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Even in Earth's high northern-latitude forest, climate change is predicted to make wildfires .

Earth's far north hosts the boreal forest, a vast green belt that stretches from North America to Siberia. The boreal forest is one of the world's largest COâ‚‚ sinks. Over the past few thousand years it has removed around from the air, storing it in the . Because of the large amount of carbon stored in the boreal forest, fires here can release much more COâ‚‚ into the air than forest fires elsewhere, amplifying climate change.

Wildfires release lots of climate-warming COâ‚‚ while they rage. But in the European part of this forest has shown that the forest's COâ‚‚ sink recovers slowly, with the burnt area continuing to release COâ‚‚ for several years after the fires die. This exceeds the amount of COâ‚‚ produced from the fire itself.

A large COâ‚‚ store is in danger

Extremely hot and dry weather in 2018 led to the largest number of wildfires in the Nordic countries in modern history. In Sweden, the total burnt area was than in an average year.

Our research concentrated on the most extensive of these fires in the very center of Sweden, near Ljusdal. We continuously measured how much COâ‚‚ was exchanged between the land surface and the atmosphere and monitored the soil and regrowth of vegetation during the first four years after the fire.

We did this in areas of forest where the trees were killed and areas where trees survived. We also compared areas that were turned into nature reserves—with charred trees left standing—and areas that underwent typical post-fire management in Sweden, with all charred trees being cut (so-called "salvage logging") and new ones planted.

Our measurements from some of the most degraded sites, namely a burnt forest after salvage logging and a young forest where all trees were killed during the fire, revealed that 650g of carbon was emitted on average from every burnt square meter during the first four years after the fire. This more than doubles the total emissions from the fire. For comparison, a similar unburnt forest would from the air per square meter during the same time.

And the emissions continue. We fed the data we collected from burnt sites and other Swedish boreal forest into a computer model that simulates tree growth. The model predicted that it would take for all the COâ‚‚ that was emitted due to the 2018 fire to be stored again in new forest.

What about elsewhere?

Fires in the boreal forest of North America , but this knowledge is less easily applicable in Europe. North American wildfires tend to rise up into the forest canopy and hence they kill lots of trees. During such fires, most of the carbon emissions happen during the fire itself.

Meanwhile, between 60 and 80% of the forested area of the Nordic countries is for commercial forestry, which changes the way wildfires affect the forest. Forest management can involve the removal of dead plant material and regular logging to ensure that the trees don't grow too close to each other, a practice known as "thinning."

Tree species too: North American boreal trees need fire to reproduce, whereas their European equivalents have evolved to resist burning.

As a result, wildfires in the managed boreal forest of the Nordic countries mostly stay near the ground and consume the soil and undergrowth, .

Post-fire management is critical

We also found that further disturbing the burnt forest—with salvage logging or by plowing the soil before replanting trees, which is a common management procedure in Sweden—slows the recovery of vegetation. If surviving trees are left standing they might than unburnt trees, but they are nevertheless continuously capturing and storing CO₂.

The way a forest is managed after a fire affects how long it takes before enough new vegetation grows to turn the forest back into a carbon sink. From this point of view, our findings dispute the effectiveness of logging surviving trees.

In the Nordic countries, especially in Sweden, intensive forest management has wildfires during the past 200 years. It is not clear whether this will still be possible with more frequent droughts and an expected in .

All of this relates to scientific understanding of how wildfires contribute to climate change. As COâ‚‚ emissions continue during the slow recovery of vegetation and soil post-fire, climate modelers should not only consider emissions from the fire itself, but the additional long-term emissions our research has uncovered.

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