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By Danielle Koopman 15 December 2020 6 min read

Plants resprout following a bushfire (Image: Dan Metcalfe, CSIRO)

Before COVID-19 seized our collective attention earlier this year, the world watched in horror as Australia burned in the Black Summer of 2019/20.

Australia is one of the most fire-prone countries on Earth. According to leader of CSIRO's Bushfire Behaviour and Risks team, Dr Andrew Sullivan, this is driven by both our vegetation and weather, which, of course, are interrelated.

“We have a large arid centre that generates a lot of heat during summer and powers most of the bushfires in Australia,” Dr Sullivan said.

“During bushfire season, that hot air regularly drains through south-eastern Australia. When it combines with an approaching cold front from the Southern Ocean, we get strong, hot, dry winds which drive fires through Victoria, Tasmania and southern New South Wales.”

The south-east is where most of us live and where the most bushfire fuel is – our forests, laden with combustible vegetation and debris. And climate change is literally turning up the heat, with the Bureau of Meteorology declaring 2019 the warmest, driest year on record.

Last summer, as the fires raged, so did the national debate: could we have done more to prevent the carnage? Should we have done more hazard reduction burning?

What is hazard reduction burning?

Also spreading like wildfire was misinformation. Commonly, and excruciatingly for experts such as Dr Sullivan, commentators confused the firefighting tactic of ‘backburning’ with the preventative ‘hazard reduction burning’ or prescribed burning.

He says three key elements constitute a bushfire hazard: weather (high temperatures, low relative humidity and strong winds), fuel and topography. Of these, fuel is the only element we can attempt to directly control.

Bushfire fuel is dead vegetation – leaves, twigs, branches – fallen from trees as well as bark, grasses and shrubs.

Organised hazard-reduction burning of our forests started around 80 years ago in Australia following devastating bushfires. Controlled fires are lit during low-risk conditions, using specific lighting patterns, to reduce the amount of fuel in our landscapes.

Low-risk conditions typically occur in early spring and late autumn in southern Australia, when the vegetation is dry enough to burn without the high temperatures and strong winds that would risk the burn getting out of control.

Dr Sullivan says the art and science of hazard reduction burning is finding the optimum balance between burning enough in the right conditions, to effectively modify wildfire behaviour without it becoming uncontrollable and escaping.

There are other ways to reduce fuel – such as grazing, chemical treatments (e.g. herbicides), and physically removing debris – but these cannot achieve the scale, time-, labour- and cost-effectiveness of hazard reduction burning.

Hazard reduction burning is effective at reducing fuel load at multiple levels, including bark on trees, often the cause of spotfires.

Hazard reduction burning remains an important tool for fire management to be used well before the onset of bushfire threat. Its application and effectiveness are complex, highly contextual and resource-intensive. More research and learning from experience are required to refine how and where it can bring best value and protection.

How does hazard reduction burning help manage bushfires?

Hazard reduction burning reduces the amount of bushfire fuel, and modifies its structure, reducing the intensity and potential severity of wildfires under a range of conditions.

It also extends the window of opportunity for firefighters to safely and effectively suppress wildfire.

“Management of vegetation well before the onset of the threat of wildfire is currently the only cost-effective tool for fire management,” said Dr Sullivan, who is also an author of the hazard reduction burning chapter of CSIRO’s Climate and Disaster Resilience Technical report.

But it does not prevent wildfire, nor guarantee a wildfire won’t get out of control and cause damage.

“Hazard reduction burning is complex and, depending on conditions, difficult to achieve,” he said.

“Its effects on fuel are not consistent and there’s always residual fire risk. This risk increases over time as the fuel reaccumulates.”

Also, while prescribed burning reduces hazard at the undergrowth level, once a fire gets established under severe conditions, its behaviour is driven by the weather. At that stage, the amount of hazard reduction burning that was undertaken is almost irrelevant.

Why is it so complex?

Dr Sullivan said it wasn’t unusual for planned burns to be cancelled if, for example, weather conditions were unsuitable (too wet or too dry and windy).

If a burn went ahead in unsuitable conditions, the fire might go out and not burn as intended or it might escape and burn more than was intended, or at a higher intensity.

And often, information about how much burned, and the quality of the burn, is not captured and shared.

Coordinating prescribed burns is challenging as responsibility usually falls upon several government agencies in each state, as well as private landowners who must apply for permits. Much of the work is carried out by volunteers.

“It can take up to five years for planned burns to happen because of the regulations and finding the window when it can burn safely,” Dr Sullivan said.

Layered over that complexity is the size and diversity of our continent: what works in Victoria doesn’t work in northern Australia or the tropical savannahs.

Dr Sullivan said the behaviour of prescribed fires was one of the least studied aspects of bushfire research, which tended to focus on the potential for higher impacts (e.g. ecological) of wildfire.

To study controlled burning there are many variables that need to be accounted for.

“Some models of prescribed burning behaviour have been developed with very specific requirements and locations. But in many instances, people are working from experience; expert knowledge as opposed to formal knowledge,” he said.

Since 1939, Australia has had at least 18 major bushfire inquiries yet there are still many unanswered questions.

Deputy Director of CSIRO Land and Water Dr Dan Metcalfe said experts have studied fire behaviour for 60 years.

“We can use near-earth and satellite-based remote sensing to tell us everything from the amount of biomass in an area to its curing level (how dry it is) to how fires are moving across the landscape,” he said.

“As we get more data, it highlights the complexities of fire. It’s the classic case of the more we learn, the more we understand we don’t know.”

Fire is an integral part of the Australian landscape. Many Australian species need fire to propagate, but at differing intervals.

Ideally, small ‘patchwork’ fires, such as those traditionally lit by Indigenous peoples, can provide different species with different opportunities to thrive across a landscape, but to achieve this through hazard reduction burning can be labour-intensive, costly and inefficient.

When Europeans established infrastructure and large-scale agriculture in Australia, they didn’t consider the integral role of fire in the natural environment. The Australian public’s discomfort with fire remains today.

“Generally, Australians don’t like lighting fires (for hazard reduction), and when we do, we err on the side of caution because they can – and have – become, large, fierce, fast-moving and out-of-control. And no one wants to be responsible for that,” Dr Metcalfe said.

In hazard reduction burning there is trade-off between risk and the amount of hazard reduced, the labour required to do it, and the ecological and air quality costs.

Hazard reduction burning necessarily creates smoke which, at best is an inconvenience that ruins the washing on the line. At worst, it can be life-threatening or fatal for those with respiratory illnesses.

CSIRO has developed a smoke hazard prediction tool (AQFx) to help guide planned burning in sites with high value or vulnerability and to minimise public impact.

Dr Metcalfe said the forecast La Niña meant Australia was unlikely to see devastating bushfires this summer.

“The areas that burned last summer are not going to burn like that again until they achieve the equivalent biomass (fuels),” he said.

“But if we have a good couple of wet years, that biomass is going to replace itself pretty quickly.”

Both Dr Metcalfe and Dr Sullivan agree that ultimately Australians are going to have to accept wildfire as a natural event and an integral part of our landscape.

“Hazard reduction burning has a really valuable role in our suite of management options for reducing the risk to life and property in bushfire prone areas,” Dr Metcalfe said.

“But it’s not the panacea. It has to be managed on an appropriate risk/benefit basis in conjunction with other policies including building codes, setbacks, vegetation management and the responsibility of property owners to adequately maintain those structures.”

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