In a recently logged clearing in the north woods near the headwaters of the St. Louis River, a crew of 13 tree planters fan out, swinging heavy tools called hoedags into the ground.
Often, they clang against rock, the sound echoing against the surrounding forest. But when they pierce quietly into the wet earth, the planters quickly stoop, stuff a tiny white pine, white spruce or jack pine seedling into the ground, tamp it down, and then move on to the next seedling.
In this way, one tree at a time in a 15-acre tract, the Nature Conservancy along with state, federal and university partners, is hoping to maintain northern Minnesota's iconic boreal forest into the next century, despite climate change models that predict that warmer, drier conditions will make it much more difficult for these species to survive in Minnesota.
"This is the test kitchen for conservation in northern forests and climate proofing our landscape," said Meredith Cornett, science director for the Conservancy in Minnesota, North and South Dakota.
Nonprofit groups, the Minnesota Department of Natural Resources and the United States Forest Service have long planted pine seedlings in northern Minnesota to try to return conifers to their former grandeur. They once blanketed nearly three quarters of northern Minnesota. Now they cover only about half that territory.
What's new with this project is that foresters are targeting specific areas of the forests, cold spots on the landscape, where temperatures have historically been cooler, or the temperature has not been rising as fast, where spruce, pine and other species may be able to continue to survive, even as the bulk of the boreal forest migrates northward.
"We're trying to better understand how we can better maintain components of this southern boreal forest as the climate warms," explained Nature Conservancy forest ecologist Mark White. "Hence the term 'conifer strongholds,' because we think that we're going to be able to find the places where they can persist and thrive for a longer period of time."
Climate change is a particularly keen threat to northern Minnesota's forests for two reasons. First, temperatures are rising more quickly in northern latitudes. Temperatures are expected to rise from 2 to 10 degrees over the next 50 years.
Second, Minnesota's boreal forest in the Boundary Waters and surrounding areas is particularly vulnerable to climate change because it is at the far southern end of its range.
"It's just so on the edge," said Lee Frelich, director of the University of Minnesota's Center for Forest Ecology. "And we've already warmed up a fair amount. One or two more degrees of warming would definitely tip the balance away from boreal."
The only question remaining, said Frelich, is how much boreal forest will Minnesota be able to retain on the colder parts of the landscape.
In several decades, assuming a "business as usual scenario for carbon dioxide emissions," Frelich expects much of the northern boreal forest will convert to a temperate hardwood forest that's common in much of central and southern Minnesota, dominated by maples, basswood and oaks.
But, he said, if projects like the Nature Conservancy's prove successful, it may be possible to preserve remnants of the boreal forests in cold spots like the north facing slopes of hills, or boggy areas where cold air drains, interspersed among more southerly species that migrate northward.
This effort to preserve "conifer strongholds" in northern Minnesota is an example of a broader climate change adaptation strategy, in which land and wildlife managers try to preserve what are known as climate "refugia," those cold spots on the landscape where threatened species may continue to thrive as the climate changes around them.
Other examples include an effort by the Minnesota DNR to protect 176 so-called "refuge" lakes in northern Minnesota, where land managers hope to preserve cold-water habitat for cisco, an important forage species for walleye and other popular game fish.
Scientists in northern New England are working to preserve parts of the spruce-fir forest there to maintain refuges for moose, Canada lynx and snowshoe hare.
"This is an important strategy of climate adaptation," said Toni Lyn Morelli, a research ecologist at the U.S. Geological Survey's Northeast Climate Science Center, who studies climate refugia.
"We have stressors across the landscape, and we only have limited resources that managers can put to dealing with those stressors," she said. So, it makes sense to prioritize funding in the places where it has the best chance for success.
Protecting these climate cold spots, or conifer strongholds, is the flip side of another strategy often called "assisted migration," in which foresters plant southerly species like red and burr oak in northern areas, to help facilitate their transition northward and ensure that the region doesn't lose its forestland.
The technique is controversial. Some land managers argue it could lead to unknown, unintended consequences. For example, it could create new invasive species.
The Nature Conservancy and U.S. Forest Service began experiments three years ago to examine how different species fare farther north. The Minnesota DNR is now looking at planting red oaks near Sawbill Lake on the southeastern edge of the Boundary Waters, said Paul Dubuque, silvaculture program coordinator for the DNR who's advising The Nature Conservancy on its project.
To make the forest more resilient to climate change, it's important to diversify the forest as much as possible, Dubuque said. "That has a lot of benefits for not only timber, but forest products, wildlife habitat and biodiversity."
Crews will be planting conifer seedlings on 30 sites across northeast Minnesota through the end of the month. The Nature Conservancy hopes to plant 400 acres of trees this year, and another 400 next year. The effort will cost about $450,000, with funding coming from the Wildlife Conservation Society.
It's a lot of money, said White, the forest ecologist, as he stooped to inspect a recently planted white pine seedling — a tiny beginning to what he hopes will grow into a more resilient future forest.
"We just think it's really important to get out in front of these issues," he added. "We don't really have a lot of time to figure it out. So we think the time is now to start adapting how we manage our forest lands."