Boreal forests comprise the world’s largest terrestrial biome—a vast landscape ringing the high latitudes of the Northern Hemisphere. The expanse of trees in this sparsely populated wilderness helps keep Earth’s climate stable.
“We’re talking about the entire Northern Hemisphere changing fundamentally because of the climate change that we have caused.”
As Earth steadily warms, however, boreal forests will witness a substantial shift in tree distribution, according to a new study published in the Proceedings of the National Academy of Sciences of the United States of America.
“We’re not just talking about changing a few patches of trees,” said coauthor Ronny Rotbarth, who was a Ph.D. candidate at Wageningen University and Research in the Netherlands when he conducted the study and is now a postdoctoral researcher at the Universität Freiburg in Germany. “Here, we’re talking about the entire Northern Hemisphere changing fundamentally because of the climate change that we have caused.”
A Glimpse into the Future
The southern, warmer parts of boreal forests are characterized by dense tree cover. As the biome stretches into the Arctic, the drop in temperature inhibits such density and turns lush forests into sparse woodlands.
In the new study, Rotbarth and his fellow researchers analyzed tree distribution across the boreal biome from 2000 to 2020. They employed a stochastic modeling approach using the Moderate Resolution Imaging Spectroradiometer (MODIS), a sensor on NASA’s Terra and Aqua satellites.
“It was a great use of long-term satellite data and aligns with other work that has been done in recent years,” said Logan Berner, a forest ecologist who studies the Arctic tundra biome at Northern Arizona University and was not involved in the study.
The researchers used the 21 years of observational data to simulate changes in the decades leading to 2100. Their models showed a tendency for tree cover to decline in warmer areas and increase in colder areas.
These changes could lead away from the temperature-related bimodal tree distribution to a unimodal, open forest state with 30%–50% tree cover before the end of the century. This distribution is much lower than the more than 60% tree cover that warmer boreal forests currently have and higher than the 5%–15% cover of colder forests.
“We didn’t really expect that,” Rotbarth said. “What was quite surprising was the speed…with which some of these more open, colder boreal forests are becoming denser.” The authors’ modeling suggests that colder forests will experience up to a 4% increase in tree cover every decade.
Carbon Emissions and Other Cascading Effects
But changes in tree cover are only one part of the story.
Temperature-driven changes in tree distribution could lead to changes in the boreal forest’s carbon storage capacity, the authors note. In the southern region, a loss of tree cover means a loss in biomass carbon storage. Meanwhile, as the permafrost underlying northern forests thaws, it awakens underground microbes that can break down organic materials, releasing carbon into the atmosphere.
This loss of carbon storage will outweigh the amount of biomass gained by tree cover in the colder region, Berner explained. The researchers projected a carbon gain of 11.4%, or 17.7 gigatons, by 2100.
The transition to an open forest will have other cascading impacts, including reductions in biodiversity and water availability, said Zoe Pierrat, a postdoctoral fellow at NASA’s Jet Propulsion Laboratory who was not involved in the study but coauthored a commentary on it.
“To me, the biggest concern is the potential impacts on wildfire regimes,” Pierrat said.
One of the biggest climatic drivers of boreal wildfires, she explained, is the lack of available water from snowmelt and increased temperature. In spring, melted snow creates a large pool of water readily available for trees to absorb as they grow in the summer. By fall, these trees could have depleted the water supply and turned dry, creating prime conditions for fire to kindle.
Moving Forward in a Changing Ecosystem
One of “the big questions we need to address in the coming years is what can we do to these forests so that they can withstand climate change.”
“Temperature alone does not fully explain how climate regulates or controls the ecosystem,” Berner acknowledged.
As novel as the study findings were, he continued, incorporating a wider collection of climate model projections (such as how climate influences the way organisms behave) could strengthen the findings and help scientists better understand the far-reaching effects of future climate.
For Rotbarth, there is still hope. “We should not stop reminding ourselves that we have such an influence on our planet,” he said. One of “the big questions we need to address in the coming years is what can we do to these forests so that they can withstand climate change and increasing pressures from environmental disturbances like fire.”
—Kristel Tjandra (@KristelTjandra), Science Writer