Aboriginal Australians, who arrived from Asia at least 65,000 years ago, burned savanna and dry forests on a rotational basis to reduce the latent fuel load and intensity of natural wildfires. These techniques fostered biodiversity and provided a better environment for the animals they hunted.
For decades, scientists have argued over just when this “firestick farming” began, and paleontological evidence has been in short supply. “Most of the sites dry out. That’s incredibly destructive to organic accumulation,” said David Bowman, a University of Tasmania pyrogeographer who has spent 40 years searching for such evidence.
Along with his colleagues, Michael Bird, a geochemist at James Cook University, scoured northern Australia’s remote Arnhem Land in vain for a suitable site. Then, looking out the window on a homebound flight, they spotted Girraween Lagoon shining beneath eucalyptus trees on the fringes of the city of Darwin. They decided to look there next. What they found is the subject of a new paper published in Nature Geoscience.
Evidence Spanning 150,000 Years
Girraween Lagoon is locally famous, as it was the filming location for a crocodile attack scene in the 1986 movie Crocodile Dundee.
Bird and his team discovered that the lagoon, a collapsed sinkhole, had been collecting sediment for 150,000 years and, crucially, had remained wet the entire time, meaning organic materials in the anoxic mud were well preserved.
“You have sort of a tape recorder from the bottom to the top,” Bird said.
They cored down 18 meters. Crucially, that time span crossed through the last ice age into the previous interglacial period, a time when northern Australia experienced a climate similar to today’s but that differed from the present in one very important way: There were no humans around.
For the first time, researchers had a baseline data set to compare with the current interglacial, the Holocene period.
Check the Charcoal
Natural wildfires are common in Australia’s Top End, so predictably, the sedimentary record was full of charcoal. The “burning” question was what portion of that charcoal came from natural wildfires and what came from fires started by humans.
Natural wildfires are typically much hotter and more intense than managed burns, so the researchers used new techniques to search for chemical evidence of fire intensity.
Stable polycyclic aromatic carbon (SPAC) is a compound created in high-intensity fires, which the team found much more frequently in prehuman charcoal deposits.
High-intensity fires burn entire trees, reducing their crowns and trunks to charcoal. They also obliterate grass, turning it to ash that scatters on the wind. However, lower-intensity fires, such as those used in firestick farming, don’t combust trees in their entirety, and they turn more of the grass to charcoal.
By analyzing SPAC content and the ratio of tree to grass charcoal, the team detected a change from high-intensity natural fires to more frequent, less intense fires starting at least 11,000 years ago. The findings suggest a shift from natural wildfires to managed burning as the ice age ended. That might be explained by geography; as sea levels rose, the coastline shifted closer to Girraween. The local climate became wetter and milder and therefore more attractive to people.
Part of the Solution
Independent New South Wales paleoecologist Mark Constantine IV, who was not involved in the study, said that the insights from Girraween Lagoon wouldn’t have been possible before recent advances in fire science. “Now we can actually describe past fires, whereas before we could just talk about their existence, or lack of existence.”
The research, he said, “is a very interesting way of using charcoal data to think about whether people actually were burning and how much.” At the same time, he cautioned that “these are all proxy data. There is no direct evidence of people setting a fire and using it to burn the landscape, up until there was [European] eyewitness evidence of these things happening in the 1700s. But I think it does provide some evidence that there was some systematic burning being done.”
Bowman, who was not involved in the study, said he was delighted with the results, although “sort of kicking myself” for not coring Girraween Lagoon himself. “I could have got a taxi to that site, if I had thought about it.”
“For the whole of the Holocene, which is a long time, we’re seeing skillful fire management.”
The study, he said, is a nail in the coffin of old ideas that firestick farming was only a relatively recent development in Aboriginal culture. “The very refreshing part of this paper is to say, ‘Hang on, for the whole of the Holocene, which is a long time, we’re seeing skillful fire management.’”
With the arrival of Europeans, traditional Aboriginal burning came to a halt in most areas. In recent decades, horrific wildfires fueled by excess fuel loads have pushed species to the brink of extinction.
After the last ice age, northern Australian savannas coevolved along with this regime of traditional burning, said Bird. “If you take that fire regime off, you get all sorts of very nasty biodiversity impacts, which we’ve seen in the last 200 years.”
Today, managed burns are again being used to preemptively tame wildfires in a warming, drying climate. Turning to Indigenous methods of land management, said Bowman, might help humans find a way to cope with the climate crisis and be part of the solution, not just the problem.
“Humans can moderate climate change, stabilize environments, and conserve biodiversity,” he said. “It’s not the humans that are the problem. It’s human culture.”
—Bill Morris, Science Writer
This news article is included in our ENGAGE resource for educators seeking science news for their classroom lessons. Browse all ENGAGE articles, and share with your fellow educators how you integrated the article into an activity in the comments section below.
