Background Paper: Land management – hazard reduction: a literature

review

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The Royal Commission into National Natural Disaster Arrangements was established on 20 February 2020 in response to the extreme bushfire season of 2019-20 which resulted in devastating loss of life, property and wildlife, and environmental destruction across the nation.

The Letters Patent for the Royal Commission set out the terms of reference and formally appoint Air Chief Marshal Mark Binskin AC (Retd), the Honourable Dr Annabelle Bennett AC SC and Professor Andrew Macintosh as Royal Commissioners.

This paper was published on 15 June 2020.

© Commonwealth of Australia 2020

ISBN: 978-1-921091-16-2 (online)

With the exception of the Coat of Arms and where otherwise stated, all material

presented in this publication is provided under a Creative Commons Attribution 4.0

International licence. For the avoidance of doubt, this means this licence only applies

to material as set out in this document.

The details of the relevant licence conditions are available on the Creative Commons

website as is the full legal code for the CC BY 4.0 licence

<www.creativecommons.org/licenses>

The terms under which the Coat of Arms can be used are detailed on the

Department of the Prime Minister and Cabinet website.

Contents Background Paper: Land management – hazard reduction: a literature review ............................................. 1

Introduction .................................................................................................................................................. 4

Literature review scope ............................................................................................................................. 4

PART ONE: Prescribed burning ..................................................................................................................... 5

A diverse evidence base ............................................................................................................................ 5

Significance of fuel ..................................................................................................................................... 7

Effect on fire intensity ............................................................................................................................... 8

Effect on fire extent ................................................................................................................................... 9

Effect on housing and infrastructure loss ................................................................................................ 10

Effect on fire severity (ecological impacts).............................................................................................. 10

Costs of prescribed burning ..................................................................................................................... 11

PART TWO: Mechanical Fuel Load Reduction ............................................................................................ 12

A limited literature base .......................................................................................................................... 12

Potential benefits .................................................................................................................................... 12

Interactions between mechanical fuel load reduction and other forms of hazard mitigation ............... 13

Effect of logging activity on fire intensity ................................................................................................ 14

Ecological implications ............................................................................................................................ 14

Costs and resourcing requirements ........................................................................................................ 14

PART THREE: Livestock Grazing .................................................................................................................. 15

A limited literature base .......................................................................................................................... 15

International evidence............................................................................................................................. 16

Next steps .................................................................................................................................................... 16

Bibliography ................................................................................................................................................ 17

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Terms of Reference (b), (f):

We... require and authorise you to inquire into...:

(b) Australia’s arrangements for improving resilience and adapting to changing climatic conditions, what

actions should be taken to mitigate the impacts of natural disasters, and whether accountability for

natural disaster risk management, preparedness, resilience and recovery should be enhanced,

including through a nationally consistent accountability and reporting framework and national

standards;

(f) ways in which Australia could achieve greater national coordination and accountability — through

common national standards, rule-making, reporting and data-sharing — with respect to key

preparedness and resilience responsibilities, including for the following

i. land management, including hazard reduction measures;

Introduction The effectiveness and benefits of different vegetation-related hazard reduction activities remain the subject of considerable debate amongst fire management authorities, fire scientists, ecologists, and the broader community.

The literature review focuses predominantly on prescribed burning, which has been used in Australia as the preferred method to reduce fuel loads and fuel continuity and thus has received the most attention in the scientific literature. However, there are a range of other methods that are used to reduce fuel. The paper also canvasses literature related to mechanical fuel load reduction and livestock grazing as fuel management techniques.

In relation to prescribed burning, this paper covers:

the methods used to understand the efficacy of prescribed burning

the significance of fuel in fire behaviour

the effect of prescribed burning on the intensity, extent and severity of bushfires, and

the effect of prescribed burning on mitigating housing and infrastructure loss.

The review also considers literature on the costs associated with prescribed burning, including the resource costs, and adverse health and ecological impacts.

Literature review scope This background paper reviews a selection of relevant literature. It is not a position paper and the views and statements contained within do not represent the views of the Commission. The Commission continues to gather information and analyse evidence relating to fuel management in the context of natural disasters, and will not make findings or draw conclusions until it has completed this process.

The paper is designed to provide a summary snapshot of issues that have been identified in the literature in relation to fuel management, and areas where there may be agreement or divergence of positions within the literature. It is not exhaustive and represents a selection of findings from a selection of scientific papers.

This paper is limited to published literature. It does not cover operational perspectives from fire authorities, or the decades of unpublished applied research and documented case studies undertaken in-house by land and fire agencies, which also contribute to a significant body of knowledge. Selected academics have also made broader public comment on these issues, which are likewise not canvassed here. Please refer to the individual research papers for relevant disclosures of any potential conflicts of interest.

A separate literature review has been prepared on Indigenous cultural burning practices.

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PART ONE: Prescribed burningPrescribed burning involves the deliberate application of fire to a pre-determined area under specific conditions (or ‘prescriptions’) to achieve resource management objectives. Where undertaken for hazard reduction purposes, prescribed burns are intended to burn slowly and ‘coolly’ (i.e. with less intensity than a bushfire) so as to reduce the mass, and alter the structure, of fuels on, or close to, the ground (Fernandes and Botelho, 2003).

The objective of these burns is to reduce the intensity and rate of spread of bushfires so as to enhance and support other measures, including fire suppression, urban planning and building regulations. Ultimately, they aim to reduce the impact of bushfires on life, property and other economic interests, and on environmental and heritage values. In addition to mitigating hazards, prescribed burning can be used to conserve biodiversity in fire-adapted landscapes and revive Indigenous cultural traditions impacted by European settlement (AFAC, 2015; Fernandes and Botelho, 2003). The practice is distinct from a back burn, which is a firefighting technique used during a bushfire event to remove vegetation on the ground and create a scorched buffer to inhibit the advancement of an active bushfire (AFAC, 2015).

Prescribed burning is undertaken in a variety of ways, including around settlements, infrastructure and other high value assets, in strategic locations in forests and grasslands (e.g. around high ignition points and known high risk fire paths) and in mosaic patterns across the broader landscape (Dixon et al., 2018; Tolhurst and McCarthy, 2016; Penman et al., 2014). Burns also vary in intensity and scale, from small fires ignited by ground crews to significant landscape fires ignited through aerial ignition (McCaw, 2013; AFAC, 2015).

The practice is used in different ecosystems, both in Australia and elsewhere, including in forests, woodlands and grasslands. In Australia, much of the literature on the use and effectiveness of prescribed burning is focused on the tall and medium eucalypt forests of southern Australia. These forests cover approximately 60 million hectares, or almost 50 per cent of Australia’s native forests (MPIGA and NFISC, 2018). Researchers describe them as both fire-prone and often fire dependent (e.g. Attiwill and Adams, 2013; Bradstock et al. 2012).

In addition to studies on the southern temperate forests, there is a significant body of literature on the use of prescribed burning in Australia’s tropical savannas (Russell-Smith et al., 2009; 2013; 2015; 2020; Fitzsimons et al., 2012; Richards et al., 2012; Murphy et al., 2015; Cook et al., 2016). Most of this literature is associated with the use of early dry season prescribed burns to reduce the adverse environmental impacts of late dry season wildfires. Studies suggest that this practice can reduce greenhouse gas emissions and improve biodiversity and Indigenous cultural outcomes, although there is geographic and practice-related variability in results (Fitzsimons et al., 2012; Levick et al., 2019; Perry et al., 2016).

Prescribed burning is not intended to prevent fires from igniting. It is also not viewed as the only way of managing fuel. Other methods of controlling fuels include mechanical clearing (mowing, slashing or thinning) and grazing with livestock.

Both the scientific and practitioner literature on prescribed burning emphasise that it can have a role to play in mitigating risk associated with bushfires, but cannot eliminate the risks entirely (Penman et al., 2011; Price and Bradstock, 2012; Howard et al., 2020). Consequently, it generally forms part of a broader strategy that incorporates, amongst other things, appropriate siting of assets away from hazards, construction of fire-resistant buildings, active community preparation and awareness, and effective suppression (Penman et al., 2011; Price and Bradstock, 2012; Florec and Pannell, 2017; Howard et al., 2020).

A diverse evidence base A range of methods are used to assess the value and efficacy of prescribed burning, and there is considerable debate on the appropriate evidence on which to base conclusions (AFAC, 2015). The methods that are used include laboratory and field experiments, statistical analysis of data on past fires, simulation modelling and case studies.

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Experimental fire studies

Experimental fire studies are based on direct observation and measurement of fire behaviour under controlled conditions, either in laboratories or in the field. These studies measure the relationship between interventions and fuel characteristics and/or fire behaviour, such as frequency, pattern, size and intensity. The most prominent, and arguably the most extensive example of an experimental fire study in Australia is Project Vesta, which involved 100 experimental fires in dry eucalypt forests in south-western Western Australia (Gould, et al., 2007; Cheney et al., 2012). A number of researchers argue that experimental fire studies are the best means to evaluate the effectiveness of fuel management programs (see, for example, Fernandes and Botelho, 2003; Cruz et al., 2018). However, there are a number of limitations of this approach, including the cost, risks associated with conducting experiments and the difficulty in isolating the impacts of prescribed burning from the other variables that affect fire behaviour (Gould et al. 2011; McCaw et al. 2012; Cheney et al. 2012; AFAC, 2015; Fernandes and Botelho, 2003).

Statistical analysis of data on past fires

Several studies have assessed the effectiveness of different fuel management techniques by analysing the relationship between prescribed burning and the behaviour and impacts of past bushfires (McCaw et al., 2012; McCarthy and Tolhurst, 2001; Boer et al., 2009; Gibbons et al., 2012; Price et al., 2015). A number of studies use this approach to examine the relationship between prescribed burning and the extent of bushfires, a relationship sometimes quantified in ‘leverage ratios’ (see below) (see for example Price et al., 2015; Boer et al., 2009). Other statistic-based studies look at the relationship between asset (e.g. house) losses and proximity of vegetation (Chen and McAneney, 2004; Crompton et al., 2010), or the relationship between asset losses and a range of variables, including weather, wind direction, topography and fuel loads (Gibbons et al., 2012). The data relied on to conduct these types of analyses include spatial information on fires, vegetation, topography and relevant assets, data on prevailing weather conditions during the fires, and information on human health impacts and property losses.

Simulation modelling

Simulation modelling is used to assess the mechanics of a bushfire or prescribed burn and the significance of various parameters that influence it. These methods are then used to predict future events under particular fuel management options. Simulation models are increasingly used to provide data for quantitative fire-risk analysis of fuel management and to guide decisions about expenditure of fire management budgets (Penman and Cirulis, 2020). One of the main benefits of simulation studies is they are relatively cheap to undertake compared to experimental studies and involve minimal risks (AFAC 2015; Florec et al., 2019).

Case study analysis

Documented case studies concentrate on specific areas or events. Case studies are widely used in Australia to examine and describe the behaviour of unplanned fires and to provide insights on the effects of different fuel conditions and interventions on fire behaviour, environmental values and suppression activities (AFAC, 2015). Like simulation modelling, case studies are a low cost and low risk way of analysing the impacts of prescribed burning.

Some researchers have highlighted that a limitation of case studies is that they rely on information from a specific situation and are not necessarily representative of other environments and circumstances (AFAC, 2015). This limits the conclusions that can be drawn from them about the effects of prescribed burning on fire behaviour in other conditions. Findings specific to a particular ecosystem, area and circumstance may not therefore necessarily apply more generally (Fernandes and Botelho, 2003; Tolhurst and McCarthy, 2016; Harris, 2018).

Fernandes and Botelho (2003) reviewed literature considering the effectiveness of prescribed burning for fuel hazard reduction across North America, Australia and Europe and found that findings from all evidence types supported the effectiveness of prescribed burning. However, there are differences in the literature about when, and the extent to which, prescribed burning can reduce the extent and impacts associated

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with wildfires. Burrows (2018) suggests some of these differences are attributable to the use of different methods.

Significance of fuel Fire behaviour is influenced by a number of variables, including temperature, humidity, wind, topography (slope) and the quantity and arrangement of fuels (Gibbons et al., 2012; McCaw, 2013). Of the relevant variables, fuel (quantity and structure of burnable materials) is the only one that can be readily modified though direct human intervention (Parliament of Victoria, 2008; Esplin et al., 2003; Tolhurst and McCarthy, 2016; Fernandes and Botelho, 2003).

Prescribed burning is seen by many to be a useful management tool because it provides a cost-effective means of altering quantity and structure of fuel (Gould et al., 2007; CSIRO, 2019; Cheney et al., 2012; AFAC, 2015). McCaw (2013) suggests that, to be effective, prescribed fire needs to:

reduce the depth, quantity and continuity of surface and near surface fuel which contribute directly to fire spread and flame depth;

reduce the height of the elevated fuel layer of understory shrubs which contribute to flame height; and

remove flammable and loose outer bark on tree stems that contributes to spotting.

A number of studies have demonstrated that long established views that equate greater fuel loads with faster rates of fire spread require more nuanced understanding. For example, Burrows (1999a; 1999b) found that the speed of fire spread in jarrah forests in Western Australia was not correlated to fuel loads; rather the key determinants were wind speed and fuel moisture. Similarly, Cruz et al. (2018) found an inverse relationship between fuel loads and the rate of fire spread in undisturbed (natural or ungrazed) grasslands; the higher the fuel loads the lower the rate of fire spread.

A significant body of research suggests that the effectiveness of prescribed burning in mitigating fire behaviour is influenced by the time intervals between treatments. This has been attributed to the fact that fuel loads generally accumulate reasonably quickly after fires and gradually reach a quasi-equilibrium state over a period of several to multiple decades, where the rate of input matches the rate of decomposition (Thomas et al., 2014). Consistent with this, a large number of studies indicate that prescribed burning is most effective when fuel age is kept below 5-10 years (see for example, McCaw, 2013; Fernandes and Botelho, 2003; Bradstock et al., 2010; Price and Bradstock, 2010; 2012; Tolhurst and McCarthy, 2016; Gibbons et al., 2012; Sneeuwjagt, 2008; Boer et al., 2009; Gould et al., 2007; CSIRO, 2019; Cheney, 2010). Different periods of time for which prescribed burns remain the most effective are related to how quickly fuel builds up again after burning. This is affected by a range of factors such as fire intensity and forest type (McCaw et al., 2002). The period of effectiveness is also impacted by severe weather. For example, analyses by Bradstock et al. (2010) and Price and Bradstock (2012) indicate that the window for effectiveness of fuel reduction via burning may be broader (e.g. 10 years) under moderate weather (where the McArthur Forest Fire Danger Index (FFDI) is less than 20), narrower (e.g. 5 years) under severe to extreme weather (where the FFDI exceeds 50) and minimal (e.g. 1 year) under catastrophic weather (where the FFDI is greater than 100).

A number of studies have demonstrated that the environmental conditions under which an area is burned, including the weather and the moisture content of fuels, can influence fire behaviour (see for example, Tolhurst and McCarthy, 2016; McCaw, 2013, Sullivan et al., 2012, Bradstock, 2010). For example, studies have found that wind directly and indirectly affects fire behaviour through its effects on fuel drying, combustion and rate of spread, and temperature, relative humidity and solar radiation determine the transfer of water vapour in and out of fine dead fuels (Sullivan et al., 2012). Not all fuel elements within an environment respond in the same way to environmental conditions. For example, McCaw (2013) suggests that in Eucalypt forests, fine fuels respond rapidly to environmental conditions (e.g. air temperature, relative humidity, and precipitation), whereas coarse fuels respond more slowly.

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Zylstra et al. (2016) contend that the primary endogenous (non-weather related) driver of fire severity is not the quantity of surface fuels but the traits of the plant species present in the environment. The influence of plant traits is said to arise from three factors:

the properties of individual leaves such as size, shape, moisture content and chemical composition determine their ignitability, combustibility and sustainability;

the three-dimensional arrangement and spacing of leaves and fine stems affect the likelihood of fire propagating via drying and heat transfer; and

the three dimensional arrangement and spacing of leaves affects the nature of environmental influences on ignition and the spread of fire among fuel elements via effects on insolation (sun exposure) and wind (Zylstra et al., 2016, p 2).

The modelling approach used in Zylstra et al. (2016) has been criticised by other researchers (Cruz and Sullivan, 2016).

Effect on fire intensity Fire intensity refers to the heat energy released by a fire. It is typically measured in watts per metre of fire front (W m-1). Other indicators of intensity include flame height, rate of spread and fire propagation through spotting (Keeley, 2009). More intense fires have larger flames that consume greater quantities of live and dead foliage on understorey shrubs, and bark on standing trees (Gould et al., 2007; Wotton et al., 2012). Fire intensity is a good measure of how likely a fire is to propagate, and how difficult it will be to suppress. In most cases, the aim of hazard reduction burns is to reduce the intensity of bushfires to a level where there is greater probability of successful suppression (Fernandes and Botelho, 2003; Howard et al., 2020).

A large body of evidence suggests that altering and reducing the fuel load through prescribed burning can reduce fire intensity (see, for example, Gould et al., 2007; CSIRO, 2019; Cheney, 2010, Price and Bradstock, 2010; Price and Bradstock, 2011; Fernandes and Botelho, 2003; Sneeuwjagt, 2008; AFAC, 2015; Boer et al., 2009; Howard et al., 2020). However, there is uncertainty about the extent of this effect. For example, Price and Bradstock (2010) found that fuel age had only limited effect on propagation of unplanned fires. Findings from the Project Vesta experiments indicate prescribed burning reduces intensity but the extent of the reduction varies across forest stands depending on the life history of the understory species (McCaw et al., 2012).

The scale of burning required to affect fire intensity meaningfully is debated (AFAC, 2015). For example, McCarthy and Tolhurst (2001) found that intense, fast-moving bushfires can readily pass over or around fuel reduced areas that are relatively small, isolated, and/or separated by long unburnt areas with far heavier fuel loads. The implication of these results is that prescribed burning would need to be undertaken across a reasonably substantial area to reduce fire intensity effectively. This is consistent with the findings from several studies that have noted that, in eucalypt forests prone to spotting, fuel reduced areas several kilometres deep are required to reduce the density of new ignitions started by spotting (McCaw et al., 2012; McCaw, 2013).

The significant scales of burning required to generate meaningful fire intensity reductions have led some researchers to conclude that prescribed burning might best be strategically located to protect assets or target areas of high risk. For example, using simulation modelling, Furlaud et al. (2017) found that to substantially reduce the intensity of wildfires at a state-wide scale in Tasmania, prescribed burning would need to be undertaken across so large an area that they considered it unrealistic. Due to this, they recommended prescribed burning be used in a more targeted manner.

While there is considerable support for the notion that prescribed burning can reduce the intensity of bushfires under low to moderate fire weather conditions, there is uncertainty about its effects on intensity under extreme conditions (AFAC, 2015). A significant body of research indicates that prescribed burning becomes less effective in aiding suppression efforts when the FFDI exceeds Very High (above 50). The literature suggests that, at this point, bushfire behaviour becomes ‘weather dominated’ and variations in fuel and topography become less significant (see for example, Fernandes and Botelho, 2003; Gibbons et al.,

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2012, McCarthy and Tolhurst, 2001, Price and Bradstock, 2012; Tolhurst and McCarthy, 2016). For example, Price and Bradstock (2012), in their analysis of the 2009 bushfires in Victoria where the FFDI was well above 100, found that intensity reductions resulting from prescribed burned areas up to 5-10 years old would be insufficient to increase the chance of effective suppression. Tolhurst and McCarthy (2016) similarly found that the effects of prescribed burning in assisting fire suppression declined substantially when the FFDI exceeded 50.

Other studies have found that prescribed burning (and other fuel reduction methods) can reduce the intensity of bushfires under extreme conditions (see, for example, McCaw, 2013; Sneeuwjagt, 2008; Bradstock et al., 2010; Price and Bradstock, 2010). For example, McCaw (2013) highlights evidence that, under severe weather conditions and extreme fuel dryness, fire intensity was reduced within prescribed burnt areas and for a distance of up to several kilometres downwind of them, even where the extent of area burned by a bushfire was not arrested by prescribed burning activity. Studies that have utilised spatial data have also found that, under extreme weather conditions, the probability of crown fire (an indicator of intensity) in eucalypt forest is significantly reduced for up to 5 years after previous fire (Bradstock et al., 2010, McCaw, 2013).

Effect on fire extent Fire extent (also known as fire scale) refers to the total amount of land burned as a result of an unplanned fire. Researchers often use the concept of ‘leverage’ to study the effect of prescribed burning on the extent of bushfires. Leverage refers to the ratio between the area saved to area treated; in other words, the unit reduction in wildfire resulting from one unit of prescribed burning (Loehle, 2004; Price et al., 2015).

A number of studies have found that prescribed burning can minimise the extent of unplanned fire (see, for example, studies by Cheney, 2010; Boer, et al., 2009; Price et al., 2015, Sneeuwjagt, 2008). For example, using 52 years of fire history in the dry eucalypt forests of southwest Western Australia, Boer et al., (2009) found that every four units of prescribed burning resulted in a one unit reduction in the extent of bushfires (i.e. a leverage ratio of approximately 0.25).

Other leverage studies have found more variable results. For example, Price et al. (2015) explored the leverage ratios across 30 bioregions in south-eastern Australia over a 25 year period and found similar leverage ratios to Boer et al. (2009) in two of the bioregions: New England Tablelands (0.36) and North Coast (0.29) in New South Wales. However, they found significantly lower leverage ratios in the Sydney Basin (0.16) and Australian Alps/South Eastern Highlands (0.09) bioregions and no or a negative leverage effect (prescribed burning increased wildfire extent) in the remaining 26 bioregions. Several other studies have similarly found a limited relationship between prescribed burning and the extent of bushfires (see for example, Price and Bradstock, 2010; Price and Bradstock 2011; Cary et al., 2009).

The findings from a number of studies suggest that the influence of prescribed burning on fire extent is reduced when the fire burns under strong winds, low humidity or during drought (e.g. Price and Bradstock, 2010; 2011). There is also evidence that the extent of bushfires is spatially variable and heavily influenced by non-fuel related variables such as weather and ignition management efforts (e.g. Cirulis et al., 2020; Cary et al., 2009).

A number of studies have suggested that strategically locating prescribed burning areas within the landscape can reduce the area that needs to be treated to achieve a given reduction in bushfire extent (e.g. McCaw, 2013; Price and Bradstock, 2010; King et al. 2006; Loehle, 2004).

As with fire intensity, there is divergent evidence and debate about the role of prescribed burning in influencing fire extent under extreme conditions (AFAC, 2015). A significant body of research suggests that weather is a stronger influence on the extent of unplanned fire than fuel loads or previous area burned (see for example Price et al., 2015; Cirulis et al., 2020; Cary et al. 2009; Penman et al. 2011; Penman et al. 2013; Price and Bradstock, 2011; Boer et al., 2009). However, several studies suggest that, even under extreme conditions, fire extent can be effectively reduced by prescribed burning (see for example, Grant and Wouters, 1993; Attiwill and Adams, 2013). An argument made in support of this proposition is that prescribed burning can provide windows of opportunity where small fires can be extinguished or controlled

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with aerial and other initial attack resources, thereby preventing them from becoming mega-fires (Attiwill and Adams, 2013).

Effect on housing and infrastructure loss There is a substantial body of research that demonstrates that vegetation and fuel reduction around a structure can significantly increase the chance of structure survival in a bushfire (e.g. Cohen, 2000; Cohen, 2004; Chen and McAnaney, 2004; Crompton et al., 2010; Gibbons et al., 2012; Syphard et al., 2014; Penman et al. 2019). Fuel management close to houses and strategic assets aims to reduce house loss by creating defensible space. This involves the clearing of vegetation and other materials near a structure both to reduce nearby fuels and enable a safe and effective fire response. It is designed to reduce flame contact and radiant heat from an approaching fire and also decrease the chance of spot fires igniting from embers (Syphard et al., 2014; Penman et al., 2019).

A number of studies suggest that fuel management activities, including prescribed burning, are most effective when undertaken in close proximity to assets (see for example, Cary et al., 2009; Ager et al., 2010; Bradstock et al., 2012; Penman et al., 2014; Gibbons et al., 2012; Penman and Cirulis, 2020; Price et al., 2015; Price and Bradstock, 2010; Furlaud et al., 2017; Cirulis et al., 2020). For example, Penman et al. (2014) found that halving the risk of high intensity fires reaching houses in Sydney was possible at rates of burning 10 per cent of the relevant area, if treatment was exclusively in the urban interface. The same rate of treatment, when confined to landscape burns, achieved a reduction in risk of 19 per cent.

There is also a considerable body of research that suggests that prescribed burning undertaken away from assets and the urban interface zone can significantly reduce the risk of asset losses (Tolhurst 2007; Florec et al., 2019; Penman and Cirulis, 2020; Cirulis et al., 2020). For example, Tolhurst (2007) indicates that more extensive prescribed burning at a landscape scale, located distant from property and settlements (including in remote areas), can mitigate bushfire threat and make it easier to suppress fires that could gain momentum and spread to settled areas. Similarly, Penman and Cirulis (2020), in their study of fuel treatments in the ACT and surrounding area, found that extensive fuel treatments reduced risk. Their research also suggested that ongoing treatments were required to reduce fire size and retain optimal risk values.

Several studies on bushfire-related housing loss demonstrate that weather has a strong effect on losses, with a greater proportion of houses lost at higher temperatures and wind speed and lower relative humidity. These studies also demonstrate benefits of modifying fuel closer to houses to reduce housing loss under extreme fire weather conditions (see, for example, Gibbons et al., 2012; Price and Bradstock, 2012). For example, Gibbons et al. (2012), who examined the impact of modifying key fuels on housing loss during wildfires that burn in extreme fire weather conditions, found that fire weather conditions were the predominant effect on losses. In severe fire weather, reducing trees and shrubs from 90 per cent cover to 5 per cent cover within 40 m of houses could potentially reduce house loss by an average of 43 per cent.

Studies also demonstrate that other factors, such as housing density and design, urban planning to establish houses away from highly fire prone areas, landscape position, proximity and type of vegetation or trees around the house (e.g. within 30 metres), irrigation and water bodies, and building construction materials, were equally or more important than hazard reduction activities such a prescribed burning (Penman et al. 2019; Gibbons et al., 2012; Penman et al., 2015; Cary et al., 2003).

Effect on fire severity (ecological impacts) Fire severity generally refers to the ecological impact of bushfires, including tree mortality, loss of biodiversity, and adverse impacts on soil and water quality (Keeley, 2009). Keeley (2009) notes that, in empirical fire studies, severity is often simply defined as the change in aboveground and belowground organic matter.

While prescribed burning is generally carried out with the intent of reducing bushfire risks to lives, property and infrastructure, it can also be used to reduce the severity of ecological impacts. The potential ecological benefits associated with prescribed burning can arise by reducing the extent, severity and rate of spread of bushfires, and by creating greater patchiness in burning patterns (see, for example, Evans and

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Russell-Smith, 2020; Tolhurst and McCarthy, 2016; Tolhurst, 2003; 2007; AFAC, 2015; McCaw, 2013; Price and Bradstock, 2012).

There is however uncertainty about the extent to which prescribed burning generates positive ecological impacts. This uncertainty stems from the uncertainties discussed above about the effectiveness of prescribed burning in reducing the extent and intensity of bushfires, particularly in extreme fire weather. Some evidence suggests that the measurable effect of reduced fuel on unplanned fire severity is limited (see for example Price and Bradstock, 2012). Other research indicates a broad scatter of fire severity data within their results, indicating that fire severity at any point in the landscape is a result of many interacting factors (see, for example, Tolhurst and McCarthy, 2016).

Costs of prescribed burning There are costs associated with all types of fuel management. In the case of prescribed burning, these can be placed in four broad categories: resource costs from prescribed burns; damage costs from fire escapes; smoke-related impacts; and environmental impacts from burns.

Resource costs refer to the labour, equipment, fuel and other costs associated with undertaking prescribed burns. There is limited information on the resource costs of prescribed burning in the published literature, although several studies have found that they are highly variable, ranging from under $100 to $10,000 per hectare, depending on the location and associated risks (see, for example, Florec et al., 2019; Penman and Cirulis, 2020; Bradstock et al. 2012; Gibson et al., 2016).

Prescribed burning comes with risks of fire escapes, which can lead to deaths and injuries, and adverse impacts on property, infrastructure and other assets (McCaw, 2013). There are a number of documented cases in Australia where damaging fires have been caused by escaped prescribed burns. For example, a prescribed burn in Lancefield-Cobaw, Victoria in 2015 broke containment lines and destroyed several homes, fence lines and more than 3000 hectares of farmland and forests (Independent Lancefield-Cobaw Fire Investigation Team, 2015). In 2011, an escaped prescribed burn close to Margaret River in Western Australia led to the destruction of 32 houses and other infrastructure, and burned out more than 3,400 hectares of land (Keelty, 2012).

There is extensive literature on the linkages between particulate pollution and respiratory illnesses. As fires are a source of particulate pollution, there is a risk that prescribed burns (and bushfires) can lead to adverse health effects, particularly amongst vulnerable populations. Consistent with this, several Australian studies have found positive associations between vegetation-fire smoke exposure and adverse health outcomes (see, for example, Haikerwal et al., 2015a; 2015b; Dennekamp et al., 2015; Dennekamp and Abramson, 2011; Horsley et al. 2018, Borchers et al. 2020; Broome et al., 2016). However, there is uncertainty about the extent of the effect, particularly where exposures are episodic. Several of these studies have indicated that the health impacts of prescribed burning must be evaluated in the wider context of the adverse impacts, including on health, of unplanned fire activity (e.g. Broome et al., 2016). In addition to the health impacts, smoke from prescribed burns can have adverse impacts on some industries, including viticulture and apiaries. This has led to conflict in some areas. For example, McCaw (2013) describes how vineyard owners in Western Australia initiated legal proceedings to recover commercial losses attributed to impacts of smoke from prescribed burning. The court ultimately ruled it would be unreasonable to impose a duty of care to avoid smoke damage to wine grapes on a public authority charged with fire management responsibilities and biodiversity conservation functions; a decision upheld on appeal. From an ecological perspective, the impact on plants from smoke generated by biomass burning and the toxicology of smoke from biomass burning on any form of native fauna have been identified, amongst other areas, as requiring additional study (see for example Bell and Adams, 2009).

The findings of research into the environmental impacts of prescribed burning are variable (see for example, Bowman et al. 2013; AFAC, 2015). There is some evidence of adverse environmental consequences, for example, the loss of obligate seeding plant species in some areas (see, for example, Cullen and Kirkpatrick, 1988; Harris and Kirkpatrick, 1991), changes in species diversity (Bradstock et al., 1998; Tolhurst, 2003), and increases in carbon losses to the atmosphere (di Folco et al., 2011; Bradstock et al., 2012b). However, several studies have highlighted the need for greater longitudinal analyses of the

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impacts of prescribed burning regimes (e.g. Burrows et al., 2019; AFAC, 2015; Lindenmayer, 2018; Tolhurst, 2003).

The necessity for ecosystem-specific understanding of the ecological impacts of prescribed burning and burning frequency is highlighted in the literature. Studies demonstrate that some flora and fauna require fire or similar disturbances to regenerate or renew their habitats (e.g. acacias). Other species have adapted survival strategies when exposed to natural stresses such as fire. Some species may require fire but also require long fire-free intervals to ensure they reach maturity and continue to flourish and some ecosystems should be protected from prescribed burning entirely (see, for example, AFAC, 2015; Lindenmayer and Taylor, 2020; Penman et al., 2011b; Dixon et al., 2018; Croft et al., 2016).

PART TWO: Mechanical Fuel Load Reduction Mechanical fuel load reduction involves use of machinery to remove or alter the structure of fuels in a landscape. It encompasses targeted reduction of understorey and dense forest and regrowth, harvesting and commercial thinning of selected trees at fixed intervals and removal of trees and vegetation to create fuel breaks or buffers in order to protect developments and infrastructure.

Mechanical fuel load reduction activities encompass a range of techniques and technologies, including (but not necessarily limited to):

use of hand held machinery such as mowers and brush cutters, targeting grass and other vegetation

use of mechanical slashers such as ride on mowers and tractor towed implements,

use of ploughs, graders and bulldozers to produce breaks

use of pruners or tree removal machinery to remove canopy, branches or full trees

reducing fine fuels and leaf litter by hand

Technology to underpin mechanical fuel load reduction often comes from traditional forest operations or agricultural operations sectors, which are well suited to handling larger size stems, as well as technology from agroforestry using equipment such as forage harvesters, mowers and grinders (Ximenes et al., 2017).

A limited literature base Mechanical fuel load reduction as a means of vegetation fuel management is widely used in other parts of the world, such as in the United States. In Australia, there is less public discussion about these activities and a lower familiarity with their use as part of a fuel management strategy (Ximenes et al., 2017). Consequently, the literature on the efficacy of these techniques in different environmental, economic and social conditions is limited (Ximenes et al., 2017; Volkova and Weston, 2019).

The majority of available literature on mechanical fuel load reduction focuses on the thinning of forests at a landscape level, rather than exploring these techniques in targeted environments such as road sides. There is some academic literature exploring the value of this technique vis-à-vis prescribed burning in more built up areas. Most literature identifies forest type, but there is little evidence comparing the same techniques across different forest environments. There are also limited academic data available to assess the trade-offs between alternative fuel reduction treatments.

Potential benefits Studies have identified a number of potential benefits of mechanical fuel load reduction, such as avoiding adverse impacts on air quality through smoke, reducing potential threat to humans, animals and property through fire escape and an ability to be conducted in an environmentally sound manner around key assets, such as conservation areas, roadsides and key strategic assets. It also addresses the progressively narrower window of appropriate weather days now available for undertaking fuel reduction burning in light of changes in climatic conditions (see, for example, Australian Forest Products Association, 2020; Ximenes et al., 2017; Deloitte Access Economics, 2014).

Mechanical thinning as a mechanical fuel load reduction technique is intended to slow the rate of fire spread, lower flame heights and improve recovery. In the United States, a central component of the

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Collaborative Forest Landscape Restoration Program involves the thinning of forest regrowth using machinery for the purposes of bushfire mitigation. A mid-term review of the program (United States Department of Agriculture, 2015) found that re-establishing desired vegetation conditions through mechanical thinning or prescribed burning makes landscapes more resilient to fire and reduces the risk of catastrophic wildfire. The review also found that these mechanical and burning treatments help preserve and restore critical wildlife habitat, protect the water supply, enable firefighters to manage fires more safely, reduce the risk to communities, and may also result in providing wood byproducts to benefit local economies.

In Australia, under the National Bushfire Mitigation Programme, $1.5 million of a total $15 million funding allocation was allocated to implementation of mechanical fuel load reduction trials. The trials aimed to assess and quantify the effectiveness and efficiency of these activities in areas where forest structure or proximity to urban interfaces make prescribed burning difficult to implement, or where values of assets could be compromised by prescribed burning. The trials also looked at whether mechanical fuel load reduction, compared to prescribed burning, is acceptable to the community, cost effective, mechanically feasible and effective in reducing fire risk across the landscape (Ximenes et al., 2017). Evaluation of the trials is not publicly available as yet.

Proctor and McCarthy (2013) examined the changes in the overall fuel hazard (influenced by the occurrence of shrub and bark fuel) in East Gippsland Victoria over a fifteen year period after thinning. Their study found that the equivalent overall fuel hazard was on average lower than the overall fuel hazard in adjacent un-thinned sites, primarily due to the reduction of elevated fine fuel (bark and shrubs especially). They concluded that thinning, by reducing the overall fuel hazard, may reduce the likely suppression difficulty by substantially reducing the potential for vertical development of fire at the flaming fire front. However, by increasing the amount of coarse woody material on the ground, thinning may increase the difficulties of complete fire extinguishment.

A study by Piqué and Domènech (2018), which examined the effects of thinning intensity and type of slash management on forest fire behaviour in Mediterranean Pinus nigra forests, also found that thinning followed by fire is the most effective treatment for reducing crown fire potential. They also found that low thinning of light intensity keeps the canopy closed, thus allowing greater control of understory growth and that heavy intensity thinning, with a greater canopy opening, is expected to develop more understory faster over time, and thus lose effectiveness.

Interactions between mechanical fuel load reduction and other forms of hazard mitigation In their research outlining the optimal design of mechanical fuel load reduction trials, Ximenes et al. (2017) note that any effort to modify fuels to mitigate bushfire risk must consider the reduction in fuel hazards in all attributes (for example, amount of material, horizontal and vertical continuity of the layer, thickness of the fuel, total weight of the fuel) in each of the four strata (surface, near surface, elevated and bark). Reducing any one attribute in isolation without addressing the hazard represented by other fuel attributes or other fuel strata would not necessarily reduce the risk posed by a bushfire. Any effectiveness comparison needs to consider the overall effect of the hazard presented by all four layers. This may involve the interactions of a number of different treatments.

A number of scientific studies have examined the effectiveness of mechanical thinning operations in combination with prescribed burning across a landscape scale. For example, Volkova and Weston (2019), examined the effect of mechanical thinning, burning and combination fuel reduction treatments on forest carbon and bushfire fuel hazard in open forests dominated by Eucalyptus sieberi in South-Eastern Australia. Their study found that a combination of mechanical thinning and burning was the most effective treatment for reducing above ground fuel hazard, with major reductions in dead trees, stumps and understorey, as well as stems removed from the sale of pulpwood. Significantly, the study also found that thinning without burning did not reduce pre-treatment fuel hazard rating due to high surface forest floor fuel loads and loose and flammable bark on the retained over-storey trees and concluded that the main fuel hazard reduced by thinning is understorey fuels.

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A review by Kalies and Kent (2016) of 56 studies addressing fuel treatment effectiveness in eight states in Western United States found that a combination of thinning and prescribed burning treatments helped to reduce fire severity, tree mortality and crown scorch. Burning or thinning undertaken by themselves had either less of an effect or no effect at all compared to untreated sites. Studies which focused on carbon storage demonstrated that application of treatments did not result in more carbon storage but did reduce post wildfire carbon emissions due to tree mortality. The treatment effects on understorey plants was highly variable and there were significant data gaps regarding the treatment effects on ecological variables such as soil, wildlife, water and insects.

Hartsough et al. (2008), in their analysis of seven forest sites in western United States, found that mechanical fuel load reduction combined with fire treatment was the most effective, followed by fire only, in reducing the modelled severity of bushfires under extreme conditions. The effectiveness of mechanical only treatments depended on how much surface fuel remained on site, and a whole tree harvesting system which removed the tops and limbs along with felled trees were more effective in reducing fire severity than methods which left debris.

Effect of logging activity on fire intensity The evidence of the extent to which human interventions such as selective and clear-fell logging (not undertaken with a fire management purpose) influence fire severity is contested (see, for example, Attiwill et al. 2013; Bradstock et al, 2014). Although commercial logging activity is not explicitly undertaken for the purposes of mechanical fuel load reduction for fire mitigation, supporters of this practice have cited its benefit as a fire hazard mitigation technique.

Several studies have found that logging activity can increase fire severity in wet eucalypt forests (see, for example, Winoto-Lewin et al., 2020). However, Attiwill et al. (2013) found no effect of logging on fire risk.

Research in some eucalypt forests subject to logging activity indicates that forests that had regenerated after harvesting were at greater risk of burning at higher intensity than older, unlogged forests (see, for example, Price and Bradstock, 2012; Taylor et al., 2014).

Studies by (Ough and Murphy, 1996) indicated that elevated fire severity in logged forests is due to:

the extensive amount of logging slash left behind, which contributes to forest fuel;

the loss of mesic understory plants such as tree ferns in logged arounds, which contributes to drying of the forest; and

the densely stocked stands created by reseeding after logging.

Ecological implications The impact of mechanical thinning treatments on biodiversity has been investigated in only a limited number of studies in Australia. Researchers such as Ryan (2013) outline the benefits of gaining a more comprehensive understanding of a wide range of bushfire management policies, including adaptive thinning, on local fauna.

There is some evidence that mechanical fuel load reduction activities may have positive impacts on biodiversity. These studies are focused on forest thinning in general, rather than thinning for the purposes of bushfire mitigation (see for example, Pike et al. 2011; Barr et al., 2011; Craig et al., 2010; Bluff, 2016).

In their research into the optimal design of mechanical fuel reduction trials, Ximenes et al. (2017) note that the design of any mechanical thinning operation must be heavily influenced by the forest conditions and that, in the interest of minimising adverse site and soil disturbance, mechanical fuel load reduction systems will need to use relatively small and agile machines. Machines with a light footprint will allow the operation to be carried out without soil damage or damage to the retained trees.

Costs and resourcing requirements There is a dearth of research and data analysis assessing the relative costs and benefits of alternative hazard reduction methods such as mechanical fuel load reduction. This can, in part, be attributed to a

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policy focus on fuel reduction burning (Deloitte Access Economics, 2014). Deloitte Access Economics (2014) argues that there are prima facie indications of public policy benefits of alternative approaches to fuel load reduction, such as mechanical fuel load reduction, but this would require additional testing.

The Deloitte study outlines the key factors affecting mechanical treatment costs as the forest structure (e.g. tree stocking, species), the types of fuel to be targeted (e.g. overstorey, intermediate, elevated, surface), harvesting technique, extraction and processing (e.g. in-field chipping), haulage distance and availability of markets. Ximenes et al. (2017) note that the involvement of heavy machinery in mechanical fuel load reduction incurs significant expense.

Ghaffariyan and Brown (2013), in their study of the most efficient harvesting methods in Western Australian eucalypt plantations, note that the size of the material to be cut, under most operation conditions, is the primary driver of the cost of the operation. Ximenes et al. (2017) identify that after the size of the trees to be removed, terrain conditions (such as slope, roughness, post-treatment tree spacing and what will be done with the resulting material after the operations (e.g. recovered for use or treated on site) will be the next most important factors to consider from both an efficacy and cost basis.

Some studies indicate that the costs of mechanical fuel load reduction can be offset to some extent by the sale of biomass (see, for example, Ximenes et al., 2017; Rothe, 2013; Deloitte Access Economics, 2014). To provide a basic estimate of the offset benefits of sale of biomass in mechanical fuel reduction activities, Deloitte Access Economics (2014) compares costs of sourcing woody biomass for renewable energy from conventional wood production forest areas. The main costs are associated with harvesting, chipping, loading and haulage. The report estimates delivered cost of biomass to be between $30 and $85 per green tonne, depending on type of material.

PART THREE: Livestock Grazing

A limited literature base Use of livestock as a fire hazard reduction technique aims to reduce and alter fuel types and abundance.

Grazing as a fire management technique, particularly the use of cattle, sheep and goats, has generated significant debate in Australia. There are differing view on the effectiveness of this technique as a means of fire management, as well as debate on the appropriateness of using this technique in national parks and the potentially damaging effects on biodiversity.

Researchers note that there have been limited scientific studies examining the capacity of cattle grazing to reduce fire hazard. As such, there is limited evidence to demonstrate that grazing is an effective tool to mitigate fire extent or severity at landscape scales under the weather conditions that lead to large fires.

An inquiry into the 2002/2003 bushfires in Victoria (Esplin et al., 2003), which drew on the evidence of fire ecologists, found that there was no scientific support for the view that ‘grazing prevents blazing’ in the alpine and sub-alpine high plains. In 2005, the Alpine Grazing Taskforce, which investigated and reported on options relating to the future of cattle grazing in the high country in eastern and north-eastern Victoria, similarly found that fire severity was predominantly determined by the prevailing weather conditions, topography, fuel loads and fuel flammability types, not whether an area has been grazed. The Taskforce concluded that cattle grazing does not make an effective contribution to fuel reduction and wildfire behaviour in the Alpine National Park (Alpine Grazing Taskforce, 2005). Other research also concludes that grazing has a limited impact on reducing fire hazard, and returns findings that livestock grazing may increase fire hazard (Williamson et al., 2014; Kirkpatrick et al., 2011; Kirkpatrick and Bridle, 2016).

Advocates of grazing as a bushfire mitigation tool refute these findings, and draw predominately on personal experiences to highlight the benefits of grazing as a bushfire mitigation technique (Mountain Cattleman’s Association of Victoria, 2010).

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International evidence Internationally, there is some evidence supporting grazing as a fuel reduction strategy combined with other fuel reduction techniques. A study by Damianidis et al. (2020) examined whether the practice of integrating agricultural crops and/or livestock (such as horses, pigs, sheep, goats and beef cattle), could be a management tool to reduce wildfires in European Mediterranean countries. The study found that agroforestry areas had fewer wildfire incidents than forests, shrub lands or grasslands, and indicated that this was evidence of the potential of agroforestry to reduce fire risk and protect ecosystems. Horses, pigs and goats have a preference for feeding on woody shrubs and young trees and will help convert land to a more herbaceous cover, where animals with a preference for less woody vegetation, such as cows and sheep, can graze. Relevant factors for controlling the biomass include the stocking rate and the type of grazing pattern. The authors identify grazing as a practice particularly relevant at the interface of urban and densely vegetated areas.

Next steps The Commission continues to gather information and analyse evidence relating to vegetation fuel load management in the context of natural disasters, and will not make findings or draw conclusions until it has completed this process.

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