What is Killing Our Oak Trees and What Can We Do to Help?

By Colleen O’Hara

Oak decline has been a growing concern in our region. In fact, many of us have experienced this outbreak firsthand in our own back yards or neighborhoods. But why are these oak trees dying and what can we do about it?

How do we know that we have a problem with our oak trees? 

The first indication of oak decline is visible in the upper canopy of the tree. Dieback begins from the tips of the branches and progresses inward and downward. Branches can die during the growing season sometimes leaving a canopy of brown leaves, but typically branches will fail to leaf out in the spring, said Patrick O’Brien, an Urban Forester with Fairfax County Urban Forest Management in a recent webinar on oak decline.

Eventually, larger branch and limb dieback will occur over several years or even decades. Other symptoms include sparse foliage, and new sprouts along the trunk and limbs called “epicormic sprouts”, O’Brien said. 

The occurrence of oak decline can vary from a few trees in an urban forest to several thousand oaks in large, forested areas. So far, there has been no scientific evidence in Virginia of Sudden Oak Death, which is a water mold pathogen, or of Oak Wilt, which is a fungal disease, he said. 

The next question is: What is killing oak trees?

 “Unfortunately, most of the time there is no clear explanation for why an oak died or what is responsible,” O’Brien said. “Tree death is the result of a combination of interacting stresses over years or decades.”

These stressors involve the interaction of “predisposing,” “inciting,” and “contributing” factors, and these factors can vary considerably from one area to another, he said. 

Predisposing stressors include age, poor soil, and density (tree spacing). Inciting factors include drought, soil compaction, high temperatures, root damage, and defoliating insects. Contributing stressors include armillaria fungal root rot, two-line chestnut borer, and ambrosia beetles. 

O’Brien notes that, generally in Fairfax County, some oaks are more vulnerable to oak decline due to their advanced age and history, which makes them more susceptible to inciting factors and more vulnerable to insects and diseases, he said.  

It’s impossible to know the entire history of a specific tree, or what factors have impacted it over the years, O’Brien said. As oaks age they have less tolerance for events such as land use changes and disturbances, and competition. 

Short-term discreet events add further stresses to oaks and initiate their decline. These include early season damage to foliage caused by a wind storm, insects, or late spring drought and frost.

During drought, defoliation, or injury “a tree must use its stored food reserves to recover,” O’Brien said. Recovery from a prolonged drought is a significant strain on the tree’s carbohydrate reserves, which can affect the tree as many as 10 years after the event. “This could leave them weakened and vulnerable to insect attack.”

As noted above, secondary insects and diseases cause further stress and damage to trees, including armillaria root rot, two-line chestnut borer and ambrosia beetles. 

Normally these insects and diseases are present in the environment and take advantage of stressed trees, but by themselves cannot initiate oak decline, O’Brien said. When contributing factors such as these are present in oak trees, they are considered “the nail in the coffin.”

Fairfax County is in a recovery period from the timber boom of the late 1800s to early 1900s, which means most trees in the forest are around the same age, are getting older, and as they grow larger, compete for resources, said Jim McGlone, Urban Forest Conservationist in the Virginia Department of Forestry. 

These factors along with drought, flooding, and unusually cold weather, cause stress on trees. “As the tree develops more and more stress, they become less able to resist endemic secondary pests and pathogens,” McGlone said during the webinar. 

So, with all of these stressors, what can we do to help our oak trees? 

First, water trees during a drought! Trees in the Mid-Atlantic are adapted to our typical summer weather and water regime, with temperatures between 80 and 92 and around 2 inches of rain per month. Use a soaker hose placed on the ground in a spiral around tree and let it run over night in a slow trickle of water. Use a soil moisture meter to check if the soil is dry 3 inches deep. If it is, then it’s safe to water deeply. Be sure not to overwater, McGlone said. How much is too much? Vincent Verweij, Urban Forest Manager with Arlington County, Virginia, says it is difficult to overwater an established tree unless the soil is already soaked. He recommends watering mature oaks only during dry periods.

Second, get rid of turf, especially under trees. Turf likes warm, dry, bacterially-dominated soils and trees like cool, moist, fungally-dominated soils. “Trees and turf are fighting with each other to create the conditions in the tree’s root zone that the tree likes,” McGlone said. 

There will be five times more tree root mass under mulch than there will be under turf, he said. Turf tends to restrict the infiltration of water and oxygen in the soil below its root zone. Pull the turf back and mulch around the tree with two to three inches of green or brown mulch. Mulch to the property line if possible, but at minimum to the tree’s “drip line,” which generally extends from the trunk to where the tree canopy or branches end. The roots however, extend much further. 

“Under a good organic mulch you will get cool, moist, fungally-dominated soil, which is what the trees like and what you will find out in the forest,” McGlone said.

One resource is Chip Drop, a service in which arborists and tree companies deliver free wood chip mulch to your house that they can’t use. But beware: they may provide large loads that you will probably need to share with your neighbors! 

When mulching, wood chips are best, and leaf litter is good too. Avoid bark mulch or mulch that creates a barrier on top of the soil and doesn’t allow water or vapor to penetrate. 

Another option is using “green mulch,” which means creating a shade tolerant native plant garden underneath the tree to keep the soil cool and increase water infiltration. This approach has the added benefit of supporting native insects and birds, which are in decline, and sequestering more carbon than turf would, McGlone said. “We have evidence that people planting native plants in their yard help support our bird and insect population,” he said. So, this is a win-win for trees and local birds and insects.

Don’t mound mulch against the trunk of the tree or above the root flare either. This creates a “mulch volcano,” which could eventually kill the tree by making it susceptible to disease and insects.

These tree-friendly approaches may also add some years to the life of an oak that is in decline. In addition, an arborist can help reduce soil compaction, and treat with growth regulator to simulate root growth. 

However, if it’s time to plant a new oak, look for one that is less than 1.5 inches in diameter, create a good size mulch ring that at least extends to the drip line, and if there is enough space, plant it in a group 10 feet apart from other trees. “If the tree is well planted, small and given room, oaks can grow pretty fast,” McGlone said. “It comes down to right tree, right place.”

To view the webinar in its entirety, go to: https://www.youtube.com/watch?v=Elq9cZyejFM&list=PLzBdBgskem3RnAUikVojAbe58jrDKmRKM

Additional links: https://www.fairfaxcounty.gov/publicworks/trees/about; https://www.arlingtonva.us/Government/Programs/Sustainability-and-Environment/Trees; https://www.arlingtonva.us/Government/Programs/Sustainability-and-Environment/Trees/Tree-Care-and-Removal/Watering.