Philip Zylstra, 24 October 2022

Our paper published in 2022 shows that, according to DBCA records, bushfires have been seven times more likely on land previously burned by them than on land that they have not burned.

Mr Jack Bradshaw’s educated guesses about the causes of karri regeneration prior to 1850 do not change this, they use speculation to distract from the hard evidence we are discussing. He argues that our findings are invalid because fire suppression was involved but offers no argument as to how this could work, how suppression would only have reduced fire size in the oldest forests. He refers to my claim that suppression is probably more intensive around houses than it is in remote areas as “a strange construction of fire management and suppression practices”.

But why does he argue this? I’m not sure how much time Mr Bradshaw has spent on a fire line, but certainly in my years of firefighting and fire management, the moment houses came under threat all major resources were redirected to protect them. All agencies prioritise “life and property” over the environment, that’s just the way it is.

It may be that suppression was actually more effective in the oldest forests, but if that’s the case, then that strongly reinforces our point that fires are more controllable in those forests where the understorey is open. If supported, Jack Bradshaw’s objections would support our findings. Again though, they are conjecture; all we know is what actually happens according to DBCA records: fires are seven times more frequent in areas that have dense understorey regrowth than they are in places where the understorey self-thinned.

In the days when it was called CALM, DBCA and CSIRO conducted around 100 experimental fires in jarrah forest, which makes up the bulk of our study area. Their most recent analysis of that work concluded: “…the best fuel descriptor from the point of view of the operational prediction of fire spread is the height of the understorey fuels. This variable is defined as the average height of both the near- surface and elevated fuels weighted by their cover on a per area basis” (Cruz et al. 2022).

The condition of the understorey drives fire. Sure, when the shrubs die, they leave sticks behind, but of course, sticks decay. What about the 50 t/ha of litter though? CSIRO and DBCA said that this was not the issue, the issue was the understorey. Dr Neil Burrows in CALM had already shown that litter had absolutely no effect on the rate of fire spread. Yes, those places can certainly burn, but fires are less likely to reach them if those fires spread more slowly and are more easily extinguished because the shrubs that drive them have self-thinned.

The issue is very straightforward. Speculation, models and guesses aside, the hard empirical evidence quite clearly shows that bushfires are much less likely in forests where the shrubs have been allowed to self-thin. We published this work seven months ago, including all of the data we used. So far, neither Mr Jack Bradshaw nor any other critic has found a flaw in our work that stands up to the independent scrutiny of peer-review. We need to move on and accommodate this reality, because it arms us with new possibilities to work smarter in the bush and better deal with fire risks.

Related stories:

The Zylstra theory: a final comment: Roger Underwood;
Philip Zylstra’s response #4 – self-thinning forest understoreys and wildfire debate;
Jack Bradshaw to Philip Zylstra #2 – self-thinning forest understoreys and wildfire debate;
Philip Zylstra’s response #3 – self-thinning forest understoreys and wildfire risk debate;
Self-thinning forest understoreys and wildfire risk debate – Roger Underwood responds;
Peter Rutherford to Philip Zylstra #2 – self-thinning forest understoreys and wildfire debate;
Philip Zylstra’s fire research: Adding value or creating risk? : Peter Rutherford;
Philip Zylstra continues the debate – self-thinning forest understoreys and wildfire risk;
Self-thinning forest understoreys and wildfire risk debate – Jack Bradshaw responds to Philip Zylstra;
‘Self thinning forest understoreys reduce wildfire risk, even in a warming climate’: Philip Zylstra responds to Jack Bradshaw;
Comment on ‘Self-thinning forest understoreys reduce wildfire risk, even in a warming climate’: Jack Bradshaw.