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Climate Change, Weather Extremes, and Oak Decline

By Leslie A. Cameron, Extension Master Gardener

Oak decline in the North Ridge neighborhood, Alexandria, VA
Oak decline in the North Ridge neighborhood, Alexandria, VA, 2019.

In 2014, Jane Seward and Lynn Gas began noticing a lot of trees dying in their North Ridge neighborhood in Alexandria, Virginia. By 2019, it seemed the losses had accelerated dramatically. On a walk through about 20 blocks of their neighborhood, they counted approximately 90 dead and 90 seriously declining trees, mostly Quercus alba (white oak) and Quercus montana (chestnut oak), and they noticed that many of the chestnut oaks along a ridge line were dying. Some of the trees had infestations of ambrosia beetles as well.

In September 2019, the University of Maryland Extension’s Total Pest Management/Integrated Pest Management (TPM/IPM) Weekly Report for commercial horticulturalists included information about large numbers of white oaks dying in Takoma Park, a heavily tree-lined community in Maryland. Extension Specialist Stanton Gill asked readers if they were seeing similar problems. Over the next two weeks, Gill was inundated with reports of trees in the white oak group declining and dying all over the area.

What’s Happening to the Oak Trees?

According to forestry experts, there does not seem to be one definitive answer, but several factors are likely. Although the problems seem sudden, some of the decline in oaks has been happening for a while. The Virginia Department of Forestry Forest Health Program regularly gets questions about dying oaks. Forest Health Manager Lori Chamberlin says sometimes the cause is definitive, like insect or chemical damage, but most of the time it’s not clear. An important factor is that many of Virginia’s oaks are aged. Many of the white oaks in Northern Virginia were mature when urban and suburban areas developed around them in the 1950s and 1960s. Being older, these oaks are more vulnerable to the stresses of human activity, poor soil, weather, and other factors. In Virginia, prolonged drought is a stress factor. Trees that are stressed by other factors are more susceptible to disease and insect pests.

White oaks with brown foliage,
White oaks with brown foliage, September 2019.
Photo © 2019 by David Clement.

Urban and suburban trees also deal with urban heat islands, competition with turfgrass, pollution, and construction. Most of an oak’s roots are in the top 18 inches of soil, and the roots typically extend well beyond the drip zone. Compaction of soil from heavy equipment or foot traffic in the root zone, grading or digging that cuts roots, and adding impervious surfaces in this root zone interfere with a tree’s uptake of oxygen, water, and nutrients.

In the fall of 2019, however, it seemed the decline in white oaks was accelerating. After they received the reports in September, University of Maryland Extension officials Karen Rane, Stanton Gill, and David Clement released Browning and Defoliation of White Oaks, summarizing their observations:

  • Affected trees were mature (40-80 years old).
  • The onset of symptoms was rapid—leaves browned quickly in August and September.
  • While other oak species showed twig dieback, the sudden browning of the crown (a sign of compromised root systems) was mostly in white oaks.
  • Most of the affected white oaks were in urban and suburban landscapes (though some were in forests and other areas with unrestricted root zones).
  • Some of the affected trees had trunk wounds, previous root damage, and previous branch dieback, but many had no obvious signs of injury.
  • Opportunistic invaders (like ambrosia beetles, Hypoxylon canker, fungal root rot) were often seen in affected trees.
Fine sawdust-like frass (arrows) from ambrosia beetles at the base of white oak with brown foliage.
Fine sawdust-like frass (arrows) from ambrosia beetles at the base of white oak with brown foliage. Photo © 2019 by David Clement.

Rane et al. speculated that the accelerated decline may be related to root problems caused by soil compaction, root or trunk damage from construction, and weather extremes of rainfall and drought. Compromised root systems are less able to support the needs of the tree. Poor pruning and repeated defoliation from opportunistic insect or disease pests can continue to weaken the tree.

Chamberlin agrees that recent weather extremes are probably a key contributor: “Oak decline has been happening in the background for a lot of trees, and it’s likely been accelerated by the extreme weather we’ve had” (Johnson, 2019). In 2018 and early in 2019 this region experienced very wet conditions, with flooding and saturated soil, which can damage the fine roots of trees and encourage root rot diseases. Then in August and September 2019, conditions turned hot and dry, trees rapidly lost moisture through their foliage, and the compromised root systems could not meet the increased need for water.

Forestry and Natural Resources Extension Agent Adam Downing has pointed out that white and chestnut oaks, the species most affected, are “upland” species, which are less tolerant of anaerobic soil conditions (such as  extended water saturation or soil compaction). Many of these trees were in effect living in “bottomland” conditions in 2018 and early 2019.

What Tree Owners Can Do

One of many oaks in Virginia that died in 2019.
One of many oaks in Virginia that died in 2019.
Photo © 2019 Lori Chamberlin, Virginia Department of Forestry.

In October 2019 Vincent Verweij (Urban Forestry Manager for Arlington County, Virginia) and Kirsten Conrad (Extension Agent, Arlington and Alexandria) gathered urban forestry experts for a meeting to talk about the declining and dying white oaks. These experts also identified the extreme storms and extended drought as key factors, as well as longer term changes in our stormwater drainage network, expanding impervious surfaces, and undergrounding streams.

They released Oak Decline in Northern Virginia, with suggestions on what tree owners can do to protect trees, including avoiding construction, compaction, and other damage to trees, watering during dry spells, and improving the soil with wood chips (at least three inches away from the trunk).

Ed Milhous (Registered Consulting Arborist and Certified Arborist in Haymarket, Virginia) recommends ample growing room for mature trees (or consider a smaller tree), native plants under trees rather than turfgrass, leaving leaves and twigs under trees (or adding compost), minimal compaction of soil, making sure roots aren’t harmed when irrigation systems are installed, and not overwatering or letting water hit stems, to discourage fungal diseases.

In winter, it may be hard to tell if a tree needs to be removed. Tree owners should get an assessment of a tree’s health from a certified arborist to make this determination. Trees that are a hazard to people, homes, or businesses may need to be removed immediately. For others, a certified arborist may recommend waiting until spring.

Alexandria’s Help Grow Alexandria’s Tree Canopy  program and Arlington’s Tree Canopy Fund and Free Native Tree programs provide trees to residents at low or no cost.

For information on preventing and managing problems with trees, and advice on planting new trees, call the Virginia Cooperative Extension Master Gardener Help Desk at 703-228-6414 or email  mgarlalex@gmail.com.

References