Climate change is driving a “fundamental shift” in tree density across vast forests in Canada, Siberia, and Alaska, forming an “open state” at a much higher risk of forest fires. That is according to new research published by Wageningen University. The research analysed two decades of satellite-gathered tree cover data and found that this shift occurs more rapidly at the southern boreal boundaries than at the northern ones.
The study, Boreal forests are heading for an open state, comes months after researchers reported that climate change is responsible for a “significant migration” northward for boreal forests, with trees dying in southern regions and instead thriving in areas previously too cold.
“Global boreal forests will undergo a fundamental transition toward an open forest state of 30 to 50% tree cover in the coming decades,” according to the abstract, with “the boreal forest biome is warming four times faster than the global average.”
Boreal forests encircle the cold, northern regions of our planet. They host a diverse set of species, regulate the global climate by storing huge amounts of carbon in the trees and soils, and provide for the livelihoods of millions of people. However, within decades, global warming rapidly changes the conditions under which boreal forests have fulfilled their roles within the ecosystem.
Understanding how boreal forests change is key to the planet’s future.
And because boreal trees grow slowly, changes in the forest take time to see: “This time lag could keep the forests in a state that doesn’t match the changing climate,” according to Ronny Rotbarth, the study’s lead author: “Forests growing at the southern boreal distribution range may not be able to maintain the current number of trees as the climate becomes drier and warmer. Therefore, they may become less dense. Other forests, in the cold north, could support more trees as conditions warm up, and in fact, become denser than the forests we currently find there.”
Why past changes to forests can offer a glimpse into the future.
Future changes in ecosystems, such as boreal forests, are typically projected by complex simulation models. In the study, the researchers used an alternative innovative approach. “By examining two decades of tree density from space, we explored whether past changes could hint at the future state to which the forests may be shifting,” says Egbert van Nes, co-author of the study. The researchers used the changes in tree density during that time to project forest conditions until 2100.
The results indicate that global boreal forests might change to a more open type, with trees covering about 30%–50% of the area. “This is very different from what we see now, which includes dense forests with more than 60% tree cover and sparse forests with around 5%–15% tree cover,” said Marten Scheffer, co-author of the study. “We were surprised by how consistent this projection is across the global distribution of boreal forests.”
Climate change is creating a perfect recipe for forest fires.
According to the model, tree cover in warmer boreal forests decreases, while cover in colder, northern boreal forests increases, moving the entire forest system to one single state. The open forest type in a warmer world could lead to more forest fires, releasing a lot of carbon currently stored in the trees and soils. “The currently dominating sparse and dense forest types burn less frequently than the open type,” said Milena Holmgren, one of the study’s co-authors.
“Sparse forests typically do not provide enough fuel for fires to burn, and dense forests create micro-climates which prevent forest fires. The open type we project, however, meets the sweet spot and could lead to more fires than we observe at present. The crucial functions of boreal forests would be at risk.”
Milena Holmgren, one of the study’s co-authors on the impact of climate change on Boreal forests.
- For more information: Ronny Rotbarth et al., Boreal forests are heading for an open state, Proceedings of the National Academy of Sciences (2024). DOI: 10.1073/pnas.2404391121. Journal information: Proceedings of the National Academy of Sciences