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Philip Zylstra’s response #3 – self-thinning forest understoreys and wildfire risk debate

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This article relates to the ongoing debate on Australian Rural & Regional News into Bushfires, Logging, Burns & Forest Management, in particular, into a debate into self-thinning forest understoreys and wildfire risk. The series of articles relating to this particular debate are all extracted on the general debate page.

Philip Zylstra, 30 October 2022

The mapped fire histories of the southwestern forests show that bushfires have been most frequent in forests with dense understoreys promoted by previous burns, and far less common in areas that have not been burned for several decades, allowing the understorey to naturally thin. Two new voices have entered the discussion on this here and made numerous claims, but their ill-informed comments have distracted from the point.

Mr Underwood says that “As Professor Zylstra himself now admits, the paper’s conclusions are based on flawed methodology, thus invalidating them.” I said no such thing. It is strange to think that I would on one hand say that (which I did not), then on the other continue to defend our work. Mr Underwood criticises the combination of open wandoo woodland with tall karri forest; but again, we did no such thing. Our methods were clear for him to see, and neither wandoo nor any other woodland was part of our study of forests. Both false claims are merely distractions.

Mr Underwood suggests that I don’t understand fire suppression in WA. Fire suppression was not part of our study. Fire suppression only came into the discussion because Mr Jack Bradshaw argued it somehow invalidated our findings, but he could not explain how. Mr Underwood’s detailed description does not explain how either. If you are going to argue that fire suppression is the reason why bushfires are so much rarer in long-unburnt forests, then you must show that more effort is made to put fires out in remote, long-unburnt forests than it is in the regularly burned bush close to houses. Mr Underwood has not done that.

Mr Rutherford is correct that two CSIRO scientists critiqued my fire behaviour model that was published in a major peer-reviewed scientific journal in 2016. The journal took the CSIRO critique seriously and sent the paper out for independent review, but the review concluded the critics were wrong. That happens in science, debate is normal. Does it alter the fact that DBCA records show bushfires have happened seven times more often in areas burnt in the previous 50-odd years? Absolutely not. It was irrelevant to our current discussion from the beginning because my fire behaviour model was not even used for our recent published new study (that appeared in 2022). Mr Rutherford’s ill-informed comment is another distraction: an attempt to discredit me using a story turned around by omitting the main point.

Mr Rutherford raised a recent study which showed that since the British colonization, bushfires had become more severe at the same time as shrubby understoreys had increased. This might sound familiar, because it is precisely our point: shrub density drives fires that are difficult to control. It is true that the authors of that study suggested that understorey thickening was a result of the lack of fire, but it is also true that they did not account for the well-proven fact that so many understoreys thin by themselves if they are allowed to age. The previous West Australian Conservator of Forests Charles Lane Poole witnessed this understorey thickening first-hand. In 1939, he said that what “the old people in Australia remember” was that “the thickening up of our forests is entirely due to fire, and the exclusion of fire will render them less susceptible to fire because it will get rid of an enormous amount of inflammable material.”

This reality is plainly visible in the 1994 work of former CALM scientist Dr Neil Burrows. The lines on his graph show that straight after fire, the shrubby understorey increases dramatically in density, but that after about 22 years it self-thins so that the understorey is as open in a forest unburnt for 50 years as it is in a forest burnt 1 year ago. That is what “the old people” used to know. I have put our findings from DBCA’s mapped fire records over the top of that in green, and the trend is remarkably similar. The oldest people – elders of the Pibulmun and Menang Noongar of these forests have also just released a paper, and there they tell us that instead of trying to burn hectares, their fire use was highly specific, contained, and localised to campsites and places where they wanted to favour particular plants. This is what we called ‘cooperation with country’ in our study. Use fire where it is useful, but if the broad forest naturally thins, then why disrupt that by burning huge areas to create the dense understoreys that now dominate them?

Fig 5-7

What are the facts here? We know from CALM/DBCA research the thing that everyone used to know – that burning the southwestern forests makes the understorey extremely dense, but that it self-thins and becomes open if left to age. We know from CSIRO/DBCA research that it is not surface litter but this understorey that drives hard to control fire. And my colleagues and I have shown from DBCA records that trends in fire size follow exactly what you would expect from this, so that old, long-unburnt forests have much less fire risk than the regrowth from burning. The critics writing here have made a lot of either false or irrelevant claims, but what they haven’t done is challenge any of these facts.

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;
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.


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