The Sawtooth Mountains in northern Minnesota are dominated by aspen and conifers. Image: Justin Meissen. Some rights reserved.
By Steven Maier
A warming climate is spurring change in Great Lakes forests.
The shift affects rainfall, wildlife and growing seasons–major factors determining forest makeup. Warmer temperatures in northern Wisconsin, Minnesota, Ontario and Michigan’s Upper Peninsula are already favoring some tree species over others, scientists say.
The losers
The northern woods of Minnesota–the Great Northwoods, as some Minnesotans call it–is an ecosystem in flux.
“A lot of these classic Northwoods species are not the ones that are going to thrive in a warmer climate,” said Meredith Cornett, director of conservation science for The Nature Conservancy in Minnesota and the Dakotas.
Northern Minnesota is dominated by aspen and conifers. Both are projected to decline. Forest cover has already thinning, Cornett said. That’s one of the things forest managers are trying to combat, along with a shrinking numbers of tree species and invasives looking to take advantage of an uncertain time in the forest’s development.
“We want to maintain, as best we can, the full diversity of mixes of species,” said Paul Dubuque, silviculture program director at the Minnesota Department of Natural Resources.
Climate change threatens diversity in age and tree species, key to forest health, scientists say.
A stand of aspens in Flagstaff, Arizona. Image: Michael Wilson. Some rights reserved.
In some places, the conifers can be preserved, but in others, that means looking to new species to fill the gaps, Cornett said. The Nature Conservancy is trying to accomplish that by planting species like red oak, bur oak and white pine.
Managing the changing forest is a balance of supporting wildlife while increasing tree diversity. Some species, like songbirds and martens, need older forest with fully developed trees and closed canopies, said Dubuque. Northern Minnesota has these, but they need to be maintained. What’s lacking is younger growth–fragmented canopies allowing room for shrubs and undergrowth.
For some Minnesotans, the change is devastating. This generation is witnessing the transformation of an iconic landscape, Cornett said. She related stories of childhood wanderings among the conifers of the northern woods.
Those woods look different now, she said. And those ecosystems will continue to change.
Until a few years ago, Cornett said, the Nature Conservancy was focused on restoring and protecting the trees from those memories. It’s only recently that they began restoring hardwood species like oak.
“We realized that a lot of the species that we were investing in were not going to be the winners in the long run,” Cornett said. “That was a pretty sobering thought.”
Now the focus is on helping the woods make a graceful transition, which means embracing the change.
The aspens won’t leave the region completely. Wetter areas will dry and could offer a refuge for them, Dubuque said.
The winners
Oaks and white pine have the potential to take over from receding conifers, but need help to do so. The climate is warming too quickly for their natural spread to keep up, so the conservancy is planting and protecting them, Cornett said.
As the climate warms, oaks are poised to take advantage of longer growing seasons and drier soil. Unlike maples, oaks grow for longer periods and withstand dry soils and droughts.
“If there’s one positive thing about climate change in this part of the world, it’s that the area would become more conducive for oak forests and oak savannas,” said Lee Frelich, director of the University of Minnesota Center for Forest Ecology.
Oaks have struggled over the last two centuries as maple trees proliferated and settlers converted oak savannas to farmland. The savannas, which support hundreds of native bees and butterflies, have dwindled from millions of acres to just a few thousand.
The Nature Conservancy is working to plant more bur oak, like this one in Indiana. Image: Justin Meissen. Some rights reserved.
The settlers aided oak forests by slashing and burning across the Great Lakes region, leaving room for oaks, which are fire hardy and can sprout from surviving root systems. Oak forests stretch across the region, but most of those are 140 years old, Frelich said. The outlook for younger oaks is far dimmer.
The early effects of climate change led to wetter climates, Frelich said, which favor maples. Faster-growing maples close the forest canopy, slowing the growth of young oaks. Many oaks are eaten by deer before they grow large enough to establish themselves. It might take four years for an oak growing in sunlight to reach that height. It can take 20 for those under a closed canopy to achieve the same feat.
While poised for success, the oaks need help. Forest managers often fence off oak seedlings and transport seeds to regions that might take decades for the oaks to reach on their own.
The future is bright for oak savanna, which thrives in shallow, dry soil. The region south of the legendary Boundary Waters in northeast Minnesota could eventually offer the perfect environment, Frelich said.
The goal is to establish the oaks while the conditions are right for it, Cornett said. Missing this opportunity could hurt diversity as already-established species take over.
“This is an ecosystem at risk,” Cornett said. “We don’t always think of it in this way. But in a way, the Northwoods is already trying to become something pretty different.
“Saving the Great Northwoods under the threat of climate change might mean helping it to transform.”