Minnesota's celebrated North Woods won't be nearly so woodsy -- and may not even stay in Minnesota -- according to a new study by University of Minnesota researchers.
Northern forests will become thinner in the next few decades. The prairie will march north and east. Climate change and drought are not the only things forest managers need to worry about, says a scientific paper to be published this week.
"Lots of other factors are going to reinforce the effect of climate change on forests, not just warmer temperatures," said study author Lee Frelich, director of the Center for Hardwood Ecology at the U.
He called the conspiring environmental pressures a "triple double whammy."
"Storms, fires, invasive insects and unsuitable climate will remove mature forests from the landscape, while other factors, such as deer and European earthworms, will prevent tree reproduction," according to the paper, published in September's issue of Frontiers in Ecology and the Environment. It references 50 previous studies about pressures on northern forests.
The impact will repaint the Minnesota landscape, it said, with the border between prairies and woods shifting 60 to 300 miles to the northeast.
Some tree species will largely die out in Minnesota, Frelich said. Jack pine, black spruce, balsam fir and aspen already are at the southern edge of their growing range in the state, he said, and can grow in the thin northern soils only because there is just enough rainfall and relatively low temperatures. As average temperatures rise only a couple of degrees, those trees will be lost.
Researchers have often looked at individual problems expected in the future such as less regular rainfall, said Frelich, but no study before this one has analyzed the cumulative impact.
Other species can tolerate higher average temperatures, said co-author and U forestry professor Peter Reich, but they are in greater danger from native and invasive pests that may survive milder winters, live longer, breed more successfully and spread further.
Insects love warmth
"Rapid warming, especially in winters, has a springboard effect for insects and diseases," he said.
Other pressures on the forest are less connected to climate change, but also will take their toll, according to the researchers.
White-tailed deer, with larger populations partly because of milder winters, are gobbling too many seedlings of northern white cedar, yellow birch, northern red oak, eastern hemlock and white pine.
European earthworms are moving across the landscape, stripping off leaves and other natural mulch that protects soil and prevents trees from drying out in late summer. Without leaf litter, soil warms and forms a hard crust. The result: Less rain soaks in, and tree roots receive very little moisture.
To be sure, some effects of warmer climate could be positive, the paper notes. More carbon in the air from greenhouse gases could make it easier for trees to grow richer foliage without losing as much moisture, and more nitrogen in the atmosphere could act as fertilizer to northern forests. But researchers said those benefits are modest, and any gains in growth will be more than offset by less regular rainfall and other changes.
Looking like Iowa, Missouri
The result, said Frelich, will be the "savannafication" of many northern areas, which will become grasslands with scattered trees and brush rather than forests, and will resemble parts of central Iowa or even Missouri.
Bob Krepps, land commissioner in St. Louis County, said he has seen unusual changes in forests in the past decade, and some of them may be related to climate change. "We can believe [climate change] or not, but we can't ignore that things are happening," he said. "We're seeing tree species here popping up that we probably wouldn't have seen 30 years ago," he said.
Krepps manages nearly 900,000 acres of forest, and said his department's employees are adapting to the changes as best they can, such as protecting pine seedlings from deer for at least five years so the trees can regenerate.
Wayne Brandt, executive vice president of Minnesota Forest Industries, said he's not sure anyone is smart enough to know what will happen to forests in coming decades.
If some tree species die out because of warmer weather, he said, there isn't much that can be done about it in the forests. Industry is innovative and can make paper, lumber and building products out of many different kinds of wood, he said. "Over time we'll be able to adapt to how the forests change," said Brandt. "If they change, then we'll have to change."
Tom Meersman • 612-673-7388