Climate change threatens our forests

For years, Minnesota climate and forest scientists have suspected that the state’s tree cover would creep northward as Earth’s climate warmed, and the prairies that predominate in the southwest of the state would take over what was previously a mix of fields, deciduous woods and pine forests.

It wouldn’t happen overnight, but the shift is already underway — prompting concern that Minnesota’s signature forests might be lost altogether.

The technical tools to predict these decadeslong changes with any specificity were always tantalizingly out of reach.

Until now.

CURRENT BIOME ZONES

ZONE KEY

 

Prairie

 

Prairie forest mix

 

Broadleaf forest

 

Forest mix

 

Boreal forest

Scientists developed worldwide climate projection computer models that were then adapted to predict local changes to Minnesota’s climate and landscape.

Those findings were presented at a series of hearings at the Legislature, with key testimony by Lee Frelich, director of the University of Minnesota’s Center for Forest Ecology and his associates. The core of their presentation was a set of maps of Minnesota and the Great Lakes region showing the current forest and prairie zones and how they will change and migrate to the northeast as the climate warms and precipitation changes.

According to Frelich, a key component of climate change is the increased amount of carbon dioxide and other greenhouse gases in the atmosphere and the way they retain more and more heat near Earth’s surface. Humans have introduced vast amounts of CO₂ into the atmosphere over the last 150 years, mainly through the burning of fossil fuels.

Because trees can be sensitive to average temperatures and rainfall, and because Minnesota contains the edges of several biomes where tree species are especially susceptible to change, Minnesota’s forest cover could change rapidly in coming decades. Oaks that are common in western and southwestern Minnesota are likely to migrate north, while the pines of the boreal forest in northeastern Minnesota are likely to recede north and perhaps disappear from the state entirely.

The result of their latest research is nine maps — three CO₂ scenarios, each with three different temperature predictions — reflecting thousands of variables that have been painstakingly entered into the computer models.

Ryan Toot, a graduate student in the U’s Natural Resources Science and Management program, said the researchers ran multiple climate models and multiple emission scenarios because “the future is uncertain,” even with most sophisticated forecasting tools. From roughly 40 computer models available to scientists today, U researchers chose three, known as BCC, CCSM4 and Miroc (shown as Warm, Warmer and Hot in graphic below). They capture the range of uncertainty for each of the concentrations of greenhouse gases by choosing cool, average and moderate potential for the region.

Toot wrote the software that calculated the predicted vegetation. The maps have a surprising level of detail, with each square representing a square kilometer.

According to Frelich, “We knew from [research], that the forest and grassland transition occurs right where the climatic water balance is zero — evaporation and precipitation are equal. Where the balance is negative [more evaporation from soil and plants than precipitation], grasslands occur, and where the balance is positive [precipitation greater then evaporation] forests occur.”

Boreal forests, which feature conifers, wetlands and deep clear lakes, occur where mean summer temperatures fall below a certain threshold, Frelich said. Temperate forests occur where the mean is above that threshold.

“With this information, it was possible to analyze the amount of evaporation and precipitation, and mean summer temperature now and in 2070, for each square kilometer,” Frelich said. “If summers are projected to be cool enough with positive water balance, then a given grid square would be boreal forest. Where summers are warm with positive water balance, we have temperate forest. And where there is negative water balance, we have prairie, regardless of temperature.”

Summer climates will, in effect, shift south-southwest by 100 to 300 miles, depending on the scenario, Frelich said. “So Voyageurs National Park could [become] St. Cloud or northwestern Iowa.”

The climate is likely to change too fast for plant species to migrate to their new locations, Toot said. “We’ll probably need to use assisted migration,” which would mean moving and replacing the plants and trees that can’t survive the new climate.

The project is part of an exhibit at the U’s Bell Museum in St. Paul, Weather to Climate: Our Changing World, which runs Feb. 2 to April 28.