In drama, the uninvited visitor is a common plot device. Everyone is getting along swimmingly until a new character arrives and upsets the apple cart. Things quickly fall apart.
Garlic mustard, a tall weed native to Europe that was introduced to the United States in the late 1800's, is a bit like that uninvited visitor. Researchers have found that it disrupts a healthy relationship between hardwood tree seedlings and soil fungi, with results that can be disastrous for a forest.
Like other scientists, Kristina A. Stinson, who studies invasive plants as a research associate at the Harvard Forest, Harvard's ecology and conservation research center in Petersham, Mass., had noticed that native trees suffered in the presence of garlic mustard. "We thought their dependence on native fungi might play a role," Dr. Stinson said.
Many plants make use of arbuscular mycorrhizal fungi, which form an elaborate network of filaments throughout the soil. These fungi are a diverse group, but they all have one thing in common: they help plants take up nutrients from the soil, getting carbon in return.
Garlic mustard is a member of the mustard family, "one of the very few families that do not need to associate with mycorrhizal fungi at all," Dr. Stinson said. These species produce chemicals that have antifungal properties. Native mustards have been around long enough, she suggested, that the mycorrhizal fungi have learned to live with them. But the fungi haven't had time to adapt to garlic mustard. "It basically is killing off the fungi," she said.
In a study using soils from a forest in Ontario, Dr. Stinson and colleagues found that sugar maple and other hardwood seedlings grew much slower when the soil came from an area infested with garlic mustard than from a mustard-free area. The findings are published in the journal Public Library of Science Biology.
In studying invasive species, scientists often see a direct effect. Invasive cane toads in Australia, for example, wipe out snakes and other predators that try to eat them. But garlic mustard displays a mechanism that, so far at least, appears to be unique. "It's really a demonstration of how 'the enemy of my friend is also my enemy,' " Dr. Stinson said. By killing fungi, "it's disrupting this longstanding native mutualism."
Garlic mustard has now spread through 30 states, from Maine to Oregon, and into Canada. "When this plant shows up in a forest, the tree species themselves that become the canopy are most at risk," Dr. Stinson said. "That could have tremendous impact by changing the composition of the forest."
While the effect might not be immediate, it will occur nonetheless. "Our experiment was on seedlings," Dr. Stinson said. "But those are the future generations of forests."