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By the time emerald ash borer beetles and city chain saws are done chewing, the Twin Cities will have lost all their public ash trees, leaving holes in the urban forest canopy that will take decades to fill.

In what arborists are calling an environmental catastrophe, Minneapolis crews plan to remove 5,000 trees a year until all the city’s public ash trees are gone. In St. Paul, more than 8,500 boulevard ash trees have been removed so far, and the pace is increasing. When the cities are done cutting, nearly 60,000 trees will be gone.

The Minneapolis Park Board has been removing ash trees whether they show signs of infestation or not because dead and damaged trees could fall and hurt people and property. And, while St. Paul officials don’t know how many ash trees are on private property there, Minneapolis officials say the crisis threatens more than 175,000 trees in the city’s yards.

“It’s been hard and emotional for the community,” said Mike Hahm, director of St. Paul Parks and Recreation. “If it hasn’t touched people directly, it will.”

The enormity of the infestation has left officials scrambling, from trying to save trees not yet infested to removing and replacing ash trees with a greater diversity. In St. Paul, the city is taking down trees so quickly the budget doesn’t have enough left to cover immediately pulling stumps and replacing trees.

So far, what local officials are calling a crisis has not spurred the Legislature to act, leaving cities across Minnesota mostly fending for themselves.

“A prevailing view [at the Legislature] is ‘the trees are going to die, so let it happen,’ ” said state Rep. Rick Hansen, DFL-South St. Paul, who has unsuccessfully pushed for state funding to help manage the problem. “I think we’re better than that. We’ve been better than that in the past.”

At one time, about 90 percent of boulevard trees in the Twin Cities were elms. When Dutch elm struck in the 1970s, whole neighborhoods of the urban forest canopy were lost. Hahm said he remembers losing more than a dozen trees just on his block in Como Park.

To help restore that canopy quickly, the Twin Cities planted thousands of ash trees, which now comprise 15 to 20 percent of all boulevard trees. Still, some areas have higher concentrations of ash trees than others, and the emerald ash borer infestation that began in St. Paul in 2009 has once again wiped out whole blocks of boulevard trees.

Leaving infested trees standing poses a risk to public safety, officials said, because trees become brittle and can fall during high winds or storms, damaging property or injuring people. In Minneapolis, the wood is sent to a Park Board processing facility where a private company grinds it up into pieces small enough to kill the beetle or larvae.

“Even the trees that still look healthy can still have the beetle in it,” said Ralph Sievert, the Minneapolis Park Board’s director of forestry.

Cost and timing are also factors in deciding to cut trees down instead of treating them with insecticide.

“If we’re jumping right on it and getting trees removed and replaced, we should never have a huge number of trees dying in a short period of time,” Sievert said. “Money we spend on treating, we can be using that to just buy more trees and replace them.”

But Karen Zumach, director of community forestry for the nonprofit Tree Trust, is unconvinced that mass removals are necessary. Cutting down trees should be the last resort, she said. It takes 60 years to regain a mature tree canopy.

“We know there are ways to keep the trees around, they just have to be willing to make that investment,” Zumach said.

St. Louis Park is injecting trees with insecticide, said Jim Vaughan, the city’s natural resources coordinator. Since conducting a tree survey in 2010, the city has been treating 300 to 400 trees a year, said Vaughan, who is also president of the Minnesota Shade Tree Advisory Committee. At the same time, St. Louis Park has been cutting down 50 to 100 lower quality ash trees a year to stave off infestation. In the end, Vaughan said, the city hopes to save 1,100 of its 2,300 boulevard ashes.

“Trees have a lot of value when they mature,” he said of their benefits for air and water quality and energy savings. “Why not try to save them if you can?”

Hennepin County has cut down about 200 infested trees and treated 100 others since 2014, said County Forester Dustin Ellis. To replace those trees with a greater diversity, the county grows 50 species of trees at the Hennepin County Adult Corrections Facility in Plymouth.

“If homeowners have ash trees,” he said, they need “to have them looked at by arborists and talk to professionals in dealing with this.”

In addition to cost, fears about pesticides harming bees and other insects may be keeping cities from considering treatment, said Zumach.

“The cities of Minneapolis and St. Paul, I know they are doing the best with what they can,” she said, adding that state financial assistance would ensure that more trees get treated.

Once trees are cut down, St. Paul turns the wood into fuel for burning at St. Paul District Energy. In Minneapolis, a private company keeps the wood at a Park Board facility after grinding it up.

Ken Holman, a forestry specialist with the Minnesota Department of Natural Resources, said there are an estimated 2.65 million ash trees on public and private land in the state and 1 billion in state forests, the highest number of ash trees of any state.

“They are all at risk,” he said.

Yet, while the DNR helps communities take inventory of their ash trees and trains tree inspectors, the state hasn’t provided money to help manage the problem, quite a change from the millions the state provided to help cities with Dutch elm disease.

“I’m baffled myself,” Holman said of the legislative silence.

In the meantime, neighborhoods keep losing trees.

“People don’t want to lose their trees,” said Dave Colling, executive director of the Harrison Neighborhood Association in north Minneapolis. “They don’t want to lose the shade they have and the look and feel of the neighborhood.”

Faiza Mahamud • 612-673-4203

James Walsh • 651-925-5041