Maintaining Healthy Oak and Sycamore Trees Following Wildfire

Fire damage to trees can range from minimal to severe. By using proper management techniques, homeowners can reduce the chance of tree decline or failure. Creating optimal conditions for the trees includes time for recovery, targeted pruning, mulching, and supplemental watering.

This article describes how fire impacts oaks and sycamores, how the trees adapt, signs of tree damage homeowners should look for, and how homeowners can properly manage fire-damaged trees.

San Diego’s recent fires burned 27,000 acres and damaged hundreds of native oak and sycamore trees, including coast live oak (Quercus agrifolia) and western sycamore (Platanus racemosa) trees located within the Escondido Creek Watershed. These native trees are important parts of the local ecosystem and are generally able to recover following fire but in some extreme instances may need assistance.

Maintaining Healthy Oak and Sycamore Trees Following Wildfire

Join Dudek’s urban foresters for a free lecture “Healthy Trees Following Wildfire” on Tuesday, July 29th from 6 to 7 p.m. at the Elfin Forest Interpretive Center, 8833 Harmony Grove Road, Escondido 92029. This talk is sponsored by The Escondido Creek Conservancy.

„ Oaks and sycamores have adapted to tolerate low to moderate-intensity fires.

„ Leading cause of fire-related death in oaks and sycamores is damage to the cambium layer and/or attacks by secondary pests and pathogens

„ Following a low- to-moderate- severity fire, an oak may look dead, but is often only an aesthetic impact.

„ Through time and proper management (i.e. supplemental watering, mulch and targeted pruning) many fire-damaged trees recover.

1. Fire-damaged coast live oak tree, 2006, exhibiting leak scorch and trunk damage. Source: Dudek.

Did You Know?

2. Minimally damaged sycamore trees following low-intensity groundfire.

Source: Dudek.

Fire Adaptations

Oak and sycamore trees, unlike many other trees in Southern California, are well adapted to tolerate fires like those that occurred in San Diego in the spring.

Oak trees, specifically coast live oak, interior live oak (Quercus wislizeni), California black oak (Q. kelloggii), canyon live oak (Q. chrysolepis), and California scrub oak (Q.

berberidifolia), have adapted to tolerate and respond to fire in several ways.

Thick Bark. Bark 1 to 2 inches thick on coast live oak and canyon live oak is a primary defense against low- to moderate-intensity fires. It protects the trees’ vital cambium layer (the living layer that transports water and nutrients) by insulating it from heat and flames.

Foliage loss. Often seen following a low- to moderate-intensity fire, foliage loss reflects the oak trees’ ability to survive. According to McCreary (McCreary 2004

), oaks suffer little long-term damage from the burning of their foliage and can survive up to 1 year with no foliage by suspending many functions and living off stored reserves.

Additional adaptations. Other adaptions to fire include extensive, deep root systems; thick acorn shells (to protect the regenerative function); and the ability to resprout from branches, root collar (the transition point between the roots and the trunk), and trunk quickly following a fire. Each oak species has adapted uniquely and will respond to fire in various ways, depending on severity of the burn.

Sycamore adaptations are not as defined as those occurring within oaks. Sycamore trees typically grow in wetter areas, where fire is less common and where they can maintain a high level of plant moisture. This moisture helps them reduce ignition during low- to moderate-intensity fires. The most common fire adaptations observed in sycamore trees include vigorous resprouting from the tree base and branches, the ability to withstand low-intensity heat, and the ability to survive low to moderate basal damage.

Like oak trees, sycamores can survive for extensive periods following fire and can sustain life with little to no foliage. However, unlike the oak tree, the sycamore does not have thick bark to protect its cambium layer and as such is more

susceptible to fire damage. Sycamore survival following fire events is dependent on the severity and location of the damage. It is not uncommon to see very old, large sycamore trees with large cavities at their trunk base from past fire damage.

1

Plumb, T.R., and A.P. Gomez. 1983. Five Southern California Oaks: Identification and Postfire Management. General Technical Report PSW-GTR-71. Berkeley, California: U.S. Department of Agriculture, Forest Service, Pacific Southwest Forest and Range Experiment Station.

2

McCreary, D.D. 2004. “Fire in California’s Oak Woodlands.” University of California Integrated Hardwood Range Management Program. June 2004.

http://www.californiaoaks.org/ExtAssets/FireByMcCreary.pdf.

4. Yellow/tan or dark color indicates death of the cambium layer.

Source: Plumb and Gomez 1983.

3. Coast live oak tree following moderate-

to high-intensity fire; exhibits extensive

foliage and trunk damage. Source: Dudek.

Fire Impacts on Oaks and Sycamores

Oak and sycamore trees have many adaptations to protect them from low- to moderate-intensity fires. However, many of these adaptations are not adequate to protect trees from high-intensity fires that expose the tree to prolonged periods of fire and high heat. This type of fire typically occurs when fuels accumulate beneath tree canopies, resulting in higher-intensity fire that burns for longer duration or when ground fire transitions to a crown fire.

When high- and even moderate-intensity fires occur within the vicinity of oak and sycamore trees, it is often the trees’

cambium layer that is significantly damaged. Severe fires can heat the cambium layer to extreme temperatures that ultimately kill the layer. The resulting damage to the trees’ cambium inhibits the trees’ ability to transport essential water and

nutrients—the larger the area affected, the harder it is for the trees to carry on basic physiologic functions.

The death of the cambium will eventually result in loss of foliage from water stress, attack by insects and disease, and potentially tree death. However, it should be noted that oak and sycamore trees can survive a partial loss of the cambium layer and continue to grow vigorously in the following years. In rare cases where extreme fire has occurred, the root system may be damaged significantly by high temperatures that extend into the soil. In extreme fires, whole trees may be consumed, including all aboveground biomass and even the roots to depths of 3 feet or more belowground.

Secondary Pests and Pathogens

Fire damage, depending on the level of severity, may leave trees exposed to secondary pests and pathogens. Specific pests, pathogens, and stressors that may be seen following fires or other tree stress conditions (such as drought) in oak trees, include the western bark beetle (Pseudopityophthorus pubipennis), ambrosia beetle (Platypus spp.), fungal pathogens such as Geosmithia pallida (a newly found fungal pathogen causing foamy bark canker disease), and sunscald, amongst others. Sycamore trees suffer from a number of pests as well, including borers. Pests and pathogens are known for attacking trees that are experiencing stress. It is these secondary factors that may determine whether a tree survives or not.

Many of these secondary issues can be minimized through the proper management and monitoring techniques discussed below in Managing Trees Following Fire.

Signs of Significant Damage

Following a low- to moderate-severity fire event, an oak or sycamore tree may look dead, but due to the tree’s adaptations and resilience, it is often still alive and the impact is primarily aesthetic. It is important to inspect the trunk and base of the tree for signs of significant damage. Signs of significant damage may include the following:

Large basal wounds on the lower trunk. Smaller wounds (less than 4 inches in diameter) will heal quickly, but larger basal (at the base of the tree) wounds (over 4 inches in diameter) may be a sign of extensive cambium damage. Additionally, larger wounds will take longer to heal and leave the tree exposed to secondary pests and disease.

Amount of fire damage to the trunk. Is the burned bark light and spotty, medium and continuous, or heavily damaged? Medium and heavily damaged bark should be inspected by an International Society of Arboriculture (ISA)- certified arborist to determine whether the cambium layer has been damaged and how significant the damage is.

Timing of the fire. The timing and frequency of the fire will impact the survival chances of a fire- damaged tree. Trees that burn in the winter and

5. Three classifications of fire-damaged

bark. Source: Plumb and Gomez 1983.

spring may recover much more quickly than those that burn in the hot/dry summer months when rainfall is nonexistent. Trees burned in May of 2014 will have a long dry period before rainfall occurs and may need supplemental irrigation on a case-by-case basis. Drought conditions throughout Southern California have resulted in lower-than-average annual rainfall levels and as such have caused stress for many trees. In years of lower-than-average rainfall, the need for supplemental irrigation for important trees may increase.

Significantly damaged canopy. As stated previously, oak and sycamore trees can withstand a loss of foliage; however, there are times when the canopy and foliage may be damaged so severely that the tree will not be able to recover.

Cambium, Twig of Branch Damage. Using a sharp knife, hatchet, or similar tool, gently scrape the bark away from small areas of selected twigs, branches, and even the trunk to determine the extent of cambium damage.

Live, healthy cambium will appear green and/or white. Damaged/dead

cambium will appear dark or tan/yellow. This is a good way to help determine which branches are dead and can be removed.

Pest infestation. Look for signs of boring beetles such as fine wood shavings,

frass (insect excrement that resembles tiny wood pellets, produced by the activity of boring insects), and holes in the bark. Moderately to heavily infested trees will not likely be able to survive and should be considered for removal so they do not become issues for adjacent healthy trees.

When inspecting your tree for signs of damage, take precautions for damaged and broken limbs, which can fall with no warning. High winds and heat may damage the canopy, leaving damaged, cracked, and split limbs exposed. If you suspect that your tree has sustained significant damage, contact an ISA-certified arborist to inspect the tree for damage.

Managing Trees Following Fire

Proper management of oak and sycamore trees following a fire event will be critical to their post-fire recovery. The good news is that most naturally growing oaks and sycamores are best left alone for several months to a year. However, important specimen trees can be inspected post-burn by an ISA-certified arborist as necessary to determine what, if any, management actions are necessary. Following an inspection of the damaged trees, the following management recommendations may be made:

Time for recovery. Trees that sustain foliage damage may not be dead and can survive for at least 1 year following a fire event. If no regrowth, budding, or new foliage has occurred after 1 year, the tree may have sustained extensive damage from the fire or may have been exposed to secondary pests and pathogens.

Pruning. Depending on the level of damage and the presence of damaged or structurally unsound limbs, pruning may be necessary. Also, oaks and sycamores near potential targets (persons or property that could be damaged by falling limbs/

trees) should be considered for pruning on a higher priority than naturally growing oaks and sycamores with no targets.

Watering. Most oak and sycamore trees will not require supplemental water following a fire event in the winter, spring, or fall. However, during the warmer and drier summer months, supplemental water may be required. If supplemental irrigation is necessary or recommended, all watering should be focused in the area under the tree crown and out to 5 feet beyond the tree’s dripline (edge of tree crown), keeping the area directly around the trunk dry. Maintain an approximately 2- to 3-foot-wide dry area at the trunk base to reduce the possibility of fungal pathogen establishment.

6. Significantly fire-damaged oak tree.

Source: Dudek.

Chemical applications. For especially high-value oak or sycamore trees, as determined by the homeowner or ISA-certified arborist, it may be considered prudent to provide insecticides and/or supplemental nutrients. Systemic insecticides can help minimize the secondary pest attacks that often follow fire damage. Likewise, soil amendments can help provide the damaged tree with necessary nutrients needed for repair, regrowth, and defense against pests.

Mulch. Following low-, moderate-, or high-intensity fire events, mulching may be necessary. Mulch will aid in soil and moisture retention, maintain soil temperatures, and provide organic soil amendment over time. If recommended, mulch should be kept away from the trunk at least 18 inches.

Monitoring. Trees may need to be monitored for signs of secondary attackers such as bark beetles. Fire damage, depending on the level of severity, may leave trees exposed to secondary pests and pathogens. If small bore holes are observed throughout the trunk, the tree may have been attacked by a bark beetle and an ISA-certified arborist should be contacted to further inspect the tree.

Most oak and sycamore trees damaged by fire will recover in time because they have adapted to do so. However, the potential management recommendations discussed herein may further aid in the long-term preservation of fire-damaged trees.

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Conclusion

Time is one of the most important factors in deciding how to manage oak and sycamore trees following fire events.

Oak and sycamore trees can survive for extensive periods following fire and can sustain life with little to no foliage for 1 year or more. Oak and sycamore trees in Southern California have adapted to naturally occurring fires and have the unique ability to withstand certain levels of damage where other non-native trees cannot. The topics discussed in this article provide a brief overview of oak and sycamore tree adaptations to fire, fire impacts on oak and sycamore trees, signs of significant damage, and the management of oak and sycamore trees following fire.

About The Escondido Creek Conservancy

TECC is a land conservancy which is dedicated to the preservation and restoration of the Escondido Creek Watershed. Since 1991 TECC has preserved over 2000 acres of open space through direct purchase or by working in partnership with its members, donors, landowners, government agencies and community groups. For more information, visit www.escondidocreek.org or contact Managing Director, Ann Hough at annhough@escondidocreek.org.

Additional Information

For additional information, please contact Dudek’s oak and sycamore specialists:

Michael Huff (mhuff@dudek.com) and Christopher Kallstrand (ckallstrand@dudek.com).

More information is available on-line at

http://ucanr.edu/sites/SAFELandscapes/Fire_in_Southern_California_Ecosystems/.