The horrendous scale of this year’s fires in Brazil has finally captured the world’s attention, but the boreal forests of the far north are burning too. These forests lack the biological richness of the Amazon, but they store 30-40 percent of terrestrial carbon making their fate crucial for Earth’s climate. A new study reveals these fires’ damage extends beneath the trees into something people are less likely to notice; the loss of organic matter in the soil.

The role of fire in the carbon cycle is complex. When trees burn they release CO₂ into the atmosphere, and the same is true for soil carbon. However, a paper in Nature notes only the top layer burns. Organic material can accumulate to much greater depths, and under historic conditions in the boreal forests of Canada and Siberia this has created a carbon sink, drawing more greenhouse gases from the atmosphere than fires release. As a result, 70-80 percent of boreal forest carbon is in the soil, not the plants above.

However, first author Dr Xanthe Walker of Northern Arizona University reports this may be shifting. “Climate warming and drying has led to more severe and frequent forest fires,” the paper notes. If this leads to so-called “legacy carbon”, which has escaped previous fires, being burned then these forests become a net source of carbon dioxide in the atmosphere, in turn leading to further warming and a dangerous loop.

Ecologists have been aware of the danger for some time, but knowing whether it is actually occurring is a different matter. Walker is part of a team that established 211 field plots to investigate the 2014 wildfires in the Northwest Territories, Canada. Thirty-two plots among black spruce, the most common sort of tree in boreal North America, were studied in great detail, with much of the legacy carbon dating back several centuries, 1,670 years at one plot.

The picture Walker produced is mixed, with the 2014 fires burning legacy carbon in forests less than 60 years old, but passing it by in older forests. The problem is that if the same forests burn twice not too many years apart, a protective layer of new matter has no time to build up above, leaving the legacy carbon vulnerable. The threat is increased as wetter forests, where organic soils build up faster, dry out in a warmer climate.

Walker and co-authors estimate the 2014 fires released 8.6 million tonnes of legacy carbon and note that, unlike when tropical savannahs burn, the boreal forests replace their stores much more slowly.

Boreal forests need to burn occasionally for their own health, but if they start to burn so often they become carbon sources, low human populations in their vicinity will make it hard to fight the fires.