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Josh Rasmussen, an arborist with the city, grinds an ash tree stump Thursday in Eau Claire. For several years, about 300 healthy ash trees have been cut annually as a pre-emptive strike against the invasive emerald ash borer. Different species eventually will replace the removed trees. View more photos at LeaderTelegram.com.

Just a few years ago, ash trees accounted for a third of Eau Claire’s urban forest along streets and in parks.

Since a proactive campaign started in 2011 to reduce that proportion — prior to the emerald ash borer discovery last year in Eau Claire — the tree species vulnerable to that invasive bug now is down to less than a quarter.

“We’ve made some progress,” said Todd Chwala, superintendent of city parks, forestry and cemeteries.

A check this week of the city’s constantly updated tree inventory showed ash are now 23 percent of the 32,450 trees managed by the city in boulevards and parks.

Previously dominated by fewer species, the city has been working toward greater diversity in its tree population with the intent to keep Eau Claire green when insects, diseases or other threats target specific kinds of trees.

The ultimate goal is to not have any single species account for more than 10 percent of the urban forest, but that will take a long time given the large number of maples in Eau Claire.

“Getting to 10 percent will take a couple generations,” Chwala said.

Maples are the most common trees in Eau Claire’s public spaces, amounting to 33 percent. Because there’s no current threat to maples, Chwala said thinning their numbers would happen through attrition, and the city does not plan to cut them down just to meet the goal. But they aren’t planting more of them, instead using other kinds of trees that grow well in our climate and require little maintenance.

After the maple and ash, other trees already are under that 10 percent mark in the city’s inventory. Linden trees are at about 9 percent, oaks are around 6 percent and elm are at 4 percent. At 3 percent and under are varieties of locust, crabapple, pine and hackberry. And then there are more recent additions to the urban forest that remain low in number, but are showing promise.

Among the varieties the city is increasingly planting includes honey locust, Kentucky coffee and ginkgo biloba. The latter has been difficult to come by, Chwala noted, because many other cities are planting them and supply hasn’t kept up to the demand. Japanese tree lilacs also are seeing more use in Eau Claire, namely under power lines and other spots where space is limited. And while Dutch elm trees were decimated decades ago by a disease, the city has been planting other elm varieties that don’t fall victim to that malady.