North Central Research Station

Oak Decline

A large number of oak trees are declining and dying in oak-dominated forests of the Missouri and Arkansas Ozarks. Scientists at the North Central Research Station are investigating this problem and identifying management solutions.

The Issue

Oak decline refers to the progressive dieback and eventual mortality of oak trees. The oak trees most susceptible to decline are

• Red oak group species such as black oak (Quercus velutina) and scarlet oak (Q. coccinea);
• Relatively old (>70 years) or large trees;
• Growing on “dry sites” such as where soils are shallow or rocky, and on broad ridges or south-facing slopes.

Oak decline is believed to be triggered by drought. Severe droughts during the 1950s and 1980s caused extensive oak decline during those decades. Even the extensive decline occurring now is associated with drought that occurred from 1998 to 2000. Other events that can incite decline include repeated defoliation by insects or injury from frost, ice, or wind.

Other Contributing Problems

Once oaks begin to decline, they become susceptible to many other kinds of diseases and insect infestations that cause further stress or damage. Some of these problems include

• Armillaria root rots, which can girdle tree roots.
• Hypoxylon cankers, which can kill stems.
• Red oak borer larvae and carpenter worms, which can damage wood.
• Two-lined chestnut borer, which can kill branches and whole trees by girdling them with their tunneling.

How Extensive is Oak Decline Today?

In 2001, the USDA Forest Service estimated that 100,000 acres had severe oak decline and wood borer damage on the Salem and Potosi Districts of the Mark Twain National Forest, Missouri. In decline areas, dozens of red oak borer attacks were found per tree, whereas healthy forests commonly have fewer than five per tree. Decline in Arkansas has been more extensive and severe. In 2001, decline in Arkansas covered approximately 300,000 acres and red oak borer attacks numbered from several hundred up to a thousand per tree.

Why is Oak Decline Such a Big Problem Now?

Even though oak decline and associated diseases and insect infestations have occurred in the past, the extent of these problems today is unprecedented. This is because red oaks are now the most common tree species on poor quality sites. These red oaks largely originated from the extensive timber harvesting and grazing practices of the late 1800s and early 1900s that favored the establishment of red oaks on sites that were once mixtures of pine and oaks. Now thousands of acres of forest land containing red oaks are reaching or surpassing maturity and becoming much more susceptible to oak decline. Moreover, new research conducted in the Missouri Ozarks is suggesting that populations of the fungi causing Armillaria root rots are increasing because of the increased availability of dead or dying root tissue from stressed oak tress. This dead or dying root tissue is the food source for these fungi. This combination of forest age, species composition, and accumulated diseases and stresses could result in oak decline changing from an episodic problem to a chronic problem.

Potential Impact to the Ecosystem

Oak decline and associated diseases and insect infestations can greatly affect oak forests as well as the animals and people who live in or near them. Some potential impacts are

• Decreased timber value because of borer holes inside of tree trunks; some mills in Missouri are reporting that defect in wood increased from 15 to 20 to 30 to 40 percent.
• Decreased acorn production in declining stands—acorns, aside from being a potential source of oak reproduction, are an important food for wildlife.
• Increased fire danger because of increased volume of dead trees, branches, twigs, and leaves.
• Reduced opportunities for hiking, camping, hunting, and other activities in forests because of esthetics or because of increased risk of injury due to falling limbs and boles of dead or dying trees.
  

What are the Research Needs?

State and Federal forest land management agencies as well as other forestry professionals and landowners managing private forest land need to know:

• The current extent and severity of oak decline and associated disease and insect problems.
• The ecology of oak decline and its underlying causes.
• The future extent and severity of oak decline and methods for predicting where it is most likely to occur.
• Silvicultural methods and prescriptions for restoring forests affected by oak decline or mitigating future episodes of decline.
• The economic impact of oak decline and costs and benefits associated with its mitigation.

What is the North Central Research Station and is Partners Doing?

Scientists from the USDA Forest Service North Central Research Station are working in conjunction with scientists from the USDA Forest Service Southern Research Station, the University of Missouri-Columbia, the University of Arkansas-Monticello, and the Missouri Department of Conservation, and with personnel from the Mark Twain National Forest to assess the extent of oak decline and red oak borer infestation and to develop and test management solutions for mitigating oak decline.

Key areas of research currently underway or in development include

• An assessment of oak decline and red oak borer infestation in the interior highlands of Arkansas, Oklahoma, and Missouri.

This work is being conducted jointly by the USDA Forest Service Southern Research Station and North Central Research Station, the Missouri Department of Conservation, the University of Arkansas-Monticello, and the University of Missouri-Columbia. This assessment will be used to develop a database and maps of the distribution of oak decline and locations of red oak borer activity.

• Methods for predicting future occurrences of oak decline in the Missouri Ozark Highlands.

This work is being conducted by the Mark Twain National Forest and the North Central Research Station, the Missouri Department of Conservation, and the University of Missouri-Columbia. These methods will be used to develop a Geographic Information System database to generate maps showing where oak decline is expected to occur.

• Understanding the role of Armillaria root rot on oak regeneration in the Ozark Highlands.

This research is being conducted by the University of Missouri-Columbia and the North Central Research Station. It will provide fundamental information needed for developing appropriate methods for mitigating oak decline.

• Silvicultural methods and prescriptions for restoring and managing pine-oak woodlands—a healthier mix of species for sites vulnerable to oak decline.

This work is being conducted by the North Central Research Station and the University of Missouri-Columbia. Silvicultural methods and prescriptions will be prepared and made available to foresters and land management agencies through management notes and publications as well as Web applications.