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.

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”.

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THIS is what a US wildfire looks like up close. Trees and embers burn on the other side of the Bidwell Bar Bridge, which spans California’s Lake Oroville in Butte county. These appear white in the image, while the lights to the right are from a small boat.

Thick smoke from the fire is responsible for the red hues. Smoke particles filter out shorter wavelengths of light, such as blue and yellow, while allowing longer, redder wavelengths through.

Its peculiar beauty is a dire warning about the power and spread of wildfires in the region. The Bear Fire in Butte County is part of a major crisis in the western US, where the most severe wildfires in two decades are destroying homes, landscapes and livelihoods.

The first wildfires started in California and Colorado in August. So far, more than 10 states have been affected, including Washington and Oregon. Collectively, some 18,000 square kilometres have been scorched.

The smoke could also have serious effects on health. The risk of lung infections from inhaling the smoke is made worse by the other pressing threat, covid-19.

Wildfires serve as a stark reminder of climate change. Longer and drier summer seasons coupled with strong winds are largely to blame for the unprecedented size and scale of the fires.

Wildfires and blazes lit by humans have been found to be sending huge amounts of carbon into our oceans via rivers every year.

When trees and other vegetation are incompletely burned, they release black carbon into the air, which can last for centuries on land and even longer in oceans. Now we have the best global picture yet of how much of the stuff is making its way to the sea: around a third of all the black carbon produced by fires.

Unlike the two-thirds that stays on the land, the carbon ending up in the oceans will stay there much longer, says Matthew Jones at the University of East Anglia in the UK. “Once it reaches the oceans, it has potential for storage over millennia and tens of millennia. That’s why we care about it. It’s almost locked up for the distant future,” he says.

Jones and his colleagues looked at 409 measurements of black carbon dissolved in the waters of 34 major rivers globally, plus many smaller rivers, including far more data from the tropics than previous efforts. They then modelled how it will travel to the oceans. “It’s quite spectacular how long-lived this material is and how much does end up in the environment,” says Jones.

The team found that the amount of black carbon being carried by rivers varies substantially around the world, with the rivers in the tropics carrying twice what those in cooler, temperate regions do.

When compared with emissions from humanity’s fossil fuel use, the actual amount reaching the oceans is relatively small – about 43 teragrams of black carbon each year, equivalent to the Netherlands’ annual carbon dioxide��emissions. Nonetheless, says Jones, understanding how black carbon is being moved around is vital for building better climate models and for our understanding of the global carbon cycle.

The amount being produced by fires has been relatively stable for the past two decades, with forest fires being offset by savannah being turned into farmland, which reduces the amount of vegetation available to burn. But Jones points out that more fires are expected as the world warms, which will deliver more black carbon into the oceans.

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