Forests’ vital carbon-storing abilities failing in areas affected by worsening wildfires, research shows

The natural world is not bouncing back from the burdens of the climate crisis, writes Harry Cockburn

Savannah Fire at Africa's Kruger National Park (Corli Coetsee/University of Cambridge)

The rising frequency and scale of forest fires around the world is reducing the ability of our planet’s woodlands to sequester carbon dioxide, alarming new research has shown.

Trees’ absorption and storage of carbon are a critical part of maintaining stable weather systems on Earth, but as temperatures rise, our forests are not able to keep pace with the changes.

Scientists have said the rise of more intense and frequent fires, combined with droughts associated with the climate crisis, are resulting in fewer trees growing in affected areas, as repeated burning means woodlands are not able to recover.

This in turn results in the forests not having as much carbon-storing capacity.

The effect is most pronounced in savannah ecosystems, and regions with extreme wet or dry seasons, the scientists said.

This is because many of the trees in these parts of the world are fast growing, but can also be the most sensitive to changes in fire frequency.

Meanwhile trees in regions with moderate climate are more fire resistant.

Repeated fires also cause less damage to tree species with protective traits like thicker bark.

"Our study shows that although wetter regions are better for tree growth, they’re also more vulnerable to fire,” said Dr Adam Pellegrini of the University of Cambridge and first author of the paper.

“That will influence the areas we should manage to try and mitigate climate change.”

Large wildfires, such as those seen in the Amazon, Africa and Australia in recent months are playing a growing role in global carbon emissions.

Fire burns five per cent of Earth’s surface every year, releasing carbon dioxide into the atmosphere equivalent to 20 per cent of our annual fossil fuel emissions.

In the past, the majority of this carbon released by the fires was recaptured as ecosystems regrew, and re-absorbed the carbon.

But the more frequent fires seen in recent years, driven by changes in climate and land use, don’t always allow time for this.

The study found the impacts of bigger, more frequent fires only emerge when studied over the course of several decades.

The effect of a single fire is very different from the effect of repeated burning over time.

The researchers said that after 50 years, regions with the most extreme annual fires had 63 per cent lower wood area and 72 per cent fewer individual trees than in regions that never burned.

This lower wood area indicates lower carbon storage than would have been available previously.

But though such changes will reduce the forest’s long-term ability to store carbon, they can also serve as a buffer to reduce the immediate effect of future fires, as lower quantities of wood burning will have less of an impact.

The researchers also said that simply planting trees in these areas must be carefully planned, with some less obvious species possibly being better candidates for longer term carbon sequestration.

"Planting trees in areas where trees grow rapidly is widely promoted as a way to mitigate climate change,” Dr Pellegrini said.

“But to be sustainable, plans must consider the possibility of changes in fire frequency and intensity over the longer term.”

Past studies have found that frequent fires reduce levels of nutrients, including nitrogen, in the soil. The new study demonstrates this can favour slower-growing tree species that have adaptations to help them survive with less nutrients.

But these tree species also slow down nutrient cycling in the soil by holding onto what they have. This can limit the recovery of the forest as a whole by reducing the nutrients available for plant growth after an intense fire.

“As fire frequency and intensity increases because of climate change, the structure and functioning of forest ecosystems are going to change in so many ways because of changes in tree composition,” said Dr Pellegrini.

“More fire-tolerant tree species are generally slower growing, reducing the productivity of the forest. As climate change causes wildfires to become more intense and droughts more severe, it could hamper the ability of forests to recover - reducing their capacity for carbon storage.”

Speaking about the research, Mike Barrett, the executive director of science and conservation at WWF-UK, said the loss of each tree counts, and warned that British consumers could be impacting deforestation around the world through the products we buy.

He said: “The natural world is our life support system, but nature is currently in freefall. Our world’s forests are vital for storing carbon and every tree that burns down is another loss in the battle against the climate crisis.

“In many regions, the combination of climate change, deforestation and burning land for agriculture – including for the products we buy here in the UK – is leading to more extreme, frequent and longer-lasting fires.

“This is causing devastation to crucial forests, such as the Amazon. We must protect nature, to protect the planet, and to protect ourselves.”

The research is published in the journal Nature Ecology and Evolution.