TheMondayGarden.com

from the NY Tinmes:
May 2, 2006
Observatory
Garlic Mustard Casts a Pall on the Forest
By HENRY FOUNTAIN
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."

http://www.nytimes.com/2006/05/02/science/02observ.html?
_r=1&oref=slogin#