Oak Decline and the Future of Missouri’s Forests

By Robert Lawrence, Bruce Moltzan and Keith Moser | July 2, 2002
From Missouri Conservationist: Jun 2002

Faint gnawing sounds come from within the trunk of a large black oak in an Ozark forest. A red oak borer, a large wood-boring beetle with very long antennae, is chewing through the bark, preparing to emerge from the tunnel where it has been living for the past two years.

Millions of oak borers emerged during the summer of 2001 in Missouri and Arkansas, and we'll likely have even higher numbers chewing through our oaks in the near future.

Populations of the red oak borer and other wood-boring insects are dramatically increasing throughout the Ozarks. Oak-attacking fungi have also become more active. Their increased attacks on oaks are part of a phenomenon collectively called oak decline. This condition has worsened in the Ozarks recently due to a few years of drought and the advancing age of many oaks, resulting in an ever-growing number of dead and dying oaks.

Although declines have occurred at times in the past, the current episode is unique. Missouri's early history of poor or no management, over-harvesting, burning, grazing and other practices set the stage for oak decline.

Trees take a long time to mature. In the late 1800s and early 1900s, Missouri's forests, many of which contained a healthy mixture of shortleaf pine, and black, white, scarlet and post oaks, were logged on a grand scale.

By the 1920s, most of our large trees had been cut. Subsistence farmers and ranchers burned the cut-over lands to encourage grass growth. Frequent burning killed pine seedlings but encouraged hardwoods to re-sprout, resulting in the hardwood forests we have today.

Most of the trees affected by oak decline are in the red oak group. the species most severely affected are black oak (Quercus velutina) and scarlet oak (Quercus coccinea). Many of these trees have managed to survive on shallow, rocky soil on broad ridges or south- and west-facing slopes, less than ideal conditions for red oaks. Often the stands are crowded with large numbers of trees that are at least 70 to 80 years old.

Oak trees growing under these conditions become physiologically stressed as they have to compete heavily for limited water and nutrients. Older trees are less vigorous than younger ones and can't withstand repeated attacks by diseases and insects. A multitude of stresses inevitably weakens a tree to the point that it only produces dwarfed, sparse foliage, and its crown becomes thin. Branches in the upper crown die back from the tips as tree growth is reduced. Various diseases and insects attack these weakened trees. In time, the combination of stressors so weakens a tree that it dies.

In 1999, the USDA Forest Service estimated severe red oak decline and mortality was occurring on approximately 19,000 acres of the Ozark National Forest in northwest Arkansas. By June 2001, the estimated area of severe damage was about 300,000 acres. The numbers of red oak borer attacks, an indicator of the level of stress of trees, had increased to unprecedented levels of 300 to 500 attacks per tree, compared to a typical rate of less than 10 for healthy trees .

The level of oak decline and mortality in Missouri was not as severe in 2001 as that observed in Arkansas, but deteriorating conditions made oaks increasingly vulnerable here, too. In December 2001, the Forest Service estimated that more than 100,000 acres on the Salem and Potosi Ranger Districts were seriously affected by decline and wood borer damage.

Many red oak stands in southern Missouri and northern Arkansas were already stressed to their limit before the late 1990s, when two to three years of severe drought hastened their decline by making them more vulnerable to disease-causing fungi, such as Armillaria, and wood-boring insects, such as the red oak borer.

Disease and Insect Factors

The most common disease agents involved in oak decline are armillaria and hypoxylon fungi. The most common insects involved include the red oak borer, carpenterworm, and two-lined chestnut borer. All of these organisms are native to Missouri.

Armillaria is a fungus that causes white rot of woody roots. It is found wherever trees grow. Most of the time, Armillaria acts as a decomposer, decaying coarse, woody debris that has fallen to the forest floor. When trees become stressed or wounded, this fungus can also act as an aggressive parasite, attacking their root system. Infected roots no longer effectively take up water, particularly in drought-stressed soils. This causes a progressive branch die-back throughout the tree crown.

The red oak borer is a reddish brown beetle that is 7/8-inch to 1 3/8- inches long and has antennae as long as, or longer, than the rest of its body. It has a two-year life cycle. After mating, the adult female deposits her eggs in crevices in the bark of host trees, usually red oaks. The eggs hatch, and young larvae chew through the bark and into the tree where they will spend the next two years.

Red oak borer larvae, which look like white grubs, create a 1- to 2-inch wide cavity in the phloem layer just under the bark during the first summer and the following spring. By the middle of the second summer, larvae begin burrowing into the sapwood and often go deeper into the heartwood of the tree.

Each larva creates about a 3/8-inch diameter tunnel that extends several inches, first angling upward away from the bark and then turning straight up through the wood.

The adult red oak borer emerges from the tree in the third summer by chewing an oval hole in the bark near where it entered the tree as a larva.

Most adult red borers emerge from their host trees in June and July of odd-numbered years. Huge numbers of these adult beetles are expected to emerge in 2003. Trees still stressed at that time will be at high risk for increased attacks.

Despite its name, the two-lined chestnut borer commonly attacks stressed oaks. Adult beetles are small, black and bullet-shaped and have two yellowish stripes down their back. Larvae of the two-lined chestnut borer are wormlike creatures that grow to about an inch long. They spend all of their time tunneling in the phloem layer and outer sapwood just under the bark. Their tunnels wind around the trunk of the tree, disrupting the flow of food, water and minerals, but they do not go deeply into the wood.

Reducing the Impacts of Oak Decline

Periods of reduced oak growth, mainly due to drought, have occurred in the Ozarks about every 14 years. Mortality due to oak decline was common in southern Missouri following droughts of the 1980s, although not on the scale of current conditions. Droughts and the resulting effects on our oak forests are always going to be with us, so we need to manage our forests so they can best withstand these stresses.

You can prevent or reduce oak decline by taking steps to improve the health and vigor of trees on your property. Start by increasing the diversity of tree species on your property. Select species appropriate for the conditions of the site. Remove poorly-formed trees and trees with unhealthy crowns, but keep some standing dead trees and trees with cavities for wildlife habitat. Some cavity nesting birds, woodpeckers for example, are important predators of wood-boring insects.

Landowners may avoid effects of oak decline by regenerating oak stands before trees become vulnerable due to old age. Harvest mature trees and allow seedlings and sprouts to re-stock the site.

Remember that "old age" varies by site condition and tree species. For example, trees growing in fertile, deep, well-drained soils on north-facing slopes can remain vigorous at much older ages than trees of the same species growing in thin, rocky soils on south-facing slopes. White oaks and post oaks grow slower and live longer than black and scarlet oaks. Under very good growing conditions, white oaks may live up to 600 years, while black oaks may live up to 150 or 200 years. Their life spans under most Missouri conditions, however, are typically much shorter.

Given the composition of many of our forests, oak decline seems inevitable. With it, we'll see reduced mast (acorn) production, degraded timber value and a higher risk of wildfires, as well as increased danger from toppling trees.

On the other hand, the dying trees will create openings in the forest canopy that may encourage a different and hardier composition of tree species to grow. Forest stands that were predominantly oak, for example, may change to mixed pine and oak, or even all pine, a species which once dominated many of these sites.

That's not to suggest that we should just let this "natural" phenomenon take its course.

Active management plans that restore and maintain the appropriate species on appropriate sites will improve forest health and reduce the need for drastic management intervention in the future.

Why Oak Decline Occurs

Trees most vulnerable to decline have one or more of the following characteristics:

  • tree age (70 to 90-plus years)
  • shallow, rocky soils
  • ridgetops
  • south- and west-facing slopes
  • Conditions that have further stressed Missouri's oaks:
  • short-term, severe drought
  • repeated insect defoliation
  • damage by frost, ice, or wind

Diseases and insects that are taking advantage of the weakened state of our red oaks:

  • Armillaria fungus
  • Hypoxylon fungus
  • red oak borer
  • carpenterworm
  • two-lined chestnut borer
  • leaf-feeding insects

Preventing Oak Decline

  • Maintain a diversity of tree species, ages and sizes. A good rule-of-thumb for larger stands of trees is the "Four Quarters Rule": No more than 25 percent of tree stocking should be in any one species. Count scarlet and black oaks together as one species. For example, you could have 25 percent in shortleaf pine, 25 percent in scarlet/black oaks, 25 percent in white oak and 25 percent in other mixed species of maples, hickories, etc.
  • Match tree species to the sites where they grow best. In the Missouri Ozarks, shortleaf pine is often better suited for ridgetops and south and west-facing slopes than are scarlet and black oaks. White oak might be an appropriate choice for mid-slopes.
  • Maintain the vigor of your forest through regular thinnings. Scarlet and black oaks should be thinned regularly after they are about 35 to 40 years old, with a final harvest before they are 70 years old.
  • During thinnings, remove low-vigor trees and infested trees.
  • Leave standing dead trees and trees with cavities for wildlife habitat, but remove dead or unsound trees near buildings or areas of human activity, where they could become hazards.
  • Avoid pruning or thinning during the growing season. Minimize wounding trees when operating heavy equipment or logging, particularly during the time of loose tree bark in spring.
  • Periodically examine trees to identify pests before they cause too much damage.

Managing Wood Borers In Landscape Trees

  • Wood borers are attracted to trees stressed by injury, drought, soil compaction and limitation of root growth due to nearby buildings and pavement.
  • Healthy trees can defend themselves against wood borers and are able to recover after a limited number of borer attacks.
  • Avoid damaging tree trunks and roots with mowing equipment and string weed trimmers.
  • Avoid pruning oaks in spring and summer, when beetles carrying the oak wilt fungus and wood borer adults are active.
  • Do not have trees topped (cutting branches back to stubs in a hat-rack manner.) Topping stresses trees, increasing their risk of tree decline and death.
  • Place mulch around the base of trees to conserve moisture. Mulch should be distributed 2 to 3 inches deep in a ring around the tree, but should not touch the trunk or be piled against it in a volcano shape.
  • Provide supplemental water during summer droughts by allowing a hose to drip slowly on the soil above the root zone at approximately two-week intervals.
  • Wood borers are difficult to control with insecticides, and few insecticides are registered for treating wood borers in landscape trees. Attempts to use insecticides are likely to be ineffective and may have harmful effects for humans or the environment.

Dealing With Oak Decline

At sites where insect borer larvae attack, you might see oozing sap or a sap stain on the bark. Oozing sap can sometimes also be a symptom of a fungal infection, sapsucker attack or a mechanical injury.

Frass (granular or powdery mix of insect excrement and wood particles) often accumulates in bark crevices or around the base of a tree attacked by wood borers. Exit holes made by various kinds of borers can range in size from about 1/16 inch to 1/2 inch in diameter. Their larval tunneling also degrades the quality of oak lumber.

  • Remove trees that have 30 percent or greater die-back (dead outer branches) in the upper and mid-sections of the crown.
  • Remove smaller scarlet and black oaks that are overtopped by larger trees.
  • If the stand does not respond with improved tree growth and vigor, you may consider regenerating it (harvesting mature trees and allowing seedlings and sprouts to re-stock the site.)
  • After regenerating a stand, consider removing some sprouts before they are 20 years old. This will allow sprouts that originate low in the stump to become dominant. This will reduce competition and lower decay losses for future stands.
  • There are no pesticides that are effective in treating oak decline or wood borer infestations in forest stands, and attempts to use pesticides in these situations are likely to have harmful environmental effects.

Dead Tree Alert

  • Be alert for dead, weakened trees, and dead limbs, especially hanging broken limbs and branches along roads and trails. Dead trees and branches can fall at any time.
  • Keep a safe distance away from dead trees, especially in windy weather. One tree height distance (often 70 to 80 feet) may be needed.
  • Be careful where you set up camp and park a vehicle. Dead oaks may fall or large branches may break off, especially under windy conditions.
  • As oaks die and fall, logs and limbs could blanket the ground, possibly fueling wildfires or making prescribed burns more difficult to control. Be careful with camp fires.
  • Report any dangerous trees or limbs to the responsible land management agency.

This Issue's Staff

Editor - Tom Cwynar
Managing Editor - Bryan Hendricks
Art Editor - Dickson Stauffer
Artist - Dave Besenger
Artist - Mark Raithel
Photographer - Jim Rathert
Photographer - Cliff White
Staff Writer - Jim Low
Staff Writer - Joan McKee
Circulation - Bertha Bainer