Ellen Phiddian
Cosmos science journalist
Shrub cover across southeastern Australia halved after the arrival of Indigenous populations, according to a new study.
The study, published in Science, attributes the change to cultural burning practices, suggesting that these changes reduced the risk of high-intensity bushfires.
Shrubs, which fill out the middle layer of a forest between grasses and trees, can be big intensifiers of bushfire.
They allow ground fires to spread to tree canopies, causing intense and hard-to-control crown fires.
Aboriginal cultural burning has been well-established as a fire control method, and is now widely practised again across northern Australia.
But records are less thorough in the southeastern corner of the continent, which saw the most intense disruption from British colonisation.
The researchers examined ancient pollen trapped in sediment cores to estimate the plant coverage in southeastern Australia.
“There are little, tiny grains of pollen that end up in lakes and wetlands, and preserve for millions of years in some cases,” co-author Dr Simon Connor, a researcher at the Australian National University, tells Cosmos.
“They’re really good ecological indicators, because they tell us what kind of plants were around the around that lake or wetland in the past, how much of them there was, and how they changed over time.”
The team examined 2,833 records from sites ranging from eastern South Australia and southern Queensland, down to the Victorian coast.
They found that in those areas, prior to the arrival of humans, shrubs covered about 30% of the land. After they arrived, this number dropped to 15%.
“We think [cultural burning] is the most likely explanation for it, because we don’t have clear climatic drivers for that kind of thing,” says Connor.
“It seems very likely that it was people systematically applying cultural practice through burning, over this huge part of the continent, that resulted in this suppression of the shrub layer.”
After British colonisation and the suppression of these activities, shrub cover increased to 35% – higher than pre-human levels.
The researchers say in their paper that their findings correlate with both Indigenous oral histories, and landscape descriptions from early European colonists.
It’s likely, although harder to establish from the fossil record, that cultural burning prevented more dangerous megafires, according to Connor.
“The evidence that we’ve got is that the really catastrophic fires that we’ve seen in the last few decades probably weren’t as common. But I have to say, that is really difficult to reconstruct,” he says.
“In a really big fire, a lot of the biomass that burns, a lot of the trees and shrubs and grasses – they get turned to ash, and that just blows away.”
Climate change has lengthened the Australian fire season, and made super-intense bushfires more likely.
The researchers, many of whom are involved with contemporary Indigenous cultural burning, say it’s “crucial” to reintroduce these practices at a wide scale in combination with Western management.
Connor says that, given the changing climate, these processes should be “carefully monitored”.
“The cultural burning practices are valid cultural practices – they work. The issue we face is that the vegetation, for one, has changed, and then the climate has also changed. So how can those cultural practices adapt in into those new realities?” he says.
“Can we bring back a particular area of forest that hasn’t been culturally burnt for 200 years and restore it to that regime? I think it’s possible.
“But what effect will that have on reducing the likelihood of big fires? That’s something that really needs to be tested and monitored.”
