28 OCTOBER 2021

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Bill McCormick and Dr Daniel May
Science, Technology, Environment and Resources Section

 

Executive summary

Following major bushfires in the past twenty years, public and political attention has been drawn to the potential for fuel reduction burning to reduce bushfire risk and damage. This paper provides a major update to a 2002 Parliamentary Library publication examining the issue. It incorporates the findings of recent research and the numerous inquiries published since then.

Fuel reduction burning remains an effective component of broader strategies to reduce bushfire damage, but it is not a panacea. The aim of fuel reduction burning is to reduce fuel in order to modify the behaviour of a potential bushfire. Generally, it is not expected to stop a bushfire, but to slow its spread and reduce its intensity to allow for more effective suppression, limit ecological damage, and reduce damage to assets such as houses. It is most effective when conditions are moderate and is of diminishing effectiveness when conditions worsen.

There are substantial differences in practice and among experts as to the optimum strategy for fuel reduction burning. Considerations of timing, frequency, size, and smoke all point to the finding that the effectiveness of fuel reduction burning varies substantially on a local basis. The environmental effects of fuel reduction burning are also complex: no single fire regime suits all ecological communities and any impacts from fuel reduction burning should be weighed against the potential impacts of future bushfires. Managers must also consider the long-term effects of fuel reduction burning and the increasing challenges posed by climate change.

Contents

Executive summary
Glossary
Introduction
Understanding fires
Effectiveness of fuel reduction burns
Fuel reduction burning and biodiversity
Other issues
Implications of climate change
Conclusion

Glossary

Term	Definition[2]
Backburn	A method that aims to stop bushfires from burning out specific areas by setting fires to consume fuel in the path of a bushfire. It is used to try to manage an active bushfire. Backburning is often dangerous and may exacerbate bushfire damage. The difference between fuel reduction burning and back burning is ‘effectively the same as the difference between elective and emergency surgery’.[3]
Crown fire (alternatively known as a canopy fire)	A fire that advances through the canopy of trees or shrubs. Crown fires are extremely difficult and dangerous to suppress.[4]
Crown scorch	Browning of the needles or leaves in the crown of a tree or shrub caused by heat from a fire.[5]
Direct attack	A method of fire attack where wet or dry firefighting techniques are used. It involves suppression action right on the fire edge which then becomes the fireline
Fine fuels	Fuels such as grass, leaves, bark and twigs less than 6mm in diameter that ignite readily and burn rapidly when dry.
Firebrand	A piece of flaming or smouldering material capable of acting as an ignition source (such as Eucalyptus bark).
Fire danger	Sum of constant danger and variable danger factors affecting the inception, spread, and resistance to control, and subsequent fire damage; often expressed as an index. The Australian Fire Danger Rating System (AFDRS) is being introduced by fire and emergency services agencies to create a consistent national fire danger rating system.[6]
Fire ecology	The study of the relationships between fire, the physical environment and living organisms.
Fire front	Unless otherwise specified, the fire front is assumed to be the leading edge of the fire perimeter. It is the part of the fire within which continuous flaming combustion is taking place.
Fire intensity	The rate of energy release per unit length of fire front usually expressed in kilowatts per metre (kW/m). The rate of energy release per unit length of fire front, defined by the equation I=Hwr, where: I = fireline intensity (kW/m) H = heat yield of fuel (kJ/kg)-16,000 kJ/kg w = dry weight of fuel consumed (kg/m2) (mean total less mean unburnt) r = forward rate of spread (m/s) The equation can be simplified to: I = w r/2 where I = fireline intensity (kW/m) w = dry weight of fuel consumed (tonnes/ha) r = forward rate of spread (m/hr)
Fire regime	The history of fire in a particular vegetation type or area including the frequency, intensity, and season of burning. It may also include proposals for the use of fire in a given area.
Fire severity	Fire severity is used to describe the effect of energy released by a fire on an ecological community. ‘Fire intensity’ usually refers to the physical properties of a fire (that is, the amount of energy released).[7]
Fire suppression	The activities connected with restricting the spread of a fire following its detection and before making it safe.
Fire weather	Weather conditions which influence fire ignition, behaviour, and suppression.
Forest Fire Danger Index (FFDI)	The Forest Fire Danger Index (FFDI) is used to provide a range of fire danger ratings. It can function as an indicator of the difficulty of suppression and be used as a basis for Fire Danger Ratings to warn the public. It uses inputs including relative humidity, air temperature, windspeed, and a drought factor. It is subdivided into a number of classes that rate the difficulty of suppression (Low, Moderate, High, Very High, Severe, Extreme, Catastrophic/Code Red). There is also a Grass Fire Danger Index that is outside the scope of this paper.[8]
Fuel load (alternatively known as fuel loading)	The oven dry weight of fuel per unit area. Commonly expressed as tonnes per hectare.
Fuel reduction burning (alternatively known as hazard reduction burning)	The planned application of fire to reduce hazardous fuel quantities; undertaken in prescribed environmental conditions within defined boundaries.
Fuel structure	The arrangement of shrubs and litter fuels. Fire will spread more easily through a continuous fuel layer. Shrubs, loose bark and vines provide a ladder for fire to climb into trees.[9]
Indirect attack	A method of suppression in which the control line is located some considerable distance away from the fire's active edge. Generally done in the case of a fast-spreading or high-intensity fire and to utilize natural or constructed firebreaks or fuelbreaks and favourable breaks in the topography. The intervening fuel is usually backburnt; but occasionally the main fire is allowed to burn to the line, depending on conditions.
Prescribed burning (alternatively known as controlled burning or planned burning)	The controlled application of fire under specified environmental conditions to a predetermined area and at the time, intensity, and rate of spread required to attain planned resource management objectives. This includes, but is not restricted to, fuel reduction burning.
Rate of spread	The speed with which a fire moves in a horizontal direction across the landscape at a specified part of the fire perimeter. Firefighters may divide this into forward, flank, and backing rate of spread.[10]
Spotting	Behaviour of a fire producing sparks or embers that are carried by the wind and start new fires beyond the zone of direct ignition by the main fire.
Spot fire	Isolated fire started ahead of the main fire by sparks, embers or other ignited material, sometimes to a distance of several kilometres.
Urban rural interface (alternatively known as the peri-urban interface or wildland-urban interface)	The line, area, or zone where structures and other human development adjoin or overlap with undeveloped bushland.

Introduction

The devastating 2019–20 bushfires focused public and policy attention once again on what can be done to limit the severity and impact of bushfires, and in particular, the role of fuel reduction burns.

Deliberate or prescribed burns can be conducted for a range of purposes, including reducing fuel. A useful definition of prescribed burning is the ‘controlled application of fire under specified environmental conditions to a pre-determined area and at the time, intensity, and rate of spread required to attain planned resource management objectives’.[11] Prescribed burning can be conducted for ecological reasons (such as to stimulate the growth of certain species) or cultural reasons, but is most commonly done to reduce bushfire risk by reducing the fuel available for future bushfires.

Fuel reduction burns influence the danger and intensity of fires by reducing the fuel element of the equation. Managing fuel is the only practical option available to fire managers to modify fire behaviour, because the other major influences – weather and topography – are beyond human control.[12]

The broad method of reducing fuel can fit within multiple strategies or doctrines of fire management. For instance, as discussed below in ‘Policy’, some fuel reduction burning is aimed at slowing the spread of existing fires across a broad landscape to give more time for suppression; other burning is conducted in a concentrated manner close to homes and other vulnerable structures to protect specific assets. These are not necessarily mutually exclusive strategies, but these distinctions are not always grasped in public discussion.[13] 

The politics of fuel reduction burning can be contentious in Australia. This is not new – fuel reduction burning has long been controversial in Australia.[14]

This controversy extends to researchers and practitioners who may have substantial differences of opinion regarding the efficacy and place of fuel reduction burning. Part of the reason for this is that, for fairly obvious reasons, it can be challenging to conduct experiments which measure the efficacy of fuel reduction burning in the extreme conditions which cause most bushfire damage. Much research thus relies on case studies, expert opinion, retrospective analyses from historical bushfires, and computer modelling.[15] Additionally, the broad spectrum of researchers with expertise over fire means there is wide diversity in methodological approaches.[16]

The general conclusion is that fuel reduction burns are effective at reducing risk in conditions that are in the Low, Moderate, or High Forest Fire Danger Index range, but increasingly less effective as conditions become more challenging. A 2003 review of the effectiveness of fuel reduction burning stated:

The best results of prescribed fire application are likely to be attained in heterogeneous landscapes and in climates where the likelihood of extreme weather conditions is low. Conclusive statements concerning the hazard-reduction potential of prescribed fire are not easily generalised, and will ultimately depend on the overall efficiency of the entire fire management process.[17]

A Parliamentary Library publication in 2002 entitled Bushfires: Is Fuel Reduction Burning the Answer? discussed the use of low intensity burns to reduce the fuel load in forests, thereby reducing fire risk, and also provided background about bushfires and outlined the complexities of fuel and fire.[18] Since that paper was published, considerable further research has been conducted into fuel reduction burning. This was due in part to a series of devastating bushfires in 2003, 2007, 2009, and 2019–20, and to a proliferation of fire research including that by the Bushfires Cooperative Research Centre and its successor, the Bushfire and Natural Hazards Cooperative Research Centre.[19]

This paper examines some of the issues surrounding fuel reduction burning in south-eastern and south-western Australia. Key issues include: the effectiveness of fuel reduction burns; their impact on biodiversity; and the implications of climate change on fuel reduction burning.

This paper’s analysis is limited to forests and woodlands in southern Australia. This paper does not analyse Indigenous burning. While some fuel reduction burning practitioners have claimed continuity between pre-colonial Indigenous fire practices and contemporary fuel reduction, this is controversial among both contemporary Indigenous cultural fire practitioners and non-Indigenous fire managers.[20]

Understanding fires

Assessing the effectiveness of fuel reduction burns needs to begin with understanding fires and the characteristics that shape how hot they burn and how fast they spread. Fire scientist Ross Bradstock has conceptualised four ‘switches’ that must be ‘on’ for bushfire to occur: sufficient biomass must have been produced; the biomass must be available to burn; there must be fire‑conducive weather; and there must be ignition.[21] Once ignited, CSIRO explain that the three factors that contribute to fire behaviour are: weather; terrain; and vegetation (fuel).

The weather component that fire danger warnings are based on considers:

• wind speed
• air temperature
• relative humidity and
• recent rainfall.

The terrain also affects fire significantly as the steeper the land, the faster the bushfire will spread up it; for every 10 degrees in uphill slope, the speed of a fire will double.[22] Conversely, fire travelling downhill will tend to move more slowly; for every 10 degrees of downhill slope, the fire will halve its speed.[23]

Fuel is measured in terms of its density and composition. It is usually conceptualised as different layers: surface fuels (for example, leaf litter), elevated fuels (for example, shrubs), bark fuels, and the canopy or ‘crown’ (in the overstorey) as shown in Figure 1 below:

Figure 1: Visually distinct fuel layers within an open eucalypt forest in southern Australia

Source: Reprinted from W. Lachlan McCaw, ‘Managing Forest Fuels Using Prescribed Fire – A Perspective from Southern Australia’, Forest Ecology and Management, The Mega-fire reality, 294 (15 April 2013): 218, with permission from Elsevier.

These three core variables interact in complex ways to shape fire behaviour, which is critical to determine how much damage a fire does and whether it can be controlled (or ‘suppressed’). Once a bushfire reaches a certain intensity, suppression becomes very difficult, if not impossible.

Early Australian fire science used the basis that fuel load (mass per unit area) was the only fuel characteristic required to predict fire behaviour. However, as described below in ‘Fuel’, more recent science has drawn attention to fuel structure as an additional factor behind fire behaviour.

Effectiveness of fuel reduction burns

Suppression

Fire intensity is a measure of the physical properties of a bushfire which greatly affects the likelihood of suppression. As a bushfire’s fire intensity grows, effective suppression of the fire becomes less likely. The upper limit for direct suppression of fires is considered 3–4,000 kW/m. Above 10,000 kW/m, firefighting actions are considered ‘futile’, including firefighting along the flanks and back of the fire.[24] The likelihood of effective suppression can also be affected by other factors, including flame height and the density and extent of spotfires.

A method of representing suppression difficulty comes through the Fire Danger Rating, shown below in Table 1. Fire Danger Ratings are plain language categories (Low–Moderate, High, Very High, Severe, Extreme, Catastrophic) intended to ‘give the general public an indication of the fire danger and the consequences and risk to life should a fire start’.[25] The terminology for these ratings varies on a state-by-state basis however the Australian Fire Danger Rating System is being introduced to create a nationally consistent system.[26] The ratings are based on the Fire Danger Index or Forest Fire Danger Index (there is an equivalent Grass Fire Danger Index which is out of the scope of this paper). Table 1 shows approximate fire suppression thresholds in a forest with an available fuel load of 20 tonnes/hectare:

Table 1: Fire suppression thresholds, Fire Danger Index, and Fire Danger Rating

FDI	Flame Height (m)	Radiant Energy Released (kW/m)	FDR/Method of attack
0–12	0–0.5	0–50	Low: fires generally self-extinguish or hand tool line will hold the fire
12–15	0.5–1.5	50–500	Moderate: Offensive operations usually possible in bush fuels. Most properties usually defendable
15–25	1.5–3.0	500–2,000	High: fire too intense for direct attack. Parallel attack recommended
25–50	3.0–10	2,000+	Very High: Crown fire at upper intensities. Indirect attack recommended
50–75	10+	12,000–18,000	Severe: The fire may be worse than anything previously experienced. Actions should be focused on safeguarding people and defensive operations. Offensive operations may be possible at night
75–100	12+	18,000–25,000	Extreme: As for Severe but crew and public safety becomes a major concern. Safeguarding refuges and defensive operations may be the only safe options
100+	15+	25,000+	Catastrophic: Fire Behaviour is extremely dangerous, devastating and difficult to predict. Expect significant ember attack. Actions must focus on safeguarding lives.

Source: NSW Rural Fire Service, ‘Fire Danger Index (FDI) and Fire Danger Ratings (FDR)’. © State of New South Wales (NSW Rural Fire Service).

Unfortunately, bushfires in Australia (particularly in the south-east) have historically reached fire intensities well beyond the upper bounds of firefighting limits. For instance, bushfires burnt over 1.6 million hectares of forests, woodlands, and alpine vegetation in Victoria, NSW and the ACT in January and February 2003. There were three particularly significant runs of fire across these months during periods of persistently Very High and Extreme fire danger (FFDI>25). For instance, on 29/30 January 2003, the fire grew by 251,964 ha in just 9.5 hours. This was aided by atmospheric instability and fire-induced lightning, with an average fire intensity of 65,000 kW/m.[27] A large proportion of this area experienced a crown fire or crown scorching.[28] During their major run, the 1961 Dwellingup bushfires in Western Australia reached an intensity of at least 15,000 kW/m.[29] The Forest Fire Danger Index reading for this period was later calculated as 31; the 2009 Black Saturday bushfires in Victoria occurred during a Forest Fire Danger Index of 172 and reached a fireline intensity that peaked at 88,000 kW/m.[30] As can be seen, historical bushfires in Australia became so large and intense that suppression was impossible, and the implication of this is that there is every chance it will be impossible to suppress future bushfires of similar intensities.

However, bushfires do not experience the same conditions throughout their life. While suppression might be difficult or even impossible if a fire grows large enough and experiences the right weather conditions to make a major run, suppression is far more achievable during the early phase of a fire or while conditions are more favourable. Consequently, one of the major principles behind incorporating fuel reduction burning into land management strategy is that it may bolster the chances of effective suppression.[31] The argument is generally not that fuel reduction burning on its own will stop a fire—though there are recorded cases where this has occurred—but that it will reduce the intensity of a fire, reducing spotting, slowing its growth and extending the window for ground crews or air attack to suppress the fire or light backburns.[32]   

Fuel

As described above (Figure 1), forest fuels are categorised into different forms: surface; near‑surface; elevated; intermediate; and overstorey. In addition to this, bark on standing trees can be an essential component when determining the ‘fuel’ available to a bushfire. Many Australian eucalypt species produce bark which can carry fire vertically into a forest canopy or which provides a ‘source of firebrands that can propagate spot fires ahead of the flame front’.[33]  

The relative contribution of different types of fuel to bushfire behaviour is an active field of research. Early Australian fire science predicted bushfire rate of spread based on the fuel load of surface fuels.[34] This research proposed that if the rate of spread was directly proportional to fuel load, reducing the fuel load by half (through prescribed burning) would halve the rate of spread, and in turn reduce the intensity of the fire four-fold.[35] This formulation, combined with endorsement from the Rodger Royal Commission in Western Australia which followed the 1961 Dwellingup bushfires, provided a ‘simple but powerful argument to support fuel reduction practices in eucalypt forest in Australia for more than 30 years’.[36] However, there was little published data to support this proposed relationship, and empirical studies have shown the predicted rate of spread and fire intensity was much lower than what was actually experienced under more extreme conditions.[37]   

Project Vesta was conceived to resolve these issues as a seven-year collaborative research project by the CSIRO, the WA Department of Environment and Conservation and other state agencies (1996–2003). The Project investigated the behaviour and spread of bushfires in dry eucalypt forest in a variety of different fuel ages and understorey structures. One of the aims of the study was to quantify the changes in fire behaviour in dry eucalypt forest over time since the last fire, to improve the models used to predict fire spread and behaviour.[38] Experimental fires were lit simultaneously, in summer and under a high drought index, at two jarrah (Eucalyptus marginata) forest sites with different understorey characteristics: one with fuels dominated by litter and low shrubs, and the other dominated by tall shrubs.[39]

The study ‘set out to establish the relativity between fire spread and fuels of different age and identify those fuel characteristics which can be best correlated with forward spread’.[40] It aimed to identify the impact of fuel structure (the spatial arrangement of different types of fuel) as well as fuel load (total amount of flammable matter in the forest) on fire spread and flame height. The fuel structure, and how it changes with age from last burn, was examined to see how it affected fire behaviour:

In order to explain the changes in fire behaviour as fuels increased with age it was necessary to first develop measures of fuel structure that were suitable for field use and then examine the relationship between fire behaviour, wind speed and fuel structure.[41]

This level of experimental burning under these fire weather conditions had not been carried out before in Australia. However, these experiments were carried out in Moderate to High fire weather conditions, with a Forest Fire Danger Index (FFDI) of less than 25, rather than in Very High to Severe conditions, where the FFDI is greater than 25.[42] As described above, it should be noted that these Moderate to High fire weather conditions are the conditions when the majority of fire suppression is able to be carried out.

Project Vesta showed that the change of fire behaviour after a fuel reduction burn will depend ‘not only on the total quantity of fuel removed but also on how the fuel structure is altered’.[43] The Project enabled the development of tools to visually assess ‘hazard scores’ across different fuel layers, taking into account fuel structure and arrangement in addition to fuel load.[44] Some researchers have argued that fuel hazard guides (and predictions of fire behaviour) should also place greater emphasis on the dominant species of the forest, as vegetation structure and plant traits (especially leaf structure) vary considerably.[45] This remains an active topic of research.

Determining the effect of different types of fuel is important for planning fuel reduction programs, as fuels recover and reaccumulate at different rates. Surface litter may recover very quickly—within 2–5 years after a fuel reduction burn. In contrast to this, bark may take much longer to reaccumulate after a prescribed burn.[46] Project Vesta, for instance, showed that the effect of the burn on fire behaviour will persist longer than that just indicated by surface fuel accumulation: in both forest types studied in these experiments, there was still a reduction in fire behaviour for 10–15 years compared to long-unburnt forest.[47]

It should be noted that a fuel reduction burn does not necessarily have to consume literally all the fuel available in the desired layer to be regarded as effective.[48] On the other hand, burns of very low intensities may fail to affect bark fuels.[49]   

Weather

Weather is a critical variable in assessing the effectiveness of fuel reduction burning as a strategy. This applies both to establishing the constraints which restrict planning and conducting a burn, and in assessing how areas subjected to previous fuel reduction burns behave when threatened by a bushfire under extreme weather conditions.

Fire managers must aim to conduct their burns in a weather envelope or window of desirable conditions. Stray too close to the lower bounds of the envelope and the fire may not consume enough fuel to be regarded as effective. Stray too close to the upper bound of the envelope and the burn may escape.[50]

As discussed above in the ‘Suppression’ section, the degree of effectiveness of fuel reduction burns in challenging weather is important as such weather is certain to reoccur in southern Australia. Unfortunately, for fairly obvious reasons, empirical experiments of fire behaviour under extreme conditions have been limited. Project Vesta was a highly significant experimental study of fire behaviour and fuel reduction, however, these experiments were carried out in Moderate to High fire weather conditions.[51] While these experiments yielded important insights and were conducted under FFDI conditions similar to when the majority of fire suppression is able to be carried out, some historical bushfires have vastly exceeded these conditions. Therefore, researchers have generally been forced to rely upon reconstructions of bushfire activity.

An earlier paragraph described the intensity of the 2003 fires in south-eastern Australia. Studies which have examined the impact of fire weather upon these fires provide an opportunity to assess the performance of fuel reduction burning. Almost half the area in north-eastern Victoria was severely burnt resulting in the top canopy of vegetation being completely removed, either through crown fire or crown scorching and resulting leaf loss.[52] About 200,000 ha of the burnt area had been burnt within the previous ten years, of which 93% was from prescribed burns.[53] As described above, the fire rapidly expanded on particular days, and this expansion coincided with periods of Very High to Extreme fire danger.[54]

Satellite imagery was used to map the severity of the fires and compare these with the areas of recent fuel reduction burns from the fire history records of the Victorian Department of Sustainability and Environment. The study found that there was a roughly 15% reduction in fire severity in areas burnt just one year prior to the 2003 fire and this decreased with increased age: ‘by the time previous fires were 10 years old, there was no consistent reduction in fire severity due to burn age’.[55] The results further showed ‘there was no clear effect of previous fires on the fire severity in areas burnt more than 10 years previously’.[56] However, it was also found that ‘as the severity of the fire behaviour decreased, the benefit to the recently-burnt area became greater’.[57]

The study concluded that the greatest effect of fuel reduction burns ‘on fire suppression and severity is while the fire is still developing and small, and when the weather conditions moderate and the fire intensity is reduced’.[58] The authors note that:

A reduction in fire severity means that fires will take longer to reach their peak fire behaviour under severe weather conditions, increases the period of time when fires are controllable and reduces their effect on soils, fauna and flora.[59]

The 2009 Black Saturday bushfires in Victoria burned nearly half a million hectares and killed 173 people on a single day. Extremely dry fuels and strong winds produced fireline intensity peaking at 88,000 kW/m, rate of spread peaking at 9.2 km/h, and spot fires igniting up to 33km from the main fire.[60] Some researchers have stressed the importance of a powerful wind change in the late afternoon that caused high fireline intensities and ‘profuse’ spotting.[61] Analysis of fuel reduced areas in the path of the Kilmore East fire on Black Saturday following the wind change showed they ‘had little impact on its overall spread’, but satellite data showed that fire intensity had been reduced both within those areas and downwind of them.[62] Some researchers argued that this ‘partial diminution’ of fire intensity and rate of spread was not enough to enable safe and effective suppression.[63] The Expert Panel advising the 2009 Victorian Bushfires Royal Commission stressed that while ‘previous burns did not mitigate the immediate impacts under the most severe conditions’, some fuel reduction burning ‘had significantly assisted in ultimate fire containment’.[64] Thus while fuel reduction burning may only have limited effectiveness for suppression and reducing ecological damage during extreme conditions, these conditions do not last forever, and fuel reduction burns are helpful before and after these conditions occur.

This led the 2009 Victorian Bushfires Royal Commission to conclude:

Extreme weather is the predominant influence on the likelihood that a crown fire will develop, followed by forest type then fuel age. In contrast, for more moderate and low weather conditions fuel age has a significant effect on the fire being confined to the understorey. This means that there is a significantly greater chance of effective suppression … The effectiveness of prescribed burns is strongly contingent on weather.[65]

This emphasis on the weighting of weather is matched by a modelling study of bushfire, which concluded:

Weather and ignition management effort were more important than fuel management approach and effort in determining total area burned in five landscape fire models.[66]

Studies are ongoing on the effect of the 2019–20 bushfires and it is likely that full analysis of fire severity will not be available for some time. However, one early study examined how the fires behaved in areas that had been subject to fuel reduction burning. It found that in approximately half of the 307 fuel reduction burns examined in NSW and Victoria ‘there was a statistically significant decrease in fire severity’ with more recent burns having a greater impact.[67] However, this decrease may not have been strong enough to make fire suppression practical and many burns had no statistically significant impact.[68]

The Royal Commission following the 2019–20 fires consistently heard expert evidence that these bushfires ‘exposed gaps in the scientific understanding and ability to predict fuel behaviour under extreme weather conditions’.[69] The Commission heard that in extreme weather conditions, ‘fuel loads do not appear to have a material impact on fire behaviour’.[70]

Frequency, season and timing

Fuel reduction burning managers and policymakers must also consider the frequency, seasonality, and timing for burns. The ‘inhibition period’, or period in which prior burning has had a measurable effect upon bushfire hazard, varies between forest types, implying that the frequency of burns may not be uniform for different forests.[71] In some vegetation types, recovery from fire ‘can be so fast that fuel management may be futile or even counter-productive’.[72] This has been theorised to apply to Allocasuarina verticillata (drooping sheoak) shrubland surrounding Hobart, as a low intensity burn will consume surface fuel but scorch low-hanging branches which will then drop their dead needles, recreating a litter bed.[73] However, such rapid reaccumulation is not universal and the effect of fuel reduction burning will persist in many Australian forest types for some years.

Project Vesta, for instance, found that while all the fuel variables (surface fuels, near-surface fuels, elevated fuels, intermediate tree and canopy and overstorey tree and canopy) increase with time since fire, some reach an equilibrium level after 7–8 years while others, such as near surface fuel hazard, continue to increase for at least 15 years after the fire.[74] The inhibition period for jarrah forests in Western Australia has been quantified at 6 years, while Victorian mountain forests can have shorter inhibition periods at 4 years.[75] The Expert Panel assembled for the 2009 Victorian Bushfires Royal Commission concurred:

Reduction in the rate of spread of fire will persist as a consequence of prescribed burning for five to eight years. Reduction in flame height, firebrand prevention, and less spotting downwind of the fire are effects of prescribed burning that last longer than five to eight years. There is congruence among the studies of vegetation for eucalypt forests suggesting that ‘the period of five years matters’.[76]

Other studies have investigated the effects of the seasonality of burns. For example, Project Vesta suggested that mild spring burns should be alternated with hotter autumn burns in order to balance fauna management with effective reduction of bark.[77]

Timing also presents complexities for fuel reduction burning, as there are limited windows in which burning is regarded as possible and these windows vary on a local basis. In 1965, a forest manager in jarrah forests in south-western Australia speculated that there were perhaps 45 days per year suitable for prescribed burning.[78] Fire scientist Phil Cheney has commented that the south-west is ‘blessed with the most benign burning conditions that you would find anywhere in Australia, if not the world’.[79] In contrast to this, an Inquiry following the 2002–03 bushfires in Victoria reported that the window in parts of Victoria averaged just 10 days per year.[80] These figures are intended as illustrative, not exhaustive, as the governing factors behind them (including smoke concerns, favourable prevailing winds, fuel moisture, and risk management) vary on a local basis. Additionally, the average window within a region may significantly vary on a year-to-year basis, with few studies available which quantify this interannual variability.[81]

One of the most enduring and controversial questions in Australian environmental history relates to the long-term effects of fuel reduction burning: does reducing fuel through burning make plant communities more flammable in the long term? This question has been passionately debated since at least the 1939 Black Friday bushfires.[82] It remains controversial and researchers debate whether the same answer applies to all forests.  

While fuel reduction burning ideally produces fires of lower severity than bushfires, there are concerns that programs of fuel reduction burning may result in feedback loops—a salient reminder that fire must be considered in terms of fire regimes, rather than individual events. Certainly, there is strong evidence that high severity fire can create feedback loops.[83] Frequent fires may cause shifts in understorey species composition to favour herbaceous species over shrubs. They may also expose the understorey to more wind by reducing the cover provided by overstorey species. Therefore:

Repeatedly burning a site at short intervals might result in a feedback loop where the site becomes more flammable and better able to support a higher frequency of fires.[84]

Much research assumes that ‘forests beyond a certain age are always more flammable than young forests’.[85] As discussed, Project Vesta showed that some fuels reach a level of equilibrium within 7–8 years after a fire, while others continue to increase for at least 15 years after a fire.[86] Critically, it found that surface fuels initially rapidly recovered within 6 years, then steadily accumulated up to 10 years, and continued to increase at a slowed rate for at least 25 years.[87]

In contrast to this, some researchers have argued that some forests, if unburnt for sufficiently long, will represent less of a fire hazard:

For example, after a period of 40 years without fire in a snow gum (Eucalyptus pauciflora) community, shrubs begin to senesce [dry out with age] and, in the following decades, a grass dominated understorey will develop … [However, if a fire reaches these communities it will lead] to increased shrub density, thereby increasing flammability.

Our results … [found] that long-unburned sites supported lower overall fuel hazard than sites with an intermediate time-since-fire.[88]

Figure 2 below shows a representation of the likelihood of fire in an alpine ash (Eucalyptus delegatensis) forest over time:

Figure 2: Alpine ash forest flammability dynamics over time

Source: Philip Zylstra, ‘Contrary to Common Belief, Some Forests Get More Fire-Resistant with Age’, The Conversation, 17 April 2018, with permission from author.

Modelled and empirical results such as those mentioned above have led some researchers to suggest:

… potential management options to reduce overall fuel hazard are: (1) to burn the landscape more frequently; (2) to manage for a transition of a greater percentage of the landscape to long-unburned; and/or (3) to focus fuel management treatment closer to assets, thereby reducing the area that overall fuel hazard needs to be intensively managed.[89]

As noted by historians, the contours of these debates are recognisable over at least the last 80 years of Australian history.[90] Part of the issue is that the timeframes proposed for forests to become more fire-resilient are on the scale of decades. Many studies are only able to use fuel data collected over timeframes of 20–30 years.[91] However, the proportion of Australian forests which have remained long-unburnt has shrunk dramatically in recent decades, allowing for fewer opportunities for comparative studies.[92]

Size and placement

In addition to considerations around frequency and timing, fuel reduction burning effectiveness is also governed by spatial dimensions. In the words of the Expert Panel advising the 2009 Victorian Bushfires Royal Commission: ‘size does matter’.[93] Fuel reduction can reduce the ‘take-up rate of fire’ through lightning or spotting.[94] This can be important as ‘spotfires are the primary mechanism by which forest fires breach barriers in fuel such as road and firebreaks’.[95] However, for a fuel reduction burn to be effective in reducing the density of ignitions started by spotting, it needs to burn a sufficiently large area. The Expert Panel advised this would require a minimum area of 1,000 hectares to capture most embers falling within three kilometres of a bushfire.[96] Additionally, fuel reduced areas that are too small can be ‘ineffective if they are readily outflanked by a bushfire burning through heavy fuel in adjacent areas’.[97]

Similarly, the placement of burns matters. On a micro scale, the intensity of a fuel reduction burn (and its resulting effect on ecological communities) can be manipulated by igniting with or against winds, burning up or down slope, igniting in lines or as points.[98] Fuel reduction burns can be strategically placed to synergise with natural barriers such as rock outcrops,[99] or with artificial barriers such as fuel breaks or roads.[100] On a macro scale, ‘the key to a burning program for wide‑scale protection is to have the blocks strategically located across the landscape in a pattern [such] that, when repeated, large fires are going to sooner or later run into one of these low fuels and be checked’.[101] This is known as the ‘encounter rate’ and is critical in assessing the effectiveness of broadscale strategies for fuel reduction burning.[102]

Differences in the effectiveness of fuel reduction burning between forest types

All the above factors interact such that the effectiveness of fuel reduction burning varies greatly on a regional basis. The differing inhibition period (how quickly fuel grows back), burn windows (when and how often burns can be conducted), topographies (how flat or rugged the terrain is, and how this complicates burning), and the relative vulnerability of assets (how many houses you need to potentially protect while conducting a burn), all contribute to this variability.

One measure that has been developed to compare the effectiveness of fuel reduction burning on a broad-based strategy between regions is ‘leverage’: a ‘regional-scale measure of the effectiveness of fuel treatment’ which seeks to establish ‘the reduction in area of unplanned fire that results from the prior treated area’.[103] Or, the quantification of ‘the total area protected from high-intensity wildfire per unit area treated by fuel reduction measures’.[104]

Leverage can greatly vary. In the savannah woodlands of Western Arnhem Land in the Northern Territory, leverage is 0.9–1.0, which means that one hectare of fuel reduction burning reduces the annual extent of bushfire by one hectare,[105] while in the jarrah forests of south-western Australia, leverage is 0.25, meaning that four hectares must be subjected to fuel reduction burning to reduce the annual extent of bushfire by one hectare.[106] A study examining 30 bioregions in NSW, Victoria, and South Australia found that in 16 of these regions, there was no evidence of leverage, and that only four bioregions had leverage—that is, that ‘a negative effect of previous fire on area burnt by unplanned fire in any given year was present’.[107] Comparisons of leverage have led some researchers to argue against the notion that fuel reduction burning is ‘universally effective’.[108]

Policy

The above factors are some of the mechanisms which underpin the variation between fuel reduction burning policies and doctrines. This variation is not always appreciated in public or policy debate.

Some researchers have argued fuel reduction should be targeted primarily to reduce house loss. As researchers Danielle Clode and Mark Elgar write, ‘most houses are lost through ignition by flying embers rather than direct flame contact’ and, critically, ‘the likelihood of ignition from embers decreases exponentially with distance from the fire front’.[109] During the Victorian Black Saturday fires of 2009, 90% of houses destroyed were within 100 metres of vegetation.[110] A study was conducted following Black Saturday using statistical analysis and modelling to determine the effectiveness of various measures in preventing house loss. This study found that an effective method to reduce the likelihood of house loss under Catastrophic fire weather conditions (that is, where FFDI=100) was to reduce the proportion of trees and shrubs within 40 metres of houses from 90% cover to 5% cover — this resulted in a 43% reduction in the likelihood of house loss.[111]

Based on this research, some researchers have found:

… the proximity to houses of prescribed burning is more important than the total percentage of the landscape that is prescribe-burnt. These results are consistent with previous research indicating the effects of prescribed burning can diminish within a short period of time (2–6 years) and in severe fire weather conditions, which are the conditions when most houses are destroyed. Our results therefore indicated that prescribed burning—when executed at the scale observed in this study—was most effective when undertaken close to houses and at least every 5 years.[112]

The study’s authors suggest that more intensive fuel reduction treatments close to houses will be more effective in reducing impacts on housing than broad-scale fuel reduction. Another modelling study also found this style of burning (which it called ‘interface burning’, referring to the urban rural interface) to be more cost effective overall for south-eastern Australia.[113]

In contrast to this style of fuel reduction, many fire managers and researchers point to the use of broad-based (also known as broadscale or landscape scale) burning. Managers of jarrah forests of south-western Australian have been conducting broad-based burning for half a century, providing a valuable data set. Between 5–10% of the area is annually targeted for burns, balancing fuel management, biodiversity conservation, and asset protection. A major analysis of this style of burning in this region found it had significantly changed the distribution and composition of fuel age across the landscape, in turn reducing the incidence and extent of large unplanned fires.[114] Simulation studies comparing landscape and interface burning in the south-west found that interface burning would reduce more damage and risk to houses but be less cost-effective than landscape burning.[115]

This context helps explain calls following major bushfires in the south-eastern states for greater fuel reduction burning. This was particularly loud following the 2009 Black Saturday bushfires in Victoria. The Expert Panel advising the Royal Commission gave a nuanced set of findings on fuel reduction burning, including that an annual target for burning would be useful as a guide but should not be a panacea.[116] The Royal Commission compressed this into Recommendation 56: ‘The State fund and commit to implementing a long-term program of prescribed burning based on an annual rolling target of 5 percent minimum of public land’.[117] The result of this is described below:

The Commission’s Final Report was concluded in July 2010, and the then-government decided to support all recommendations—including Recommendation 56. Over the next few years, the Department of Environment, Land, Water and Planning (DELWP) ramped up its prescribed burning commitment higher than at any point in Victoria over the last two decades; however, it never came within 100,000 hectares of the 380,000-hectare target. After internal and public criticism that the 5 per cent target was not ‘achievable, affordable or sustainable’, in 2015 the Inspector-General for Emergency Management (IGEM) was asked to review the policy. IGEM recommended the hectare-based target be replaced with a strategy based around risk reduction. It argued DELWP now possessed both greater capability to determine the value of burning different areas (at least partly through the aid of computer simulation program PHOENIX RapidFire) and a greater level of ecological knowledge around the effects of prescribed burning on fire-sensitive vegetation and fauna. In 2016, DELWP introduced a risk reduction target which aimed to reduce ‘residual risk’; by 2017, this shift was complete. A simple state-wide measure based on area burned had been replaced with a system where Victoria was divided into seven regions, allowing a complicated measure of risk to be more precisely targeted to each region based on simulation modelling to compare scenarios involving different levels of prescribed burning. The hectare-based target had lasted less than a decade.

There is no doubt that the 5 per cent policy had increased the amount of land prescribed burnt in Victoria; however, it has been argued it led to the wrong kind of burning happening in the wrong places.

…

This perverse policy outcome had been warned against by the Royal Commission’s expert panel. As IGEM noted, the shift to a risk-based strategy represented ‘a shift in focus from activity to outcome’.[118]

The Royal Commission into National Natural Disaster Arrangements reviewed the strategies for fuel reduction burning used by each state as of 2020:

WA, while maintaining a significant focus on the urban interface, highlights the role of a landscape-scale approach designed to create a mosaic of fuel loads across the landscape, driven by fuel age targets…this equates to a nominal 200,000 hectare target…

NSW has a state-wide target to treat ‘135,000 hectares a year at a five year rolling average’. Queensland has a planned burn target of greater than 5% of the total protected area and forest estate…

NSW, SA and Queensland base their approach on historical data on ignitions and fire spread, and judgments on identification and prioritisation of fuel reduction and fire management activities. This does not involve a quantitative calculation of residual risk after mitigation activities.[119]

In contrast to these approaches Victoria, Tasmania, and the ACT use a residual risk approach:

Residual risk is the amount of risk that remains after controls are accounted for – it works to determine a level of remaining acceptable risk. In fuel management it involves calculating bushfire risk using computer modelling by simulating fires and calculating the remaining risk ‘left over’. Victoria, for example, assesses risk by simulating 11,500 fires over the whole landscape and sets a percentage risk target of 70% against which to measure activities.[120]

Such residual risk targets have only been possible via increased knowledge of fire rates of spread and the development and implementation of bushfire prediction, typically through software packages.[121] Such simulation software is used both for predicting bushfires and increasingly to guide fuel reduction burning.[122] Agencies (and many post-bushfire inquiries) have been attracted to such predictive capacity for its potential to enable further risk management, assessments of cost effectiveness, and protection from liability, but prediction software still requires a degree of human judgement and is poor at predicting bushfire behaviour in extreme conditions.[123]

Some researchers have suggested the disagreement between advocates of broad-based and more targeted burning may partly be explained through how different disciplines measure fire damage: it ‘is not a question of whether or not [fuel reduction burning] is effective, but what exactly it is effective at protecting’.[124] Additionally, calls from commentators and the public to increase targets for broad-based burning represent a relatively simple metric to a government agency responsible for public lands. These researchers argue such calls potentially divert public and policy attention from measures which are more complex but more effective at reducing loss of life and houses, such as buy-backs, reforms of planning laws, or changes to building codes.[125]

Fuel reduction burning and biodiversity

Any fire regime, including those with or without fuel reduction burns and those that exclude fire, will have an impact on ecological communities. As bushfire scientist Neil Burrows puts it, ‘Unlike many processes that threaten biodiversity, fire is a natural environmental factor that can threaten or benefit biodiversity’—depending on the fire regime and other interacting factors (such as invasive species).[126] The key question is whether fuel reduction burning (which ideally comprises low intensity, moderate frequency fire) has impacts on ecological communities worse than those from large scale high intensity wildfires.

The negative effects of fire are recognised in Australian legislation: both high frequency fires and inappropriate fire regimes are listed as potentially threatening processes under the Victorian Flora and Fauna Guarantee Act 1988 and the ecological consequences of high frequency fires are listed as a key threatening process under the New South Wales Biodiversity Conservation Act 2016.[127] Fire regimes that cause biodiversity decline were nominated for listing as a key threatening process under the Commonwealth Environment Protection and Biodiversity Conservation Act 1999 in 2007; finalised advice from the Threatened Species Scientific Committee (TSSC) was provided to then Minister Tony Burke in May 2013 and revised advice following consultation from state and territory governments was due in November 2013.[128] In a statement to The Guardian published on 8 May 2020, the Department of Agriculture, Water and the Environment stated that the TSSC ‘is currently updating the draft assessment of “fire regimes that cause biodiversity decline” as a key threatening process in response to the 2019–20 bushfires’.[129]

While a substantial amount of research has been carried out on fire ecology in forest ecosystems, the application of this research to direct fuel reduction burning programs is still limited.[130] Four themes about the ecological effects of fire in south-west Australian forests, where fuel reduction burning research has been carried out for several decades, were outlined in a 2008 paper:

• the biota have evolved in a fire prone environment whereby some plant species are cued or enhanced by fire and specific fire regimes are required to maintain plant communities. However, the response to fire is variable, with some fire resilient communities recovering quickly while other are sensitive to the intensity or frequency of fires, taking decades to recover
• therefore, no single fire regime suits all plant communities
• a mosaic of patches of vegetation of a particular plant community at a variety of different stages of fire recovery benefits biodiversity. This is because such a mosaic provides greater structural and habitat diversity associated with the diversity of successional stages
• there is a need to protect fire-sensitive plants and communities from frequent or intense fires.[131]

While there is limited long-term evidence for positive or negative effects of fuel reduction burning on fauna and flora and it is likely to vary depending on the ecology of the area,[132] a number of general points can be made.

Biota—whether flora or fauna—are not so much adapted to fire, as they are adapted to fire regimes.[133] The overall interactions between fire and biodiversity remain an area of active research and debate. A popular theory is that pyrodiversity promotes biodiversity and thus fire managers should mostly aim to create a diverse spectrum of fuel age (often conceptualised as a ‘mosaic’), though this concept has been disputed.[134]

Fire intervals are important for biodiversity. Fire intervals that are too short or too long at a particular site can lead to local extinctions of plants and, if sustained, inappropriately high or low rates of fire will lead to loss of plant species, changes in vegetation structure and corresponding loss of animal species.[135] Many plant species require a regular cycle of fire to stimulate recruitment of new plants to maintain their population which may be low or absent due to the absence of fire.[136] 

For example, there can be local extinctions of plants referred to as ‘obligate seeders’ (which depend on seeds to regenerate) if two severe fires that kill populations of such plant species occur within the time period necessary for the plants to produce sufficient seed to re-establish a viable population. [137] An example is Eucalyptus delegatensis (Alpine ash) in the Australian Alps, where repeated high-severity fires in 2002–2003, 2006–2007, and 2009 have greatly reduced the number of juvenile trees.[138] This may not happen with species referred to as ‘resprouters’ that survive fires by the activation of dormant vegetative buds to produce regrowth, such as Eucalyptus tricarpa (red ironbark).[139] Even if extinction of obligate seeders does not occur, short fire intervals may cause significant changes in species abundance.[140] Researchers must also consider whether fuel reduction burns might trigger feedbacks and shifts in forest composition, as described above.

Unburnt areas may be disproportionately important, both to protect floral species which are vulnerable to fire, and as refuges for faunal and invertebrate species.[141]

Some animal species may be threatened by infrequent but severe fire, as happened to a colony of New Holland mice (Pseudomys novaehollandiae) in Anglesea, Victoria. This colony had been recently discovered in heathland and there were efforts to introduce a regime of ecological prescribed burning to aid its preservation. However, the 1983 Ash Wednesday fires destroyed the entire habitat and the species became locally extinct.[142] Large bodied mammals are strongly sensitive to fire intensity because they may have few places to shelter from intense fire. As a result, their populations may be depleted or eliminated by large, intense fires.[143]

Even fuel reduction burns of low intensity may have effects on fauna. A study into the collapse rates of hollow-bearing trees following low intensity, prescribed burning concluded that such fires could cause substantial levels of destruction of these trees and therefore significantly impact on animals that depend on such hollows for shelter or as nesting sites.[144] A long-term study in the Wombat State Forest in Victoria found that small mammal populations recovered within two seasons after a low-intensity burn if sufficient refuges were available.[145]

Another aspect to consider is whether fuel reduction burning may act to help or hinder the spread of invasive species.[146] A small scale experiment in the Otway Ranges in Victoria found that by consuming understory cover, a prescribed burn increased habitat suitability for red foxes and feral cats and made native mammals more vulnerable to predation.[147] On the other hand, prescribed burning can be successfully used in isolation or combined with pesticides or manual removal to successfully control invasive plant species.[148] For example, fire can reduce thickets of the invasive species Gorse (Ulex europaeus) and also stimulate seed germination which, in concert, allows for effective spraying which will significantly reduce the Gorse seedbank.[149]

Where intense fires deplete or eliminate populations of species that are sensitive to fire, recovery will depend on immigration of species and their re-colonisation of the area.[150] This takes time and, in turn, depends on the degree of vegetation cover remaining after the last fire.[151] This means the timing and frequency of fires, plus the area of available habitat and the typical size of fires, will influence long-term persistence of species. Isolation from sources of recolonization by these species may also be altered by patterns of human development that modify natural vegetation (such as roads, urban areas, or farms).[152]

There are also concerns around whether prescribed burning has an impact on sediments and water, which can be particularly concerning for those forests used as catchments for urban populations. This impact largely occurs through post-fire erosion and elevated surface runoff, due to the ability of runoff to ‘transport pollutants including sediment, macronutrients, or other volatilised organic compounds into water systems’.[153] It is well documented that bushfires have resulted in erosion and disruption to water supplies in Australia.[154] However, the impact of fuel reduction burning (usually intended to be of lower intensity than bushfires) upon sediments and water is less clear. A 2020 review found that fuel reduction burning had a ‘generally low impact on sediment exports from forested environments’ and that any changes in water pH values as a result of fuel reduction burning returned to baseline within 1–2 years.[155] While fuel reduction burning was found to have increased phosphate concentration and movement, this was far lower than nutrients in runoff from agricultural lands.[156] As with all assessments of the impact of fuel reduction burning, practitioners argue that the impact upon soils and water must be weighed against the impact from unplanned high intensity fires.[157] Impacts from fuel reduction burning can be minimised by careful timing of burns while fuel moisture is relatively high, limiting fire severity, and conducting burns outside wet seasons, allowing vegetation to recover before rainfall.[158]

Researchers have proposed various ways to guide the weighting of such issues including modelling of successional stages and ‘multi-criteria decision making’.[159] Nevertheless, the above factors mean that, in the words of some fire ecologists:

In fire-prone environments, determining the management strategy most likely to achieve an overarching objective is complicated.[160]

This might be called an understatement.

Other issues

Some fire managers have perceived a decline in fuel reduction burning rates in southern Australia and this has been attributed to a range of factors.[161] In addition to the three factors discussed below, changes in suitable weather patterns and ‘a general increase in aversion to risk arising from management activities’ have been proposed as explanatory causes.[162]

Smoke

Some managers have attributed such declines to community concerns around the public health and economic impacts of smoke.[163] As described in another Parliamentary Library publication:

In general, bushfires can cause a range of health impacts beyond death. These can include burns from radiant heat, dehydration and heat exhaustion, smoke inhalation, and the immediate and ongoing effects from trauma—both physical and psychological. The hazard from bushfire smoke and air pollution is of particular concern. In addition to containing pollutants such as carbon monoxide and ozone, bushfire smoke can contain a large amount of small particulate matter—including fine (under 2.5 microns; abbreviated to PM2.5) and ultrafine (under 1 micron) particulate matter. This can disperse far from the fire itself, cause eye irritation and, when inhaled, can penetrate into lungs and enter the bloodstream, inducing physiological responses such as inflammation.

There is evidence to show bushfire smoke causes increased visits to doctors and hospital admissions for respiratory symptoms, particularly for asthma, bronchitis, dyspnea (shortness of breath), and chronic obstructive pulmonary disease. There are also concerns around the effects of bushfire smoke on the cardiovascular system, although evidence to date is inconsistent. Similarly, there is partial evidence of PM2.5 exposure having negative impacts on neurological functions and birth outcomes. However, there are significant knowledge gaps, particularly around the long term effects of bushfire smoke exposure.[164]

All the above impacts apply to bushfires and must be weighed against any assessment of smoke impacts from fuel reduction burning. Unfortunately, most research has tended to focus upon the health impacts from large bushfires rather than from fuel reduction burning.[165] A study investigating PM_2.5 concentrations in the Yarra Valley of Victoria during prescribed burning in 2013 found that there were very high exposures in short-term peaks (some reaching as high as 15 times higher than the daily advisory standards), but that this exposure to smoke was usually of short duration.[166] As with any assessment of fuel reduction burning risks, trade-offs must be considered: does the potential health impact of smoke from fuel reduction burning present an acceptable or unacceptable risk compared to the potential health impact of smoke from a bushfire?[167] A study published in 2016 argued that this kind of evaluation was not yet possible with available data.[168]

Ironically, the very conditions that allow for safe, predictable fuel reduction burning—such as cool, still days—may also cause smoke to concentrate and become trapped. As NSW Rural Fire Service Commissioner Rob Rogers told the Royal Commission into National Natural Disaster Arrangements in 2020:

So it’s called the Sydney basin for a reason, and that is that it’s like a basin and the smoke goes in there and it gets trapped often by an inversion layer overnight and the next morning there’s a heavy layer of smoke over the city.[169]

Smoke from fuel reduction burning also has other economic impacts. Industries such as viticulture, apiculture (beekeeping), and tourism may be affected by this smoke. In Western Australia, fuel reduction burning was challenged in court ‘during litigation to recover commercial losses attributed to the impacts of smoke from prescribed fires on neighbouring vineyards’.[170] In this case, the court determined that it would be ‘unreasonable to impose a duty of care to avoid smoke damage … where it is not always possible to avoid some smoke during the sensitive stages of grape production’.[171]

Responsibility and escaped burns

This paper is not intended to provide an overview of who is responsible for fuel reduction; merely to signal that it is a complex issue. Confusion over responsibility for fuel reduction has long been a feature of Australian bushfire history—a trend confirmed by the 2020 Royal Commission into National Natural Disaster Arrangements.[172] The vast majority of civilian fatalities from bushfire occur on private property, yet most studies overwhelmingly focus upon public lands—indeed, discussion on fuel reduction burning on private land was excluded from the 2009 Victorian Bushfires Royal Commission.[173]

Over time there has been growth of the population exposed to potential bushfire risk. Some fire managers have stated that population growth in the urban rural interface adjacent to public land has caused constraints for agencies.[174] Some agencies have sought to encourage fuel reduction burning on private land through programs such as Hotspots in New South Wales.[175]

Fuel reduction burning also carries the risk of escaped fires. For instance, a prescribed burn at Margaret River in Western Australia escaped containment lines in November 2011, destroying 32 houses and causing hundreds of residents to evacuate.[176] Fire managers point out that as ‘serious as the consequences of these escapes have been, they must in the end be balanced against the risk of not intervening to manage fuels’.[177] Similarly, ‘the more prescribed burning that is done, the easier and safer’ it becomes to do more.[178]

Multiple inquiries have pointed to the need to engage local knowledge of bushfire behaviour in a broad sense.[179] The Australasian Fire and Emergency Service Authorities Council also lists engagement of local knowledge as an important component of its ‘Best Practice Principles for Prescribed Burning’.[180]

Implications of climate change

In 2009, Australian fire scientist David Bowman made the comment that, in light of the Black Saturday bushfires in Victoria and the extreme weather events that led to them, climate change may be driving the greater incidence of extreme weather that promotes fires and that ‘Earth’s life zones’ are set to be reconfigured:

A new concern is that global climate change is driving the greater incidence of the kind of extreme weather that promotes fire …

Those recent wildfires highlighted how closely coupled fire and weather extremes are. A real worry is that such sustained fires are changing fire regimes and thereby changing vegetation types – by selecting for more flammable and fire-tolerant species – and contributing to climate change via massive CO2 emissions and regional climate effects.

The relationship between climate and human activity is hard to untangle, but there are things we know now which, with further research, will help improve models for the Intergovernmental Panel on Climate Change. Among these is that since the start of the industrial revolution, all types of landscape fire combined produced CO₂ emissions equal to 20 per cent of those from burning fossil fuels. Fire also influences climate by releasing black-carbon aerosols which absorb heat from the sun. These may have the strongest effect on global warming after CO₂ levels …

Fire begets fire. Fundamentally, adaptation demands we rethink our place in flammable landscapes. Like it or not, fires are going to change the way we live and where we live.[181]

Australia’s climate has warmed, on average, by 1.44 ± 0.24°C since 1910.[182] There has been a substantial decline of rainfall in southwest and southeast Australia, along with declines in streamflow. Without significant reductions in global greenhouse gases, Australia’s climate is projected to continue to warm.[183]

There has been a long-term increase in fire weather in the last half-century, with an increase in the frequency of dangerous fire conditions and the most significant changes occurring during spring and in the southern half of Australia.[184] One study sought to disentangle the influence of climatic drivers including the El Niño Southern Oscillation (ENSO), the Southern Annular Mode (SAM) and the Indian Ocean Dipole (IOD) upon this observed trend; this analysis determined that the upward trend in fire weather is ‘most likely due to anthropogenic climate change’.[185] As the Royal Commission into National Natural Disaster Arrangements reported, ‘Climate change has already increased the frequency and intensity of extreme weather and climate systems that influence natural hazards’.[186]

As discussed above, fuel reduction burning is of diminishing effectiveness as weather conditions grow more severe. Climate change is projected to further increase the number of days with FFDI indexes of Severe or greater.[187] This includes projected increases in the conditions which lead to pyrocumulonimbus (pyroCB) firestorm events, where fire-generated thunderstorms cause unpredictable fire behaviour.[188] Therefore, fuel reduction burning may be less helpful in an overall sense, and fire managers may need to emphasise other strategies over fuel reduction burning.

While declines in rainfall might lead to less fuel accumulation through reduced vegetation growth, this calculation must also take into account that increased carbon dioxide concentrations can change the rate and amount of vegetation growth.[189] Furthermore, hotter and drier conditions dry out fuels and expand the areas that can burn. This was vividly illustrated by the 2019–20 bushfires, where prolonged drought led to a bushfire season which burned more than 21% of Australia’s temperate broadleaf forests.[190] This was a globally unprecedented proportion of any continental forest type to burn in a single season.[191]  

Climate change is also expected to lead to shifts in species distribution, which will further impact fuel reduction strategies. For instance, modelling of the distribution of two Eucalyptus species in Tasmania under future climate scenarios showed profound changes, projecting some expansion in some areas but much greater contraction in other areas.[192] This will, in turn, have conservation implications in terms of whether species regrowth and recovery will be possible. Additionally, species that have traits favourable to fuel reduction burning may be replaced by species for which fuel reduction is less favourable. While species distribution will also be affected by factors other than climate change, this suggests that fire managers will need to be flexible and adaptive to changes in species distribution.

The fire season has also lengthened in many areas.[193] A paper published in 2007 noted that the fire season in Melbourne, Adelaide, Canberra and Wagga Wagga had increased by between 2–6 days each year.[194] In 2020, the Bureau of Meteorology gave evidence to the Royal Commission into National Natural Disaster Arrangements that, for eastern Victoria and south-coastal NSW, fire weather has ‘started arriving three months earlier in the year this century, when compared to [the] middle of last century’.[195] The most recent set of simulations project that fire seasons in southern and eastern Australia will continue to lengthen.[196] This might also result in competition for resources: personnel with fire expertise may be diverted from igniting controlled fires to fighting uncontrolled fires.

Evidence for how climate change might affect the window for suitable conditions for hazard reduction burning is more ambiguous and highly regionally variable. One study projected that some regions of south-eastern Australia would experience declines in suitable conditions (such as central and northern coastal NSW during autumn) but that other regions would experience increases in suitable times. The researchers stressed that interannual variability would increase under their modelled scenario, increasing the complexity of planning and executing fuel reduction burning.[197] Another study projected that between 2060–2079, south-eastern Queensland, coastal NSW, and parts of coastal South Australia would experience ~50% reduction in average burn windows, but that much of Victoria would experience an increase in its April and May burn windows.[198] Such variation in burn windows across regions illustrates how the effects of climate change are projected to be diverse across Australia: different regions may face different fire futures.

Conclusion

A number of general conclusions emerge from the vast accumulation of research and experience:

• Fuel reduction burns are effective in reducing the intensity of unplanned fires. Effective fire suppression is typically enhanced by fuel reduction burning.
• Fuel reduction burning is most effective when conditions are moderate.
• Recent fuel reduction burns will modify fire behaviour even under extreme or catastrophic conditions, but under such circumstances, this modification may not be strong enough to assist fire suppression efforts. However, it may have a measurable impact upon fire severity, and reduce ecological damage.
• The persistence of the effect of fuel reduction burning on different fuel layers is variable. Surface fuels may recover more quickly than bark.
• There are significant knowledge gaps, especially about the long-term effects of fuel reduction burning, and about fire behaviour under extreme conditions.
• An effective method to reduce the likelihood of house loss from bushfire is to reduce vegetation close to houses.
• There are distinct differences of opinion among researchers and practitioners about whether it is more desirable to concentrate fuel reduction burning close to assets (such as houses) or to conduct broad-based fuel reduction to slow the spread of fires.
• The effectiveness of fuel reduction burning varies greatly by region.
• The Australian biota is adapted to fire regimes. Fuel reduction burning will have an impact on biodiversity—as will high-intensity bushfires or the complete suppression of fire.
• There has been an increase in fire weather in Australia in the last half-century, and this is projected to accelerate. Similarly, the fire season has lengthened, and is projected to continue to lengthen.
• The extension and intensification of the fire season may reduce the opportunities for fuel reduction burning to be used and impact its effectiveness. Climate change may therefore present substantial challenges to fuel reduction burning and fire management more generally.

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[1] All links valid as at June–July 2021.

[2] Unless otherwise noted, definitions come from: Australian Rural and Land Management Group, ‘Bushfire Glossary’ (Australasian Fire and Emergency Services Council, January 2012).

[3] David Bowman, ‘Explainer: Back Burning and Fuel Reduction’, The Conversation, 8 August 2014.

[4] NSW Rural Fire Service, ‘Fire Danger Index (FDI) and Fire Danger Ratings (FDR)’, 24 April 2017.

[5] Some researchers argue forcefully that crown scorch is not synonymous with crown fire; see Grant Wardell-Johnson, James Watson, Michelle Ward, and Philip Zylstra, ‘Native Forest Logging Makes Bushfires Worse – and to Say Otherwise Ignores the Facts’, The Conversation, 20 May 2021.

[6] Australian Institute for Disaster Resilience (AIDR), ‘Understanding the Australian Fire Danger Rating System’, AIDR website, n.d.; Royal Commission into National Natural Disaster Arrangements, Mark Binskin, Annabelle Bennett, and Andrew Macintosh, ‘Royal Commission into National Natural Disaster Arrangements Report’ (Commonwealth of Australia, 2020), 291–95.

[7] Jon E. Keeley, ‘Fire Intensity, Fire Severity and Burn Severity: A Brief Review and Suggested Usage’, International Journal of Wildland Fire 18, no. 1 (2009): 116–26.

[8] NSW Rural Fire Service, ‘Fire Danger Index (FDI) and Fire Danger Ratings (FDR)’; M. P. Plucinski, A. L. Sullivan, and W. L. McCaw, ‘Comparing the Performance of Daily Forest Fire Danger Summary Metrics for Estimating Fire Activity in Southern Australian Forests’, International Journal of Wildland Fire 29, no. 10 (2020): 926–38.

[9] NSW Rural Fire Service, ‘Standards for Low Intensity Bush Fire Hazard Reduction Burning (for Private Landholders)’, n.d.

[10] NSW Rural Fire Service, ‘Fire Danger Index (FDI) and Fire Danger Ratings (FDR)’.

[11] Australasian Fire and Emergency Service Authorities Council and Forest Fire Management Group, ‘Overview of Prescribed Burning in Australasia’, Report for the National Burning Project - Subproject 1, 2015, 9.

[12] Paulo M. Fernandes, ‘Empirical Support for the Use of Prescribed Burning as a Fuel Treatment’, Current Forestry Reports 1, no. 2 (2015): 118.

[13] Daniel May, ‘Shallow Fire Literacy Hinders Robust Fire Policy: Black Saturday and Prescribed Burning Debates’, in Disasters in Australia and New Zealand: Historical Approaches to Understanding Catastrophe, ed. Scott McKinnon and Margaret Cook (Palgrave MacMillan, 2020).

[14] Daniel May, ‘To Burn or Not to Burn Is Not the Question’, Inside Story, 17 January 2020; Stephen J. Pyne, The Still-Burning Bush (Melbourne: Scribe Short Books, 2006).

[15] James M. Furlaud, Grant J. Williamson, and David M. J. S. Bowman, ‘Simulating the Effectiveness of Prescribed Burning at Altering Wildfire Behaviour in Tasmania, Australia’, International Journal of Wildland Fire 27, no. 1 (2018): 15–28; Fernandes, ‘Empirical Support for the Use of Prescribed Burning as a Fuel Treatment’.

[16] Adam Leavesley, Mike Wouters, and Richard Thornton, eds, Prescribed Burning in Australasia: The Science, Practice and Politics of Burning the Bush (East Melbourne: Australasian Fire and Emergency Services Council, 2020).

[17] Paulo M. Fernandes and Hermínio S. Botelho, ‘A Review of Prescribed Burning Effectiveness in Fire Hazard Reduction’, International Journal of Wildland Fire 12, no. 2 (2003): 117.

[18] Bill McCormick, ‘Bushfires: Is Fuel Reduction Burning the Answer?’, Current Issues Brief 8, 2002–03 (Canberra: Department of the Parliamentary Library, 10 December 2002).

[19] Victoria. Bushfires Royal Commission, Bernard Teague, Ron McLeod, and Susan Pascoe, ‘2009 Victorian Bushfires Royal Commission, Final Report, Volume II: Fire Preparation, Response and Recovery’ (Melbourne: Government Printer for the State of Victoria, 2010): 392–397; SGS Economics and Planning, ‘The Value of the Bushfire and Natural Hazards Cooperative Research Centre’ (June 2020): 12–13, 19–20.

[20] Dean Freeman, ‘Aboriginal Burning in Southern Australia’, in Prescribed Burning in Australasia: The Science, Practice and Politics of Burning the Bush, ed. Adam Leavesley, Mike Wouters, and Richard Thornton (East Melbourne: Australasian Fire and Emergency Services Council, 2020), 239–41; Victor Steffensen, Fire Country: How Indigenous Fire Management Could Help Save Australia (Hardie Grant Travel, 2020); Timothy Neale, ‘What Are Whitefellas Talking about When We Talk about “Cultural Burning”?’, Inside Story, 17 April 2020.

[21] Ross Bradstock, ‘A Biogeographic Model of Fire Regimes in Australia: Current and Future Implications’, Global Ecology and Biogeography 19, no. 2 (2010): 145–158.

[22] Bianca Nogrady, ‘Bushfire Basics: What You Need to Know’, CSIROscope, 17 December 2015.

[23] Country Fire Authority, ‘How Fire Behaves’.

[24] Fernandes and Botelho, ‘A Review of Prescribed Burning Effectiveness in Fire Hazard Reduction’, 117; Furlaud, Williamson, and Bowman, ‘Simulating the Effectiveness of Prescribed Burning at Altering Wildfire Behaviour in Tasmania, Australia’.

[25] NSW Rural Fire Service, ‘Fire Danger Index (FDI) and Fire Danger Ratings (FDR)’.

[26] Royal Commission into National Natural Disaster Arrangements et al., ‘Royal Commission into National Natural Disaster Arrangements Report’, 291–95.

[27] Kevin G. Tolhurst and Greg McCarthy, ‘Effect of Prescribed Burning on Wildfire Severity: A Landscape-Scale Case Study from the 2003 Fires in Victoria’, Australian Forestry 79, no. 1 (2 January 2016): 6.

[28] Tolhurst and McCarthy, ‘Effect of Prescribed Burning on Wildfire Severity’.

[29] Fernandes and Botelho, ‘A Review of Prescribed Burning Effectiveness in Fire Hazard Reduction’, 117–20.

[30] Kevin G. Tolhurst, ‘Report on the Physical Nature of the Victorian Fire Occurring on 7th February 2009’, 15 May 2009, 14; Fernandes, ‘Empirical Support for the Use of Prescribed Burning as a Fuel Treatment’, 121.

[31] Peter M. Attiwill and Mark A. Adams, ‘Mega-Fires, Inquiries and Politics in the Eucalypt Forests of Victoria, South-Eastern Australia’, Forest Ecology and Management 294 (April 2013): 51.

[32] Owen F. Price and Ross A. Bradstock, ‘The Effect of Fuel Age on the Spread of Fire in Sclerophyll Forest in the Sydney Region of Australia’, International Journal of Wildland Fire 19, no. 1 (2010): 35–45; W. Lachlan McCaw, ‘Managing Forest Fuels Using Prescribed Fire – A Perspective from Southern Australia’, Forest Ecology and Management, The Mega-Fire Reality, 294 (15 April 2013): 218; Victoria. Bushfires Royal Commission et al., ‘Final Report, Volume II’, 284.

[33] Neil Burrows and Lachlan McCaw, ‘Prescribed Burning in Southwestern Australian Forests’, Frontiers in Ecology and the Environment 11, no. s1 (August 2013): e27.

[34] J. S. Gould, W. L. McCaw, N. P. Cheney, P. F. Ellis, I. K. Knight, and A. L. Sullivan, Project Vesta: Fire in Dry Eucalypt Forest: Fuel Structure, Fuel Dynamics and Fire Behaviour (Perth: CSIRO Publishing and Department of Environment and Conservation, 2007), 2.

[35] Gould et al., 3.

[36] Gould et al., 3; Stephen J. Pyne, Burning Bush: A Fire History of Australia (New York: Henry Holt and Company, 1991); G. J. Rodger, ‘Report of the Royal Commission Appointed to Enquire into and Report upon the Bushfires of December, 1960 and January, February and March, 1961 in Western Australia: The Measures Necessary or Desirable to Prevent and Control Such Fires and to Protect Life and Property in the Future, and The Basic Requirements for an Effective State Fire Emergency Organisation’ (West Australian Parliament, 1961).

[37] Gould et al., Project Vesta, 2.

[38] Gould et al., Project Vesta.

[39] Gould et al., 5, 32–33.

[40] Gould et al., 4.

[41] Gould et al., 4.

[42] W. Lachlan McCaw, James S. Gould, N. Phillip Cheney, Peter F. M. Ellis, and Wendy R. Anderson, ‘Changes in Behaviour of Fire in Dry Eucalypt Forest as Fuel Increases with Age’, Forest Ecology and Management 271 (1 May 2012): 175.

[43] McCaw et al., 180; Gould et al., Project Vesta, 79.

[44] G. W. Morgan, K. G. Tolhurst, M. W. Poynter, N. Cooper, T. McGuffog, R. Ryan, M. A. Wouters, N. Stephens, P. Black, D. Sheehan, P. Leeson, S. Whight, and S. M Davey, ‘Prescribed Burning in South-Eastern Australia: History and Future Directions’, Australian Forestry 83, no. 1 (2 January 2020): 13; Fernandes, ‘Empirical Support for the Use of Prescribed Burning as a Fuel Treatment’, 121.

[45] Philip Zylstra, Ross A. Bradstock, Michael Bedward, Trent D. Penman, Michael D. Doherty, Rodney O. Weber, A. Malcolm Gill, and Geoffrey J. Carey, ‘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, no. 8 (16 August 2016): e0160715.

[46] McCaw et al., ‘Changes in Behaviour of Fire in Dry Eucalypt Forest as Fuel Increases with Age’, 180; Fernandes and Botelho, ‘A Review of Prescribed Burning Effectiveness in Fire Hazard Reduction’, 122.

[47] Gould et al., Project Vesta, 79–80.

[48] Fernandes and Botelho, ‘A Review of Prescribed Burning Effectiveness in Fire Hazard Reduction’, 118.

[49] Gould et al., Project Vesta, 79.

[50] Hamish Clarke, Bruce Tran, Matthias M. Boer, Owen Price, Belinda Kenny, and Ross Bradstock, ‘Climate Change Effects on the Frequency, Seasonality and Interannual Variability of Suitable Prescribed Burning Weather Conditions in South-Eastern Australia’, Agricultural and Forest Meteorology 271 (June 2019): 149.

[51] McCaw et al., ‘Changes in Behaviour of Fire in Dry Eucalypt Forest as Fuel Increases with Age’, 175.

[52] Tolhurst and McCarthy, ‘Effect of Prescribed Burning on Wildfire Severity’; some researchers dispute the appropriateness of crown scorch vs crown fire; see Wardell-Johnson et al., ‘Native Forest Logging Makes Bushfires Worse – and to Say Otherwise Ignores the Facts’.

[53] Tolhurst and McCarthy, ‘Effect of Prescribed Burning on Wildfire Severity’, 6.

[54] Tolhurst and McCarthy, 7.

[55] Tolhurst and McCarthy, 10.

[56] Tolhurst and McCarthy, 7.

[57] Tolhurst and McCarthy, 10.

[58] Tolhurst and McCarthy, 12.

[59] Tolhurst and McCarthy, 12–13.

[60] Fernandes, ‘Empirical Support for the Use of Prescribed Burning as a Fuel Treatment’, 121.

[61] M. G. Cruz, A. L. Sullivan, J. S. Gould, N. C. Sims, A. J. Bannister, J. J. Hollis, and R. J. Hurley, ‘Anatomy of a Catastrophic Wildfire: The Black Saturday Kilmore East Fire in Victoria, Australia’, Forest Ecology and Management 284 (15 November 2012): 282.

[62] McCaw, ‘Managing Forest Fuels Using Prescribed Fire – A Perspective from Southern Australia’, 220.

[63] Owen F. Price and Ross A. Bradstock, ‘The Efficacy of Fuel Treatment in Mitigating Property Loss during Wildfires: Insights from Analysis of the Severity of the Catastrophic Fires in 2009 in Victoria, Australia’, Journal of Environmental Management 113 (December 2012): 153.

[64] Victoria. Bushfires Royal Commission et al., ‘Final Report, Volume II’, 283.

[65] Footnote references have been omitted from this quotation and can be viewed in the source document; Victoria. Bushfires Royal Commission et al., 283.

[66] Geoffrey J. Cary, Mike D. Flannigan, Robert E. Keane, Ross A. Bradstock, Ian D. Davies, James M. Lenihan, Chao Li, Kimberley A. Logan, and Russel A. Parsons, ‘Relative Importance of Fuel Management, Ignition Management and Weather for Area Burned: Evidence from Five Landscape–Fire–Succession Models’, International Journal of Wildland Fire 18, no. 2 (3 April 2009): 154.

[67] S. Hislop, C. Stone, A. Haywood, and A. Skidmore, ‘The Effectiveness of Fuel Reduction Burning for Wildfire Mitigation in Sclerophyll Forests’, Australian Forestry 83, no. 4 (2020): 255, 263.

[68] Hislop et al., 263.

[69] Royal Commission into National Natural Disaster Arrangements et al., ‘Royal Commission into National Natural Disaster Arrangements Report’, 372.

[70] Royal Commission into National Natural Disaster Arrangements et al., 373.

[71] Kelly M. Dixon, Geoffrey J. Cary, Graeme L. Worboys, Julian Seddon, and Philip Gibbons, ‘A Comparison of Fuel Hazard in Recently Burned and Long-Unburned Forests and Woodlands’, International Journal of Wildland Fire 27, no. 9 (3 October 2018): 610; Matthias M. Boer, Rohan J. Sadler, Roy S. Wittkuhn, Lachlan McCaw, and Pauline F. Grierson, ‘Long-Term Impacts of Prescribed Burning on Regional Extent and Incidence of Wildfires—Evidence from 50 Years of Active Fire Management in SW Australian Forests’, Forest Ecology and Management 259, no. 1 (2009): 134, 140.

[72] Fernandes and Botelho, ‘A Review of Prescribed Burning Effectiveness in Fire Hazard Reduction’, 122.

[73] Drooping sheoak’s fuel structure also hinders the effectiveness of fuel reduction burning, as its low branches can act as a ladder for fire to reach the canopy. R. J. Fensham, ‘The Management Implications of Fine Fuel Dynamics in Bushlands Surrounding Hobart, Tasmania’, Journal of Environmental Management 36 (1992): 316.

[74] Gould et al., Project Vesta, 79–80.

[75] Boer et al., ‘Long-Term Impacts of Prescribed Burning’: 135, 140.

[76] Footnote references have been omitted from this quotation and can be viewed in the source document; Victoria. Bushfires Royal Commission et al., ‘Final Report, Volume II’, 284.

[77] Gould et al., Project Vesta, 79–80.

[78] W. R. Wallace, ‘Fire in the Jarrah Forest Environment’, Journal of the Royal Society of Western Australia 49, no. 2 (1965): 33–44.

[79] P. Cheney in ‘Transcript of Proceedings, Tuesday 23 February 2010’, 2009 Victorian Bushfires Royal Commission, 15349.

[80] State Government of Victoria, Report of the Inquiry into the 2002–2003 Victorian Bushfires (Melbourne: State Government of Victoria, 2003), 20.

[81] Clarke et al., ‘Climate Change Effects on the Frequency, Seasonality and Interannual Variability of Suitable Prescribed Burning Weather Conditions in South-Eastern Australia’, 149.

[82] May, ‘To Burn or Not to Burn Is Not the Question’; Tom Griffiths, Forests of Ash (New York: Cambridge University Press, 2001).

[83] James W. Barker and Owen F. Price, ‘Positive Severity Feedback between Consecutive Fires in Dry Eucalypt Forests of Southern Australia’, Ecosphere 9, no. 3 (March 2018); David M. J. S. Bowman, Brett P. Murphy, Dominic L. J. Neyland, Grant J. Williamson, and Lynda D. Prior, ‘Abrupt Fire Regime Change May Cause Landscape-Wide Loss of Mature Obligate Seeder Forests’, Global Change Biology 20, no. 3 (March 2014): 1008–15.

[84] T. D. Penman, R. A. Bradstock, and O. Price, ‘Modelling the Determinants of Ignition in the Sydney Basin, Australia: Implications for Future Management’, International Journal of Wildland Fire 22, no. 4 (2013): 475.

[85] Philip John Zylstra, ‘Flammability Dynamics in the Australian Alps’, Austral Ecology 43, no. 5 (August 2018): 578.

[86] Gould et al., Project Vesta, 79.

[87] Gould et al., 26–27.

[88] Footnote references have been omitted from this quotation and can be viewed in the source document; Dixon et al., ‘A Comparison of Fuel Hazard in Recently Burned and Long-Unburned Forests and Woodlands’, 610–16.

[89] Dixon et al., ‘A Comparison of Fuel Hazard in Recently Burned and Long-Unburned Forests and Woodlands’, 618.

[90] Pyne, Burning Bush; Griffiths, Forests of Ash.

[91] For instance, in the areas for Project Vesta the site with the longest time since fire was 16 years. Gould et al., Project Vesta, 8; Dixon et al., ‘A Comparison of Fuel Hazard in Recently Burned and Long-Unburned Forests and Woodlands’, 609.

[92] Dixon et al., ‘A Comparison of Fuel Hazard in Recently Burned and Long-Unburned Forests and Woodlands’, 609.

[93] Victoria. Bushfires Royal Commission et al., ‘Final Report, Volume II’, 284.

[94] Victoria. Bushfires Royal Commission et al., 284.

[95] McCaw et al., ‘Changes in Behaviour of Fire in Dry Eucalypt Forest as Fuel Increases with Age’, 171.

[96] K Tolhurst in ‘Transcript of Proceedings, Monday 22 February 2010’, 2009 Victorian Bushfires Royal Commission, 15191.

[97] McCaw, ‘Managing Forest Fuels Using Prescribed Fire – A Perspective from Southern Australia’, 219.

[98] McCaw, 219; Furlaud, Williamson, and Bowman, ‘Simulating the Effectiveness of Prescribed Burning at Altering Wildfire Behaviour in Tasmania, Australia’.

[99] McCaw, ‘Managing Forest Fuels Using Prescribed Fire – A Perspective from Southern Australia’, 219.

[100] These also provide opportunities for firefighters to light backburns towards an advancing fire. Tiago M. Oliveira, Ana M. G. Barros, Alan A. Ager, and Paulo M. Fernandes, ‘Assessing the Effect of a Fuel Break Network to Reduce Burnt Area and Wildfire Risk Transmission’, International Journal of Wildland Fire 25, no. 6 (22 June 2016): 619–32.

[101] Victoria. Bushfires Royal Commission et al., ‘Final Report, Volume II’, 284.

[102] Owen F. Price, Trent D. Penman, Ross A. Bradstock, Matthias M. Boer, and Hamish Clarke, ‘Biogeographical Variation in the Potential Effectiveness of Prescribed Fire in South-Eastern Australia’, Journal of Biogeography 42, no. 11 (2015): 2235.

[103] Price et al., 2235.

[104] Boer et al., ‘Long-Term Impacts of Prescribed Burning’, 133.

[105] Fernandes, ‘Empirical Support for the Use of Prescribed Burning as a Fuel Treatment’, 123; Trent D. Penman, Luke Collins, Thomas D. Duff, Owen F. Price, and Geoffrey J. Cary, ‘Scientific evidence regarding the effectiveness of prescribed burning’, in Prescribed Burning in Australasia: The Science, Practice and Politics of Burning the Bush, ed. Adam Leavesley, Mike Wouters, and Richard Thornton (East Melbourne: Australasian Fire and Emergency Services Council, 2020), 104.

[106] Boer et al., ‘Long-Term Impacts of Prescribed Burning’.

[107] Price et al., ‘Biogeographical Variation in the Potential Effectiveness of Prescribed Fire in South-Eastern Australia’, 2239.

[108] Price et al., 2241.

[109] Danielle Clode and Mark A. Elgar, ‘Fighting Fire with Fire: Does a Policy of Broad-Scale Prescribed Burning Improve Community Safety?’, Society & Natural Resources 27, no. 11 (2014): 1194; some researchers emphasise that ‘significant’ risk reduction can be achieved by creating ‘defensible space’ to further reduce the risk of ignition from direct flame contact, see Sandra H. Penman, Owen F. Price, Trent D. Penman, and Ross A. Bradstock, ‘The Role of Defensible Space on the Likelihood of House Impact from Wildfires in Forested Landscapes of South Eastern Australia’, International Journal of Wildland Fire 28, no. 1 (13 February 2019): 4–14.

[110] Clode and Elgar, ‘Fighting Fire with Fire’, 1194.

[111] Philip Gibbons, Linda van Bommel, A. Malcolm Gill, Geoffrey J. Cary, Don A. Driscoll, Ross A. Bradstock, Emma Knight, Max A. Moritz, Scott L. Stephens, and David B. Lindenmayer, ‘Land Management Practices Associated with House Loss in Wildfires’, ed. Rohan H. Clarke, PLOS ONE 7, no. 1 (18 January 2012): e29212.

[112] Footnote references have been omitted from this quotation and can be viewed in the source document; Gibbons et al., 4.

[113] T. D. Penman, R. A. Bradstock, and O. F. Price, ‘Reducing Wildfire Risk to Urban Developments: Simulation of Cost-Effective Fuel Treatment Solutions in South Eastern Australia’, Environmental Modelling & Software 52 (February 2014): 167.

[114] Boer et al., ‘Long-Term Impacts of Prescribed Burning’.

[115] Veronique Florec, Michael Burton, David Pannell, Joel Kelso, and George Milne, ‘Where to Prescribe Burn: The Costs and Benefits of Prescribed Burning Close to Houses’, International Journal of Wildland Fire 29, no. 5 (2020): 440–58.

[116] May, ‘Black Saturday and Prescribed Burning Debates’, 149.

[117] Victoria. Bushfires Royal Commission, Bernard Teague, Ron McLeod, and Susan Pascoe, ‘2009 Victorian Bushfires Royal Commission, Final Report: Summary’ (Melbourne: Government Printer for the State of Victoria, 2010), 35.

[118] Footnote references have been omitted from this quotation and can be viewed in the source document; May, ‘Black Saturday and Prescribed Burning Debates’, 150–52.

[119] Footnote references have been omitted from this quotation and can be seen in the source document; Royal Commission into National Natural Disaster Arrangements et al., ‘Royal Commission into National Natural Disaster Arrangements Report’, 376.

[120] Royal Commission into National Natural Disaster Arrangements et al., 376.

[121] Timothy Neale and Daniel May, ‘Fuzzy Boundaries: Simulation and Expertise in Bushfire Prediction’, Social Studies of Science 50, no. 6 (13 February 2020): 837–59.

[122] Morgan et al., ‘Prescribed Burning in South-Eastern Australia’, 15.

[123] Neale and May, ‘Fuzzy Boundaries’; Furlaud, Williamson, and Bowman, ‘Simulating the Effectiveness of Prescribed Burning at Altering Wildfire Behaviour in Tasmania, Australia’.

[124] Clode and Elgar, ‘Fighting Fire with Fire’, 1197.

[125] Clode and Elgar, 1196–7.

[126] N. D. Burrows, ‘Linking Fire Ecology and Fire Management in South-West Australian Forest Landscapes’, Forest Ecology and Management 255, no. 7 (April 2008): 2395.

[127] Department of Environment, Land, Water & Planning, ‘Flora and Fauna Guarantee Act 1988: Processes List (December 2016)’, 2016; NSW Government Office of Environment & Heritage, ‘Threats’, Threatened Species; Biodiversity Conservation Act 2016 (NSW).

[128] ‘Extensions to EPBC Act Listing Assessment and Decision Timeframes’, Australian Government Department of Agriculture, Water and the Environment; S Ley, ‘Answer to Question in Writing: Bushfires’, [Questioner: R Sharkie], Question 275, House of Representatives, Debates, 4 February 2020.

[129] Lisa Cox, ‘Australian Government Stops Listing Major Threats to Species under Environment Laws’, The Guardian, 8 May 2020.

[130] Burrows, ‘Linking Fire Ecology and Fire Management in South-West Australian Forest Landscapes’, 2395.

[131] Burrows, 2396.

[132] Michael F. Clarke, ‘Prescribed Burning: Expert Opinion of Assoc Prof Michael F. Clarke’, 2009 Victorian Bushfires Royal Commission, EXP.016.001.0001, Item EXH-0735 in P0001, VPRS 16497, Public Record Office Victoria, 9; see also Australasian Fire and Emergency Service Authorities Council and Forest Fire Management Group, ‘Overview of Prescribed Burning in Australasia’, 44–51.

[133] Bowman et al., ‘Abrupt Fire Regime Change May Cause Landscape-Wide Loss of Mature Obligate Seeder Forests’, 1008.

[134] Robert E. Martin and David B. Sapsis, ‘Fires as Agents of Biodiversity: Pyrodiversity Promotes Biodiversity’, in Proceedings of the Symposium on Biodiversity of Northwestern California, ed. R. R. Harris, D. C. Erman, and H. M. Kerner, Wildland Resources Center, Report 29 (Berkeley: University of California Press, 1992), 150–57; Gavin M. Jones and Morgan W. Tingley, ‘Pyrodiversity and Biodiversity: A History, Synthesis, and Outlook (Early View)’, Diversity and Distributions: A Journal of Conservation Biogeography, 2021; Catherine L. Parr and Alan N. Andersen, ‘Patch Mosaic Burning for Biodiversity Conservation: A Critique of the Pyrodiversity Paradigm’, Conservation Biology 20, no. 6 (2006): 1610–19.

[135] Kevin Tolhurst, ‘Report on Land and Fuel Management in Victoria in Relation to the Bushfires on 7th February 2009’, 2009 Victorian Bushfires Royal Commission, EXP.013.001.0001, Item EXH-0737 in P0001, VPRS 16497, Public Record Office Victoria, 33.

[136] Ross Bradstock, ‘Questions for Experts Land and Fuel Management’, 2009 Victorian Bushfires Royal Commission, EXP.012.001.0001, Item EXH-0733 in P0001, VPRS 16497, Public Record Office Victoria, 20. See also David McKenna, ‘Environmental Effects of Prescribed Burning’, in Prescribed Burning in Australasia: The Science, Practice and Politics of Burning the Bush, ed. Adam Leavesley, Mike Wouters, and Richard Thornton (East Melbourne: Australasian Fire and Emergency Services Council, 2020), 143–53.

[137] Bowman et al., ‘Abrupt Fire Regime Change May Cause Landscape-Wide Loss of Mature Obligate Seeder Forests’; Zylstra, ‘Flammability Dynamics in the Australian Alps’.

[138] Bowman et al., ‘Abrupt Fire Regime Change May Cause Landscape-Wide Loss of Mature Obligate Seeder Forests’.

[139] Luke Collins, ‘Eucalypt Forests Dominated by Epicormic Resprouters Are Resilient to Repeated Canopy Fires’, Journal of Ecology 108, no. 1 (January 2020): 310–24.

[140] Burrows and McCaw, ‘Prescribed Burning in Southwestern Australian Forests’, e31.

[141] Kelly M. Dixon, Geoffrey J. Cary, Michael Renton, Graeme L. Worboys, and Phillip Gibbons, ‘More Long-Unburnt Forest Will Benefit Mammals in Australian Sub-Alpine Forests and Woodlands’, Austral Ecology 44, no. 7 (November 2019): 1150–62.

[142] Tolhurst, ‘Report on Land and Fuel Management in Victoria in Relation to the Bushfires on 7th February 2009’, 34.

[143] Bradstock, ‘Questions for Experts Land and Fuel Management’, 21.

[144] Harry Parnaby, Daniel Lunney, Ian Shannon, and Mike Fleming, ‘Collapse Rates of Hollow-Bearing Trees Following Low Intensity Prescription Burns in the Pilliga Forests, New South Wales’, Pacific Conservation Biology 16, no. 3 (2010): 209–20.

[145] Kevin G. Tolhurst, ‘Fire Severity and Ecosystem Resilience – Lessons from the Wombat Fire Effects Study (1984–2003)’, Proceedings of the Royal Society of Victoria 124, no. 1 (2012): 33.

[146] McKenna, ‘Environmental Effects of Prescribed Burning’, 149.

[147] Bronwyn A. Hradsky, Craig Mildwaters, Euan G. Ritchie, Fiona Christie, and Julian Di Stefano, ‘Responses of Invasive Predators and Native Prey to a Prescribed Forest Fire’, Journal of Mammalogy 98, no. 3 (29 May 2017): 835–47.

[148] For example, see Lotte Richter, Debra Little, and Doug Benson, ‘Effects of Low Intensity Fire on the Resprouting of the Weed African Olive (Olea Europaea Subsp. Cuspidata) in Cumberland Plain Woodland, Western Sydney’, Ecological Management and Restoration 6, no. 3 (December 2005): 230–33; David B. Lindenmayer, Jeff Wood, Christopher MacGregor, Yvonne M. Buckley, Nicholas Dexter, Martin Fortescue, Richard J. Hobbs, and Jane A. Catford, ‘A Long-Term Experimental Case Study of the Ecological Effectiveness and Cost Effectiveness of Invasive Plant Management in Achieving Conservation Goals: Bitou Bush Control in Booderee National Park in Eastern Australia’, PLOS ONE 10, no. 6 (3 June 2015): e0128482.

[149] Council of Heads of Australasian Herbaria, ‘Ulex europaeus L.: Gorse’, Weeds Australia – Profiles, last updated 12 October 2020.

[150] Bradstock, ‘Questions for Experts Land and Fuel Management’, 19–22.

[151] Bradstock, ‘Questions for Experts Land and Fuel Management’, 19–22.

[152] Bradstock, ‘Questions for Experts Land and Fuel Management’, 19–22.

[153] Kipling Klimas, Patrick Hiesl, Donald Hagan, and Dara Park, ‘Prescribed Fire Effects on Sediment and Nutrient Exports in Forested Environments: A Review’, Journal of Environmental Quality 49, no. 4 (2020): 794.

[154] Ian White, Alan Wade, Martin Worthy, Norm Mueller, Trevor Daniell, and Robert Wason, ‘The Vulnerability of Water Supply Catchments to Bushfires: Impacts of the January 2003 Wildfires on the Australian Capital Territory’, Australasian Journal of Water Resources 10, no. 2 (January 2006): 179–94.

[155] Klimas et al., ‘Prescribed Fire Effects on Sediment and Nutrient Exports in Forested Environments’, 796, 802.

[156] Klimas et al., 80.

[157] McCaw, ‘Managing Forest Fuels Using Prescribed Fire – A Perspective from Southern Australia’, 218.

[158] Klimas et al., ‘Prescribed Fire Effects on Sediment and Nutrient Exports in Forested Environments’, 800.

[159] K. M. Giljohann, M. A. McCarthy, L. T. Kelly, and T. J. Regan, ‘Choice of Biodiversity Index Drives Optimal Fire Management Decisions’, Ecological Applications 25, no. 1 (2015): 264–77; Don A. Driscoll, Michael Bode, Ross A. Bradstock, David A. Keith, Trent D. Penman, and Owen F. Price, ‘Resolving Future Fire Management Conflicts Using Multicriteria Decision Making’, Conservation Biology 30, no. 1 (2016): 196–205; Tim Gazzard, Terry Walshe, Peter Galvin, Owen Salkin, Michael Baker, Bec Cross, and Peter Ashton , ‘What Is the “Appropriate” Fuel Management Regime for the Otway Ranges, Victoria, Australia? Developing a Long-Term Fuel Management Strategy Using the Structured Decision-Making Framework’, International Journal of Wildland Fire 29, no. 5 (10 September 2019): 354–70.

[160] Giljohann et al., ‘Choice of Biodiversity Index Drives Optimal Fire Management Decisions’, 273.

[161] Mark Adams and Peter Attiwill, Burning Issues: Sustainability and Management of Australia’s Southern Forests (Acton, ACT: CSIRO Publishing and Bushfire CRC, 2011), 110.

[162] McCaw, ‘Managing Forest Fuels Using Prescribed Fire – A Perspective from Southern Australia’, 221.

[163] Burrows and McCaw, ‘Prescribed Burning in Southwestern Australian Forests’, e32.

[164] Footnote references have been omitted from this quotation and can be viewed in the source document; Daniel May, ‘2019–20 Australian Bushfires — Frequently Asked Questions: A Quick Guide (Updates)’, Research paper series, 2020–21, (Canberra: Parliamentary Library, 2021).

[165] Anjali Haikerwal, Fabienne Reisen, Malcolm R. Sim, Michael J. Abramson, Carl P. Meyer, Fay H. Johnston, and Martine Dennekamp, ‘Impact of Smoke from Prescribed Burning: Is It a Public Health Concern?’, Journal of the Air & Waste Management Association 65, no. 5 (4 May 2015): 595; G. J. Williamson, D. M. J. S. Bowman, O. F. Price, S. B. Henderson, and F. H. Johnston, ‘A Transdisciplinary Approach to Understanding the Health Effects of Wildfire and Prescribed Fire Smoke Regimes’, Environmental Research Letters 11, no. 12 (December 2016): 9.

[166] Haikerwal et al., ‘Impact of Smoke from Prescribed Burning’.

[167] For instance, the 2019–20 Australian bushfires ‘caused repeated exposure to substantial amounts of bushfire smoke across many weeks’; see May, ‘2019–20 Australian Bushfires — Frequently Asked Questions: A Quick Guide (Updates)’.

[168] Williamson et al., ‘A Transdisciplinary Approach to Understanding the Health Effects of Wildfire and Prescribed Fire Smoke Regimes’.

[169] Royal Commission into National Natural Disaster Arrangements et al., ‘Royal Commission into National Natural Disaster Arrangements Report’, 375.

[170] McCaw, ‘Managing Forest Fuels Using Prescribed Fire – A Perspective from Southern Australia’, 221; Southern Properties (WA) Pty Ltd v. Executive Director of the Department of Conservation and Land Management [2012] WASCA 79 (4 April 2012).

[171] McCaw, 221; Southern Properties (WA) Pty Ltd v. Executive Director of the Department of Conservation and Land Management [2012] WASCA 79.

[172] May, ‘To Burn or Not to Burn Is Not the Question’; Royal Commission into National Natural Disaster Arrangements et al., ‘Royal Commission into National Natural Disaster Arrangements Report’, 368, 377–79.

[173] Clode and Elgar, ‘Fighting Fire with Fire’, 1196.

[174] Burrows and McCaw, ‘Prescribed Burning in Southwestern Australian Forests’, e32.

[175] Morgan et al., ‘Prescribed Burning in South-Eastern Australia’, 18; Hotspots Fire Project, ‘Home’, 2021.

[176] Australian Disaster Resilience Knowledge Hub, ‘Bushfire — Margaret River, WA’.

[177] McCaw, ‘Managing Forest Fuels Using Prescribed Fire – A Perspective from Southern Australia’, 222.

[178] Neil Burrows, ‘The Great Escapes’, Fire Australia, 2017, 37.

[179] See P. J. Kanowski, R. J. Whelan, and S. Ellis, ‘Inquiries Following the 2002–2003 Australian Bushfires: Common Themes and Future Directions for Australian Bushfire Mitigation and Management’, Australian Forestry 68, no. 2 (2005): 76–86.

[180] Australasian Fire and Emergency Service Authorities Council, Best Practice Principles for Prescribed Burning, National Burning Project No. 8 (AFAC: Melbourne, 2017), 5.

[181] David Bowman, ‘The World on Fire’, New Scientist 204, no. 2729 (10 October 2009): 28–29.

[182] CSIRO and Bureau of Meteorology, ‘State of The Climate 2020’ (Commonwealth of Australia, 2020).

[183] CSIRO and Bureau of Meteorology, ‘State of The Climate 2020’; CSIRO, ‘The 2019–20 Bushfires: A CSIRO Explainer’ (CSIRO, 3 February 2021).

[184] Sarah Harris and Chris Lucas, ‘Understanding the Variability of Australian Fire Weather between 1973 and 2017’, PLOS ONE 14, no. 9 (2019): e0222328.

[185] Harris and Lucas, 27; see also Geert Jan van Oldenborgh, Folmer Krikken, Sophie Lewis, Nicholas J. Leach, Flavio Lehner, Kate R. Saunders, Michiel van Weele, Karsten Haustein, Sihan Li, David Wallom, Sarah Sparrow, Julie Arrighi, Roop K. Singh, Maarten K. van Aalst, Sjoukje Y. Philip, Robert Vautard, and Friederike E. L. Otto, ‘Attribution of the Australian Bushfire Risk to Anthropogenic Climate Change’, Natural Hazards and Earth System Sciences 21, no. 3 (11 March 2021): 941–60.

[186] Royal Commission into National Natural Disaster Arrangements et al., ‘Royal Commission into National Natural Disaster Arrangements Report’, 55.

[187] CSIRO and Bureau of Meteorology, ‘State of The Climate 2020’, 22.

[188] Royal Commission into National Natural Disaster Arrangements et al., ‘Royal Commission into National Natural Disaster Arrangements Report’, 58–59, 64–65; see also Giovanni Di Virgilio, Jason P. Evans, Stephanie A. P. Blake, Matthew Armstrong, Andrew J. Dowdy, Jason Sharples, and Rick McRae, ‘Climate Change Increases the Potential for Extreme Wildfires’, Geophysical Research Letters 46, no. 14 (28 July 2019): 8517–26.

[189] CSIRO and Bureau of Meteorology, ‘State of The Climate 2020’, 5.

[190] Matthias M. Boer, Victor Resco de Rios, and Ross Bradstock, ‘Unprecedented Burn Area of Australian Mega Forest Fires’, Nature Climate Change 10, no. 3 (March 2020): 171–72.

[191] Boer et al, ‘Unprecedented Burn Area of Australian Mega Forest Fires’.

[192] Jessica Lucas and Rebecca M. B. Harris, ‘Changing Climate Suitability for Dominant Eucalyptus Species May Affect Future Fuel Loads and Flammability in Tasmania’, Fire 4, no. 1 (7 January 2021): 1–17.

[193] Adams and Attiwill, Burning Issues: Sustainability and Management of Australia’s Southern Forests, 62–64; CSIRO and Bureau of Meteorology, ‘State of The Climate 2020’.

[194] C. Lucas, K. Hennessy, G. Mills and J. Bathols, Bushfire Weather in Southeast Australia: Recent Trends and Projected Climate Change Impacts, consultancy report prepared for the Climate Institute of Australia, Bushfire Cooperative Centre, Melbourne, 2007, 45.

[195] Karl Braganza, The Influence of Climate Variability and Change on the 2019–2020 Australian Bushfire Season, Bureau of Meteorology, 25 May 2020, BOM.502.001.0060.

[196] CSIRO and Bureau of Meteorology, ‘State of The Climate 2020’, 22; Morgan et al., ‘Prescribed Burning in South-Eastern Australia’, 21.

[197] Clarke et al., ‘Climate Change Effects on the Frequency, Seasonality and Interannual Variability of Suitable Prescribed Burning Weather Conditions in South-Eastern Australia’.

[198] Giovanni Di Virgilio, Jason P. Evans, Hamish Clarke, Jason Sharples, Annette L. Hirsch, and Melissa Anne Hart, ‘Climate Change Significantly Alters Future Wildfire Mitigation Opportunities in Southeastern Australia’, Geophysical Research Letters 47, no. 15 (16 August 2020): e2020GL088893.

 

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