Trees Migrating North Due to Warming

Bruce Dorminey, National Geographic News 9 Feb 09;

Other than the Ents of Lord of the Rings fame, trees generally aren't known for their mobility. So news that some tree species may be headed north at an average clip of 62 miles (100 kilometers) a century may come as a surprise.

At that rate, stands of yellow birch in the U.S., for example, may move well north of the Canadian border by the early 2100s.

That's the finding of a new study led by the U.S. Forest Service, which concludes that a few dozen tree species in the eastern U.S. are moving north at an unexpected rate, likely due to global warming.

In a paper appearing this month in the journal Forest Ecology and Management, the study authors documented the northward march of 40 major tree species over 30 eastern states based on the distribution of seedlings versus mature trees.

Previous studies of plant migrations had been done using only computer simulations, or they focused on how some species are climbing up hills and mountains, said co-author Chris Oswalt, of the Forest Service's Southern Research Station in Knoxville, Tennessee.

By contrast, the new study looked at movement based on latitude, using a sampling of the forest service's most recent ground-based data.

The finding confirms a link between global warming and forest migration, said lead study author Chris Woodall, of the Forest Service's Northern Research Station in St. Paul, Minnesota.

"This is no longer conjecture," he said.

Trees on the Move

Woodall and colleagues studied data from 15 northern species, 15 southern species, and 10 species found in both regions. They compared the latitudes of seedlings—trees less than 20 years old, on average—with those of their older counterparts.

Eleven of the 15 northern species appear to have shifted more than 12 miles (20 kilometers), on average, from their historic ranges.

Among the species headed north are the northern white cedar, American basswood, sugar maple, black ash, bigtooth aspen, and yellow birch.

The basswood and maple appear to have moved the most, perhaps as much as 30 miles (50 kilometers).

"This is the first serious attempt at documenting a forest shift for a wide array of species across a broad geographical setting," said Mark Schwartz, a plant-conservation biologist at the University of California, Davis, who was not involved in the study.

"I find it very significant that a cohesive climate change signal emerges from the data."

"Baseball Bat" Trees

Northern trees don't do well in very warm conditions, so the hope is that the climate won't change faster than the species can move.

Some tree species, however, are at the mercy of intervening wildlife when it comes to where and when their seeds take root.

Unless a cedar's pinecone is snatched up by a waiting blue jay, for example, the seed likely won't fall far from the tree.

Meanwhile, cottonwoods, poplars, ashes, and maples have seeds that are light enough to be dispersed by the wind over several miles.

Such highly mobile seeds might allow some species to migrate at rates that even exceed the Forest Service estimate—creating the potential for economic busts—said Dan Botkin, an ecologist at the University of California, Santa Barbara, who was also not a part of the research.

"Northern Pennsylvania and southern New York State are where the best white ash for baseball bats are grown," Botkin noted, "so few [people in those states] would be happy if the trees head north."