Frequent fire makes the bush more likely to burn, UOW study finds

Challenging assumptions: Fire crews performing a hazard reduction burn in Wiseman's Park on behalf of Wollongong council in May. Picture: Adam McLean.
Challenging assumptions: Fire crews performing a hazard reduction burn in Wiseman's Park on behalf of Wollongong council in May. Picture: Adam McLean.

As firefighters prepare for an earlier than normal bush fire season, hazard reduction burns have again become a fixture in the Illawarra.

But new research out of the University of Wollongong has revealed that, in the study area of the Australian Alps, fire has made the bush more likely to burn.

And while Dr Philip Zylstra says his findings don’t mean hazard reduction burns should be stopped, he says it does challenge “the central assumptions in Australian fire management”.

He calls these ever-present burns “prescribed burning” as “it’s only hazard reduction if it does reduce the hazards”.

“Traditionally, people think ‘oh, that forest hasn’t burnt for a long time, so it’s flammable’,” he said. “We now know that that’s just not true in the alps, and it’s not true in a lot other other areas – it is just an assumption.”

“As far as it goes for prescribed burning, it’s still not enough to tell us that we should or shouldn’t do it – but on average, fires across the Australian alps have been much more like to burn in regenerating forests rather than old forests.”

Dr Zylstra’s research, published this month, measured 36 million locations over 1.5 million hectares from 58 years of mapped fires in the 12 national parks.

It found that the different types of forest in the alps did have a low flammability after fire in the very short term, because there was nothing left but bare ground.

However, in the short to medium term – ranging anywhere between two years to 25 years depending on the type of forest  – the bush was much more flammable if it had been burnt.

Mature forest – aged from about 20 years old – was much more fire resistant, and in some cases was eight times less likely to burn than bush which had been exposed to frequent fire.

“There’s a hump in the middle where things are regrowing and restoring, and that’s when the forest is most likely to burn,” Dr Zystra said.

“As a forest ages, one of the general trends is that those plants get larger. If they’re not on fire, [trees are] kind of on our side; they slow the wind speed down so the fire spreads more slowly.

“And if you get enough rain to wet the fuel on the ground it will stay wet for longer, because they’re shaded by the canopy of the large trees, and that means less likelihood of the fire occurring.”

Dr Zylstra said more work was needed before Australia would be able to break with the “tradition” of burning to stop future fires, but said he would recommend at least one immediate change.

“Without knowing anything more than we do now, we can say that we can reduce flammability by finding areas of forest that we can nurse into old age, because the more of the old growth forest, the less flammable it is,” he said.

“We love fire in Australia – people love burning things, and people like to think there is a simple answer [to stopping fires], but this tells us that isn’t necessarily the case.”

“People have a look at a prescribed burn, and they can see that a wildfire burned into it and stopped.

“And while that might be true for two years for that spot, a prescribed burn that is four years old might not have been as effective. And you can’t keep all the bush at two years old.”

Dr Zylstra’s findings are similar to others arising across the dry eucalypt forests of southeast Australia.

Over the past few years, researchers have found fires become more likely in many forests that have recently burnt, with similar findings in America and New Zealand.

“The message from many ecosystems across the world is that while we’ve been assuming otherwise, fire has been breaking their defences and feeding more fire,” he said.

“This may not be universal, but as climate change gives us a drier landscape, we can no longer afford to simplify these communities into a fuel load.

“There is vast complexity at work.”

The research findings on fire behaviour in the Australian Alps were published in the August 2018 issue of the journal Austral Ecology.