Thousand-year lake core ties southeast Australia's thicker forests to the end of Indigenous burning

Purghagoolah, also called Lake Elusive, lies in Croajingolong National Park. A team led by Michael-Shawn Fletcher, a palaeoecologist at the University of Melbourne ^[3], drew a metre-long core from its bed that spans a millennium ^[12]. The surrounding forest is largely closed to people and left without active management ^[13], so its recent mud is less likely to record logging or planned burns. Cut at half-centimetre intervals ^[14], the core yields about 200 samples averaging roughly five years each, if deposition was even ^[24]. The 30 years over which settlers removed the GunaiKurnai span about six of them ^[25].

For eight centuries before that, the pollen shows open woodland ^[4], in country the GunaiKurnai had managed mainly with cultural burning for thousands of years ^[5]. How large the change was depends on which report you read. New Scientist's tree-cover figures amount to a fourfold rise ^[19]. Nature's news team says eucalypt coverage doubled around 1840 ^[2]. The two may be different measures, all trees against eucalypts alone.

The fire-temperature record is the cleverest part of the design. The team burned plants at known temperatures in the laboratory, identified the molecules each temperature leaves in charcoal, and then searched the sediment layers for them ^[6].

The authors tie the vegetation and fire records into one feedback. Hot fires clear competing plants, and eucalypts sprout back fast, adding fuel and thickening the canopy ^[7]. Once crowns connect, flames travel more easily, and a shrub layer lets fire climb from the forest floor into the canopy ^[8]. "Fire temperature went up, fire amount went up, eucalypt cover went up, and it trapped this system into a hot, catastrophic fire loop," Fletcher said ^[9].

A single core gives sequence and timing. Causation takes more. Colonisation changed several things here at once. Early settlers reported driving herds of cattle easily through what was then open woodland ^[20], so grazing sits in the same window as the end of burning. The authors attribute the shift to the loss of regular, small Indigenous fires ^[10], and Nature's report calls the suppression of burning the probable cause ^[11]. Supporting evidence comes from other places: a 2022 pollen study Fletcher worked on found the region's forests were more open and grassy before colonisation ^[21], and he points to further southeastern sites with the same pattern ^[28].

Fletcher's policy argument is that fuel can be managed. "It gives us another variable when we're trying to tackle this problem, rather than just focusing on climate change," he said ^[26]. He expects climate change to keep driving fires but says widespread cultural burning would reduce their intensity and impact ^[27]. He links the 2019-20 Black Summer, which burned 24 million hectares, to heat and drought acting on thickened forest ^[17]. Nature puts the forest burnt in the southeast alone at more than 8 million hectares ^[18]. The core cannot apportion that season between fuel and weather, or show that restored burning would return today's forests to open woodland.

Fletcher also rejects these forests as safe carbon stores. "It's a myth that south-east Australian eucalypt forests can store carbon safely for a long period of time managed as they are today," he said ^[29]. Michael Bird at James Cook University called the study "probably the clearest example yet that pre-invasion landscapes were substantially altered by the removal of Indigenous fire management" ^[23]. David Bowman at the University of Tasmania said: "Australians have a lot of research and development to do to get into a safe space with climate change-enhanced wildfires." ^[22]