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, 18 November 2022

The critique of our study of fire history in southwestern forests illustrates the difficulty of discussion around such emotive issues. We reported that according to Departmental records, bushfires were seven times more likely in areas of forest that still had the dense understorey that had been germinated by prescribed burns than they were in other areas where the understorey had self-thinned because it had been left alone.

These are the facts, but they leave us at an impasse. Reality does not change to suit our feelings, but this reality is the exact opposite of claims repeated countless times by certain lobby groups. It is not surprising then that critics from two of these groups have attempted to discredit the study. The problem is that because their beliefs are at odds with reality here, they cannot criticise our work using reasoned, evidence-based discussion. 

Leaf litter loads may well increase to 50 t/ha, but these are simply inputs to a long-discredited model built on assumptions from American forests. They do not change the measured reality that fire in southwestern forests is far more frequent in places where there is a dense understorey from burning. When a model disagrees with observable reality, reasoned discussion sides with reality.

Reasoned discussion requires those involved to consider new thoughts, not to ignore them. I showed why we can’t just assume that suppression is the reason why there is so much less bushfire in long-unburnt forest than there is in prescribed burnt areas. Reasoned discussion would engage with those arguments, not simply repeat itself as if I had said nothing.

Reasoned discussion requires those involved to value facts, not hide them. When Mr Rutherford raised the example of the massive fires in the Blue Mtns, he implied that this was because those National Parks were left to “the self-thin model”. He knew this was not true. When in the past I’ve said that there had been twice as much prescribed burning in National Parks in the decade leading up to those fires than there had been in the preceding decade, Mr Rutherford responded with this article. The data in that article though make my point very clearly (see graph), and the ABC sent my statement out to 3 different sets of experts who all independently reviewed it and backed me up. The confusing thing is that when fires occur in NSW, pressure is placed on National Parks to burn more, but rates of burning decrease elsewhere. As a result, the rate of burning in NSW overall has fallen, while the rate in National Parks such as the Blue Mtns has risen dramatically – as shown by Mr Rutherford’s data. The biggest fire that ignited in that World Heritage Area (Gospers Mtn) started in a 100,000ha patch burned 5 years earlier. Analysis by the Bushfire and Natural Hazards CRC after the event found that the bushfire was more severe in that 5-year-old area than it was in the long-unburned, self-thinned forest. This is hard reality, and we either adjust to it or fade into irrelevance.

I have attempted to present such hard facts in a reasoned way. I know that these simply cannot be accepted by lobby groups that exist to deny and obscure them, but I hope I’ve provided enough information for readers to see through the spin. I will be leaving the debate here.

Related stories:

The Zylstra theory: a final comment: Roger Underwood;
Peter Rutherford to Philip Zylstra #2 – 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;
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.