The Free Press, Mankato, MN

Local News

December 1, 2012

Tree survey to help prepare for ash borer

MANKATO — It’s increasingly likely that the Emerald Ash Borer cannot be prevented from eventually infesting ash trees in Mankato and elsewhere.

Counting the trees in an effort to be prepared may be the best defense.

This summer, a University of Minnesota-lead group of 24 volunteers surveyed all the trees in Mankato. The effort was mostly paid for by the U.S. Forest Service.

They estimate there are 27,709 trees in the city of Mankato. Of those, about 17 percent are ash, a figure toward the middle and lower end of the range seen in Minnesota, said Gary Johnson, a professor at the U of M’s department of forest resources.

Of the 17 cities surveyed, the percentage of ash trees ranged from 8.8 percent to about half.

Public property in Mankato had a higher proportion of ash trees (26 percent) than private property (15 percent).

The survey didn’t merely count trees; it also identified types and used multiple measures for tree health.

Mankato has a wide variety of tree sizes, measured 41⁄2 feet from the base, which is good.

“What you don’t want is a skewed distribution,” where most of a city’s trees are too young or old, Johnson said.

The survey didn’t count each and every tree. Instead, it estimated the population similarly to how a poll estimates the opinion of a larger population.

The team mapped the city in zones, and randomly selected a number of blocks in each zone. This method has demonstrated to be accurate to within 10 percent in previous studies, he said.

Johnson also said Mankato offered the most volunteers of any city. The much larger city of Rochester, for example, had eight.

Emerald ash borers were discovered in Michigan in 2002, in Minnesota by 2009 and in Winona County in southeast Minnesota by August of 2011.

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