from their base, trunk, and even from their roots. Once established, they spread rapidly, displacing species (like redwoods) that had arrived before them, forever changing prehistoric landscapes.
Many oak species bear both male and female flowers on the same tree, so they are easily wind-pollinated. (How fair to other tree species is that?) The fruit of all oak trees is the acorn, which is edible for some animal species, toxic to others. Birds and small rodents, with their habit of digging holes to hide acorns, deserve perhaps the most blame for the incredible birth rate of oaks.
But oaks also resprout massively from trees either cut down or ravaged by fire. Immediately after a fire such as the 1991 Oakland-Berkeley Tunnel fire, an area may look completely free of oaks, but, in a few years, clones or descendants of those fire-destroyed oaks will be back, as big as ever, looking as if they had had nothing to do with what happened here.
Oaks have an astonishing growth rate, averaging 2 feet each year. When fully grown they may be 60-100 feet tall.
EAST BAY OAKS
The most common oak species in the East Bay is the coast live oak. Live oaks are no more alive than other oaks. They are called “live” because they are evergreen. Live oaks keep most of their leaves in the winter although they carelessly drop some leaves all year round, causing some unsightly and flammable buildup of litter no matter what the season. Coast live oak can look either shrubby and somewhat scruffy, or it can be a tall tree with a thick trunk and many branches reaching like tentacles up and sideways. It tends to grow in shady canyons and places where its roots can find some water even in the summer.
Another often-seen evergreen live oak in the East Bay is the canyon oak. It has gray bark and tiny yellow hairs on its leaves and acorns. Evergreen oaks, like all evergreen trees, tend to be more flammable than their deciduous relatives. But the thick, gnarly trunk of the coast live oak, like all thick tree trunks, is difficult to ignite.
Deciduous oaks look more like eastern oaks. They all have lobed leaves. Common deciduous oaks in this area are blue oaks and Oregon white oaks. Although deciduous oaks are not “live” oaks, they are not dead oaks. They just look dead because they lose their leaves in the winter.
Oak woodland, sometimes called oak-bay or oak-laurel woodland or oak-bay laurel woodland, is made up of a community of trees dominated by oaks, with subordinate, usually smaller native trees―bay laurels, buckeyes and redbuds―that grow so close together that their crowns may form a canopy. Some of the subordinate tree species in an oak woodland may have existed in this area before oaks invaded the Americas.
Oak savanna is considered a specific type of oak woodland, but it is not a forest at all. It is a fire-dangerous community of tall grass and shrubs (chaparral brush) with lone oak trees scattered in shaded places or along creeks, monopolizing whatever water their roots can reach.
In 2006 the California Department of Forestry and Fire Protection, in its “General Guidelines for Creating Defensible Space,” stated that “Oak woodlands, the combination of oak trees and other hardwood species with continuous grass groundcover, are found on more than 10 million acres in California. Wildfire in this setting is very common, with fire behavior dominated by rapid spread through burning grass.”
A lowly relative of coast live oak, scrub oak is a Quercus chaparral shrub that forms dense, impenetrable thickets with other chaparral species (such as baccharis, aka coyote bush) on sunny slopes. Unlike other oaks, scrub oak has several stems at ground level. Scrub oaks are evergreen, but are more flammable than coast live oaks because their leaves and branches overhang close to or even touch the ground.
Scrub oaks typically contain layers of fire-dangerous dry dead wood surreptitiously hidden below branches with evergreen leaves. Research has shown that, in wildland fire, the sharply pointed leaves of scrub oak ignite at those sharp points and eject brands causing spotting ahead of the fire front. (See Steven Smith and others, with David Wiese, “Ignition Behavior of Live California Chaparral Leaves.”)
PROBLEMS FOR OAKS, HAZARDS FOR HUMANS
If and when UC and EBRPD are successful in taking down most non-native trees in the East Bay, there is no evidence that either oak woodland or oak savanna will replace them in the areas where the non-natives now grow. (See Neil G. Sugihara, Fire in California’s Ecosystems.) However, since the odds seem to favor a continued explosion of the oak population, it is important to know about some problems and diseases that afflict oaks. (See James Downer, “Diagnosing Your Oak Tree,” Landscape Notes; another source is here.)
If you have ornamental oaks in your garden, and you suspect that any of these problems and diseases afflict them, you should seek advice from a licensed arborist. Some of these diseases/ problems will also be found in oak-bay woodlands and oak savannas.
- Sudden Oak Death. The greatest problem for oaks these days and far into the future. [Note: We will publish more about this another time.]
- Summer watering. May cause problems where disease is already present. Oaks are adapted to rainy winters and dry summers. They should not be planted on lawns or in gardens that require regular watering or sprinklers. Too much watering may cause oak root rot or oak fungus. Signs of both problems: thinning foliage and defoliation. When oak fungus strikes, all the leaves may die on the tree. With root rot, dark cracks may appear on the trunk, with associated “bleeding” and cankers.
- Mistletoe. Often parasitic on oaks, especially on trees weakened by overwatering or other stresses. Signs: dying branches or stunted growth.
- Oak worms. These tiny caterpillar critters are always present in oak trees; in most years they cannot be seen. But cyclical occurrences of oak worms can be bad enough to cause some folks to forego a walk in the woods. Signs of this problem: Hundreds of wriggling worms dropping from trees on one’s head, in one’s hair, and down one’s neck. To prevent rampant oak worms from ruining a hike, carry an umbrella.
- Powdery mildew. Can kill an oak. Signs of that problem: white or yellow leaves.
- Wood decay. Starts at the base of the trunk. Signs of that problem: large mushrooms on infected roots and trunk.
- Raw acorns. Can be toxic for people and some animals. (It’s not a problem for the tree, of course.) Humans should not eat raw acorns. The Native Americans who depended on acorns as a staple food first leached out the tannic acid by soaking the acorns in water. Dogs have been poisoned by drinking water that has had acorns soaking in it.
- Contorted trunks. Early loggers found the contorted trunk of the coast live oak difficult to shape into useful timbers. Cutting down such large trees required special skill and equipment. Some speculate that difficulties in logging twisted oaks could have been responsible for the rapid disappearance of redwoods that had grown up straight and narrow in the East Bay. Redwoods were easier to log, so they were cut first. Not as resilient and tough as oaks, redwoods thrive only under cool, foggy conditions. Sadly, they never returned in sufficient number to be able to compete successfully against the oaken aliens.
- Oaks planted in the wrong place. Oaks do not make good windbreaks. They should not be planted close to homes because they may set them afire if the fire-dangerous crown of the tree looms above the roofline. If an oak is overwatered, the roots and wood in the trunk and branches may decay quite rapidly. Any tall tree, including oaks, may drop limbs or topple over without warning especially if the tree has been stressed by overwatering or by freezing temperatures. Oaks have been called “orphan-makers.
- Allergies to oaks. Oak trees release vast amounts of pollen in the spring. The pollen can be carried miles away to trigger allergies in people that are sensitive to it. The pollen affects the nose and sinuses, causing sneezing, runny eyes, a sore throat and even mental dullness.
Without active management, oak woodlands can become overgrown with dense, highly flammable undergrowth. Oaks tend to grow in close proximity to California bay laurels. This intimacy among tree species now considered “native” (but who knows when they immigrated) can be detrimental because bays carry Sudden Oak Death, and may be contagious without any signs of ill health. Poison oak (not a relative of Quercus) is often part of the vegetation that thrives under oaks. Hikers have reported seeing poison oak unashamedly hugging oak trunks.
Oak management can entail removing the undergrowth (which may be dry in the fire season), and removing lower branches up to 8 feet from the ground. This is especially important in situations where oaks are surrounded by tall dry grassland, brush or shrubs such as coastal scrub, broom, and coyote brush. Coyote brush is especially flammable; its growth close to oaks should not be encouraged.
Native Americans who lived in the East Bay before the arrival of the Europeans managed oak woodlands by burning them regularly. Oak leaves and fallen bark are easy to ignite. “Indians regularly burned oak groves to make acorn gathering easier, increase green fodder for game animals such as deer, and encourage the growth of new straight shoots of hazelnut, sourberry, redbud and buckbrush for basketry.” (Laura Cunningham, A State of Change: Forgotten Landscapes of California.)
SOME PRACTICAL USES FOR OAKS
- Oaks make great firewood; the wood splits easily; oak fires last a long, long time.
- Oak wood makes good furniture, wine barrels, floors and buckets.
- Oak wood is often used in house construction and shipbuilding.
- California Department of Forestry and Fire Protection, “General Guidelines for Creating Defensible Space,”2006.
- Carle, David, Introduction to Fire in California, UC Press, 2008.
- Cunningham, Laura, A State of Change: Forgotten Landscapes of California, Heyday Books, 2010.
- J. Douglas Doran and others, “Fire in the Wildland-Urban Interface: Selecting and Maintaining Fire-Wise Plants for Landscaping,” IFAS Extension.
- Downer, James, “Diagnosing Your Oak Tree,” Landscape Notes, Vol. 19, No. 4. [PDF]
- Keator, Glenn, Plants of the East Bay Parks, Mount Diablo Interpretive Association, 1994.
- “Quercus Agrifolia,” US Forest Service Database,
- Smith, Steven and others, with David R. Wiese, Forest Fire Laboratory, USDA Forest Service, Riverside, CA, “Ignition Behavior of Live California Chaparral Leaves,” Google document, 2003.
- Sugihara, Neil, ed. with others, Fire in California’s Ecosystems, UC Press, 2006.
- Tudge, Colin, The Tree, Crown Publishers, 2006.