Detroit — While Belle Isle Park draws big crowds each year, some visitors are more welcome than others.

One arrival that is clearly unwanted is an invasive species taking an unhealthy interest in the island park’s oak trees. It’s a fungus-based disease called oak wilt, and it has the capacity to kill wide swaths of oaks if not put in check.

To date, Michigan Department of Natural Resources crews have identified roughly 120 disease-ridden trees that will need to be removed throughout Belle Isle, considered one of the jewels of Michigan’s state park system. It’s a discouraging finding since Belle Isle’s parks represent one of the last large remaining flatwoods areas in Michigan.

“It’s a disease that is always fatal to oaks — always,” said Heidi Frei, a natural resource steward with the DNR.

The disease previously has been identified in parts of Michigan. It has caused problems in areas as diverse as Traverse City, Livingston and Washtenaw counties, and state officials have worked to educate the public about how best to stop its spread.

In the days before Christmas, crews on Belle Isle circled infected trees with a small tractor-style vehicle equipped with a downward-facing, five-foot blade. The machine severs roots below the surface to prevent the disease from spreading tree to tree.

Oaks as far as 50 feet apart above ground can infect each other — meaning one tree with oak wilt can be bad news to dozens more.

In addition, oak wilt can also be transmitted through moving infected firewood into areas that have not seen oak wilt infestation, and by sap-feeding beetles carrying the disease to trees that have been previously damaged.

The susceptibility of Belle Isle’s oaks to oak wilt has been heightened by the arrival years ago of another invasive species. The emerald ash borer felled scores of trees across the park.

“As the ash trees started to fall down, there was considerable wounding of other trees,” said Ray Fahlsing, manager of DNR’s Parks and Recreation Stewardship Unit. “The gashes and broken limbs caused by dying trees increased the likelihood of oak wilt fungus spreading.

“That allowed the picnic beetle to be attracted to those wounds and bring the fungus to the oak trees,” Fahlsing said.

The combination of emerald ash borer and oak wilt has the potential to drastically change the make-up and appearance of Belle Isle’s forests in the coming years.

Oaks, for instance, tend to hold their leaves well into late fall and early winter. And the colors of the leaves are one more thing that draws many visitors each year during the fall.

With oak wilt taking hold, many of the island’s trees won’t have leaves by the time color watchers make their way to the park.

“An infected tree, we see the leaves almost suddenly, in the middle of summer, turn brown,” Frei said. “And that brown kind of seeps through the leaves. It usually starts from the margins, and it will slowly overcome the leaf. Then the tree will suddenly drop all of its leaves.

“So in September, it suddenly becomes naked. It’s dropped all of its leaves while the other tree species around it are nice and green.”

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