By Sharon Oosthoek

The invasive garlic mustard plant is waging underground “chemical warfare ”in Ontario’s woods, and the casualties – hardwood trees – are mounting.

New research by University of Guelph soil ecologist John Klironomos, along with U. S. and German scientists, shows that once garlic mustard take s over an area, the vast majority of maple, ash and other hardwood seedlings die. Moreover, mature trees in such areas grow at about one-tenth the normal rate. “There are entire carpets of this weed in some places, and it’s very persistent,” says Klironomos.

Garlic mustard can be found in hard wood patches throughout the province, from southern Ontario’s Carolinian forests to the edge of the boreal forest in the far north where forests are dominated by softwood trees. This invasive species is native to Europe and was first brought to North America as a seasoning and medicinal plant in the late 1800s.

Klironomos and his colleagues have discovered that the plant releases a chemical into the soil that poisons the fungi that hard woods need to help them grow. Arbuscularmy Corrhizal fungi have long microscopic threads that extend in underground networks, penetrating tree roots and lengthening their reach. The fungi rely on the trees for sugar, produced during photosynthesis, and the trees rely on the fungi to access small pockets of nutrients.

Klironomos first noticed the strange absence of saplings while walking through wooded areas around Guelph and Waterloo. He observed few native tree seedlings in forests where garlic mustard was present and suspected the plant was to blame for this lack, as it is known to produce antifungal chemicals. “Forests are highly competitive environments and all the spots are taken. There are very few examples of invasive species in forests. So this one stuck out. “The researchers collected soil samples from five wooded areas around Waterloo that contained both native hard wood trees and garlic mustard . They planted seedlings in the soil collected from the f o rests and in soil free of garlic mustard. Most of the seedlings planted in the garlic-mustard -infested soil – as much as 71 percent – did not grow. The results were the same even when the team took soil from woods where they had hand-weeded the garlic mustard for two years. Even reintroducing Arbuscular Mycorrhizal fungi into the soil failed to encourage growth of tree seedlings.

“Just removing garlic mustard isn’t necessarily going to be effective in restoring the soil,” notes Klironomos. He now worries that the garlic mustard plant could significantly change the character of Ontario’s forests.

“We could see a different age structure – mature trees with nothing [no saplings] coming up behind,” he says. In its native Europe, garlic mustard does not seem to have the same destructive effect on forests. Klironomos and his colleagues are trying to ascertain why that is and what can be done to stave off further casualties here in Ontario.