Stinson and her colleagues found that garlic mustard targets arbuscular mycorrhizal fungi (AMF), which form mutually beneficial relationships with many forest trees. These fungi have long filaments that penetrate the roots of plants, forming an intricate interwoven network that effectively extends the plant's root system. AMF depend on plants for energy and plants depend on the fungi for nutrients. When tree seedlings, which depend strongly on AMF, began to decline in the presence of garlic mustard, the researchers suspected that the invasive plant might thwart this symbiotic relationship.
To test this possibility, they collected soil from five forests in Ontario dominated by four species of native hardwoods. First, the researchers tested seedlings' ability to form mycorrhizal relationships in soil with a history of garlic mustard invasion. Three species -- sugar maple, red maple, and white ash -- had significantly less AMF root colonization and grew only about one-tenth as fast in the infested soil. Seedlings grown in sterilized, AMF-free soil taken from invaded and pest-free locations showed similar reductions, suggesting that diminished microbial activity had suppressed tree growth. Other experiments showed that adding garlic mustard extracts to soil impaired AMF colonization and seedling growth, implying that the weed uses phytochemical poisons to disrupt native plants' mycorrhizal associations and stunt their growth.
When the study was subsequently replicated with seedlings of 16 other native plants, only the hardwoods and other woody plants were harmed by the presence of garlic mustard.
"This suggests garlic mustard invades the understory of mature forests by poisoning the allies of its main competitors," Stinson says. "By killing off native soil fungi, the appearance of this weed in an intact forest could stifle the next generation of dominant canopy trees. It could also invite other native and