American Beech Under Attack from Beech Bark Disease
February 26, 2009
Exotic forest pests continue to reshape the forests of North America. Perhaps you remember the loss of the American elm and American chestnut caused by Dutch elm disease and chestnut blight. These trees once dominated the landscapes of many cities and forests.
Currently, the emerald ash borer is expanding its range, threatening an entire genus of trees containing 21 species of ash. A less publicized, but no less important forest health threat is beech bark disease (BBD).
Beech bark disease was first introduced in Nova Scotia from Europe in 1890. By 1932, it was killing American beech throughout the Maritime Provinces and Maine. It now infests most states in the northeastern United States.
In 2000 BBD made a jump to the Midwest and, according to Dr. Robert Heyd, Forest Health Management Program leader for the Michigan Department of Natural Resources in the Upper Peninsula, the disease likely was brought to this state in infested firewood.
"The arrival of beech bark disease in Michigan marked the beginning of another major shift in the ecology of our northern hardwood forests," said Heyd.
BBD is caused by two pests, a scale insect and a fungus, Nectria. The beech scale is a tiny, immobile insect which gives heavily infested beech bark a white, fuzzy appearance. Only after the scale infests the bark can the Nectria fungus successfully invade the tree.
"The fungus begins by killing areas on the tree trunk," Heyd said. "Disease weakened tree trunks of seemingly healthy trees often break in wind or ice storms."
This, Heyd explained, is called "beech snap."
Because of beech snap, scale infested beech are a sign of a highly hazardous environment.
"This public safety hazard has forced the untimely removal of these trees in campgrounds and parks," Heyd said.
Not all beech trees will snap, but most succumb to BBD within a few years after being heavily infested by the scale.
Both the scale insect and the fungus are carried on the wind. Scales also are moved to new areas by birds and black bears foraging for beech nuts in the fall.
"Because all scale insects are females, it only takes one insect to start an infestation and populations build very rapidly," Heyd said.
The American beech is easily recognized by its smooth, light gray bark which even the oldest trees retain. This smooth bark presents a tempting surface for the carving of initials and names. Beech bark carvings persist for the life of the tree, as do bear claw marks produced as black bears forage for nuts in the fall.
Beech is the only nut producer in the northern hardwood type. The distinctive triangular nuts are eaten by people and are an important food for wildlife. Beech mast is palatable to a large variety of mammals and birds, including mice, squirrels, chipmunks, black bears, deer, foxes, ruffed grouse, ducks and blue jays.
Beech wood is used for flooring, furniture, veneer, plywood, railroad ties, baskets, paper pulp, charcoal and lumber, and is especially favored for firewood.
Michigan has about 7 million acres of beech-containing forest. Forest inventories report 138 million beech trees of all sizes containing 1.67 billion board feet of lumber. Beech trees commonly live for 300 to 400 years. About a million of Michigan's majestic beech trees are 24 to 45 inches in diameter.
In cooperation with research projects underway at Michigan State University, Michigan Technological University and University of Michigan, the DNR is investigating the impacts of BBD on our forests.
"This research is helping to determine scale spread rates, and to predict short- and long-term impacts of beech bark disease on forest composition, productivity and wildlife values," said Roger Mech, a DNR Forest Health Monitoring Program leader based in Lansing.
"To date, the spread of beech scale to new areas is fairly variable, averaging six miles per year. Once the fungus catches up to the scale, the first trees to snap or die are larger beech with a diameter greater than 10 inches," Mech said.
As BBD impacts our beech resource, an informed, measured response is needed.
"Keeping a beech component in the forest has little effect on forest productivity, and offers desired wildlife habitat and an extended period of mast production," said Mech.
As BBD moves through Michigan it is estimated that 7.5 million of these larger trees will be lost. This equates to about 800 million board feet of sawtimber.
"Salvage of scale infested trees prior to the onset of mortality is often necessary due to high risk of beech snap," said Mech.
Eventually, after BBD has moved throughout the range of beech, a large portion of the remaining mature, large beech will be lost.
On a positive note, one to three percent of American beech are at least partially resistant to the scale, and, therefore, resistant to BBD.
Identification and protection of resistant trees offers hope for a future healthy beech resource. The DNR has joined forces with USDA Forest Service Research to produce resistant beech seed and seedlings.
"The long-term goal is to return American beech to both urban and forest landscapes," said Ron Murray, supervisor of the DNR's Forest Health Inventory and Monitoring Unit.
Although BBD is primarily spread by the wind and wildlife, Murray said people can prevent artificial, long-distance spread of the beech scale by not moving infested firewood or logs between July and November.
"Preventing BBD and other exotic pest introductions is best," Murray said. "The more people become aware of the problems associated with moving plants and plant products like firewood, the better chance we have of stopping the growing threat of exotic insects, diseases and invasive plants."
For more information on BBD in Michigan, ask your local Michigan State University Extension Office for Bulletin E-2746, "Biology and Management of Beech Bark Disease."