War in the North Woods
UMaine forest pathologists are working to understand bark disease that is decimating beech

About the Photo:  Fagus grandifolia, or American beech, is an important hardwood in a variety of forest types from Maine to Michigan and south as far as Georgia and Texas. Mature trees produce large quantities of triangular nuts that are an important food for wildlife. Beech trees can live to be more than 300 years old, and can reach heights exceeding 100 feet. The thin, gray bark of the beech makes it highly susceptible to damage by fire, sunscald and sucking insects such as scale. Secondary pathogens, including more than 70 species of decay fungi, often enter the tree through openings in damaged bark. In addition to being an important tree ecologically in Maine, it also is economically important, providing high-density wood for use as flooring, furniture, veneer and fuel. Links related to this story

Maine's forests are at war. Facing new foreign invaders at every turn, the signature species of the state's vast forestlands are falling, one by one, to imported pathogens that not only kill individual trees, but change entire ecosystems in the process.

First to fall was the American chestnut, whose stately branches were wiped from the landscape by a fungal blight near the turn of the last century. The chestnut was soon followed by the American elm, which fell by the thousands after an epidemic of Dutch elm disease. Right now, Maine's beech trees are facing a second wave of disease by foreign invaders, leaving landowners and scientists alike worried that the beech may follow in the footsteps of the chestnut and the elm.

Enter Matt Kasson.

Kasson, a master's degree candidate in the University of Maine School of Forest Resources, is a key player in the battle to assess the future of Maine's beech trees, having made it his mission to understand the disease that is rapidly turning one of the state's top trees into so much cordwood. Logging in thousands of miles on dusty dirt roads and seemingly endless stretches of northern highway, Kasson, with the help of a few dedicated undergrads, has bushwhacked his way to beech stands in more than 20 townships, drilling core sections and gathering bark samples that he hopes will lead to new information that could help explain the deadly blend of afflictions that cause beech bark disease.

"What we're seeing is not just a pathogen, it's a complex," says Kasson, arranging a set of crimson-colored cultures in an incubator in the lab. "There are a number of factors that can lead to the same result. We're trying to determine what factors contribute to high mortality in these stands so that we can better understand how this disease is affecting the trees and the forest."

Kasson has collected more than 2,200 tree cores — pencil-size cylinders of wood that provide a record of growth, from the tree's days as a sapling to its most recent annual ring — in which he hopes to find important clues about how the disease kills. Beech trees can live as long as 300 years, despite hosting more types of decay fungi than almost any other American tree. By comparing cores from beech and other tree species unaffected by the disease, Kasson may be able to determine what environmental conditions contribute to both the spread of beech bark disease and the likelihood that it will kill its host.

"By looking at the tree rings in the cores, we can determine whether environmental conditions such as drought play a role in tree mortality," says Kasson. "We can also better understand what factors may have determined why some beech stands were hit harder than others. We found evidence of lower mortality in north-facing slopes, for instance, which may offer us some insights into the conditions that are necessary for the disease to kill."

Pathogens causing beech bark disease were introduced to North America in the 1890s, arriving in Nova Scotia in a shipment of contaminated beech seedlings from England. From there, the pathogens spread west and south, reaching Maine's coastal region by the 1930s. The primary pathogen is an exotic scale insect, Cryptococcus fagisuga, which provides access channels into the host tree's tissues for the second pathogen, an invading fungus.

"The scale weakens the tree, but the fungus kills it," said Kasson. "The roots usually survive, sending up dense thickets of young shoots that have no resistance to the pathogens."

Strangely, while Neonectria faginata, one of the fungi that helped cause the disease, was introduced from Europe, the other, Neonectria ditissima, is a native of North America, just like the beech.

Kasson is examining the unique relationship between the two fungi to see how their interaction may affect the occurrence and expression of the disease across the landscape. He collects small bark discs from both healthy and infected trees, removes any fungal spores that are present and then grows the fungi in the lab. Once the cultures are identified using morphological characteristics and DNA analysis, Kasson plans to set up trials to see how the fungi interact with one another. Initial observations suggest that the foreign species may be replacing the native fungus in the disease complex.
 

Healthy beech tree

When the first wave of the disease spread across Maine in the middle portion of the last century, many large trees and beech populations in more northerly stands were spared. A second "killing front" that has been slowly moving through the North Woods during the past decade has been much less merciful. Kasson believes that the relationship between the native and European fungi may hold the key to the virulence of the current outbreak.

Kasson also is looking at Fusarium, another fungi common throughout Maine's forests. Kasson and his research colleagues have observed unusually high concentrations of two Fusarium species in the bark of trees infected with beech bark disease — a fact that he believes may be significant in the complex interrelationship between the two Neonectria fungi and the scale insect causing the disease.

"When we take bark samples, Fusarium is everywhere. It's in more than 80 percent of the samples," says Kasson. "We plan to explore the role of Fusarium as a side project. If it is selective toward the native species of Neonectria, that could explain some of what we are seeing in the second wave of beech bark infection. Fusarium is ubiquitous, but it's occurring so often, it seems to be more than just background noise."

Working under the guidance of UMaine forest resources professor Bill Livingston, Kasson is taking a comprehensive approach to studying the disease, examining not only how the disease has spread across the state and the forest, but how it moves through the stand and the individual tree. The ongoing research, funded by the U.S. Forest Service, has benefited greatly from the cooperation of the Maine Forest Service and northern Maine's landowners, many of whom are understandably concerned about the effects beech bark disease may have on the state's woodlands.

According to Kasson, less than 2 percent of the state's beech population has shown resistance to the disease, and nearly a third of the beech trees in northern Maine have died.

Kasson hopes that what he learns through his research can be used to save the surviving beech trees in Maine and across the country.

"The fact that beech bark disease came into Maine first means we are on the front lines," said Kasson. "We need to find out as much as we can now. It's moving into Pennsylvania, Virginia, Michigan and elsewhere, and what we have here may just be a preview of coming attractions."

by David Munson
January-February, 2007

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