Debate over native forest logging and bushfire severity continues to rage in Tasmania and Victoria

Since then, another scientific debate has become more intense: the influence of native forest harvesting on bushfire severity.

Last month, landscape ecologist Professor David Lindenmayer from the Australian National University co-authored another paper, published in EcoSphere, which built upon previous work in this area. The relationship between fire weather and site slope and aspect had long been established in terms of fire severity, but this study triangulated those with other factors including "time since previous major disturbance", which, in most cases, means time since logging.

Using data from 995,000 hectares of the 2019-20 Victorian bushfires, the study analysed wet, dry and lowland forest, old growth, late mature, previously burned and previously logged forest. They wanted to further determine the probability of crowns of trees being burned or scorched - an indicator of fire severity.

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The "best fitting model" for crown burn and scorch was found to be a three-way complex interaction: fire weather, forest type and time since major disturbance.

And when it came to time since disturbance - or logging - a finding appeared to emerge. Fire severity peaked in forests aged 10 to 40 years.

"Such elevated risks of high-severity fire in stands of relatively young forest are of key concern. This is because the nominal rotation age for logging is 80 years," the study reads.

"Given relationships between fire severity and time since previous major disturbance for some forest types and under some fire weather conditions, we suggest that stands managed for timber production near settlements may be at increased risk of high fire severity.

"This is because clearcut logging resets stand age to zero, after which there is a subsequent period of increased probability of high-severity fire, particularly under extreme fire weather conditions."

Forests younger than 10 years, and those progressively older than 40 - particularly old growth into the hundreds of years - were seen as presenting a significantly reduced fire severity risk.

While the study occurred in Victoria, Dr Lindenmayer believed there were relevant findings for Tasmania where there has been a comparative research gap.

Both states carry out logging in eucalyptus regnan forest - known as mountain ash in Victoria and swamp gum in Tasmania, common in the North-East and Styx Valley. One factor - long argued by environmental groups in Tasmania - is the even age at which regrowth occurs following harvesting which leaves sections of uniform forest in that 10 to 40-year-old window.

Dr Lindenmayer said Tasmania needed to properly investigate the risks, but had concerns about growing links between research bodies and the forestry sector that could stifle the independence of studies.

His latest study built upon similar research in this area by himself and other ecologists - the vast majority of which is heavily disputed by the forestry industry and various research bodies.

Their back-and-forth has continued in recent years.

In July, Professor Rod Keenan from the University of Melbourne co-authored another paper, published in Australian Forestry, rebutting Dr Lindenmayer's studies and public commentary on forestry activity.

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This paper stated that wet eucalypt forest - similar to that found in Tasmanian logging areas - was only 3 per cent of the area burnt in the 2019-20 Victorian bushfires.

It cited a paper which disputed the relationship between past logging and fire severity, instead stating that topography and fire weather were the most dominant factors.

"Analysis indicated an overwhelming dominance of broad spatial factors (mostly topographic), followed by fire weather (expressed as Forest Fire Danger Index), as the factors driving complete scorch or the consumption of forest canopies in natural and plantation forests," the paper reads.

"The drivers of severe canopy damage were broadly the same across the three study regions. Recent harvest status, in combination with or separate from recent burning, was ranked low in importance as a driver of fire severity in each region."

A study of Victoria's 2003 alpine fires by Keenan's co-authors "found no link between timber harvesting and bushfire extent or severity, with these fires driven almost entirely by weather conditions, slope, aspect, fuel levels, atmospheric stability and the scale of the fire".

The Keenan paper went on to cite government inquiries after the bushfires, including an assertion that the timber industry provides increased firefighting capacity, and that the phasing out of native forestry could reduce this capacity.

The Australian Forest Products Association used the paper as conclusive evidence there was "scientific consensus" that logging was "not to blame" for the severity of the bushfires.

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When asked about Dr Lindenmayer's latest co-authored study, Sustainable Timber Tasmania also pointed to the Keenan paper. STT did not respond to questions about how it factors in bushfire risk in post-harvest, regrowth forest.

In turn, Dr Lindenmayer was critical of these studies.

In Victoria, the case against native forest harvesting prompted the government to transition away from the practice by 2030.

But in Tasmania, STT maintains a legislated requirement to make 137,000 cubic metres of "high quality eucalypt sawlogs and veneer logs" available per year, although it routinely falls short of this target.

And the debate around logging and bushfire severity reached a flashpoint last year when forestry groups demanded an investigation into a research paper that found unlogged forests were less flammable than logged forests, focusing on a bushfire in the Huon Valley.

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The paper had been retracted due to its use of incorrectly categorised geographic data from the Tasmanian Government.

The University of Tasmania complied with the industry's request for an investigation, but it ultimately cleared the two conservation scientists of research misconduct.

One of the two researchers, Dr Jennifer Sanger, said the entire episode was a demonstration of the power of forestry industry bodies.

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"Being subjected to a research misconduct investigation is serious. It's the kind of thing that destroys careers," she said. "Scientists retract papers all the time, so we were concerned when the university listened to a lobby group."

She said Dr Lindenmayer's latest paper supported her findings.

"A lot of the research that David Lindenmayer does is in wet eucalypt forests and we have the same forest types, so this is relevant," Dr Sanger said.

"Victoria is warmer and drier in certain parts, so it's a good example of what Tasmania could look like in 10 to 20 years time."

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