Raging bushfires started 60 million years ago

Bushfires may have been a dominant feature of the Australian outback for much longer than experts thought.

FREQUENT BUSHFIRES WERE ripping across the Australian landscape nearly 50 million years earlier than previously thought, a new study suggests.

By examining fossilised gum tree pollen, researchers at the Australian National University in Canberra have dated the unique regenerative capacity of eucalypts to around the same time that Australia's rainforests began to recede 62 million years ago – shortly after the extinction of the dinosaurs.

Up until now, most studies have argued that bushfires have only been common for the past 15 million years.

Smoking gun

"Our work suggests the ancient ancestors of the eucalypt moved out of the rainforest and into the woodlands that you see today, at about the same time as this [regenerative] adaptation arose," says study author Professor Michael Crisp. "It's a smoking gun of evidence that...both things originated at the same time as fire became a presence in Australian vegetation."

Many trees have dormant 'epicormic' buds underneath the bark that sprout when the tree is damaged by storms or drought – but eucalypts have evolved a unique variation of this tissue. "This tissue is very deep in the wood of the eucalypt tree and is very resilient, springing into life with damage from fire," Michael says.

"In other parts of the world that have similar environmental conditions to Australia, such as California with its oak-dominated forests, a severe bushfire will kill everything above ground. Eucalypts, however, have developed a unique capacity to recover from fire," he says.

Michael's team argue that the evolution of this ability in gum trees may have been part of a cycle which – along with the changing climate – caused Australia to become much more arid and led to the rainforests being replaced by dry eucalypt forests.

"We think that there was a feedback loop between the eucalypt surviving fires and creating an environment that favoured the eucalypt," he told Australian Geographic.

Super wet environment

Professor Robert Hill, a botanist at the University of Adelaide isn't convinced by the findings, however.

He warns against using fossilised pollen of just a few species to make far-reaching conclusions about the spread of bushfires, and adds that it's highly unlikely that bushfires were frequent 62 million years ago. "At this period of time the whole of Australia was dense rainforest – a super wet environment," he says.

Robert goes on to question the idea that unique regenerative ability of eucalypts first evolved as an adaptation to fire. "It could have been frost or drought or insect damage; there are all sorts of things that damage foliage," he says.

The results of the study were recently published in the journal Nature Communications.

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