Climate change could push North America’s forests to a ‘tipping point’

When we think about forests and climate change, our minds often see an image of rainforests burning in the Amazon or tropical jungles being scorched in Indonesia for palm oil.

But new research reveals that climate change has also dramatically affected more temperate, colder forests in North America, potentially further accelerating the effects of global warming as these trees store significant amounts of carbon.

A trio of studies published Wednesday in the journal Nature look at the different ways climate change threatens the forests of the continent. Their conclusions suggest that the prospects for long-term survival are bleak if we do not act and reduce the amount of greenhouse gas emissions we release into the atmosphere. Reverse breaks down the three big takeaways from these studies and what they mean for our planet.

“We humans, through our fossil fuels, have unwittingly done the equivalent of throwing a burning match on a pile of arctic carbon that has been tied up in frozen soil and water for millennia,” Roman Dial, professor of biology and mathematics at Alaska Pacific University, Narrator Reverse. Dial is the lead author of one of the studies that discusses climate change’s impact on forest expansion.

Warmer temperatures mean less tree growth

Deciduous trees, such as maples, are not growing faster due to climate change, new research finds.Getty

The first of these studies assesses the impact of global warming on temperate broadleaf forests in eastern North America. Unlike evergreen pine or palm trees, deciduous trees shed their leaves in the fall until they grow back in the spring. They are the ones we associate with the picturesque changing autumn colors of New England.

“Studying the effects of climate change in these forests is particularly important because of how closely the timing of leaf growth appears to be linked to temperature,” Cameron Dow, lead author of Nature study, tells Reverse. Dow researches the impact of climate change on trees at Purdue University.

Previous studies showed that warmer spring temperatures caused leaves to appear on trees earlier – and therefore stay longer. Scientists refer to the spring renewal of leaves on the trees as “leaf up” or “green up”. They also assumed that longer leaf periods would lead to greater tree growth, but in fact scientists have in the latter Nature study found the exact opposite: Earlier leaf renewal has “no consistent effect” on the growth of hardwood trunks.

So why is it a problem? Trees act as “carbon sinks”, meaning they store huge amounts of carbon and prevent it from being released into the atmosphere – a phenomenon that helps offset global warming. This process is also known as “carbon sequestration.”

Therefore, if deciduous forests do not store as much carbon as scientists previously expected, our current models may overestimate our ability to counteract climate change through carbon sequestration.

“As climate change intensifies and temperatures continue to rise, these negative summer effects are likely to continue to offset the earlier green upswing, leading to lower amounts of carbon sequestration as trees suffer during hot summers,” says Dow.

Boreal species are moving – and that’s a problem

Two white spruce trees sit in the arctic tundra of remote Alaska. Climate change is forcing these trees into the arctic tundra. Roman disc

The second study examines the impact of North America’s boreal forests, particularly their unprecedented expansion into Alaska’s arctic tundra. Boreal forests – often referred to as “taiga” – occur in colder northern climates with sub-zero temperatures, and they contain both conifers such as pine and spruce and hardwoods such as maple. According to a 2021 study, boreal forests store up to a third of global terrestrial carbon, so they are hugely important to mitigating climate change.

In this latest study, the researchers studied how a few tens of thousands of young white spruce trees migrate north past established tree lines and penetrate the frozen Arctic tundra.

“The trees can ‘jump’ kilometers away from the established tree lines and start new populations there,” says lead author Dial.

The researchers found juvenile white spruce trees have spread across the mountain boundary that separates boreal and arctic ecosystems, likely due to winter winds carrying the seeds long distances.

“[Global] warming leads to a reduction in the ice cover in the Arctic. The increase in open water generates stronger winds that help spruce seeds travel farther, Dial explains.

A more open sea ice also increases snowfall on land, which “protects young trees from being blown over in the winter,” adds Dial. Therefore, young trees are more likely to reach full maturity. Finally, global warming affects the soil, leading to more nutrients.

All of these factors combine to increase the survival of white spruce trees in their new home in the arctic tundra—a place “where they haven’t grown in thousands of years,” according to Dial. We can think of these trees as perhaps the first white spruce colonizers of the Arctic tundra since the Last Glacial Maximum – the last Ice Age – around 27,000 years ago.

“These colonists growing miles away from tree lines have more nutrients in their needles, grow faster and respond more positively to warming than trees at established tree lines,” says Dial.

The northward advance of the boreal forest is a problem because it can reduce habitat for migratory birds and change the migration patterns of animals that indigenous people depend on for food, such as caribou. But increasing Arctic tree cover is also a major concern as the planet warms. Boreal forests do not store as much carbon as the Arctic tundra, so converting the tundra to forest “can add carbon and heat to the atmosphere as the warm soil is released [carbon dioxide] and absorb more sunlight, says Dial.

“It signals, perhaps like heat waves, floods and wildfires, that the climate is really changing,” adds Dial.

The future of North America’s southern boreal forests is uncertain

Boreal forests are threatened due to climate change, a new study shows. Pictured here is a boreal forest in Alaska’s Denali National Park.Getty

A third Nature the study takes a broader look at North America’s southern boreal forests, and provides serious conclusions about their survival under climate change. According to the study, southern boreal forests may be approaching a troubling “tipping point” where they transition from being a boreal forest to an unknown mix of vegetation – such as shrubs and temperate trees – that are less ecologically resilient.

The southern boreal forest refers to the southern half of the boreal ecosystem that stretches across the Atlantic Ocean to the Rocky Mountains of Canada. That’s an area the size of “10 Texases” according to lead author Peter Reich. Reich is the FB Hubachek Senior Chair in Forest Ecology and Tree Physiology at the University of Minnesota.

The paper looks at the survival and growth of nine boreal tree species under an increase in global temperatures of 1.6 °C, as well as an increase of 3.1 °C.

Warmer temperatures led to greater juvenile mortality for all tree species, but conifers – those with cones such as pines – that dominate the southern boreal forests were particularly hard hit. Four of these conifer species – balsam fir, white spruce, paper birch and northern white pine – lost a full three-quarters of their biomass under a scenario of 3.1° Celsius warming and little precipitation. Biomass refers to the total weight of organic matter in a specific ecosystem.

“Because they’ve adapted to grow in cooler, wetter conditions, they don’t have the capacity to cope with warmer, drier conditions,” Reich explains.

The loss of the southern boreal forest’s most abundant species may further contribute to global warming since boreal forests store carbon dioxide and prevent its release into the atmosphere.

“Forests that grow poorly will produce less timber and less carbon storage,” says Reich.

On the other hand, several tree species with temperatures such as maple and oak were not as affected and were actually more likely to experience growth during global warming. But the study says these resilient tree species are rare in the southern boreal forest, so they won’t be able to quickly replace the conifers that are dying from climate change.

“Governments may consider seeding or planting more of these temperate species in the boreal forest, but currently there are no funds available to do this on a widespread basis,” says Reich.

Reich adds that it is “impractical” to expect that we can fully adapt to climate change in North America’s boreal forests by planting new temperate trees. Instead, he says “it will be far more efficient economically and environmentally to simply move away from fossil fuels and stop climate change.”

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