DULUTH, Minn.-Invasive earthworms from Europe that came over with early settlers and have been moving across North America ever since are causing sugar maple trees to decline in northern Minnesota forests.
That was the conclusion of a research project published in the latest issue of the journal Biological Invasions - the second major project in as many years pointing to earthworms as the culprit in northern Minnesota forest problems.
Scientists in the latest study, led by Michigan Technological University biologist Tara Bal, looked at plots of maples in Michigan's Upper Peninsula, Northeastern Minnesota's Superior National Forest and northern Wisconsin, and found maple trees experiencing "dieback" due to disturbances on the forest floor.
That disturbance was worms eating the leaves.
Dieback is when top branches that should be full of leaves instead had bare spots. That's a sign of tree decline, where trees stop growing or die. In some areas, timber companies said maples were dying and becoming worthless even before they could be harvested.
From 2009 to 2012, Bal made annual visits to the same sites to measure what was happening.
"We went into this not knowing what was causing it, thinking it could be (forest) management or insects or something else," Bal told the News Tribune. "But going out to over 120 sites over several years, you could see it was the worms and the lack of duff. We didn't see any older maple leaves because they're the leaves worms like to eat first."
That invasive earthworms are a problem isn't new. Scientists in northern Minnesota have been reporting issues for years. But the maple situation appears to be worse than others.
"It's not necessarily a death knell for maples. You can have trees and worms," Bal said. "But when you have worms and then you add on a drought or high temperatures or heavy deer browsing, they are combining to impact maples pretty severely. It's changing the forest out there."
Many people mistakenly believe that brown earthworms, including angleworms and night crawlers, are native. But there are no native earthworms in North America; they all were taken out during the last ice age. The only native worms are the whitish, skinny worms you might find under rocks. North American forests developed over the past 10,000 years without earthworms.
Last September a study published in journal Global Change Biology found that the invasive earthworms are reducing species diversity, changing the makeup of forests wherever the worms are prevalent. That study included research by more than two dozen scientists, including Lee Frelich, director of the University of Minnesota's Center for Forest Ecology and an expert on mortheastern Minnesota forests.
Frelich said the non-native worms consume the duff - the decaying leaves on the forest floor - as they eat their way across the continent's forests. The worms alter the physical and chemical properties of soils, changing the pH, nutrient and water cycles and disrupting symbiotic relationships between soil fungi and tree roots.
The recent studies back past findings by UMD professor Cindy Hale that found the worms' impact especially intrusive on hardwood forests. They seem to have less impact on evergreen species, probably because needles are more acidic and less tasty to the worms.
As worms eat the leaves on the forest floor, big trees survive, but many young seedlings perish, along with many ferns and wildflowers. That changes what kind of trees are in a forest, scientists say.
Scientists have been studying night crawlers on a peninsula at Leech Lake for years and determined that worms were first introduced in the 1950s, probably as they became popular for fishing bait. Now, there are 30,000 to 100,000 night crawlers per acre in that area.
In some parts of southern Minnesota, with better soils, there are up to 200,000 night crawlers per acre. And that doesn't include angleworms, which can be as dense as 400,000 per acre - all of them digesting plant material on the forest floor.
Bal and others say anyone who goes into the forest should make sure they are not releasing night crawlers into the woods because there still are some areas not impacted by invasive worms.
For more information on foreign earthworms and their impact on forests, go to greatlakeswormwatch.org.