It was previously known that forest fires in boreal regions can survive the winter by smoldering in peaty soils: so-called 'zombie fires'. But a group of scientists from Vrije Universiteit Amsterdam, among others, discovered that fires can also overwinter by smoldering in tree boles and roots. Their study has been published in Nature Ecology & Evolution.
Helicopters
Zombie fires were previously detected via satellite images. Until now, no research team had investigated them directly in the field. Climate scientists Thomas Hessilt, Rebecca Scholten, Max van Gerrevink and Sander Veraverbeke (VU Amsterdam) investigated the phenomenon together with Canadian and American scientists in remote areas of the Northwest Territories, Canada, and Alaska, United States.
They were able to build on prior research to refine detection methods. Hessilt used high-resolution satellite imagery to locate field locations. "Previous studies relied on satellite imagery with a spatial resolution of about 1 kilometer. That is not precise enough to locate overwintering fires," he explains. The team therefore developed an algorithm that can identify fire locations with a much finer 30-meter resolution. These locations became the basis for a helicopter-supported field expedition.
Prolonged smouldering
During this field expedition, the geoscientists examined local topography, drainage, and ecosystem and fire characteristics. Veraverbeke: "We expected that zombie fires would primarily occur in peatlands and burn deeper in organic soils than typical wildfires."
“Somewhat unexpectedly, we found that overwintering fires were not limited to peatlands,” said forest ecologist Jennifer Baltzer (Wilfrid Laurier University), lead author of the study. “They also occurred frequently in dry forests, where fallen tree boles and roots provided fuel for prolonged smoldering throughout the winter.”
Fire management
The study shows that zombie fires do not emit more greenhouse gases than conventional wildfires and that forests recover just as well after these fires. However, the discovery that smoldering can persist in both peat and woody debris also suggests that zombie fires may be more widespread than previously thought. This poses new challenges for fire managers in boreal forests, which have experienced increasingly severe fire seasons in recent years.
The VU team received support for the research from two grants under the European Union’s Horizon 2020 research and innovation programme (European Research Council; grant agreement No. 101000987; Marie Skłodowska-Curie grant agreement No. 847504)