The trees are growing faster thanks to the increase in carbon dioxide in the atmosphere linked to global warming.
Aspen trees, the backbone of Minnesota's paper industry, are liking the extra carbon dioxide in the air linked to global warming.
New research published Friday found that aspen growth rates increased by 53 percent during the past half-century, as carbon dioxide in the atmosphere increased about 20 percent.
"Trees eat carbon dioxide for a living," said Don Waller, study author and University of Wisconsin-Madison botany professor.
As carbon dioxide increases in the air, he said, plants can extract more of it and convert it to sugar through photosynthesis. That speeds up their growth.
The results could be especially important for Minnesota and Wisconsin, where aspen is the dominant species on about 7.3 million acres of timberland.
"It's the most abundant and the most-used species by the forest products industry," said Tim O'Hara, vice president of forest policy at Minnesota Forest Industries. It is the main species used for paper and certain construction board, he said, and is also used for pallets and other products.
The Wisconsin study is one of the first to research aspen and outdoor carbon dioxide levels in their native forest environment.
Waller and other scientists chose 919 aspen trees, ranging from 5 to 76 years old, from three regions of Wisconsin. They took core samples from each and analyzed their growth rings. Even accounting for different ages of the trees and rainfall rates over the decades, Waller said, carbon dioxide was the major cause of boosted growth rates.
The study included researchers at the University of Minnesota, Morris, and was published Friday in Global Change Biology, a national journal.
Waller said that the findings do not mean that aspen will continue to grow at ever-faster rates.
The species may already be reaching a saturation point on the limits of how much carbon dioxide it can absorb.
"Aspen may have a relative growth edge right now because of this carbon dioxide fertilization effect," he said. "But that doesn't mean necessarily that they're going to continue growing better forever."
'Sudden aspen decline'
What happens in the woods will be much more complicated and unpredictable, said Waller.
Rapid aspen growth and expansion could trigger an insect infestation or set off a plant disease. It could also reduce the number of other tree species, he said, and change wildlife habitat.
Studies by other researchers have found correlations between carbon dioxide and tree growth, but have used growth chambers or greenhouses that piped in elevated concentrations of the gas. Other scientists studying oak and pine have not observed faster growth rates.
Waller said that carbon dioxide is a more powerful influence on growth than he expected.
Still, he said, nothing can counteract drought, which is killing millions of aspen in Canadian prairie provinces, northern plains states and parts of the West.
Colorado alone has lost half a million acres of aspen in this decade -- a phenomenon known as "sudden aspen decline" -- in large part because of acute drought and increased vulnerability to insects.
"The big lesson here is that there are surprises in nature, and that global climate change isn't just about temperature," Waller said. "It would be very difficult and unwise to predict all the ecological consequences of this one research result."Tom Meersman • 612-673-7388