Proceedings of the conference held 8-9 October 1994, Footscray, Melbourne
Biodiversity Series, Paper No. 8
Department of the Environment, Sport and Territories, 1996
1. The effectiveness of fuel reduction burning for fire management
CSIRO Division of Forestry
In 1994, reviews of fire management strategies and practices by State fire authorities and land management agencies in Western Australia, Queensland, Tasmania and New South Wales all revealed disturbing trends in fire management in Australia. All recognised a role for prescribed burning to reduce fuel load and the impact of fire, and to assist fire suppression. Governments, however, are providing fewer resources to land management agencies to undertake fire management and are placing greater demands on volunteers. There is a lack of expertise to ensure management burning meets prescriptions.
Despite the repeated conclusions of professional review teams there continue to be claims from people who are opposed to the use of fire for fuel management that prescribed burning is ineffective in reducing the spread and threat of wildfires. Some claims reflect a poor understanding of fire behaviour and may be due to over-simplified assumptions about the relationship between fuels, spread and fire suppression. Recent research has shown that simple direct relationships between fuel load and fire spread cannot be applied to all fuel types. More detailed characterisation of the fuel bed is required to better understand and predict fire behaviour. There is good qualitative evidence that fuel reduction assists fire suppression by reducing the rate of development and the spotting potential of forest fires. Further research is required to quantify the magnitude of these effects under difficult fire weather and the time that they will persist after burning.
Key words: fuel reduction, fire management, fire behaviour, prescribed burning, fire suppression
Reviews of fire management practices were already in place in Western Australia (Lewis et al. 1994), Tasmania (Bale 1994) and the Commonwealth (Anon. 1994a) before the bushfires in January 1994 in coastal New South Wales (NSW). The fires stimulated an Inter-Departmental Committee audit of fire management in Queensland (Anon. 1994b), a parliamentary committee in South Australia, and in NSW a Cabinet Committee investigation as well as a Coronial inquiry. While the reviews were put in place for different reasons and had a different focus in each State they reveal a number of common trends that should give concern to land management agencies and people concerned with the role of fire in maintaining bio-diversity.
Over the ten years prior to 1994 there has been:
- reduction in funding to government land management agencies (LMA) for fire management;
- decline in manpower in government LMA (all States);
- sharp decline in expertise in fire management, particularly at line and sector level (all States);
- decline in the area prescribed burnt for fuel or habitat management;
- shift away from LMA-directed fire suppression towards suppression by Rural Fire Services in Qld, ACT, Tas and NSW;
- increased reliance on volunteers to undertake suppression on government lands; and,
- increased funding to emergency management agencies.
In Tasmania there have been substantial changes in the rural population demography and in fire philosophy. The involvement of rural landholders in volunteer bushfire brigades co-ordinated by the Tasmania Fire Service is declining as is the general level of expertise in forest fire fighting outside forestry employees (Cheney 1993). Volunteers are concerned by the creation of major fire hazards caused by changing land use (e.g. withdrawal of grazing, logging, cessation of fuel reduction burning) and are questioning their volunteer role in fighting fires on land where they perceive the owner or agency has contributed to the fire hazard (Bale 1994). The manpower resource of the Forestry Commission to fight fires is aging and there are probably insufficient fire fighters trained to crew leader or sector boss level to supervise fire fighters if multiple fires occur or fires extend beyond 2 work periods (Cheney 1993; Bale 1994).
In WA on the other hand, the efficiency of the Department of Conservation and Land Management (CALM) practice of prescribed burning to prevent the development of major forest fires has been so effective in the south-west of the State that local government authorities and residents in surrounding shires have developed a false sense of security and relaxed planning constraints on rural developments. CALM, however, has allowed manpower and equipment levels to decline so that suppression crews are having difficulty in maintaining the prescribed burning programs and are concerned about their physical safety when suppressing wildfires (Lewis et al. 1994).
Similar trends are occurring in other States and have been accelerated by governments reducing public sector manpower and expenditure. For example in NSW prior to the January bushfires, State Forests had reduced their central fire management branch to one person, and there is a continuing decline in experienced forestry field staff (Dicker, I. 1994 pers comm). In Victoria fire protection branch staff of the Department of Conservation and Natural Resources are concerned that recent staff redundancies have reduced the capacity to fill key fireline positions with experienced staff.
After the NSW fires the dominant view of governments, media, review committees and the public was that extensive high-intensity bushfires that cause loss of life and major property damage are unacceptable, regardless of their biological consequences in natural areas. The NSW events also demonstrated that if bushfires were not quickly contained during prolonged periods of extreme fire weather, they would burn large areas and eventually threaten life and property adjacent to parks and reserves - allowing fires to develop to such an extent was also unacceptable.
Other trends are: vocal opposition to prescribed burning has reduced (at least temporarily); an increase in public demand for more fuel reduction burning; and, some increase in funding for fire management in WA and NSW. There has been extensive lobbying to increase Australia's fire suppression capacity through the introduction of expensive water-scooping aircraft. A Senate Committee recommended that the Commonwealth government in conjunction with State Governments trial at least four CL-215 water bombing aircraft (Anon. 1994a), despite the view of the Australian Fire Authorities Council that this was not the most cost-effective alternative.
Spectacular potential solutions to our bushfire problems like large water scooping air tankers are attractive to some politicians, but the main conclusion of many reviews after is that mitigation of risk is an essential facet of bushfire management. An integral part of this mitigation in our native forests and plantations is to change the structure and reduce fine fuel loads by prescribed burning or other means (Anon. 1977; Miller et al. 1984; Lewis et al. 1994; Bale 1994). Even in North America, where vast sums (by our standards) are spent on sophisticated suppression techniques, the US Forest Service has recognised the need for a greatly increased prescribed burning program to assist suppression operations (Thomas 1994).
However, there are still some public concerns that using repeated low-intensity fire to reduce fuel loads does not make fire suppression easier. Furthermore, it is thought that repeat burns may have an adverse impact on biodiversity and other forest values such as soil fertility and forest productivity. In WA where prescribed burning is carried out extensively through both the dry jarrah forests and the wetter karri forests public concerns about CALM's burning program were divided. There was considerable opposition to spring burning, but for differing reasons. Farmers generally were concerned about the lower fuel removal and the possibility of re-ignition in summer before crops are harvested. They favoured hot burns in autumn which reduced the maximum amount of available fuel. Conservationists felt that spring burning was biologically more damaging than autumn burning, particularly to nesting birds, despite the lower fuel removal and generally lower heat release. The review panel found no evidence to substantiate the claims that repeated burning had caused a long-term change in either plant or bird populations in the jarrah forest. It was recommended that the necessary funding be provided to the prescribed burning program to increase the diversity of season, intensity, frequency and sequence of fires on any single area to ensure that no species will become threatened or extinct (Lewis et al. 1994).
The continuing trends of reduced expenditure by government land management agencies, reduced expertise, and greater reliance on suppression by volunteers on public lands, will make it increasingly difficult to carry out prescribed burning programs. Burning for biodiversity will require more specific prescriptions and a wider range of intensities and burning conditions than is required for fuel reduction alone. This in turn will require better knowledge of fire behaviour and better practical application of fire which needs to be carried out by properly trained groups in the agencies who are charged with managing biodiversity.
The biological impacts of prescribed burning are discussed elsewhere in this conference record. In this paper I examine the physical basis for prescribed burning, look at claims about its efficacy in assisting fire control, and examine the research basis for
Reduced to the most basic outline, most land managers who take a responsible attitude towards fire management have three platforms in their fire management policies. These are:
- to manage fire so that damage to forest, environmental and economic values is minimised;
- to manage fires so that as far as possible they are prevented from spreading to other lands; and,
- to provide fire suppression on a least cost plus loss basis.
These policies have long been recognised and the conclusion reached in various Royal Commissions (Tucker et al. 1900; Stretton 1939; Rodger 1961) was that burning-off was essential in Australian forests to limit the spread and damage of bushfires. In the 1950s, A.G. McArthur set out to establish the relationship between fuel and fire behaviour, to define quantitatively the fuel and weather conditions under which burning-off could be carried out safely, and to set limits on prescribed fire intensity so that damage to forest values would be minimised.
McArthur (1958) correlated the rate of spread of a surface fire in dry sclerophyll eucalypt forest having little understorey with fine fuel load, dead fuel moisture content and wind speed. He defined the fraction of the fuel load that determined the spread of eucalypt fires as leaf, twig, bark and grass material <1/4 inch (6 mm) in diameter and claimed that:
- other factors being equal the rate of head fire spread was directly proportional to the load of fine surface fuels;
- in eucalypt forests crown fires were dependent on the presence of an intense surface fire and if surface fuels were less than about 7.8 t ha-1 crown fires would not occur even under the worst recorded conditions.
McArthur also concluded that: because of the spotting phenomena of fires in eucalypt forest, any fire management system based only on fire breaks would not meet the policy objectives outlined above; fire management needed to become fuel management. In his work Control Burning in Eucalypt Forests, McArthur (1962), set out the benefits of reducing the quantity of fuel available for combustion; viz:
- the intensity of a fire burning under any set of meteorological conditions is reduced;
- the rate of spread of the fire is reduced;
- the difficulty of constructing effective control lines is reduced, so any fire becomes easier to control; and,
- the likelihood of fires starting is reduced.
Most experienced fire fighters can verify the change in fire behaviour and the increased ease of suppression following fuel reduction burning. Few people argue with the definition of fire intensity as the rate of heat release from a fire front. This is the product of the heat of combustion, fuel load burnt and rate of spread (Byram 1959); it is also agreed that the lower the fuel load the lower the fire intensity.
There is, however, considerable uncertainty about how fuel characteristics determine fire behaviour, and about the efficacy of reducing surface fuel loads to modify fire behaviour particularly under extreme fire weather. Incorrect arguments have been put on both sides of the debate to justify stances. Anti-burners have claimed that under extreme weather conditions crown fires will continue to burn across recently (usually not defined in terms of fuel age) burnt areas. Pro-burners have included large fuel components to claim excessive fuel loads (>200 t ha-1) and justify burning. Knowledge about the subject is inadequate and many involved in the debate are confused.
A key problem in predicting fire behaviour is to define the fuel components that contribute to the flame front of the fire and hence to its rate of spread. The characteristics of a fuel bed such as the size of individual components, its continuity, fuel height and fuel load vary markedly depending on the vegetation type. It is therefore most important, when describing fire behaviour, to specify precisely the type of fuel concerned. The fraction of the total fuel load that is consumed in the flaming front of the fire clearly cannot be measured before the fire. Therefore, we must estimate the fraction of the fuel bed and the size of the fuel particle that is likely to burn in the flaming zone of the fire; in the smouldering zone behind the front; and, what may not burn at all under different weather conditions. In simple fuel types, such as grasslands, the fuels involved in the flame front appear quite straightforward, yet different proportions are burnt by flaming or smouldering combustion. This depends on the type of fire and the wind speed driving the fire.
In stratified fuel types, such as forest fuels, some fuel strata burn in the flame front when they are pre-heated by convection from fire in fuels below them. Thus as the weather conditions worsen, and fire intensity increases, increasingly elevated layers of fine fuels eventually including the tree crowns will be involved in the flame front. Some texts do not illustrate this process very clearly (e.g. Brown & Davis 1973; Artsybashev 1985) promoting the concept of independent crown fires. Independent crown fires do not occur in tall eucalypt forests (Luke 1961; Luke & McArthur 1978) because fire in the crown alone cannot preheat adjacent crowns by convection and lateral heat transfer by radiation is insufficient to maintain combustion.
The characteristics of the flame front depend then on the volume of fuel being consumed behind the leading edge of the fire, and the rate of spread will depend on the spatial continuity of the fuels and how they influence the heat flux from the fire. For example, a narrow fire trail will prevent a low-intensity fire from spreading. At progressively higher intensities, increasingly large discontinuities in the fuel bed will be overcome by the flame front so that local variations in fuel characteristics have less and less influence on the volume of the flame front. The characteristics of the fuel bed that influence fire behaviour must, therefore, be averaged over increasing spatial scales.
Most research into the influence of fuel characteristics on fire behaviour has been carried out in the laboratory or with relatively low-intensity experimental fires. High-intensity fires are more difficult to study; but recent research has given more insight into fuel characteristics that influence fire behaviour and into additional factors that need to be considered, when assessing the effectiveness of fuel reduction burning.
1.4.1 Fuel size
Provided sufficient heat is available to maintain combustion the residence time of a fuel particle, (i.e. the time that it remains flaming) is dependent only on the fuel size (Clements & Alkidas 1973). Cheney et al. (1990) confirmed this and determined the relationship between residence time and initial diameter of E. sieberi twigs and logs to be:
Tr = 1.7d1.67
where Tr is the residence time in minutes and d is the diameter of the fuel particle in centimetres.
The time taken for the fuel bed to burn out then depends not only on the average distribution of the sizes of the fuel particles that make up the fuel bed but also the depth and compaction of the fuel bed (Cheney 1990). Individual grass particles burn out in a few seconds and grass fires have residence times of between five and ten seconds. Thus in grasslands fuel particles greater than perhaps 3 mm will burn behind the flaming front of the fire.
In forest fuels where the bulk of the fuel bed is less than 6 mm in diameter the residence time is around 45 seconds. The above formula indicates that only a small proportion of the larger fuel components will be burnt in that time. Because surface fuels are compacted a fire will propagate across the loosely compacted surface layers and burn downwards into more compacted fuel layers (Cheney 1990). Thus in deep fuel beds the residence time of the flame front will depend on the time taken to burn down into the fuel bed and will be longer than the time taken to burn out individual fuel particles. Large fuel components embedded in the fuel bed will not ignite immediately the flame front arrives but will ignite when the flame front burns down to them.
McArthur's selection of a size limit of 6 mm was appropriate to define the fine fuels that burn in the continuous flame front of a surface fire in eucalypt fuels. However, it must be recognised that this size limit should be lower in fuel beds where very fine fuels dominate the fuel components, such as grasses and shrublands, and higher in fuel beds where larger fuel components, such as slash fuels, make up a greater proportion of the continuous fuel bed. Reduction of fine fuels only will substantially reduce fire behaviour although it may be desirable to reduce large fuels to assist suppression (e.g. mop-up).
1.4.2 Fuel continuity
Several authors (Burrows et al. 1991; Griffin & Allan 1984) have found that rate of fire spread in hummock grasslands was influenced by the patchiness (defined in different ways) of the fuel clumps and bare ground.
Fuel bed continuity is difficult to define in heterogenous forest fuels. The rate of spread of low-intensity fires in young E. sieberi forests with a distinct layer of wire grass understorey was directly related to the continuity of the near-surface fuel layer – the fuel layer above the surface litter layer which was composed of shrubs, grasses and suspended litter (Cheney et al. 1992). Continuity expressed as the percent cover was, however, largely a subjective estimate. A reasonable correlation was found between rate of spread of these fires and the average height of the near-surface fuel layer; a fuel characteristic which was easier to define and use by others in a burning guide than fuel continuity.
The bark on trees can be an increasingly important factor contributing to frontal flame dynamics as fire intensity and the density of trees with flammable bark increases. When fire intensities are low, the bark on the trees burns more or less independently of the surface fire and contributes little to the fire spread. Mild fires consumed 7 t ha-1 of bark in a forest where 55 per cent of the trees were stringybarks (Tolhurst & Flinn 1992): this represented more than 50 per cent of the load of surface fuels. As fire intensity increases the combustion of bark on the trees becomes an integral part of the flame front, contributes to the heat flux of the fire and lifts the fire into the tree crowns. As yet we have no adequate way of quantifying the contribution that bark combustion makes to the forward spread of a fire.
1.4.3 Fuel height and curing
The height of the fuel bed obviously contributes to the height of the flames. However, the influence on rate of spread is not straightforward. The spread of low-intensity (<1000 kW m-1) fires under E. sieberi forest was directly proportional to the height of the near-surface fuel layer (Cheney et al. 1992). In continuous open grasslands, however, the rate of spread of the fire could not be shown to be dependent on grass height (Cheney et al. 1993). The height of most fuel beds can be difficult to define for repeated measurements, particularly when there is a change in bulk density within the fuel bed.
Fire spread in grasslands is strongly dependent on the state of curing or the fraction of dead material in the pasture sward (McArthur 1966). There is observational evidence that the fraction of green shrubby materials in the near-surface fuels and the shrub layer of forest fuels modifies the behaviour of the fire. This has not been adequately quantified to include in any fire spread prediction system.
1.4.4 Fuel load
Fuel load has been the single fuel characteristic used to predict the rate of spread and fire behaviour within a dry eucalypt forest (McArthur 1962; Peet 1965). Although a direct relationship between rate of spread and fuel load has been demonstrated for low-intensity fires in fuels of different ages (e.g. McArthur 1967) no such relationship has been found in fuels of the same age. The rate of spread of head fires was not correlated with fuel load in uniform grass swards that had been harvested to different levels to change fuel load (Cheney et al. 1993).
However, in eucalypt forests, fuels re-accumulating after fire exhibit both changes in mass and fuel structure over time. Structure will change during the first five to ten years as litter accumulates, and grasses, fine shrubs and other fine fuel components develop. Although fuel load may not directly influence rate of spread it may be a surrogate measure of fuel age and thus represent a range of structural factors such as height, continuity and greenness, which do influence fire behaviour. These concepts require field testing. The characterisation of fuel loads burnt by moderate to high-intensity fires and quantification of the rate of spread of fire in fuels of different ages are major aspects of a research program currently planned between CSIRO Division of Forestry and CALM, Western Australia.
A substantial fuel load is required to produce an intense surface fire with strong convection to heat and maintain combustion in tree crowns. Burning bark contributes to crown fire initiation but, in the absence of strong convective heating, the crowns of eucalypt forests are too sparse and have insufficient dead material to allow fire to propagate horizontally.
1.4.5 Fire size
Recent research on high-intensity fires in grasslands and woodlands in northern Australia has shown that the potential rate of spread of a fire is dependent on the length of the fire ignition line (Cheney et al. 1993). As the fire progressively develops spread also depends on the width of the headfire (Cheney & Gould, in preparation). The width of the headfire front at which fires attain a quasi-steady-state rate of spread increases as wind speed increases. At high wind speeds this quasi-steady-state rate of spread may not be achieved until the fire front has achieved a width of more than 150 m. Although experimental data is limited it appears that similar relationships may also hold true for fires in jarrah forest fuels. The time taken for a fire lit at a point to develop a head fire width of >150 m depends primarily on fluctuations in wind direction and is virtually impossible to predict in advance. Under moderately severe burning conditions grass fires took between ten and 60 minutes to reach their potential rate of spread. In forests where the wind reaching the fire front is modified by the tree canopy the time taken for fires to grow from a point ignition to a width of 150 m or more can be very much longer. In these circumstances the fires may not achieve their maximum potential rate of spread before burning conditions moderate. Any fuel treatment that reduces the rate of spread and the responsiveness of fires to changes in wind direction will further delay the development of a fire to its full potential and provide advantages to initial attack crews.
1.4.6 Spotting effects
In the past, spot fires have been thought to cause substantial increases in the rate of spread of a fire (McArthur 1967). It now appears that the main influence of spotting is not to increase the rate of spread directly, because most spot fires are overrun by the main front before they can develop significant dimensions, but to enable the fire to overcome discontinuities in fuel and topography. The convection column of a fire burning on to a firebreak collapses when surface fuels burn out and produces a shower of spot fires directly across the break into fresh fuels. These spot fires rapidly coalesce and reform a fire front of a size similar to that existing before the break, and the fire progresses with little impediment to its rate of spread. A similar process occurs when a fire burns over a ridge.
Regardless of its influence on fire behaviour the onset of spotting in forest fires is the key factor which causes fire suppression efforts to break down and can trap and kill fire fighters (eg McArthur et al. 1966). There is good evidence that hazard caused by eucalypt bark (Wilson 1992a, 1992b) is reduced by prescribed burning and, in some fuel types, fire fighters consider reduced spotting can assist suppression for at least seven to ten years (Grant & Wouters 1993).
Although there are few direct quantitative data on the effect of fuel reduction burning on the rate of spread of high-intensity fires, observations of fire fighters and wildfire case histories have convinced Royal Commissions, Committees of Enquiry and Coroners that fuel reduction, by prescribed burning, is an essential component of fire management in eucalypt forests.
Recent research has shown that the definition of fine fuels used for prescribed burning guides is valid but that the suggested relationships between fine fuel load and rate of spread do not apply to all eucalypt fuel types.
Several fuel factors probably determine the rate of spread of intense forest fires. Both fuel mass and structure change after prescribed burning and may modify fire behaviour. The broad benefits of prescribed burning in modifying fire behaviour and assisting fire suppression are generally accepted but more research is required to quantify the changes in fire behaviour as fuels re-accumulate after burning.
The importance of changing the spotting potential of forest fuels has been generally under-rated. Spotting is the phenomenon which causes fires to overcome fuel discontinuities and is often the reason that fires escape control. There are many field reports but few case studies that quantify the effects that reducing spotting potential has on suppression effort. In some fuel types prescriptions for burning to reduce the height of elevated fuels and the quantity of flammable bark may be more important for fire suppression than prescriptions aimed at reducing only surface fuel loads.
It is likely that future prescriptions for burning for fuel reduction or biodiversity management will be more detailed and may extend over a wider range of environments than in the past. This will require more fire expertise and better practical training with fire in LMAs. Current trends in funding and staffing of LMAs mitigate against skilful use of fire.
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