Oaks ’n’ Folks – Volume 6, Issue 3 – December 1991
The recent fires in the Oakland and Berkeley hills that claimed several thousand homes and caused several billion dollars in property damage, occurred partly in oak woodlands dominated by the coast live oak (Quercus agrifolia). As homeowners, open space managers, and arborists assess fire damaged oaks, should accurately evaluate the extent of fire damage, and the likelihood for continued survival. Coast live oaks contribute to the overall aesthetic beauty of the East Bay hills, provide stabilization of the soils on steep hillsides, add property value to homesites, and contribute to wildlife habitat value.
Effect of Fire on Oaks
Coast live oaks are native trees in the Bay Area, and are well-adapted to fire conditions. Prior to European settlement, oak woodlands were subject to regular natural fires as well as controlled fires by Native Americans. Fire effects range from complete tree consumption, to wounding of the cambium area (see Figure 1 – conducting tissue under the bark), to death of the leaves, to no visual effects at all. Longer term effects may include introduction of fungi causing rot to the structural part of the tree due to wounding and increased probability of tree failure in the future. Oaks have a tremendous capacity to resprout from the base of the tree, the trunk, and the branches to allow recovery of the tree following fire.
The major effect of fire on oaks is determined by the temperature reaching the living cells of the cambium beneath the bark. Coast live oak, noted for its thick bark, provides good insulation for living cambium cells. Bark thickness increases with tree diameter at breast height (d.b.h.), so that larger trees are much more resistant to high temperature fires than thin-barked small trees. The texture of the bark also effects the severity of the fire injury. Coast live oak has relatively smooth-textured bark, which actually inhibits the fire from being carried up the trunk of the tree.
Assessing Fire Damage
Burned tree leaves and small stems are not a major source of concern, as new leaves can be regenerated if the cambium area is not severely damaged. The most critical factor to evaluate is the amount of damage to the cambium of the trunk. This is done by the classifying the level of char from the fire into one of the following three classes: light – spotty char or scorch with scattered pitting of bark; medium – continuous charring with minor reduction in bark thickness; heavy – continuous charring with pronounced reduction in bark thickness with underlying wood sometimes exposed. Light and medium char on the trunk of the thick-barked coast live oak usually indicates that fire intensity was not sufficiently high to seriously injure the cambium. For bark with heavy char, further evaluation will be necessary. If the bark is completely consumed, or has cracked or separated from the wood, then the cambium is dead. If the bark is still intact and firmly attached, carefully peel away the bark in a small area under the char. If the cambium area under the bark has a yellow tinge, rather than a healthy white or pink, then the cambium is either dead or seriously injured. It may be possible to detect a fermented aroma when the cambium is seriously damaged.
Management of Oaks After Fire
Coast live oak trunks can survive fire damage, even when completely charred. The size of the tree influences how much insulation is provided by the bark. The amount of charring also indicates how intense the fire was at the tree. Research has shown that most coast live oaks over 6 inches d.b.h. will survive even heavy charring conditions. However, an evaluation of all trees with heavy charring is recommended in the East Bay hills following the procedure outlined above because of the possibility of tree failure in the future due to rotting fungi entering through damaged cambium. If the circumference of the tree is over 40 percent circled with dead or seriously damaged cambium, then it would be wise to remove the tree to prevent future tree failures, especially in areas where hazards of personal injury are high if the tree should fall. If you do cut a damaged tree, there is an excellent possibility that the stump will resprout and form a new tree. The table below gives some general recommendations which might be followed to guide your evaluation of damage to coast live oaks from the fire.
Trees that are left should continue to be evaluated for several years to determine if they in fact have been able to survive the fire’s effects. Once the tree has resprouted from the branches or trunk, you will have a good idea of how extensive the final damage is. Resprouting may begin within a few week, or may take until this spring. You should plan to prune off dead wood of the tree to minimize future safety concerns.
It may be wise to seek professional assistance to guide your evaluation of the tree’s damage, follow-up pruning, and continued monitoring of tree health. A professional arborist is trained in the biology and safety issues involved in these evaluations. The International Society of Arborists certifies its members who have taken advanced training in tree care, and have passed a rigorous examination testing them on their knowledge. You would be best to hire a Certified Arborist to assist you.
Richard B. Standiford
Univ of California, Department of Environmental Science, Policy, and Management
Plumb, T.R.; Gomez, A.P. Five southern California oaks: identification and postfire management. General Tech. Report PSW-71. Berkeley, CA: Pacific Southwest Forest and Range Experiment Station, Forest Service, U.S.D.A.; 1983. 56 p.