Severe and Short Interval Fires Rearrange Dry Forest Fuel Arrays in South-Eastern Australia

Round 1

Reviewer 1 Report

I found your contribution very interesting, well-written a nd carefully designed. I consider however that it is necessary for the broader readership two explanatory sentences regarding your perception of the terms 'severity' and 'hazard'. These two sentences will clear your approach from uncertainties in understanding of metrics. For example, 'severity index' is often used as the ratio of area burned/frequency of fire, which is certainly not the case in this paper. 

Author Response

Reviewer 1, Comment 1: I found your contribution very interesting, well-written and carefully designed. I consider however that it is necessary for the broader readership two explanatory sentences regarding your perception of the terms 'severity' and 'hazard'. These two sentences will clear your approach from uncertainties in understanding of metrics. For example, 'severity index' is often used as the ratio of area burned/frequency of fire, which is certainly not the case in this paper. 

Response: We thank the reviewer for their comments. We now specifically define fire severity in the introduction section of the manuscript, with reference to a key citation defining it.

Lines 40 – 44: “and increases or decreases in fire severity (i.e. loss of above- or below-ground organic matter in response to fire) [1] can disadvantage species with physically dormant seeds that require specific fire intensities to stimulate germination.”  

This definition of fire severity is included one top of the descriptive definition which was provided in the original manuscript.

Lines 152 – 158: “Fire severity was defined by the degree of overstory canopy consumption during the Black Summer fires using the New South Wales Fire Extent and Severity Mapping (FESM) maps [41,42]. In our study, low fire severity represented sites burned by understory fire that did not extend to the overstory tree canopy (low, moderate FESM classes), and high fire severity represented sites burned by overstory fire that caused tree canopy scorch or consumption (high, extreme FESM classes).”

We agree that fuel hazard needs to be properly defined. We now do this in the introduction section of the manuscript, with reference to a key citation defining it.

Lines 123 – 125: “For our purposes, fuel hazard refers to fuel arrays that may potentially facilitate a spreading fire given an ignition. Greater fuel hazard ratings are associated with greater potential fire behaviour such as flame height, rate of spread and spotting potential [2,3].”      

Reference [1]: Keeley, Jon E. "Fire intensity, fire severity and burn severity: a brief review and suggested usage." International journal of wildland fire 18.1 (2009): 116-126.

Reference [2]: Gould, James S., W. Lachlan McCaw, and N. Phillip Cheney. "Quantifying fine fuel dynamics and structure in dry eucalypt forest (Eucalyptus marginata) in Western Australia for fire management." Forest ecology and management 262.3 (2011): 531-546.

Reference [3]: Finney, Mark A. “The challenge of quantitative risk analysis for wildland fire.” Forest ecology and management 211.1-2 (2005): 97-108.

Reviewer 2 Report

The text is well structured, the text is written succinctly and objectively. Furthermore, the text presents an important contribution to the topic of studies. Given this, I believe that the text should be accepted for publication.

Author Response

Reviewer 2, Comment 1: The text is well structured, the text is written succinctly and objectively. Furthermore, the text presents an important contribution to the topic of studies. Given this, I believe that the text should be accepted for publication.

Response: We thank the reviewer for the comments.

Reviewer 3 Report

I have to say I was confused both by the methodology (rather methodologies), presentation (especially the figures), and conclusions of this paper.

Fuel connectivity (as you define it) is not a standard fuel measurement and it can be confusing with fuel escalation, when applied to vegetation up to two meters high, as you did.

You did not explain why is connectivity higher in areas with high severity fires. In a first look, it can be confusing, or contradictory to what would be logically expected. 

Your statement 'rates of fuel accumulation are the greatest 2.5 years after the fire and by these we can project changes in the future.' is at least dubious and certainly needs further justification.

The canopy (elevated) fuel attributes and their connection with catastrophic crown fires is not sufficiently addressed, measured and discussed. Yet, these fires are the most disasterous in Australia.

In general, to my opinion, you use too many variables and too many sampling schemes in order to test too many hypotheses that are common logic and cannot be assesses how they will evolve in the future. The presentation and conceptualization with figures, is confusing.  Figures should be reconsidered in this paper.

Author Response

Reviewer 3, comment 1: I have to say I was confused both by the methodology (rather methodologies), presentation (especially the figures), and conclusions of this paper.

Response: We are somewhat perplexed by this comment as two other reviews of the manuscript were extremely positive about the methodology, study design and writing. As the reviewer has not identified any examples of specific issues or offered any direct suggestions to improve the manuscript, we found ourselves unable to address this comment.  

Reviewer 3, comment 2: Fuel connectivity (as you define it) is not a standard fuel measurement and it can be confusing with fuel escalation, when applied to vegetation up to two meters high, as you did.

Response: We argue that “fuel connectivity” is a very important component of our study and critical to understanding wildfire more generally. This is because fuel connectivity and structure are increasingly being recognised as key drivers of fire – fuel interactions (see references 1 – 4) and considered in contemporary fire behaviour models (see references 5 – 6). For example, the VESTA-2 fire behaviour model – which is routinely used in fire management operations across east Australia – requires explicit representation of vertical continuity [see reference 6) below].  

Although the term “fuel connectivity” may not be traditional, we feel that it accurately describes the field measures. For example, we cannot call the variable “fuel structure” because we specifically assessed how vertically fuels are connected. We agree that the term “fuel connectivity” may be too ambiguous as our field measures only assessed vertical fuel connectivity. Therefore, we now refer to “vertical fuel connectivity” throughout the manuscript.   

References [1]: Collins, Luke, et al. "The effect of antecedent fire severity on reburn severity and fuel structure in a resprouting eucalypt forest in Victoria, Australia." Forests 12.4 (2021): 450.

References [2]: Barker, James W., Owen F. Price, and Meaghan E. Jenkins. "High severity fire promotes a more flammable eucalypt forest structure." Austral Ecology 47.3 (2022): 519-529.

References [3]: Wilson, Nicholas, Ross Bradstock, and Michael Bedward. "Detecting the effects of logging and wildfire on forest fuel structure using terrestrial laser scanning (TLS)." Forest Ecology and Management 488 (2021): 119037.

References [4]: Karna, Yogendra K., et al. "Indications of positive feedbacks to flammability through fuel structure after high-severity fire in temperate eucalypt forests." International Journal of Wildland Fire 30.9 (2021): 664-679.

References [5]: Cruz, M. G., Cheney, N. P., Gould, J. S., McCaw, W. L., Kilinc, M and Sullivan, A. L. “An empirical-based model for predicting the forward spread rate of wildfires in eucalypt forests.” International Journal of Wildland Fire 31.1(2021):81-95

References [6]: Zylstra, P., Bradstock, R. A., Bedward, M., Penman, T. D., Doherty, M. D., Weber, R. O., et al. “Biophysical mechanistic modelling quantifies the effects of plant traits on fire severity: species, not surface fuel loads, determine flame dimensions in eucalypt forests. ” PloS One 11.8(2016): e0160715

Reviewer 3, comment 3: You did not explain why is connectivity higher in areas with high severity fires. In a first look, it can be confusing, or contradictory to what would be logically expected.

Response: We describe why fuel connectivity is higher in areas with high severity fires in the below lines of the original manuscript (first paragraph of Discussion). We feel that this description is enough.

Lines 345 – 352: “Midstory (elevated) fuel cover and fuel hazard scores were higher in areas burned by high rather than low severity fire, presumably because high severity fire stimulated the mass recruitment of shrubs requiring high temperatures to facilitate seed germination [6]. Elevated fuel cover, connectivity, max. height and fuel hazard scores were also lower in forest burned by short rather than long fire intervals, presumably because multiple short interval fires killed obligate seeding shrubs before maturation, and hence exhausted seed banks and regeneration vigour [50].”

Reviewer 3, comment 4: Your statement 'rates of fuel accumulation are the greatest 2.5 years after the fire and by these we can project changes in the future.' is at least dubious and certainly needs further justification.

Response: The reviewer does not provide a specific line number for this comment, but appears to be referring to Lines 91 – 93 of the original introduction:

“We also focus on quantifying fuel recovery 2.5-years following fire because this is when rates of fuel accumulation are greatest [36], and hence provides a good indicator for planning appropriate interventions in subsequent years.”

We understand that our original text did not explicitly link why or how our 2.5-year postfire field data could be used to predict future fuel states and management interventions. To further consolidate this point, we now include a further description of postfire fuel accumulation in the introduction section of the manuscript and link this to why assessments at 2.5-years can be used to predict future fuel states:

Lines 91 – 98: “We also focus on quantifying fuel recovery 2.5-years following fire because this is when rates of fuel accumulation are greatest. For example, the Olsen curve predicts surface fuel loads to increase by 2.87 t/ha between 2.5- and 5-years postfire, but only 0.5 t/ha between 12.5- than 15-years postfire (here for Sydney coastal dry sclerophyll forest)[36]. Therefore, understanding fuel recovery 2.5-years postfire provides a good indicator of future fuel states (using predictive approaches such as the Olsen curve), which will be key for planning appropriate interventions in subsequent years.”

In addition to this, we extensively discuss how fuel accumulation may continue in the future in the discussion section of the original manuscript, including stating caveats about our predictions. See lines 467 – 475 of the here resubmitted manuscript.

Reviewer 3, comment 5: The canopy (elevated) fuel attributes and their connection with catastrophic crown fires is not sufficiently addressed, measured and discussed. Yet, these fires are the most disasterous in Australia.

Response: The connection between elevated fuels and extreme fire behaviour are well researched and formalised in Australian forest fire behaviour models. For example, Phoenix [1], Vesta 1 [2] and Vesta 2 [3].

We discussed the importance of elevated fuels in promoting high severity and intense fire in the introduction section of the original manuscript, including some of the references mentioned above. For example:

Lines 70 – 72: “Elevated fuels and bark fuels are important drivers of fire intensity and severity because they allow flame transfer from the ground to the tree canopy.”

We also discuss how elevated fuel responses to the Black Summer fires will impact current and future fuel hazard and fire likelihood in the discussion section of the original manuscript (see lines 383 – 388 and 418 – 430). In response to the reviewer’s comment, we have added additional information to the discussion section of the here resubmitted manuscript specifically discussing how elevated fuel responses to the Black Summer fires will impact the likelihood of larger catastrophic fire events.

Lines 426 – 430: “It is important to note that although elevated fuel hazard may be relatively high in areas experiencing high severity or short interval fire, fuel load and connectivity will still be lower than those present at older fuel ages. Therefore, it is unlikely that large un-controlled wildfires (like the Black Summer fires) will occur until fuel arrays increase and bypass fuel limitations on fire spread.”

References [1]: Tolhurst, Kevin, Brett Shields, and Derek Chong. "Phoenix: development and application of a bushfire risk management tool." Australian Journal of Emergency Management, The 23.4 (2008): 47-54.

References [2]: Cheney, N. Phillip, et al. "Predicting fire behaviour in dry eucalypt forest in southern Australia." Forest Ecology and Management 280 (2012): 120-131.

References [3]: Cruz, Miguel G., et al. "An empirical-based model for predicting the forward spread rate of wildfires in eucalypt forests." International journal of wildland fire 31.1 (2021): 81-95.

Reviewer 3, comment 6: In general, to my opinion, you use too many variables and too many sampling schemes in order to test too many hypotheses that are common logic and cannot be assesses how they will evolve in the future. The presentation and conceptualization with figures, is confusing.  Figures should be reconsidered in this paper.

Response: We disagree with the reviewer’s points, as per our responses below.

• Point 1) “you use too many variables”: Many previous studies have focussed on just one or a few aspects of fuel (e.g., litter). However, fuel contributing to fire behaviour is more complex than just one or a few elements, and fire managers do not measure fuel as one or two variables alone. Therefore, we believe it was essential to capture as many elements contributing to the fuel load in a forest as possible. This allowed us to look at how all elements are responding together rather than in isolation, and was a key factor leading to the final state-and-transition model we propose in the discussion.
• Point 2) “you use too many…. sampling schemes”: We only used one very simple sampling design where sites were stratified by two levels of fire severity and interval. Therefore, we do not understand what the reviewer means when referring to “sampling schemes”.
• Point 3) “in order to test too many hypotheses that are common logic”: We provide three very simple hypotheses in the introduction. We do not feel that this is “too many hypotheses”, especially given the complexity of the fuel array. In reference to the hypotheses being “common knowledge”, there is little to no empirical evidence showing how fire regimes impact fuel arrays in the years following fire. Therefore, our study provides novel insights into how these processes impact fuel arrays.
• Point 4) “cannot be assesses how they will evolve in the future”: We provide hypotheses and predictions about how fuel arrays may change into the future. We very clearly and purposefully provide these as predictions and not fact, because we cannot predict future fuel states. However, we feel it is important to provide these hypotheses so that they can be tested using similar empirical datasets.
• Point 5) “The presentation and conceptualization with figures, is confusing. Figures should be reconsidered in this paper.”:  As the reviewer did not identify specific issues with figures to be addressed or propose any alternatives, we were unable to address this comment. We feel that the figures are well presented, as did the other two reviewers of the manuscript. Therefore, we have chosen to keep them as is.

Round 2

Reviewer 3 Report

I am satisfied by your answers. Although I do not fully agree with you at some points, your revised version solved most of my questions regarding the manuscript.