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News Article

Could earthworms be the cause of sugar maple decline?


Increased crown dieback symptoms in the trees prompted the investigation

Sugar maple (Acer saccharum) trees in the northern Great Lakes region have recently been reported as having increased crown dieback symptoms, prompting further investigation across the region.  Upon evaluation of the dieback over a period of 3 years across a 120 plot network, researchers at the Michigan Technological University suggest that the disturbance of the forest floor from non-native earthworms was significantly related to maple dieback.

In Michigan, the sugar maple is a tree species of significant economic importance for timber and commercial syrup production.  Several years ago, foresters started to notice that the crowns of the maple trees appeared to have bare limbs, with little new growth.

“They were losing trees before they could harvest them,” said study co-author Tara Bal.  “We wondered what was causing it.” The findings of the study were published in the journal Biological Invasions.

The researchers suspected that drought was the primary reason, as the traditionally damp northern Great Lakes region was in the middle of a dry period. Other potential causes included forest management practices, climate change, soil types and the mixture of different species in the area.

Between 2009 and 2012, Bal visited more than 100 sites in Michigans upper Peninsula, northern Wisconsin and Minnesota.  Bal noticed that the condition of the forest floor stood out each time, something which can be affected significantly by earthworm activity.

Earthworms are not native to the Upper Midwest.  “All the earthworm species here are from Europe or Asia, brought in when humans transplanted plants,” said Bal.  They are spread in a similar way to invasive plants, their egg cases are dispersed by shoes and car tyres and when anglers release their bait in waterways and woods.

Before the earthworms inhabit a forest, the soil is typically covered by a thick layer of organic material.   When the worms arrive, they rapidly consume the leaf litter and expose the soil.  “Earthworms really like maple leaves.  They are sugary, soft and have fewer tannins than other trees, like oaks.”

Around ninety percent of the roots of a sugar maple are located in the top few inches of the soil, relying on the litter to prevent the soil from drying out.  Once the litter has been removed in this way, the maple trees can die due to lack of water.

While it appears that the condition of the forest floor is driving the maples’ dieback, it is possible that other factors could be involved as well.  “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 the trees.”

However, maple trees are not the only victims of these invaders.  A number of other forest inhabitants 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," said Bal. "You lose wildflowers, young seedlings and many ferns." The worms can reduce the abundance of ground-nesting birds, insects, amphibians and fungi who live amongst the forest litter. "I've never seen a salamander on the Michigan Tech Trails, but I've seen a lot of nightcrawlers.”

According to Bal, “Predictions are that with 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.  Forest managers will have to start thinking outside the box to keep forests and trees healthy and regenerating.”

One possible option would be 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 said.  “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 lost our hardwood timber industry.”

Looking for further information on earthworms and sugar maple?  A search on the Forest Science database using the terms "Acer saccharum" AND earthworms yields 40 records.  This includes a paper by Drouin et al (2016) that explores the evidence which suggests that the spread of earthworms in sugar maple stands in Southern Québec may be linked to human activity.  Holdsworth et al (2012) compares the disappearance of leaf litter in monocultures including sugar maple, by changing the composition of the litter.  A study by Larson et al (2010) involves the use of tree rings to identify the timing of the earthworm invasion and the effects of earthworm activity on Acer saccharum overstory of two recently invaded sites in northern Minnesota.  They found that trees growing in invaded areas were more sensitive to drought than trees growing in conditions that were free from earthworms.

CABI’s open access resource, the Invasive Species Compendium contains datasheets on two species of invasive earthworm present in North America, Lumbricus terrestris and Lumbricus rubellus.

Journal Reference

Tara L. Bal, Andrew J. Storer, Martin F. Jurgensen. Evidence of damage from exotic invasive earthworm activity was highly correlated to sugar maple dieback in the Upper Great Lakes regionBiological Invasions, 2017; DOI: 10.1007/s10530-017-1523-0

Article details

  • Author(s)
  • Stephanie Cole
  • Date
  • 12 September 2017
  • Source
  • Michigan Technological University
  • Subject(s)
  • Environment
  • Forest products