November 1, 2024

How colonisation led to more intense and dangerous bushfires in Australia

Researchers used ancient mud and vegetation records to piece together the puzzle of fuel loads, shrub cover, and human occupation

Fire has always been intrinsic to the Australian landscape, part of the country’s unique alchemy of climate, terrain, and vegetation.

But a new paper, by a team of global researchers, has revealed that the onset of British colonisation, after which Indigenous Australians were prevented from performing traditional fire management practices, changed the country’s ecosystems and led to a greater proliferation of shrubs in the landscape. This change has directly led to fires that are more intense and much more dangerous.

‘Shrub cover declines as Indigenous populations expand across southeast Australia’, published overnight (Friday 1 November) in Science, is led by Dr Michela Mariani of the University of Nottingham and features Dr Haidee Cadd from the University of Wollongong’s (UOW) School of Earth, Atmospheric and Life Sciences. 

Researchers from the University of Cambridge, the Australian National University, Monash University, University of Greifswald in Germany, University of Melbourne, University of New South Wales, and University of Tasmania contributed to the paper.

The team used ancient mud collected from across Southeast Australia to develop multiple records of vegetation cover, past climate, and fire, to piece together the puzzle of the country’s ecological history. They then compared this data to archaeological information to investigate how different phases of human occupation influenced the availability of fuels for fires.

Dr Cadd, a palynologist who studies plant pollen, spores and micro-organisms, said the records helped the researchers to measure how much fuel was available before human arrival in Australia (over 100,000 years ago), through periods of Indigenous occupation, and during the post-colonial phase.

“We found that Indigenous population expansion and cultural fire use around 6,000 years ago resulted in a 50 per cent decrease in shrubs, an increase in grasses and a decline in high-intensity fires,” said Dr Cadd, a researcher in the Australian Research Council Centre of Excellence for Australian Biodiversity and Heritage, headquartered at UOW.

“When we look at the timeline, starting from unpopulated Australia to the present, we found that the suppression of cultural fires after British colonisation led to more shrubs in the understorey of forests.

“This extra layer of fuels provided a ladder for flames to climb and generate high-intensity fires.”

Ladder fuels allow fires to spread quickly up the rungs of the vegetation. For example, eliminating shrub cover removes a critical step on the ladder, preventing a lower-intensity ground fire from evolving into a high-intensity crown fire.

^{Dr Haidee Cadd in the laboratory. Photo: Michael Gray} 

Crown fire, in which fire burns through the tree canopies, is the most extreme type of bushfire behaviour and virtually impossible to control.

These fires  can be extreme in the  eucalypt forests of the Australia landscape, with the unique and explosive oils present in eucalypt species providing ideal fuel for crown fire.

Research has shown that much of the forest that burnt during the Black Summer Bushfires in 2019/2020 experienced crown fires.

Dr Cadd said the European approach to fire over the past two centuries has been suppression, an approach that has disrupted the traditional fire regimes that Indigenous Australians had practiced for tens of thousands of years.

This, compounded with the significant changes in the Australian landscape, including more pastures and more shrubs in forests, has created a tinderbox that erupts with immense ferocity every few years.

Indigenous Australian fire management techniques are complex and varied. However, in many parts of the continent they included regular, controlled burns, helping to manage vegetation biomass and reduce the risk of high-intensity fires.

The growing impact of climate change has further stoked the increasingly fraught conditions. Higher temperatures and prolonged droughts have created the ideal environment for fires to spread.

“It is the combination of increased biomass and a warming climate that has resulted in fires of unprecedented scale and intensity, posing a significant threat to lives, property, and ecosystems.

“Australia’s fire landscape is complex and thorny. There is no one solution to this alarming crisis, which poses an immense threat to people, property, and our unique and diverse flora and fauna.”

Dr Cadd and the team of researchers behind the paper have advocated for a return to Indigenous fire management practices, as part of a multifaceted approach to mitigating the terrifying impact of fire on the Australian landscape and psyche.

“If we are going to face the future of fire in Australia, particularly under the growing influence of climate change, we must understand the past. Fire has always been part of the Australian ecosystem, but the ferocity and scope we are experiencing today is new and is only going to get worse.

“We need to learn from and work with Indigenous practitioners to develop more effective and sustainable fire management strategies. Rekindling cultural burning could help with Australia's growing fire crisis.”

About the research

‘Shrub cover declines as Indigenous populations expand across southeast Australia’, by Michela Mariani, Alastair Wills, Annika Herbert, Matthew Adeleye, Anna Florin, Haidee Cadd, Simon Connor, Peter Kershaw, Martin Theuerkauf, Janelle Stevenson, Michael-Shawn Fletcher, Scott Mooney, David Bowman, and Simon Haberle, was published in Science: https://www.science.org/doi/10.1126/science.adn8668