Lovely as the ash trees may be along the Chain of Lakes, the Minneapolis Park and Recreation Board will begin cutting them down to battle the emerald ash borer and revive some of the city's prized green space.

Minneapolis is in the sixth year of its $1.7 million annual plan to tear down and replace 40,000 ash trees, part of the city's attack on the invasive ash borer that's taking down millions of trees across the eastern United States and Canada. Other cities in Minnesota, including St. Paul, also are wrestling with the disease.

Ralph Sievert, the Park Board's forestry director, said passersby will notice the work around Lake Harriet, Bde Maka Ska/Lake Calhoun, Cedar Lake, and to a lesser degree Lake of the Isles. When crews are on site, the lake paths may be temporarily rerouted for a few hours.

"Most people will probably think things look a little better because you can see the water more clearly" after removal of the brush, Sievert said.

Most of the ash trees to be removed are about 10 inches in diameter, while some are as big as 30 inches. Many of them are volunteers, meaning they self-seeded, and have grown in the underbrush near the lakes.

Injections can hold off the disease, but eventually the metallic-green beetle will kill any ash tree it infests. Its larvae gnaw on a tree's inner bark, limiting the tree's ability to transport nutrients and water and eventually killing it.

Injections can hold off the disease, but eventually the metallic-green beetle will kill any ash tree it infests.

A large ash tree die-off would leave city staffers unable to keep pace with cleanup and replacement, so Minneapolis officials decided to remove the trees rather than wait for them to die. Removing live trees also decreases the risk of damage and injury from limbs falling off dying trees.

"There'll still be some ash trees out there that you'll miss, but the idea is you won't have them all die at once," Sievert said.

The city has kept on track with removals and replantings, even when long winters altered the spring planting schedule, he said. Projections are on pace to wrap up removal in 2021, with 2022 being the final year of replanting, Sievert said.

As the city replaces ash trees, foresters are methodically diversifying the city's tree canopy. Sievert has mapped every tree in Minneapolis on public and private land with a computer program. Guidelines call for no more than 10% of any tree genus in one neighborhood or park area.

Common replacement trees are disease-resistant elms, burr and bicolor oaks, seedless Kentucky coffee trees, hackberries, American lindens and river birches.

Even though they're popular, maple trees aren't a good option because they already make up some 20% of the public canopy, Sievert said. The city used to plant hundreds of maples a year; now it's down to about 70, he said.

"That's what we're trying to get people to understand — we're trying to diversify," he said.

Diversification was the lesson from the Dutch elm disease that ravaged Minneapolis' canopy in the late 1970s. Before that, elms accounted for 90% of the city's public trees; now they're at 15%, Sievert said. The number of elms that die every year has dropped to 500, down from 2,000 to 3,000 annually, he said.

Barring storms that might pull staffers in another direction, this year's tree removal could be completed in early August. Work has already started around Bde Maka Ska and should be swift at Lake of the Isles, which doesn't have as many ash trees. Cedar Lake, however, has a significant amount of ash, Sievert said.

The work in the fall turns to pruning because the young trees planted in the spring need help to grow right, he said.

Funding for the program comes from a special Park Board levy. Sievert said he's hoping the levy can be continued a bit longer to pay for continued maintenance of the developing trees.

Rochelle Olson • 612-673-1747 Twitter: @rochelleolson