Thursday, July 24, 2008

Illinois ash trees imperiled by imported beetle

Illinois ash trees imperiled by imported beetle

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If you’ve been struck by the odd sight of purple boxes tied to trees this summer you’re not alone. Those boxes are traps used by the Illinois Department of Agriculture to monitor the spread of the emerald ash borer, an insect native to Asia that was first detected in North America only six years ago, when it arrived in the Detroit area by means of wooden shipping material. (Click here to visit the information-packed IDA website devoted to emerald ash borer.) Last week brought the grim news that two emerald ash borers had been found in Bloomington, which represents the first appearance of the beetle so far south in Illinois.

Of course, many residents of Michigan, Ohio, Indiana, and northeastern Illinois have already experienced the devastation caused by the emerald ash borer. North American ash trees possess no natural defenses against this insect, which in its larval stage, feeds on the active layer of wood just beneath the bark of a tree, killing it by cutting off its supply of nutrients.

So far some 30 million ash trees have been lost to emerald ash borers in the Midwest.

In light of this development, I checked in with Jeff Dawson, a U of I professor in the Department of Natural Resources and Environmental Sciences, regarding ash trees and what they mean to people.

Dawson pointed out that although we tend to lump them together, five different species of ash trees, adapted to varying conditions of soil and climate, help to make up forests in Illinois. (Click here for a helpful online guide to ash identification.) The species that most people are familiar with as a shade tree is the green ash, originally an inhabitant of river bottoms. [Photo: A line of green ash trees shades Kenwood Road adjacent to Centennial Park in Champaign.] The adaptations that enable green ash trees to flourish in that environment also allow them to thrive near pavement and in the compacted soils typical of urban and suburban settings.

Dawson emphasized that ash wood is at once lightweight, strong, and flexible, making it useful for products ranging from tool handles and commercial flooring to canoe paddles, guitars, and of course baseball bats.

While it is important to delay the spread of emerald ash borer so that cities, forest preserves, and park districts can manage the enormous and staggeringly expensive task of replacing ash trees, no one anticipates an effective fix to this problem. Homeowners and others can preserve prized ash trees through the use of systemic insecticides, but such measures are not practical on a larger scale. It’s depressing to say so directly, but ash trees are likely to go the way of American elm trees and chestnuts within the foreseeable future. We’ll be able to grow them under highly managed conditions, but they’ll be lost as a functional component of our natural areas and a source of wood for many products.

What can we learn from this experience? For one, diversity is essential to the maintenance of urban forests. It is sad but true that some streets left devoid of trees with the loss of American elms a generation ago were replanted with only ash trees, and are thus once again starting from scratch.

The introduction of the emerald ash borer to North America also reminds us that globalization can entail great costs, both economic and ecological. Many forests will be forever altered without a healthy ash population, and many municipalities will struggle to pay the bills for ash removal and replacement in the years to come.

Finally, it’s important to recognize that individuals have an important role to play. We can slow the spread of emerald ash borer by purchasing firewood near where it is to be used rather than transporting it long distances. Emerald ash borers do not fly very far themselves, and by not giving them a lift we can buy ourselves important time.