Woodlands - Fall 2017 - 12
EARTHWORMS AT THE ROOT OF SUGAR MAPLE DECLINE
Non-Native Worms are Eating Up the Forest Floor,
Causing Sugar Maples to Die and Perhaps Harming Other Forest Dwellers
The worms crawl in, the worms
crawl out, and corpses aren't the only
casualties. A Michigan Technological
University scientist has fingered nonnative earthworms as a primary culprit in
the decline of an iconic American tree.
Sugar maples are prized as much
for their valuable lumber as for their
sugary sap and dazzling fall colors. In
Michigan alone, they are the basis of a
multi-million-dollar industry. But several
years ago, foresters began noticing that
the crowns of the big trees appeared
unhealthy, with bare limbs and little new
"They were losing trees before they
could harvest them," says Tara Bal, a
research assistant professor of forest
resources and environmental science.
"We wondered what was causing it."
Her findings were published July 26,
2017, in the journal Biological Invasions.
Drought was a suspect - the
traditionally damp northern Great Lakes
region was in the midst of a dry spell.
Other potential causes were forest
management practices, soil types,
climate change and the mix of species
in the area.
From 2009 to 2012, Bal made
annual visits to more than 100 sites in
Michigan's Upper Peninsula, northern
Wisconsin and Minnesota. She found
one factor that stood out: the condition
of the forest floor. And nothing affects a
forest floor quite like earthworms.
While they may seem as American as
maple syrup, no earthworms - including
the nightcrawler - are native to the
Upper Midwest. If there ever were any,
the last ice age wiped them out over
11,000 years ago. "All the earthworm
12 WOODLAND * Fall 2017
species here are from Europe or Asia,
brought in when humans transplanted
plants," Bal says. Now, they and their
egg cases hitch rides on tires and
muddy boots and spread when anglers
dump bait in woods and waterways.
"It's the same way invasive plants get
around," she says.
Before worms invade, the soil in a
typical northern forest is blanketed by a
thick layer of leaf litter and other organic
material. When worms arrive, they
quickly gobble it up and expose bare
dirt. "And earthworms really like sugar
maple leaves," Ball says. "They are
sugary, soft, and have fewer tannins
than other trees, like oaks."
Sugar maples have a particular
characteristic that makes them even
more vulnerable to earthworms. Ninety
percent of their roots are in the top few
inches of soil, so the trees rely on the
litter to keep the soil from drying out.
With the litter gone, maples can slowly
die of thirst.
While the condition of the forest
floor appears to be driving the maples'
dieback, there could be other factors
as well, Bal says. "At this point, we
don't really know if earthworms are
causing damage directly or making the
soil and litter conditions so poor that
drought and other things are getting to
Maples aren't the only victims of
these wriggly invaders. Many other
forest creatures depend on forest litter
for their survival. "I was just looking at
the health of the trees, but earthworms
are really affecting the whole forest,"
Bal says. "You lose wildflowers, young
seedlings and many ferns." The worms
can reduce ground-nesting birds,
insects, amphibians and fungi whose
lives are intertwined with the forest litter.
"I've never seen a salamander on the
Michigan Tech Trails, but I've seen a lot
of nightcrawlers," she noted.
People can slow their spread by
throwing bait in the trash and washing
mud off vehicles and boots before
traveling into the woods. "If you are a
forest manager, you should power-wash
your equipment when you go from one
place to another and use local road fill as
much as possible," Bal says. "That's a
good practice for stopping the spread of
all exotic species, not just worms."
It's highly unlikely that the earthworms
will go away, however. "Predictions are
that within 100 years, 95 percent of our
sugar maple forests will be invaded by
earthworms, and there's no worm-icide."
"This means change for the forest,"
says Bal. "Forest managers will have
to start thinking outside the box to
keep forests and trees healthy and
That could mean a return to diversity;
sugar maples did not always dominate
northern hardwood forests. "We have
pure maple stands because it's a
valuable wood," she says. "If some of
the sugar maple dies back, you could
have the return of other species, like
basswood, birches and ironwood.
"That could make the forest more
resilient," she suggested. "The way
things stand now, if something that loves
sugar maple, like the Asian longhorn
beetle, ever got here, we could lose our
hardwood timber industry."
Source: Michigan Technological University
Table of Contents for the Digital Edition of Woodlands - Fall 2017
Tools and Resources
Forests and Families
A Legacy to Keep
From Forests to Fermentation
Feathering a Forested Nest
Woodlands - Fall 2017 - intro
Woodlands - Fall 2017 - cover1
Woodlands - Fall 2017 - cover2
Woodlands - Fall 2017 - 3
Woodlands - Fall 2017 - Overstory
Woodlands - Fall 2017 - 5
Woodlands - Fall 2017 - 6
Woodlands - Fall 2017 - 7
Woodlands - Fall 2017 - 8
Woodlands - Fall 2017 - 9
Woodlands - Fall 2017 - 10
Woodlands - Fall 2017 - 11
Woodlands - Fall 2017 - 12
Woodlands - Fall 2017 - 13
Woodlands - Fall 2017 - 14
Woodlands - Fall 2017 - 15
Woodlands - Fall 2017 - 16
Woodlands - Fall 2017 - 17
Woodlands - Fall 2017 - Tools and Resources
Woodlands - Fall 2017 - 19
Woodlands - Fall 2017 - Forests and Families
Woodlands - Fall 2017 - 21
Woodlands - Fall 2017 - 22
Woodlands - Fall 2017 - 23
Woodlands - Fall 2017 - A Legacy to Keep
Woodlands - Fall 2017 - 25
Woodlands - Fall 2017 - 26
Woodlands - Fall 2017 - 27
Woodlands - Fall 2017 - 28
Woodlands - Fall 2017 - 29
Woodlands - Fall 2017 - From Forests to Fermentation
Woodlands - Fall 2017 - 31
Woodlands - Fall 2017 - 32
Woodlands - Fall 2017 - 33
Woodlands - Fall 2017 - Feathering a Forested Nest
Woodlands - Fall 2017 - 35
Woodlands - Fall 2017 - 36
Woodlands - Fall 2017 - 37
Woodlands - Fall 2017 - 38
Woodlands - Fall 2017 - cover3
Woodlands - Fall 2017 - cover4