Earthworms may threaten hardwood forests

DULUTH (AP) -- Earthworms, a welcome sight to the gardeners and anglers that helped spread the wriggling invaders around the state, are threatening Minnesota's hardwood forests, according to a forestry expert.

Cynthia Hale, a researcher at the University of Minnesota-Duluth, says earthworms are killing tree seedlings by consuming their preferred growth medium -- the layer of decaying plant matter on the forest floor, called duff.

Hale, a graduate student and worm expert in the Department of Forest Resources, said the duff is usually four to five inches thick.

"The worms can eat it all in a year or two, right down to bare soil," Hale said.

In a worst-case scenario, a worm-infested forest prevented from repopulating itself with seedlings might eventually disappear altogether, though few Minnesota forests are in such danger.

Worms have made their way into most of the hardwood stands in the southern part of the state, and 10 to 20 percent of them are heavily infested, Hale said. Half of northern Minnesota seems free of worms, as in the Chippewa National Forest. But 5 to 10 percent are so heavily infested that they could be in danger of disappearing in the next century.

A worm-infested forest looks deceptively healthy because the tree crowns stay green. But the telltale sign of the problem is that nothing new is growing on the forest floor -- no seedlings, and none of the other indigenous plants such as trilliums, violets and Solomon's seal.

Minnesota's hardwood forests are vulnerable because they never had to cohabitate with earthworms before, Hale said. In fact, there were virtually no worms left in the state after the last glacier froze them about 15,000 years ago.

Humans brought earthworms back in potted plants and bait buckets. Farmers tilling or moving the soil then spread them around the southern part of the state during the past several centuries.

"They invaded from the fishing resorts, the boat ramps and the lakeshores," Hale said. Emptying a bait can of worms into a lake or onto the shore is guaranteed to establish a worm colony there. "Worms can swim," she said, "and we can identify new worm populations spreading out radially from lakes."