Controlled fires burnt at Mount Solitary in the Blue Mountains range in Australia in 2018.Credit: lovleah/Getty
The suppression of Indigenous burning practices in Australia after British colonization probably transformed swathes of the southeast into dense, flammable forests that contribute to bigger, hotter bushfires today, according to a reconstruction of fires in the region over the past 1,000 years1 published in Nature.
The researchers suggest that restoring Indigenous approaches to forest management could lower the risk of devastating fires, particularly as the climate warms. “It gives us another variable when we’re trying to tackle this problem, rather than just focusing on climate change,” says Michael-Shawn Fletcher, a Wiradjuri scholar and palaeoecologist at the University of Melbourne, Australia.
The team set out to understand why southeastern Australia is experiencing larger, more-destructive fires, such as the unprecedented Black Summer fires in 2019–20, which burnt more than 8 million hectares of land of forest in that region.
In 2022, a team that Fletcher was part of used pollen records to show that forests in the region tended to be more open and grassy before British colonization at the end of the eighteenth century2. Soon after the colonization, forests changed, says Fletcher. “You can see right across the landscape there was an increase in woody fuels following the British invasion,” he adds.
A millennium of wildfire
To investigate this pattern in more detail, Fletcher and his colleagues collected a sediment core from a lake at the southeastern tip of Australia in Croajingolong National Park, part of the traditional lands of the Gunaikurnai people. Reconstructions of the landscape as well as historical evidence show that they regularly burnt the land until their violent removal by the British in the mid-nineteenth century.
Like in the 2022 work, the researchers analysed pollen in the extracted sediment to reconstruct the make-up of plant species and the structure of the surrounding forest over the past 1,000 years. For 800 years, this record showed an open woodland, with grasses and few trees. Then, around 1840, they found that the coverage of eucalyptus (Eucalyptus spp.) doubled and woody shrubs spread across the area.
The team also analysed charcoal in the sediment to estimate the temperature of fires over the study time frame. They identified molecules produced by burning plants at different temperatures in the laboratory, then searched for those molecules in the sediment samples. They found an increasing fraction of high-temperature charcoals from the mid-nineteenth century onwards, suggesting more-intense fires.
The researchers say that the changing vegetation and fire regime are linked: hotter fires spur the growth of more-flammable vegetation, more fuel and a denser canopy, creating what they call a “catastrophic wildfire loop”. Eucalyptus trees rapidly sprout after a blaze clears out other competing plants, says Fletcher.
The authors attribute this change to the end of regular, small fires ignited by Indigenous people to protect certain areas from burning or to promote vegetation growth in other areas for animals to eat.