"This Oak is the slowest growing and the longest-lived oak in the Big Woods and Minnesota," reads a sign along a trail in Nerstrand Big Woods State Park near Faribault. Nearby towers a stately, ancient white oak (Quercus alba), whose species is the subject of the posting. There is just one problem. The tree is dead.

"Last year it had a couple leaves left on it, and it just—it went," says Neil Slifka, area resource specialist for the Minnesota Department of Natural Resources. The dead old giant is one tragic character among many in a dramatic saga slowly unfolding in Minnesota's forestlands. The villain in this story? Climate change.

As Minnesota's climate grows warmer and wetter, forests are facing a new host of challenges. Persistent flooding drowns trees in some parts of the state, while in other places climbing temperatures could create thirsty conditions for forests in between rains. Longer summers and a growing absence of deeply cold winters favors the proliferation of pests that kill trees. Changing conditions favor some tree species more than others, which may lead to transformation of the forests Minnesotans are used to.

By the end of the century, our forests could look substantially different—to hunters, loggers, hikers, wildlife watchers, forest foragers, and anyone else who spends time in the north woods. While big change may seem far in the future, keen observers may already be seeing changes in their favorite state park or their own "back forty." Forest managers throughout the state are observing climate challenges, too, and they are in a race against time to discover ways to protect our forest treasures now and into the future.

High and … Wet

Nerstrand Big Woods State Park is a rare jewel in southern Minnesota.

"It is a remnant of Big Woods that's isolated out here in a sea of corn and beans," says Slifka. "People could see that it was finite and worth preserving."

Once a hodgepodge of private woodlots, grazing space, and commercially owned timber land, the park was established in 1945 to preserve this unique example of the Big Woods ecosystem, which historically covered an expansive area of central Minnesota. Today, the canopy of mature hardwoods, miles of trails, and proximity to the Twin Cities metropolitan area make the park a popular spot to search for woodland wildflowers on a cool spring morning, or take in the vibrant foliage on a sun-drenched fall afternoon.

Around 2013, park staff began to notice a mysterious ailment creeping into the overstory in the southern portion of the park. Patches of upland forest—oak, sugar maple, hickory, basswood—were dying. Strangely, while the affected patches had nearly total canopy loss, some adjacent areas remained unscathed. Staff puzzled over the issue. Was it a fungus? An insect?

The culprit, as it turned out, was water. "What we're experiencing is a fairly large-scale die-off that we've associated with increases in growing-season precipitation that have occurred over about the past 10 years," says Slifka.

The current decade is the wettest the park has seen in recorded history. And the extra precipitation is coming in deluges—huge dumps of rain greater than an inch at a time. In this part of the park, that extra water has no place to go. Despite being at a higher elevation than surrounding areas, the terrain here is flat and the thin topsoil sits on clay beds or glacial till that inhibit draining. So "the highest point in the park is drowning," Slifka remarks. "We're just not getting chances to dry out."

More than 200 acres of the 1,600-plus-acre park have been affected, and Slifka fears more will follow if the rainfall trends continue. The good news is that the northern half of the park, with iconic park features like the rare dwarf trout lily and Hidden Falls, and some of the southern half may be safe because of better drainage and steep topography. But large areas of the southern portion remain at risk, so park staff are thinking creatively about how to respond.

In a cruel bit of irony, the only species that seems to be doing well in the wet conditions is green ash. With emerald ash borer, a notorious killer of ash trees, lurking only 20 miles away from the park, a forest of pure green ash does not seem like a viable replacement.

So DNR resource managers are experimenting with "assisted migration"—planting tree species that are not traditional residents of upland areas in the Big Woods, but may fare better under wetter conditions. These species include silver maple, hackberry, swamp white oak, cottonwood, and disease-resistant elm cultivated by the University of Minnesota. Managers planted four 1.25-acre plots and will monitor the seedlings' progress over the next five years.

The approach is part of a new way of thinking about natural resources protection. "This isn't a restoration project," says Slifka, "because as the landscape evolves under a changing climate, we can't go backwards in time."

Bugged

North of Interstate 94 in Minnesota you will find a special conifer that might be mistaken for a pine or spruce in the summer, but reveals a secret when the air turns crisp and its needles turn yellow.

"It's our only native deciduous conifer in the state, so it's very unique in that way," explains Megan O'Neil, DNR forest health specialist. "It's beautiful in the fall ... That golden tamarack bog picture is iconic of northern Minnesota."

Minnesota's native larch species, known as tamarack (Larix laricina), often grows in swampy, stagnant conditions that few other tree species find hospitable, dominating the canopy almost completely in some places. For nearly two decades, DNR forest health experts have been documenting large and growing dead zones in that canopy.

"Back in 2001 they started noticing a serious decline of tamarack and attributed that, after some investigation, to eastern larch beetle," says O'Neil. Through aerial forest health surveys between 2001 and 2019, the DNR documented more than 666,000 acres of accumulated death or damage in tamarack forests caused by this beetle (Dendroctonus simplex), which gnaws its way throughout the living tissue below the bark, cutting off water and nutrient flow. That is nearly half of all tamarack acreage in the state, and the beetle's destructive march has only escalated in recent years.

The Eastern larch beetle is a native Minnesota insect that has co-existed with tamarack for millennia. In the past, outbreaks were limited by natural defenses that can allow the trees to survive a beetle attack. The duration and intensity of the current outbreak, however, bewildered forest managers and researchers alike, until researchers at the University of Minnesota uncovered an important change in the beetle's life cycle, fueled by Minnesota's longer growing seasons and warmer winters: They found the beetles were managing to squeeze in an extra generation.

This increase in reproductive activity means more beetles, but it also means that tamarack trees may experience multiple attacks in a single season—a one-two punch from the bugs. More attacks, on top of other potential stressors such as drought, are overwhelming Minnesota's tamarack trees.

Forest managers' best tool for dealing with the outbreak is to harvest tamarack ahead of it whenever possible. Tamarack requires open, sunlit conditions to grow. If a stand is harvested before it dies, select live trees can be left standing to disperse seeds for the next generation. If the trees have already died, which is often the case with the fast-moving outbreak, managers can spread collected seed from helicopters after a harvest to encourage quick regeneration.

However, these methods rely on the relatively small market for tamarack wood—a market that is currently flooded by ever-increasing supply. Lack of sufficient demand "takes away our major tool for reforestation," says Mike Reinikainen, DNR silviculture coordinator.

The DNR has partnered with the University of Minnesota, with funding from the U.S. Forest Service, to study what happens to tamarack stands that die and regenerate naturally. Initial data provides some hope.

"We're finding that, preliminarily, we're getting trees back in these damaged sites," says Reinikainen. It is unclear if natural regeneration can occur as quickly and consistently as managed regeneration, so he emphasizes that more research is needed. "We need to get more sites and better geographic spread. But early on, it's encouraging."

North Woods of the Future

From the hardwood-covered hills of southern Minnesota to the wild sub-boreal landscape of the north, climate change is causing our state's forest ecosystems to shift and change, favoring some tree species more than others.

One species projected to decline under warmer conditions is quaking aspen (Populus tremuloides), a fast-growing tree that covers large swaths of the state and regrows in dense stands after disturbances such as fire or timber harvest. Because of its wide availability and quick regeneration, quaking aspen has come to be the most important timber species in the state—the backbone of pulp, paper, and engineered wood products industries. Aspen forests also provide important habitat for all manner of Minnesota wildlife.

Suitable aspen habitat is projected to decline by 36 to 70 percent across the northeastern part of the state by the end of the century, depending on climate change severity, says Stephen Handler, climate change specialist for the U.S. Forest Service and the Northern Institute of Applied Climate Science. "That's not saying that aspen is going to decline by 70 percent," he says. "But what it tells you is that the aspen that's left behind is going to be growing further outside of its comfort zone."

Handler cites "longer growing seasons, warmer conditions during the growing season, and more erratic precipitation" as the main risks to aspen. "All that can add up to drought stress," he explains.

In drier, western states, aspen is already showing signs of decline. "We've started to get an advanced warning of how quickly the switch can flip for aspen if you get more drought," says Handler, referring to observations of sudden aspen die-off in Colorado. "It can be really dramatic."

So what can Minnesota forest managers do to prepare while aspen is still healthy and abundant? Handler suggests identifying refuges for aspen that will remain resilient under a warmer climate, and beginning now to transition to more climate-ready species in areas with less suitable future habitat. He also suggests managing aspen for a mix of age classes. "The most important thing is to not wait until you feel like you have the correct answer, " he says. "We know that climate change is going to throw us some curve balls … I don't think we should let that be a barrier to action."

Some landowners are already implementing this advice. UPM Blandin has been making paper from Minnesota trees for more than 100 years. The company also owns and manages more than 188,000 acres of forest land, most of it under a conservation easement, near its paper plant in Grand Rapids. "Our mission is to produce sustainable forest products and manage a healthy and vibrant forest here in Minnesota," says Sawyer Scherer, the company's forest ecologist.

UPM Blandin uses aspen in its paper mix, along with spruce and balsam fir, which are also projected to decline in Minnesota as the climate warms. Despite these threats, Scherer's outlook is positive. He says the company is already taking steps to make forests more resilient.

"Over the last 15 to 20 years, there's been a real conscious effort to move toward ecological forestry principles and encourage diversity in all of our forest types," he says.

This approach allows the company to nudge future forests toward more climate-resilient species like red oak, bur oak, and white pine, while still maintaining the northern species that are doing well today.

When those climate-adapted forests mature, Scherer is confident the industry will have adapted to use them: "The forest products industry is pretty innovative. They've got a lot of very smart people working on ways to develop new products or utilize species that might be more abundant in the future."

One of the new products for sale is an increasingly valuable commodity: carbon. UPM Blandin's efforts to diversify aspen forests with longer-lived species traps more carbon on the landscape. In turn, they are selling this stored carbon as credits on the growing global carbon market.

By storing carbon in trees and soil, forests help reduce the carbon-based "greenhouse" gases in our atmosphere that are warming the planet and fueling climate change. So as we help our forests adapt to change, they can also be part of the solution.

Lessons

Forests operate at a different speed than humanity. In comparison to our fast-paced lives, a forest seems to stand still, growing and changing at a rate that we frequently cannot observe except by looking backward in time. Decades from now, when Minnesotans look back on how our forests have changed, what will they notice? How will they experience the forest then versus now?

Some changes are easier to discern. When Nerstrand Big Woods State Park's interpretive trail signs were installed in 2012, they marked 12 healthy examples of prominent tree species of the historic Big Woods forest. Eight years later, all but three of those trees are dead. "What [the signs] demonstrate currently is the speed at which the mortality occurred," says Alexander Watson, DNR naturalist.

Watson isn't in a hurry to remove the signs, though. As he sees it, they teach a different lesson now—one about the rapid transformation the park is undergoing due to climate change. "It does help remind us that we've witnessed a change in our lifetime," he says. Research will continue in the park to better understand what the evolving ecosystem might look like in the future. But for now, the signs act as a placeholder for a lingering question that, ultimately, only the forest can answer.