Research suggests that bigger, hotter wildfires are turning Canada's vast boreal forest into a source of climate-changing greenhouse gases.
Soil in the forest, which crosses six provinces and two territories, has long been considered a storehouse of carbon from centuries of growth and decay.
In old-growth forests, the carbon remains in the soil even after it is burned over.
But a new paper by scientists at four Canadian universities concludes that larger fires are burning through more of the old growth, making the overall forest younger.
During fires, carbon is more likely to be released from younger forests.
Merritt Turetsky from the University of Guelph says the boreal forest may be at the point of becoming a carbon source rather than a carbon sink.
She says the finding may mean firefighters will have to change how they decide which fires to fight and what to protect.