Alaskan forests may store more carbon after being burned by wildfire

By Ibrahim Sawal

As the boreal forests of Alaska recover from wildfires, they may shift from containing mostly coniferous trees to a deciduous-coniferous mix – and this change could ultimately offset some of the carbon emitted during the fires.

Climate change is making wildfires more frequent and intense in certain parts of the world, such as the boreal forests of the Arctic. These forests typically act as carbon sinks, but if fires burn deep into their soil, they could begin to release more carbon into the atmosphere than they store through new wood growth, accelerating the effects of climate change.

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Michelle Mack at Northern Arizona University and her team assessed the Alaskan boreal forest, which is experiencing more frequent fires, to see how the blazes are affecting forest recovery and carbon storage.

Around 2.7 million hectares of land was burned there in 2004 – the area’s worst wildfire season on record – due to extreme temperatures and frequent lightning strikes. The team monitored 75 sites across this forest for 15 years after this fire year.

Before 2004, records showed that the forest contained mainly black spruce trees, a conifer species. In 2017, this spruce was the principal species at 28 per cent of sites, while 72 per cent were dominated by deciduous trees, like aspen and birch, or had a mix of deciduous and conifer.

“The fire burned more deeply at these sites, exposing the deeper, nutrient-rich layer of soil,” says Mack. Fast-growing deciduous seeds dispersed from further afield could develop rapidly in this soil layer, which might be why so many sites changed composition, she says.

Because deciduous trees take in more carbon dioxide than conifers to grow their denser wood, the team estimated that sites shifting towards deciduous species could ultimately store around five times as much carbon as those where spruce remained. This means that if deciduous trees replace conifers following a fire in a boreal forest, the new mix of tree species could more than compensate for the carbon released during the wildfire, providing a negative feedback to climate change.

Mack says this could offer another way to mitigate wildfire spread. “Deciduous species are more resilient and less flammable, which may allow the trees to stop fires spreading,” she says. Planting deciduous trees in boreal forest might be a useful strategy, she says, “almost like a type of herd immunity”.

Journal reference: Science, DOI: 10.1126/science.abf3903

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