Recent research has shown how a bad fire season in Canada’s North released about half as much carbon into the air as plants across the entire country were able to take up that year.

But a new follow-up to that research suggests more of the carbon that went up in smoke may have previously been sequestered there, even through past fires, for thousands of years — that is, until now, as forest fires in those areas become more frequent.

The new study was published Aug. 21 in the journal Nature by an international team including researchers from four Canadian universities and the Northwest Territories government, among others.

The research suggests that when untouched boreal forests in Canada’s North go up in a bad fire season, more of the “legacy carbon” or “old carbon” they’ve kept locked up for millennia will soon enter the atmosphere also.

“This study underscores why more frequent burning in the boreal forest due to wildfires is bad from a climate perspective,” University of Saskatchewan PhD graduate Xanthe Walker, now a researcher at Northern Arizona University and the paper’s lead author, said in a release.

The research began after a severe 2014 fire season in the Northwest Territories. The study team’s goal was to better understand what happened to boreal forest soils during those fires, to help improve forest and fire management.

Boreal forests store about a third of the world’s terrestrial carbon, mainly in soils, the researchers said — making those soils a “globally significant” carbon sink in between forest fires. The pools of “old carbon” in those soils have mainly been safely tucked away for all that time, since only some of that carbon is released in forest fires.

But as the forests’ climate warms and wildfires become larger — and more frequent — then more of that long-sequestered carbon is released or “mined” from the soil.

Taking soil samples at more than 200 sites in the Northwest Territories’ 2014 burn area, the research team found that in spots known to host older stands of trees, the “legacy” carbon remained protected from combustion — but in stands younger than 60 years old, the legacy carbon was burned and released.

“In older stands that burn, this carbon is protected by thick organic soils,” said Walker. “But in younger stands that burn, the soil does not have time to re-accumulate after the previous fire, making legacy carbon vulnerable to burning. This pattern could shift boreal forests into a new domain of carbon cycling, where they become a carbon source instead of a sink.”

The landscape in the Northwest Territories is made up mainly of old forests, so legacy carbon stayed protected there, Walker said — and young stands, lacking accumulated fuel, can act as a kind of fire break.

However, “the frequency of boreal forest fires is projected to increase even more with expected climate warming and drying, and as a result total burned area is expected to increase 130 to 350 per cent by mid-century,” the study authors wrote.

“By defining and analyzing ‘legacy carbon,’ this paper offers a new way to think about long-sequestered carbon stocks in boreal forests and how vulnerable they are to being burned during increasingly frequent and severe wildfires,” said Brendan Rogers, a scientist at Woods Hole Research Center who co-authored the study.

The carbon-dating work done in the new study helps show what happens when burning “goes ‘outside the norm’ from a historical perspective and begins to combust carbon stocks that survived past fires,” he said.

Graphic: This Northern Arizona University chart lays out how shorter and shorter intervals between fires in Canada’s boreal forests can lead those fires to cough up more “old carbon” than ever before.




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