The cover shows a wildfire in Alberta, Canada, in 2016. Naturally occurring fires in boreal forests emit large amounts of carbon into the atmosphere, largely through the combustion of organic soil. But during each fire, a proportion of the soil escapes combustion and over subsequent fires this forms a store of ‘legacy’ carbon locked up in the soil. This helps to make such forests a net carbon sink, holding some 30–40% of terrestrial carbon. In this issue, Xanthe Walker, Michelle Mack and their colleagues reveal that loss of legacy carbon occurred in dry, younger forests (60 years or less) following wildfires in the Northwest Territories in Canada. The researchers’ findings suggest that with boreal wildfires increasing in size, frequency and intensity, young forests may become a net source of carbon to the atmosphere over consecutive fires and may switch the boreal carbon balance from a sink to a source.