Some cities could lose half of trees to borer

Alexandria is familiar to many Minnesotans as a pleasant, shady city where they leave the interstate for the lake.

Its rich foliage and central location make it more vulnerable to the emerald ash borer than most cities.

Sixty percent of the trees along Alexandria's streets are ash, one of the highest urban concentrations in the state. That's triple the density of ash in Minneapolis and double what it is in St. Paul, where the state's first borer was detected two weeks ago.

"Really?" said City Administrator Jim Taddei, when informed of the statistics from a 2006 Department of Natural Resources (DNR) tree survey in 789 cities. "Imagine 60 percent of the trees in a community gone. It could really be a tough thing."

Metro cities may have the most ash -- 300,000 in Minneapolis and St. Paul alone -- but residents along Alexandria's Ash Street, and elsewhere in outstate Minnesota, face the prospect of losing a greater share of their shade as the ash borer continues its inexorable spread across the eastern and central United States.

The threat ranges from International Falls (50 percent), where ash enjoys the wet landscapes of northern Minnesota, to Luverne (50 percent), where ash is also one of the few shade trees to thrive in dry, alkaline prairie soils of the southwest.

"When all those street trees go, it's those greater Minnesota areas that are really going to feel this," said Val Cervenka, a forest health supervisor for the Minnesota DNR.

Since its discovery in suburban Detroit in 2002 -- which was possibly a decade after its arrival in the United States from China -- the invasive emerald ash borer has destroyed tens of millions of trees in the Midwest. The tree was the preferred replacement for elms after they were ravaged by a beetle-borne fungus a generation ago.

Most foresters think that, barring the discovery of an economical insecticide or a no-risk natural control, the borer will wipe out all or most of Minnesota's 900 million ash trees. They grow predominantly in forests from the Lake Mille Lacs area northward to the Canadian border. Minnesota has more ash trees than any state other than Maine.

Statewide, ash trees make up more than 7 percent of rural forests, where they will be difficult to protect or remove. Most foresters believe the pest will ultimately leave swaths of bare, dead trees in Minnesota's rural woods. A shrubby understory of short-lived ash will be the likely successor in the short term, followed by other tree species, said Jeff Gillman, associate professor of nursery management at the University of Minnesota.

The ash borer itinerary

Had the Detroit area been a wilderness in 2002, the ash borer would now be working its way across northern Indiana at a speed of only a few miles per year, noted Alan Siewert, a regional urban forester for the Ohio DNR.

Instead, the pest has been fast-tracked along the interstate highway system, apparently aided by debris haulers, or campers toting infested firewood, or vehicles it has simply fallen on. Another 135 miles up the road and it'll be in Alexandria.

In cities and towns such as Alexandria, ash tends to have been planted in long, street-side rows, where it was used to replace diseased elm trees over the past generation.

Dead ash will have to be removed from urban streets because they're brittle and dangerous, Siewert noted. In Alexandria, that work could consume a third or more of the city's parks budget for several years, Taddei said.

How cities pay those bills will be tricky. Some have elected to treat their ash trees with insecticides. Milwaukee spends $40 per tree, even though there is no guarantee the treatments will work. Voters in Ann Arbor, Mich., in 2005 turned down a proposed $2 million annual tax increase to deal with its ash borer infestation. Without extra money and staff, cities small and large will have to let other needs slide while they steadily remove and replace ash trees, public forestry officials said. The other option is to let them all die and face having to remove them all at once, likely about five years after the bug's arrival.

"It's a political nightmare," said Mike Pisarsky, community forester for Sandusky, Ohio, speaking Wednesday to an "Ash Borer Summit" in Hopkins. Pisarsky and Siewert also said they believe that treating public trees has more political value than tree-saving value, because it allows elected officials to say they tried to save a tree before it needs to be cut down.

Restoring the shade

If cities want their shade back, they'll be looking for some kind of tree -- tall, fast-growing and cheap -- to replace the ash. But even that might have problems. In International Falls, foresters have planted Canadian chokecherry along with ash along boulevards, but now a fungus is attacking them, said streets and parks commissioner Steve Johnson.

In the short term, the DNR and the League of Minnesota Cities are expected to announce a new model ordinance cities may use to attack emerald ash borer. The ordinance will expand on measures adopted to deal with Dutch elm disease by allowing foresters access to private property. It would also cover other pests, including the ash borer, gypsy moth and Asian longhorn beetle.

State and local forestry officials have been holding information sessions with residents in the weeks since the ash borer was found in St. Paul. A final meeting has been set for 6 p.m. today at the St. Louis Park Recreational Center, 3700 Monterey Drive.

Bill McAuliffe • 612-673-7646