Carbon emissions from fire are controlled by soil composition

Fires are becoming more frequent and more intense in the North American boreal forest, and experts are urgently working to estimate how much carbon is being released as a result. Researchers at Northern Arizona University have found that the carbon stored in soil organic matter underground is the most important predictor of how much carbon will be released in a fire.

The study suggests that carbon emissions from fire are primarily determined by the amount of fuel available, rather than by the weather conditions. 

The team studied the diverse composition of the Western Boreal forest conditions by analyzing field data from 417 burn sites in Canada and Alaska. The research revealed that the biggest predictor of how much carbon would combust was the amount of carbon that was stored in the soils, and that soil moisture was also a good indicator of carbon release.

“In these northern forests, soil, not trees, can account for up to 90 percent of carbon emissions, so we expected that these organic soils would be a significant driver,” said study  lead author Xanthe Walker. “But we were surprised that fire weather and the time of year a fire starts proved to be poor indicators of carbon combustion. It’s really about the fuels that are there when a fire starts.”

This discovery is particularly important considering that the Fire Weather Index, which measures weather conditions, is one of the main tools that is used to model carbon emissions in boreal forests.

“When we think of climate change and wildfires, we often instinctively think of extreme weather conditions,” said study co-author Marc-André Parisien. “But our study shows that vegetation also matters – a lot! Predicting future vegetation is a tough nut to crack, but this study emphasizes the need to keep chipping away at it.”

The vegetation patterns that were uncovered in the study were very complex. The researchers found that factors such as soil moisture and tree species composition interact to predict combustion amounts. For example, highly flammable black spruce was generally a predictor of carbon combustion, and the presence of this species increased with site moisture.

Climate change alters the species composition of a forest. As fires become more frequent, black spruce trees will gradually be replaced by deciduous trees and jack pine, which grow in soils that are shallow and release less carbon during fires, according to the study authors. 

“We really need to move beyond the misconception of the boreal forest as a monotonous stretch of forest,” said study co-author Professor Sander Veraverbeke. “While only a few tree species occur in the boreal forest, its diversity in ecosystem structure, forest age, topography, peatland occurrence and permafrost conditions is enormous, and our paper shows that these features dictate the carbon emissions from boreal fires. The good news is that we can map aspects of this fine-scale ecosystem variation with current tools from NASA and other space agencies. Now we need to do this at the continental scale.”

“We are figuring out that fire-vegetation feedbacks a lot stronger than we thought they were just a few years ago,” said Parisien. “Of course, we’ll never be able to manage all of vast boreal biome – nor should we want to – but this helps us know what targeted actions, such as fire management or modifying forest vegetation, we can take to limit carbon loss.”

The study is published in the journal Nature Climate Change.

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By Chrissy Sexton, Earth.com Staff Writer