Climate change may 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 accelerating the impacts of global warming even further 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 forests on the continent. Their conclusions suggest the prospects for long-term survival are grim if we don’t get our act together and reduce the amount of greenhouse gas emissions we release into the atmosphere. Inverse 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 tossing a burning match onto a pile of Arctic carbon that's been tied up in frozen soils and water for millennia,” Roman Dial, professor of biology and mathematics at Alaska Pacific University, tells Inverse. Dial is the lead author of one of the studies discussing climate change’s impacts on forest expansion.

Warmer temperatures mean less tree growth

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

“Studying the effects of climate change in these forests is particularly important because of how closely the timing of leaf growth seems to be linked to temperature,” Cameron Dow, lead author of the Nature study, tells Inverse. 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 on longer. Researchers refer to the springtime 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 in this latest Nature study found the precise opposite: Earlier leaf renewal has “no consistent effect” on the growth of deciduous tree stems.

So why is that a problem? Trees act as “carbon sinks,” which means they store enormous amounts of carbon and prevent their release into the atmosphere — a phenomenon that helps offset global warming. This process is also known as “carbon sequestration.”

Therefore, if deciduous forests aren’t storing as much carbon as scientists previously expected, our current models may be overestimating our ability to counteract climate change through carbon sequestration.

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

Boreal species are moving — and that’s a problem

The second study turns to the impact of North America’s boreal forests, specifically, their unprecedented expansion into Alaska’s Arctic tundra. Boreal forests — often referred to as “taiga” — occur in colder northern climates with freezing temperatures, and they contain both coniferous trees like pine and spruce and deciduous trees like maples. According to a 2021 study, boreal forests store up to one-third of global terrestrial carbon, so they’re hugely important for curbing climate change.

In this latest study, the researchers studied how some tens of thousands of young white spruce trees are migrating northward beyond established treelines and encroaching into the frozen Arctic tundra.

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

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

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

A more open Arctic Ocean also increases snowfall on land, which “protects young trees from being wind blasted in winter,” Dial adds. Therefore, juvenile trees are more likely to reach full maturity. Finally, global warming affects soil decomposition, leading to more nutrients.

All these factors combine to increase the survival of white spruce trees in their new home in the Arctica tundra — a place “where they have not grown for 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 — some 27,000 years ago.

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

The northward advance of the boreal forest is a problem because it can reduce the habitat for migrating birds and change the migrating patterns of animals that Indigenous people rely upon for food, such as caribou. But increasing Arctic tree cover is also a major concern as the planet warms. Boreal forests don’t store as much carbon as the Arctic tundra, so converting tundra to forests “can add carbon and heat to the atmosphere as the warming soils release [carbon dioxide] and absorb more sunlight,” Dial says.

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

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

A third Nature study takes a broader look at North America’s southern boreal forests, offering grave conclusions about their survival under climate change. According to the study, southern boreal forests may be nearing an unsettling “tipping point” where they transition from being a boreal forest to an unfamiliar mix of vegetation — such as shrubs and temperate tree species — that is less ecologically resilient.

The southern boreal forest refers to the southern half of the boreal ecosystem spanning the Atlantic Ocean to the Rocky Mountains in Canada. It’s an area the size of “10 Texases” according to lead author Peter Reich. Reich is the F.B. 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 by 1.6 °Celsius as well as an increase of 3.1 °Celsius.

Warmer temperatures led to greater juvenile deaths for all tree species, but coniferous trees — those bearing cones like pine trees — dominating the southern boreal forests were especially affected. Four of these coniferous tree species — balsam firs, white spruce, paper birch, and northern white pine — lost a whopping three-quarters of their biomass under a scenario of 3.1 °Celsius warming and low rainfall. Biomass refers to the total weight of organic materials in a specific ecosystem.

“Because they have adapted to growing in cooler, moister conditions they don’t have the capacity to deal with warmer, drier conditions,” Reich explains.

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

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

On the other hand, more temperature tree species like maples and oaks were not as affected, and, in fact, were more likely to experience growth under 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 coniferous trees that perish from climate change.

“Governments could consider seeding or planting more of these temperate species into the boreal forest, but at present, funds are not available to do this in a widespread fashion,” Reich says.

Reich adds that it’s “impractical” to expect 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 effective economically and environmentally to simply transition away from fossil fuels and stop climate change.”