Australasian Science: Australia's authority on science since 1938

Opinions Under Fire

By Phil Gibbons

An analysis of Victoria’s Black Saturday fires has provided important evidence about which factors save houses. The study highlights the difference between opinion and evidence.

When a big bushfire hits a community and brings about loss of life and property you can be sure emotions will be running high in the aftermath. A range of strongly held opinions will be expressed on what should have been done to prevent the disaster. What you rarely see, however, is a long hard examination of the evidence of what factors in the landscape are contributing to the loss of homes and lives. Do strongly argued opinions on appropriate land management fit with the available evidence? This was the question we sought to answer when we examined the evidence left in the aftermath of the 2009 Black Saturday fires in Victoria.

Following a large, damaging fire, the general debate is usually about how to appropriately manage vegetation. There are always strident calls demanding more prescribed burning. There are also claims we should increase general levels of logging or clearing. Some, however, believe that during episodes of extreme fire weather, as was experienced on Black Saturday, it doesn’t matter what is done.

The behaviour of wildfires is primarily determined by weather, terrain and fuel. Of these, fuel load (in terms of the surrounding vegetation) is the easiest to manipulate. Common fuel-reduction treatments employed are clearing, prescribed burning, grazing and mechanical removal of biomass. These treatments are often undertaken at broad-scales and quite a distance from peri-urban communities.

Fuel reduction can be very expensive, and it can have undesirable environmental impacts. It can also undermine the aesthetics that attract residents and tourists to bushland environments, and it carries its own risks. So, given that these treatments are expensive, damaging and risky, you’d think there would be compelling evidence demonstrating their effectiveness. However, evidence that these treatments mitigate impacts on peri-urban communities from wildfires remains extremely limited.

There are many reasons for this. Wildfires are a difficult phenomenon to study: large, destructive fires cannot be lit experimentally; house loss during wildfires is often aggregated, preventing replication of landscape-scale variables; and adequate pre- and post-fire data are not always available.

But this is where the Black Saturday fires provide a rare opportunity to examine and compare the many factors at play. This event destroyed a large population of houses in landscapes with a mix of housing densities, terrains and fuel types, and occurred in landscapes where there were adequate pre- and post-fire data.

The evidence I and my colleagues (a collaboration between ten fire, landscape and statistical scientists from Australia and the US) have collected was recently published in the international journal PLoS One. It’s taken nearly 3 years to collect and analyse, and then undergo peer review. It’s based on 12,000 measurements at 499 houses.

Our analysis suggests that six factors have the potential to make a big difference to house loss. They are, in order of importance:

1. the cover of trees and shrubs within 40 metres of houses;

2. whether trees and shrubs within 40 metres of houses was predominantly remnant or planted;

3. the upwind distance from houses to groups of trees or shrubs;

4. the upwind distance from houses to public forested land (irrespective of whether it was managed for nature conservation or logging);

5. the upwind distance from houses to prescribed burning within 5 years; and

6. the number of buildings or structures within 40 metres of houses.

All fuel treatments were more effective if undertaken closer to houses.

In essence, we found that prescribed burning is not the most effective way to protect houses in severe bushfires. Prescribed burning helps to protect houses, particularly when it is undertaken closer to them, but clearing vegetation close to houses was twice as effective.

And whether your house was near a national park or a logged forest didn’t make that much difference. On Black Saturday, houses were at similar risk whether they were adjacent to a National Park or a State Forest. Similarly, logging native forests does not reduce the risk of house loss – we found no significant relationship between house loss and the amount of logging.

Fuel treatments do play an important role in saving homes (and lives). Our research predicted that minimising several fuels could theoretically reduce house loss by 76–97%, which would translate to considerably fewer wildfire-related deaths. However, to achieve anything near this level of protection we need a shift in emphasis away from broad-scale fuel reduction to intensive fuel treatments close to property.

Evidence is not as timely or entertaining as opinion, but I know which I’d rather use to protect my house and my family from bushfires.

Dr Philip Gibbons is a Key Researcher with the National Environmental Research Program Environmental Decisions Hub, which forms part of the Environmental Decisions Group.