Indigenous Australians have managed land with fire for 11,000 years

Indigenous Australians have been managing the environment with fire for at least 11,000 years, according to an analysis of sediment cores retrieved from an ancient lake.

Michael Bird at James Cook University in Cairns, Australia, says the findings suggest that a return to an Indigenous regime of more frequent but less intense fires could reduce the risk of catastrophic bushfires and improve environmental management.

It has long been known that Australia’s first peoples, who are thought to have been on the continent for 65,000 years, carefully managed the landscape with fire to make it easier to move around and hunt prey. They also figured out that this benefited some animals and plants that they preferred and reduced the risk of more dangerous fires.

However, it has been difficult to establish how long this has been happening for, says Bird. That is because most waterways completely dry out in the dry season each year and the carbon in their sediments is destroyed.

Girraween Lagoon, near Darwin in the Northern Territory, is a massive sinkhole covering an area of about 1 hectare that has stayed permanently wet for at least 150,000 years. As the climate changed over millennia, so, too, did the vegetation around the sinkhole. “From Girraween Lagoon, we have got 150,000 years’ worth of sediment that has never dried out,” says Bird.

By analysing sediment cores from the lagoon’s bed, Bird and his colleagues were able to study three key metrics: the accumulation of micro-charcoal particles, the proportion of burnt material in the charred vegetation matter and a measure of the amount of the different kinds of carbon that remain after burning.

The first two metrics allow researchers to infer the intensity of fires, while the third indicates whether fires were cool enough to leave traces of grasses preserved.

Prior to the arrival of people, natural fires in the savannahs of northern Australia were ignited by lightning late in the dry season, when vegetation and the landscape had almost fully dried out. This kind of higher-intensity fire combusts biomass more completely, particularly fine fuels such as grass and litter, leaving less charred remains from grasses.

Indigenous fire regimes, on the other hand, burn frequently but with much less heat, affect small areas and are limited to the ground layer, promoting a mosaic of vegetation and helping to protect biodiversity.

Bird says the more recent layers in the cores show clear evidence of more frequent fires and grasses that haven’t been fully combusted, indicating cooler fires. These kinds of fires are a sharp departure from the previous natural pattern of fires and provide the tell-tale fingerprint of Indigenous fire management, he says.

This signal can be seen in sediments dating back to at least 11,000 years ago, the study found, but before that point the metric for the proportion of grasses and tree remains becomes harder to study. Bird says there are hints of a human burning signal from as early as 40,000 years ago, but the evidence isn’t as clear-cut.

“It means that for at least 11,000 years, the savannah has grown up with humans,” he says. “The biodiversity has grown up with that fire regime. Take that kind of burning away and you start to see significant problems with biodiversity.”

David Bowman at the University of Tasmania, Australia, says the paper highlights the twin importance of climate and humans in shaping fire regimes.

“Separating climate from anthropogenic – and importantly Indigenous – fire management is a hugely important topic,” he says. “We are battling to counteract climate-driven wildfires globally and such a deep-time perspective will be an invaluable addition to current research and development of sustainable fire management.”