CLOQUET, Minn. – Minnesota’s northern forests will look much different in coming decades as a warming climate encourages tree species like oaks and maples and pushes others, including spruce and fir, out of the region.
That was the finding of a University of Minnesota study published this week in the journal Nature Climate Change that used growing plots near Cloquet and Ely and added the amount of warmth expected later this century.
The study was the first of its kind in a northern forest to use ceramic heaters to warm the air near newly planted seedlings as well as heating cables in the ground.
The results were that species like spruce and fir that thrive in cooler areas suffered poorer growth and survival when warmed just a few degrees – up to a 40 percent decline in growth.
The study compared trees exposed to the actual temperature over three growing seasons to trees that were exposed to temperatures 3 degrees Fahrenheit above the actual air temperature and other trees that got a nearly 6 degree warm-up. That’s the warming range climate scientists say we can expect by the end of the century if climate change continues at the current pace.
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Meanwhile trees like oaks and maples did better under warmer conditions – up to 15 percent better under the warmest conditions. The study found that aspen, birch and pines had little reaction to increased temperature.
The results were consistent at both the Cloquet and Ely test plots, said professor Rebecca Montgomery, a University of Minnesota forest ecologist and co-author of the study.
“We saw some pretty substantial differences as heat was added, not just because of the warmth but because that impacts how much water the trees use,” Montgomery said. “We used northern Minnesota nursery stock to simulate how the temperatures are going to affect the forest of the future.”
The results are similar to other studies that forecast rapidly changing forests in Minnesota as some species move north and out of the state and southern species become more prevalent. Nearly the same predictions were made in a study released last June by the U.S. Forest Service’s Northern Research Center. But while most studies use computer models to predict future forests, the University of Minnesota effort actually warmed real, growing trees in the forest to see how they reacted during the growing season – from April to November.
Led by professor Peter Reich of the university’s Forest Resources Department, scientists simulated the effects of a warmer climate on 11 species on 72 plots containing about 4,100 young trees of local Minnesota origin. Researchers monitored growth rates of the trees as well as how efficiently they converted sunlight into energy, the process known as photosynthesis.
The added heat was constantly adjusted, the scientists noted. If it was 35 degrees outside, the experimental trees saw temperatures of 38 and 41 degrees. And if it was 90 degrees, they saw 93 and 96.
In addition to being directly affected by warming, researchers found that spruce and fir might also struggle to compete for sunlight and water with neighboring trees and plants as the climate changes.
The project did not examine how warmer winters might affect trees and other plants, but the researchers note that winter conditions could amplify the effects being seen in this study.
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The results also indicated that a warmer climate is likely to accelerate the northward invasion of non-native species like buckthorn.
Scientists say it’s unclear if the advance of southern species will occur fast enough to fill-in for species that can’t handle the warmer temperatures.
“In the best-case scenario, oaks and maples will become more dominant as boreal species decline, and we will have a different, but still functional forest,” Reich said in announcing the results. “In the worst-case scenario, oaks and maples will not replace the declining species fast enough, and our forests will be patchy and perhaps filled with invading buckthorn.”
Some groups, including the Nature Conservancy, already are working to plant more species like oak and maple in recently logged areas of the Northland to give them a head start for warming conditions.
Reich predicts the change in the forest “will influence everything from the supply of timber to habitat for wildlife to its allure for recreational use and tourism.”
