Philip Zylstra, 14 October 2022

Mr Bradshaw has expressed concerns about the methodology in our paper, but unfortunately, he seems to have fundamentally misunderstood what we did. Without going into that complexity, we will address his core argument here: that the proportion of an area burnt each year is dependent upon the likelihood that a fire will start there, and the amount and effect of fire suppression that occurs there. We thoroughly agree, but the question is whether this reality biases the trends in a way that will cause our results, as he has suggested. For this to be the case, it would mean for example that fires in remote, old forests are far more aggressively attacked than fires close to towns, where prescribed burning is most frequent, and long-unburnt forests much less likely. That’s not what happens though – fires that start near towns have brigades close on hand that quickly respond to smoke sightings and get to them with the best firefighting equipment available. Remote ignitions on the other hand may not be seen for days, and when they are found, their distance from roads and Western Australia’s lack of specialist remote area fire-fighting teams means that they are rarely controlled using aggressive means (hoses, and rake-hoes). Instead, it is these fires burning in remote areas where long-unburnt forest is more likely that are often contained using the backburns that Mr Bradshaw has rightly said can actually make fires larger. More effort goes into defending homes than it does remote trees; so, while we agree that these factors can affect the burnt area, accounting for them would likely make our findings stronger, not weaker. As we measured, there was about 7 times less fire burning very long-unburnt forests than in forests still recovering from prescribed burns, all despite the fact that these remote, old forests probably received far less fire suppression effort.

Mr Bradshaw also argued that a study by Rayner (1992) found that fire regeneration occurred every 36 years in karri forests prior to 1850, so our figures showing rare fire in old forests could not have been real. We’ve discussed this study at length with Mr Bradshaw, who was unable to point to any part of it making that claim. In fact, the study reported an “almost complete absence of fire scars” prior to 1850, and a later study led by Mr Bradshaw actually concluded that stands in this period were instead initiated when “age induced mortality” killed the older cohorts. Put simply, these trees died of old age, almost none of them ever experiencing a fire. Mr Bradshaw said that we did not consider the occurrence of mild-intensity fires that don’t cause datable fire scars, but didn’t mention that the Rayner report specifically said even prescribed burns scar the trees. It seems that such fires are more theoretical than real; all we really know is that the evidence of fire all but disappears prior to 1850.

As Mr Bradshaw has published himself, fire in these forests causes dense understorey regrowth that lasts for decades. This understorey eventually self-thins to recreate the open, park-like conditions so often documented in the early years of colonisation. Major research projects conducted by DBCA and CSIRO have confirmed what firefighters know so well – such dense understoreys are what drive severe, uncontrollable fire. These facts are not contested; our work was simply to show that DBCA’s own fire records match the trend you would expect from them: there has been almost no wildfire at all in those forests that have been allowed to age and self-thin. Nothing in Mr Bradshaw’s letter calls that into question.

This is a response to: Comment on ‘Self thinning forest understoreys reduce wildfire risk, even in a warming climate’: Jack Bradshaw.

Jack Bradshaw replies to this response: Self-thinning forest understoreys and wildfire risk debate – Jack Bradshaw responds to Philip Zylstra

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The Zylstra theory: a final comment: Roger Underwood;
Philip Zylstra’s response #4 – self-thinning forest understoreys and wildfire debate;
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.