Repeated fires dampen forests’ ability to absorb carbon dioxide, study finds

In an analysis of decades’ worth of data on the impact of repeated fires on ecosystems across the world by a Cambridge University team, repeated fires have been shown to drive long-term changes to tree communities and reduce their population sizes and associated carbon stored within.

The study found that the effects only emerge over the course of several decades, with the effect of a single fire very different from repeated burning over time.

After 50 years, regions with the most extreme annual fires had 72 per cent lower wood area - a surrogate for biomass - with 63 per cent fewer individual trees than in regions that never burned. Such changes to the tree community can reduce the forest’s long-term ability to store carbon, but may buffer the effect of future fires, the study found.

While wetter regions of the planet are better for tree growth, they were also shown to be more vulnerable to fire. Savannah ecosystems and regions with extreme wet or dry seasons were found to be the most sensitive to changes in fire frequency.

Trees in regions with moderate climate are more resistant. Repeated fires also cause less damage to tree species with protective traits like thicker bark.

“Planting trees in areas where trees grow rapidly is widely promoted as a way to mitigate climate change, but to be sustainable plans must consider the possibility of changes in fire frequency and intensity over the longer term,” said Dr Adam Pellegrini in the University of Cambridge’s Department of Plant Sciences, first author of the paper.

He added: “Our study shows that although wetter regions are better for tree growth, they’re also more vulnerable to fire. That will influence the areas we should manage to try and mitigate climate change.”

Past studies have found that frequent fires reduce levels of nutrients - including nitrogen - in the soil. The new study demonstrates that this can favour slower-growing tree species that have adaptations to help them survive with less nutrients. These tree species also slow down nutrient cycling in the soil: they hold 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.

Wildfires are playing an increasingly important role in global carbon emissions, with recent cases such Australia’s bushfires or those in the Brazilian Amazon providing stark examples of the severe consequences of climate change.

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

In the past, the majority of carbon released by wildfires was recaptured as ecosystems regenerated. However, the more frequent fires of recent years, driven by changes in climate and land use, don’t always allow time for this.

“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 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.”