Forest and soil type determine how much carbon is released during forest fires in the northern hemisphere

Even more than the weather conditions, the amount of carbon in the soil determines how much carbon is released during forest fires in the northern hemisphere. This is the conclusion of research led by Northern Arizona University in collaboration with forest fire specialist Sander Veraverbeke of VU Amsterdam.

10/15/2020 | 12:15 PM

Climate change is resulting in longer fire seasons and more severe fires in the northern forests of North America, so it is all the more important to know how much carbon is released during these fires. The research, published this week in Nature Climate Change, reveals that the amount of carbon released depends to a large extent on the type of forest that burns and the moisture content of the soil. Weather conditions at the time of the fire, such as temperature and rainfall, are much less important.

Stored carbon
In a field study that analysed a wide variety of forest fires in large parts of western Alaska and Canada, the international research team found that the amount of carbon stored in the organic soil layer has the greatest impact on the amount of carbon released during a fire. They collected soil samples and recorded the vegetation structure in 417 locations where fires had raged between 2004 and 2015. Based on the field measurements, the researchers conclude that the amount of carbon in the soil is the most important factor for determining how much carbon is released. 

Soils responsible for emissions
“In these northern forests, it is not the trees, but soils that are responsible for about 90% of carbon emissions. Although we expected soils to play an important role, we were very surprised that weather conditions had such a minor influence on the fires,” says lead author Xanthe Walker of Northern Arizona University’s Center for Ecosystem Science and Society. “The matter that burns is much more important than the conditions when it burns.”

Black spruce being replaced
The forest types in the study were complex; soil moisture, tree species, and the age of the forest at the time of the fire all played an important role in carbon emissions. For example, the prevalence of the highly flammable black spruce generally led to high carbon emissions. Black spruce typically grow in moist and old forests, which may become less common as a result of climate change. Due to the increasingly severe fires, black spruce is slowly being replaced by forests of deciduous trees and pine trees. These forests have shallower organic layers, which produce less carbon emissions when they burn. The detailed field analyses in this study allowed the researchers to establish these complex relationships between forest types and carbon emissions, and at the same time offered a glimpse into the future.

Charting ecosystem structures
“You cannot consider boreal forest areas as one great uniform forest,” says Sander Veraverbeke, forest fire specialist at VU Amsterdam and co-author of the article. “Although only a few tree species dominate boreal forests, there is an enormous diversity in the forest structure, the age of the trees, the topography, and the occurrence of peat and permafrost. Our research reveals that these landscape features largely determine the carbon emissions of boreal forest fires. With the current capabilities offered by satellite imagery from NASA and other space agencies, we can chart various aspects of the ecosystem structure in detail. We now need to scale this up to the entire continent.”

The research was funded by the NASA Arctic Boreal and Vulnerability Experiment (ABoVE) project, the National Science Foundation’s RAPID programme, Bonanza Creek LTER, the Natural Sciences and Engineering Research Council of Canada, the Government of the Northwest Territories Cumulative Impacts Monitoring Program, Polar Knowledge Canada’s Northern Science Training Program, and the Fires Pushing Trees North Vidi project of the Dutch Research Council (NWO).<br>

CO2-uitstoot bij bosbranden
Image credit: Victor Leshyk, Center for Ecosystem Science and Society, Northern Arizona University