Menace of Norway Maples

September 27 2005 - Mackinac Island in Lake Huron may be on the verge of being overrun by an exotic European import, Norway Maple, according to a Michigan Technological University scientist. The island's signature horses may also be playing a crucial role in spreading the seeds.

Norway Maples are pretty, fast-growing shade trees available from many garden catalogs. They are common in local gardens but, taken out of the backyard, they are big, green invaders.

"In the eastern U.S., where they've become established, diversity is much lower than in forests made up of native trees," said Chris Webster, assistant professor of forest resources and environmental science. Given a chance, Norway Maples can take over. "It leafs out one week earlier than the sugar Maple, and its leaves stay on one week longer. It outcompetes native species, and it really hurts the spring flowering plants."

They breed like rabbits and grow like weeds - and they can be very hard to kill. Webster compares them to the mythical Hydra, which grew two snake heads every time one was cut off, its roots send up many suckers after the main trunk is severed, so mere logging doesn't slow it down much. The application of powerful herbicides to the stump is the only effective way to kill it back.

Webster and his team of students began studying the distribution of Norway Maples on Mackinac Island in 2003. The trees were introduced about a hundred years ago but islanders never thought of them as a problem until they came down with the sticky, unattractive tar-spot fungus, to which Norway Maples are particularly vulnerable.

"We measured and mapped all the trees on the island with GPS," Webster said. The team also determined the age of the trees, which allowed them to look back over the decades and figure out how the population had spread around the island since its introduction during the 1920s.

In 1945, they were confined to urban areas. "They still didn't have any in the forest," Webster said. "In 1955, there were a few. In 1965, they were scattered like dust specks." But by 1995, that had changed, with Norway Maples growing in a halo around developed areas and also springing up in clumps within the forest.

This unusual distribution pattern caught the researchers' attention. As the trees turned up in rural areas, they didn't just slowly spread outward from town. They were growing in clusters, most of them near roadways.

This was no accident. "There's a unique circumstance on Mackinac Island," Webster said. "It has a lot of horses, and they produce copious amounts of manure. Residents used to sweep it up in town and haul it out to compost in the forest."

Norway Maple seeds ended up getting mixed with manure and other street sweepings before being carted into the forest. "Inadvertently, folks on the island were seeding Norway Maple into the forest and providing it with a good dose of fertilizer," Webster said.

Norway Maples have not overrun Mackinac Island's native vegetation - yet. But Webster believes that as this generation of trees matures and sets seeds, the area could see an explosion in their numbers and maybe even the end of the natural landscape.

"They are so difficult to control," Webster said. "You'd have a hard time predicting what they'll do in the next few decades."

Nevertheless, Webster thinks that a wait-and-see attitude could prove disastrous to the island's native vegetation. "We shouldn't lull ourselves into a false sense of security," he said. "If you wait until you prove it's a problem, it could be almost impossible to stop."