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Garlic mustard disrupts intimate symbiosis between native species and fungi
BY LORI BONA HUNT
An invasive weed that grows abundantly in Canada and the United States is engaging in under- ground chemical warfare, jeopardizing native trees by poisoning their best allies, a U of G researcher has discovered.
The finding by Prof. John Klironomos, Integrative Biology, Kristina Stinson of Harvard University and a team of other researchers from Guelph, the United States and Germany was published in the May issue of the Public Library of Science.
The scientists discovered that the weed garlic mustard (Alliaria petiolata) releases chemicals under the soil that are detrimental to the fungi that native trees — including the beloved Canadian maple — depend on for growth and survival.
It's the first study to show that invasive plants are hurting indigenous species by thwarting the ecological relationship between roots and certain fungi, says Klironomos.
“This noxious weed is disrupting an intimate symbiosis between native species and fungi that has been going on for millions of years.”
Garlic mustard targets and poisons arbuscular mycorrhizal fungi, the main fungal allies of native trees such as maple, ash and other hardwoods. The fungi have long microscopic threads that create a subterranean network, allowing for the exchange of nutrients with indigenous trees. The fungi rely on the trees for energy, and the trees rely on the fungi for food.
Klironomos, who holds a Canada Research Chair in Soil Biology, noticed that some native tree seedlings had declined in forests where garlic mustard was present and suspected that the alien weed was the culprit. The researchers tested the theory by collecting soil from five Ontario forests containing both native hardwood trees and garlic mustard.
They planted seedlings in both infested soil and garlic mustard-free soil and studied the young trees' ability to form relationships with fungi. The seedlings planted in infested soil grew at about one-tenth the rate of the other trees and had fewer fungal-root connections.
The researchers conducted the test numerous times to mimic different conditions in the wild: established forests, open fields, roadways, ditches, etc. Each time, they found growth was stunted due to the diminished microbial activity caused by the presence of garlic mustard.
“This is affecting current and future generations of trees and changing the habitat,” Klironomos says.
Mature forest systems are normally highly resistant to invasive weeds, largely because of the strength of fungi-tree relationships, he says. Fungi are so efficient at extracting nutrients from the soil that weeds like garlic mustard, which don't form symbiotic partnerships with fungi, typically don't stand a chance.
But garlic mustard's guerrilla tactics have allowed it to get a foothold in some well-established forests.
“There are entire carpets of this weed in some places,” Klironomos says. “It has found a way to survive by knocking out the competition.”
The researchers next plan to determine what chemicals in garlic mustard are killing the fungi, how these chemicals interact with other soil microbes, and how plants and fungi co-exist with the noxious species in its native European habitat.
