ANN ARBOR, MI – The boreal forests of North America could be destroyed by even modest climate change, new research shows.
A recently published scientific study led by a University of Michigan ecologist found an overall temperature increase of less than 3 degrees (1.6 Celsius) will cause major problems for tree species that now thrive in the most northern forests of the continent, particularly when combined with reduced rainfall.
Boreal forests of North America are among the planet’s largest remaining nearly intact woodland ecosystems, where vast amounts of terrestrial carbon remain sequestered in trees and plants. Climate changes could harm both biodiversity and the ability for these forests to produce timber, researchers warn.
These Northwoods are primarily found in Canada and Alaska, but also in parts of northern Maine, northeastern Minnesota, and across Michigan’s northern forestlands. Boreal forests are made up of mostly conifer trees such as spruce, fir, and pine, which can survive latitudes with freezing conditions during at least half the calendar year; they are bordered by tundra to the north and temperate forests to the south.
A five-year experiment involved the use of infrared lamps and soil heating cables to study expected impacts of climate change on thousands of seedlings from nine tree species found within the boreal range. Scientists warmed young trees at two University of Minnesota forest sites around the clock from early spring through late autumn in open-air conditions, not inside greenhouses or growth chambers.
The researchers studied two climate outcomes: an increase of about 2.9 degrees (1.6 C) and 5.6 degrees (3.1 C) above ambient air temperatures. They also used tarps above half the test plots before storms to catch rainwater and mimic shifts in precipitation from a changing climate.
The experiment involved more than 4,500 seedlings of nine native tree species, including five broadleaf and four conifer species: balsam fir, bur oak, jack pine, paper birch, red maple, red oak, sugar maple, white pine, and white spruce. They were planted into existing herb, shrub, and fern vegetation at the study sites in Minnesota.
Temperature and rainfall conditions replicated in the study are not expected to be seen for at least another 40, 50, or 60 years, researchers said.
The study found even an increase of 2.9 degrees (1.6 C) caused many of the tree seedlings to grow less robustly and die more frequently, especially among balsam fir, white spruce, and white pine. Lacking rainfall amplified those negative effects.
These results spell problems for the health and diversity of future regional forests, said Peter Reich, U-M forest ecologist and director of the Institute for Global Change Biology at the university’s School for Environment and Sustainability. He was the lead author for the study published Aug. 10, 2022, in the journal Nature.
Reich said the southern reaches of the boreal forest – like those in Michigan, Minnesota, and Maine – may reach a tipping point from even modest temperature increases, which could diminish the makeup of the forests. Those changes could reduce the capacity of forests to produce timber, maintain biodiversity, dampen flooding, and sequester carbon.
“Forests in Michigan that might not be growing as well aren’t going to be scrubbing as much carbon dioxide out of the atmosphere and holding it in their wood or in the soil,” Reich said. “And the same failure to thrive is going to be happening for similar forests in Wisconsin and Minnesota and in Ontario and Quebec and in Siberia, and all over the world. And that’s going to be causing the climate to change faster.”
While the changing climate is expected to cause negative impacts for conifer trees in boreal forests, the study showed the opposite may happen among hardwood tree species more common in temperate forests to the south.
Modest warming can be expected to enhance the growth of several types of broadleaf hardwoods, including some oaks and maples. But those hardwood species are too scarce in the southern boreal forest to fill the void expected to be left by vanishing conifer trees, researchers say.
Reich said what likely will remain should global warming increase temperatures as little as three degrees is an altered landscape – neither boreal nor temperate forest – with abundant invasive woody shrubs already common at the temperate-boreal border.
“What’s most likely to happen is that the most vulnerable trees will start to do poorly. Individual trees will slow down their growth, they won’t be very healthy, they might be susceptible to drought or insects, and they won’t regenerate, meaning that if their seeds shed, the seedings that come from those tree species … will suffer higher mortality and won’t grow very well,” he said.
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