Jack Bradshaw, 16 October 2022

In his response to my critique of his paper, Zylstra agrees that the proportion of an area burnt is influenced by the likelihood of a fire starting and the effect of the fire suppression effort. He also agrees that these factors were not taken into account. The degree and direction of bias in their results in therefore unknown, invalidating the results. This is exacerbated by the fact that they have treated the dry north-eastern jarrah forest and the wet karri and tingle forests, with their very different fire behaviour, as one type.

Zylstra however argues that if they had been accounted for it is likely that it would make their findings stronger. This is pure supposition based on a strange construction of fire management and suppression practices in WA, none of which has any factual basis.

The refutation of fire occurrences prior to 1850 seems to stem from a lack of understanding of the relationship between karri regeneration and fire. For karri regeneration to become established the seed must be able to reach mineral soil and the seedlings must be temporally free of competition from established understorey. In natural conditions this is only achieved by fire. The existence of a cohort of regrowth is irrefutable evidence of a fire occurring at the date of the regrowth establishment. This does not preclude the possibility of other fire that does not result in established regeneration and which may or may not leave fire scars.

Rayner’s (1992) study showed the existence of cohorts of regeneration originating before 1850 at an average age of 36 years. i.e. fires occurred at least this often in this area prior to 1850.

Zylstra claims that almost none of these trees (presumably meaning the karri forest) died of old age without “ever experiencing a fire”. The Bradshaw and Rayner (1997) structural mapping of the virgin karri forest, shows that about 45% of the virgin forest (at 1997) was regenerated (i.e. burnt) between 1770 and 1850, and 30% of the that age group is multi-aged (i.e. multiple regeneration (burning) events have occurred since the original establishment). This is not theoretical as claimed by Zylstra. The virgin karri forest has been mapped, aged and the physical evidence is there to be seen in the field.

There has been no prior discussion on these details with the authors and the Bradshaw & Rayner (1997) study did not conclude, as stated by Zylstra, that “stands prior to 1850 were initiated when aged induced mortality killed the older cohorts” and is a misunderstanding or misrepresentation of the issue.

There is no dispute that the understorey that regenerates after fire thins out and dies over time. However it does not disappear but simply adds to the litter layer on the forest floor, accumulating until it reaches equilibrium at about 50 tonnes per hectare in the case of the karri forest. Fires burning in fuel of this magnitude causes severe damage and are effectively unstoppable in severe conditions. To suggest that these areas are unlikely to burn is ludicrous.

I do not intend to comment on the fire behaviour of different aged understorey since this has been dealt with by others. My principal point is that this paper does not provide any valid contribution to this argument because of the shortcomings in methodology. Claims that the karri forest was not burnt prior to 1850 is a fantasy which is not supported by evidence in the field.

References

Bradshaw FJ, Rayner ME. 1997a. Age structure of the karri forest: 1. Defining and mapping structural development stages. Australian Forestry. 60(3):178-187.
Bradshaw FJ, Rayner ME. 1997b. Age structure of the karri forest: 2. Projections of future forest structure and implications for management. Australian Forestry. 60(3):188-195.

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.