Fire behaviour in spinifex fuels on the Gibson Desert Nature Reserve, Western Australia

Journal of AridEnvironments (1991) 20: 189-204

Fire behaviour in spinifex fuels on the Gibson Desert Nature Reserve, Western Australia Neil Burrows*, Bruce Wardt & Alex Robinson] "Department a/Conservation andLand Management, 50 Hayman Road, Como, Western Australia 6152, and tDepartment a/Conservation andLand Management, Manjimup Research Centre, BrainStreet, Manjimup, Western Australia 6538 (Received 4 December 1989, accepted 11 May 1990) Fire management proposed for desert conservation reserves in Western Australia will aim to recreate the fire-induced diversity that is believed to have existed for thousands of years during Aboriginal occupation. Since Aboriginal people no longer practise their traditional nomadic lifestyle, there has been a reduction in the diversity of fire regimes. There exists vast areas of long unburnt and sensescing spinifex (Triodia sp. and Plectrachne sp.) and vast areas burnt infrequently by lightning-caused wildfire. This has probably contributed to the reported decline in native fauna. In order to implement a patch burn strategy to increase habitat diversity, it is necessary to understand fuels and fire behaviour. In the Gibson Desert Nature Reserve, spinifex is the dominant fuel type and its distribution and cover were found to be closelyrelated to landform soils. Detailed measures of fuel characteristics and the behaviour of experimental fires allowed functional models for predicting fire spread to be developed. Spinifex fires were wind-driven and spread rapidly when wind speed exceeded 12-17 kmh- 1 at 2 m above ground. The patchy nature of the fuel prevented the development of a continuous fire perimeter. Crescent-shaped head fires burnt a finger-like pattern through the spinifex, unlike the typically elliptical shapes common to fires burning in continuous fuels. Direction of fire spread, hence fire scar pattern, was very responsive to shifts in wind direction.

Introduction Land classified as desert occupies about 1 million km 2 of the eastern half of Western Australia (Beard, 1969). Of this, about 8 million ha exist in recently gazetted nature conservation reserves administered by the Western Australian Department of Conservation and Land Management. Remoteness, absence of direct disturbances generally associated with European settlement and the recent gazettal of desert reserves has meant little intensive management of these reserves. However, there has been an alarming decline in mammal fauna (Burbidge et aI., 1975; Burbidge & Fuller, 1979; Saxon, 1984; Burbidge et al., 1988). Burbidge et al. (1988) proposed that the decline in mammals is a result of the combined effects of a changed fire regime in recent years, of predation by introduced foxes and cats and competition from exotic herbivores, such as rabbits. Until recently, Aborigines and lightning-caused fires maintained much of the desert landscape as a patchwork of different post-fire successional states (Bolten & Latz, 1978; Suijdendorp, 1981; Burbidge, 1985). This spatial and temporal mosaic was the fire regime 014Q-1963/911020189

+ 16 $03-00/0

© 1991 Academic Press Limited

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N. BURROWS ET AL.

under which desert wildlife has probably existed for thousands of years. However, over the last five or six decades the fire regime has changed. There are now large areas of pyric uniformity due to the absence of patch burning by Aborigines and large areas infrequently burnt by lightning-caused wildfire (Griffin et al., 1983). Large-scale uniformity of disturbances such as fire has probably contributed to the decline in mammal fauna, many of which depend on diverse vegetation and irregular fire-induced boundaries for suitable habitat (Saxon, 1984). The fire management strategy proposed by the Department of Conservation and Land Management for desert reserves in Western Australia aims to provide a diverse range of post-fire vegetation states on a small-grain mosaic across the landscape. An interlocking patchwork of recently regenerated herbfields and long unburnt patches of spinifex and mulga should provide suitable habitat for a suite of animals (Saxon, 1984). This strategy will also reduce the frequency and size of wildfires especially in the Great Victoria Desert. Fire management in the populated and well resourced areas of the south-west of Western Australia has evolved to a high level of sophistication. Fire breaks, aerial detection systems, well trained and well equipped fire fighting crews, fuel reduction burning and organised suppression techniques are part of contemporary fire management (McCaw & Burrows, 1989). Conventional fire management techniques are not appropriate for remote and poorly accessible desert reserves. Methods of achieving a patch burn strategy in desert reserves must be cheap, not require machinery such as bulldozers and water tankers and should be suitable for implementation on a broad scale, given that there are about 8 million ha of such reserves in Western Australia. Aircraft are routinely used for ignition of prescribed fires in the forested areas of the south-west (Packham & Peet, 1967) and offer a number of advantages for operations in desert areas. Aircraft can operate in remote areas with relatively little ground support, they can cover a considerable area in a short time and relatively cheaply. However, in order to achieve burnt patches of the desired shape and size to serve as buffers against wildfire and to provide optimum habitat, it is critical to understand how fire will behave under various conditions of fuel and weather. In this paper we report on the findings of a study of fire behaviour in various vegetation types common in the Gibson Desert Nature Reserve. This study is part of a larger multidisciplinary fire ecology project which will culminate in the reintroduction of local extinct animals such as burrowing bettongs (Bettongia Iesueur) and rufous hare wallabies

(Lagorchestes hirsutus).

Description of the study area The Gibson Desert stretches from about 22·5 to 27°S latitude and from 123 to 128°E longitude. Beard (1969) described it as 'characterised by laterite plains, a monotonous and gently undulating topography floored with ironstone gravel and vegetated with poor spinifex and stunted mulga, relieved only by low mesaform hills'. The stony hills and duricrust support mulga (Acacia aneura) and other woody shrubs such as minni ritchi (Acacia grasbyt). Ground cover consists of scattered clumps of spinifex (Triodia basedowii and Plectrachne schinzii) and soft grasses. The laterite plains are mostly covered with spinifex. Plectrachne schinzii more frequently occurs on the deep red sands (dune crests and swales and sandy depressions) with T. basedowii most common on the shallower sands packed with pisolites (buckshot). Scattered low shrubs (species of Acacia, Grevillea and EremophiIa) grow on the plains in association with spinifex and scattered Eucalyptus trees (E. centraIis, E. microtheca and E. ferriticoIa). The north-western portion of the reserve is characterised by claypans and mulga growing on loamy soils. The laterite plains are a mosaic of mulga belts in the loam-filled depressions and spinifex (Triodia) where the plains rise gently and the shallow sandy soils are heavily loaded with buckshot. A more detailed description of vegetation is provided by Beard (1974). The Gibson Desert Nature Reserve

FIRE BEHAVIOUR IN SPINIFEX FUELS

191

of 1·89 million ha lies from about 24·5 to 25·5°S and 124·5 to 127°C.The study area was located within the reserve and to the west of Gary Highway. The region has a desert climate, as defined by Beard (1969), based on a classification using ombromthermic diagrams. Annual average rainfall is about 220 mm (from Giles Meteorological Station) most of which falls in summer. Rainfall is erratic and undependable and long periods of drought are not uncommon. Beard (1969) reported a drought of 40 months in the early 1960s which caused widespread plant death. The summers are long and hot, the winters cool and mild. The weather is controlled by a succession of eastwardmoving high-pressure systems with resultant heat and dryness. Convergent mid-level north-westerly flow contributes a major source of rain in the area. Occasional tropical cyclones and low-pressure disturbances result in heavy rainfall. Most summer rainfall comes from thunderstorm activity. More detailed characteristics of the climate are given by Arnold (1963), Beard (1968) and Gentilli (1972). The geology of the area has been mapped and explained by van de Graff (1974) and Jackson (1976). Beard (1974) also provides a general description. Geological maps were used here as a primary basis for stratifying vegetation sampling. The major physiographic units are described in Fig. 1. Details of the fire history of the study area are sketchy. Early explorers reported the frequent use of fire by Aborigines for hunting, signalling and for ritual ceremonial purposes (Carnegie, 1898). More recently, Burbidge (pers. comm.) and Kimber (1983) have documented information relating to the use of fire by Aborigines based on interviews with Aboriginal people to the north and north-east of the Gibson Desert area. During the course of this study, we observed signs of past fire, such as fire scarring on Eucalyptus trees and highly-weathered charcoal. Our impression was that most of the vegetation had not been burnt for a very long time (probably in excess of 40 years) and there were no signs of recent fire. Unlike the sandy deserts to the north (Great Sandy Desert) and to the south (Great Victoria Desert) the stony Gibson Desert showed little evidence oflarge, lightningcaused wildfires, even though summer thunderstorms are common. The main reason for this probably lies in the distribution and structure of the vegetation (which becomes fuel). Fuels are fragmented by natural fire barriers such as poorly vegetated laterite plains, breakaways, pediments and claypans. This contrasts with the vast continuous expanses of flammable fuels commonly found in the sandy deserts to the north and south. However, a patch burn strategy to provide a diverse habitat is necessary before the reintroduction of animals can proceed.

'Methods

Describing vegetation asfuel Vegetation becomes the fuel for a bushfire, so to understand fire behaviour it is first necessary to understand and describe the vegetation in terms of its properties as fuel. Beard's (1974) vegetation maps for the study area are too general and therefore were not suitable for classifying vegetation as fuels at the level of resolution we required. Two other sources of information were used to assist with fuel classification: geological survey sheets (Jackson, 1976; van de Graaf, 1974) and Landsat Satellite Thematic Mapper (TM) imagery at a scale of 1:100,000. Relations between landforms and geology, as described by Jackson (1976) are summarised in Fig. 1. The major physiographic units of Jackson were then further subdivided using Landsat TM imagery which was manipulated to maximise discrimination between vegetation or soil types (depending on the dominant reflecting surface) by selecting various band combinations (D. Pearson pers. comm.). There was a high degree of similarity between Jackson's map of major physiographic units and major patterns on the Landsat

192

N. BURROWSETAL.

Undulating laterite plains

Mesas, buttes and pediments

Brood depressions separated by gentle rises. Ground covered with pisoliths and sand.

Steep to undercut cliff with siliceous and ferruginous copping. Pediments strewn with rocks.

Sand plains and dunes Featureless sand plain covered with spinifex and sparse trees. Longitudinal dunes to 5 m.

Major valley floors

Salt lakes clay pans

Flat,low sandy and clayey depressions.

Flat, bore expanses of salt and/or cloy commonly flanked with gypsiferous dunes.

1I dI~ : ~:~ ~ ~ ~ ~ ~ ~:~: : : : :~: ~ ~:~:~ :~ : : : : ~:~:~: :m : : ': : : : : : : :m : : : :!: : :

Figure 1. Schematic cross section showing topographic relief and major physiographic units of the study area, Gibson Desert Nature Reserve (adapted from Jackson, 1976).

TM image, which is perhaps not surprising as the geological maps were derived from black and white aerial photographs. A field inspection of the five major physiographic units (see Fig. 1) revealed that three of the units were so sparsely vegetated as to warrant no further investigation as fuels. Therefore, we concentrated on the sand plains and dunes and lateritic plains. The Landsat TM imagery revealed considerably more detail about soils and vegetation than the broad physiographic maps of Jackson and from the imagery, we identified and verified in the field, four subunits: (i) sand plains and dunes covered with Triodia basedowii and Plectrachne schinzii, the latter forming pure swathes on deeper sands (fuel type 1); (ii) stony plains carrying a diverse array of woody shrubs (including mulga), low trees and a sparse ground cover of Triodia basedowii and soft grasses (fuel type 2); (iii) light buckshot plains with Triodia basedowii and Plectrachne schinzii over red quartz sands with loosely packed pisolitic ironstone (fuel type 3); (iv) heavy buckshot plains, characterised by Triodia basedowii on red quartz sands densely packed with pisolitic ironstore (fuel type 4). There are a number of important and readily measurable characteristics which determine the ease of ignition of vegetation and the way in which fire behaves. These characteristics and methods for measuring them are described below.

Fuelparticle dimensions

Size and shape of the individual fuel particles will affect ease of ignition and rate of combustion of the particle (Rothermel, 1972; Luke & McArthur, 1978). Fine fuel particles, such as spinifex leaves, will ignite more readily than coarse fuel particles, such as logs. Diameter, thickness and surface area-to-volume ratio offuel particles are measures of fuel fineness. Spinifex blades are essentially cylindrical and we measured the diameter (±0·01 mm) of about 100 blades of each species using a micrometer. Rothermel's (1972) equation was used to calculate surface area-to-volume ratios. I

Fuelparticle arrangement

Arrangement of fuel particles also affects combustion rate and efficiency of heat transfer from particle to particle (Luke & McArthur, 1978). Densely packed fuel particles restrict

FIRE BEHAVIOUR IN SPINIFEX FUELS

193

the availability of oxygen and hence, burn slowly, but heat transfer is inefficient if particles are very sparsely arranged. For a given dimension offuel particle, there is an arrangement which is optimal for combustion (Rothermel, 1972). Packing ratio, the ratio of the fuel bulk density to the fuel particle density, is a measure of the compactness, or aeration, of the fuel particles (collectively termed fuel array). The bulk density of spinifex clumps was calculated by measuring the dimensions of 30 clumps to calculate volume, then clipping out the measured volume to determine oven dry weight (dried at 105°C for 48 h). The density of fuel particles was determined using the water displacement technique.

Fueldistribution

Spatial continuity of fuel will also affect fire behaviour. Fire will spread more rapidly through a continuous fuel bed than one which is patchy or separated. Projected ground cover and patchiness (Pielou, 1976; Griffin & Allan, 1984) are measures of horizontal continuity. Spinifex fuel can be considered to be a single stratum or simplex fuel, without complex vertical development. Here, we adopted the wheel point transect technique described by Griffin & Allan (1984) to estimate fuel patchiness. In each of the four major fuel types, 4000 m of wheel point transects were sampled at l-m intervals. A team of two people was able to move along the transects at a rate of about 1 km h -1 recording information into a Husky Hunter data logger. At each point, observers recorded whether a contact was made with vegetation or whether the ground was bare. When vegetation was contacted, plant species and maximum height at the point of contact were recorded. Plants which could not be identified were recorded as 'other'. These data were used to calculate the size (distance) of bare ground patches and of continuous vegetation (fuel) along the transects. The frequency distribution and patchiness ratio (variance: mean patch size ratio) were determined for each of the four major fuel types (after Griffin & Allan, 1984).

Fuel quantity

Fuel quantity has been shown to be important in affecting the rate of fire spread and intensity in most fuel types (Byram, 1959; McArthur, 1962; Peet, 1967). For each fuel type, we estimated fuel quantity by clipping out all vegetation in 30 I-m 2 quadrats located along wheel point transect lines. Samples were bagged and returned to the laboratory for oven drying and weighing (dried at 105°C for 48 h). While most samples contained only spinifex, there were occasions when woody shrubs or mulga trees were-sampled. In these instances, only particles smaller than 4 mm in diameter were bagged. From earlier observations, large particles are rarely consumed in the flaming zone of fires burning in predominantly spinifex fuels.

Fuelmoisture content

Spinifex fuel consists of both live and dead plant material. The moisture content of fuels such as grasslands and forests has a significant effect on fire behaviour (Rothermel, 1972; McArthur, 1966; Peet, 1967) and is likely to affect fire behaviour in spinifex fuels similarly. Initially, we attempted to separate the live and dead components of spinifex for moisture content determination by gravimetric analysis. However, this proved to be difficult, very time-consuming and tedious. Consequently, we elected to sample a profile throughout the entire clump, including live and dead components. We assumed that the proportions of live and dead material were constant for a given spinifex species of a given age. Samples were clipped out, placed in air-tight containers and returned to the laboratory for oven drying (l05°C for 48 h). Prior to and at hourly intervals during experimental fires, five samples each of about 100 g, were taken.

Fire behaviour Linear rates of fire spread, fire perimeter, fire shape, fire intensity and flame dimensions are all measures of fire behaviour and are influenced by conditions of fuels, weather and

194

N. BURROWS ET AL.

topography (slope and aspect). In terms of the practical application of a fire behaviour prediction system for spinifex fuels, the threshold conditions for when fire will spread, and an ability to predict head fire rate of spread, are necessary for implementing a patch burn strategy. An understanding of fire shape, fire perimeter and area of burnt patches is also necessary to reconstruct the habitat mosaic maintained by Aborigines in the past. In August 1987 and February 1989, a total of 41 experimental fires were lit at the study site using drip torches to set lines of fire up to 200 m long. Fuels were measured in the manner described above and an on-site weather station provided readings of air temperature, relative humidity, wind speed and wind direction at lO-min intervals. The position of the flames was marked by placing numbered metal tags near the flames at intervals of 14 min. Time of ignition was recorded and the ignition point marked with metal tags. The time of placement of metal tags at the base of the flames was also recorded to the nearest second. Fires were allowed to spread until they self-extinguished, either as a result of a fall in wind speed or lack of fuel (such as on sand dunes, breakaways, pediments and recently burnt areas). Hand-held anemometers were carried by observers who made regular (12 min) wind readings at 2 m above ground and within 50-100 m of the headfire. Wind gusts were also recorded this way. When fires had extinguished and the ground had cooled, the metal tags marking fire perimeter at various time intervals were surveyed using a hip chain and compass. This information enabled fire rates of spread to be calculated and provided information about fire shape. No topographical measures were made due to the flat nature of the spinifex plains. Fuel, weather and fire behaviour data were then analysed using multiple linear regression techniques (Norusis, 1985) to develop equations for predicting fire rate of spread. Multiple readings from each fire resulted in a total of 77 observations. We tested the Griffin & Allan (1984) fuel factor and weather factor equations (see below) as inputs to the models. We also compared rates of spread measured during this study with those predicted from equations developed by Griffin & Allan (1984) for spinifex fuels near Alice Springs, Northern Territory.

Results

Vegetation asfuel Live and dead spinifex (Plectrachne schinzii and Triodia basedowii) formed the dominant ground cover and fuel. Characteristics of the major types recognised here are summarised in Table I. Spinifex formed a single stratum, discontinuous fuel, and with the exception of fuel type 2, the contribution to cover of other species was very low «3%). The average height of spinifex clumps was significantly different (Students it', p < 0·05) between all fuel types except between types 2 and 3. Mean fuel quantity was also significantly different between all fuel types (Table 1), with the highest mean quantity being found on type I, occurring on the deep red sands. The frequency distributions of the size classes of spinifex clumps and of bare ground patches are shown in Fig. 2. In the field, there appeared to be noticeable visual differences in the horizontal structure of each of the four fuel types. However, this was not reflected in the data which showed no significant differences in the frequency distributions of spinifex patch sizes using the Runs test (Siegel, 1956) for fuel types I, 3 and 4. Fuel type 2 (mulga country) was significantly different to the other fuel types with regard to the frequency distribution of bare ground patch sizes. This fuel type was characterised by a number of large bare patches, some up to 40 m across.

-

8'3-13-5

-

10·6-28'6 1,2-26,6 6'0-23-4 0'4-16-0 47'3-57-8 0-0'3 0'2-2'4 0·29-{)·38 1'28-1-72

Range

Type I

140

5·7 51-3 0·1 1·8 0·36 1·46 28'3 4·9 10-2 0·0098

iz-r

19-7 9·1

Mean

i-o-u-z

-

0'3-5-4

-

-

1·0-9·2 0-4-9-8 60'2-85 '4 8'6-12-8 1-0-9-6 0,27-1' 10 9·86-20·9

1·0-10' 0

Range

Type 2

140

1·9 4·8 2-1 1·9 74·4 U·4 3'6 0·32 14-9 24·6 4·8 2·8 0'0110

Mean

-

7·0-10·6

-

-

6-2-26-4 2-4-23'4 1-8-16'0 1'4-14' 4 47·6-58 '0 0-1-2 0·8-4'4 0-26-{)-58 0'93-1,31

Range

Type 3

140

16·0 13·8 8-6 7·3 51·9 0·4 2·2 0·43 1·11 25'9 5·6 8·4 0·0096

Mean

-

3'4-7·6

-

0-3'8 30·4-38'8 0-1'2 3'2-9-2 51'0-61'2 0·4-2·6 1·0-4·0 0'37-{)'85 0'95-1'4 6

Range

Mean

140

0·6 33'4 0·2 6·6 56'6 1·7 1·9 0·56 1·25 18'6 5·6 5·4 0·0132

Type 4

Fuel type 1 found on deep red sands between dunes; fuel type 2 found on red loamy plains strewn with ironstone and quartz pebbles; fuel type 3 found on light buckshot (pisolitic nodular laterite) plains; and fuel type 4 found on heavy buckshot plains. Patchiness ratio = variance/mean of patch size (after Pielou, 1976; Griffin & Allan, 1984).

Ratio (cm" ') of blades

Plectrachne cover dead (%) Triodia cover dead (%) Bare ground (%) Mulga (%) Other species cover (%) Spinifex patchiness ratio Bare ground patchiness ratio Height of spinifex (em) Bulk density (kgm"? Fuel quantity (tha- I Packing ratio Surface area to volume

Triodia cover live (%)

Plectrachne cover live (%)

Characteristic

Fuel type

Table 1. Characteristics ofthe majorfuel typeson the Gibson Desert Nature Reserve studysite

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N. BURROWS ET AL.

196

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Length of continuous potch (rn)

80

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(c)

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Length of continuous potch (m)

Figure 2. Percentagedistribution ofsizeclasses ofspinifexclumpsand bareground measuredalong transects in four fuel types. (a) Fuel type 1; (b) type 2; (c) type 3; and (d) type 4. _, Spinifex; mbare ground. Mean values for spinifex and bare ground are (a) 1'44 m, 2·38 m; (b) 1·29 m, 7'11 m; (c) 1'36 m, 2'08 m; (d) 1,53 m, 2·29 m, respectively.

Fire behaviour Conditions of fire behaviour, weather and fuel moisture during the study are summarised in Table 2. The equations presented here are bounded by data in Table 2. Fuel type 2 did not carry fire under the conditions of this study and given the sparse nature of the fuel, it is unlikely that fire would spread even under severe weather conditions. In other fuel types, combustion of spinifex was complete, all above-ground parts of the plant being consumed in the flaming zone. The actual rate of spread of fire was plotted against predicted rate of spread using the model of Griffin & Allan C1984);

ROS = -0,419 + 1,125 x 3YCfuei factor x weather factor) where:

(1)

FIRE BEHAVIOUR IN SPINIFEX FUELS

197

Table 2. Summary offire behaviour, fuel moisture and weather conditions from 102 observations made during 41 experimental fires ontheGibson Desert NatureReserve. Datafrom fires which did notspread are also included

Fire behaviour Distance travelled by fires (rn) Head fire spread rate (mh") Flame height (m) Fire intensity CkWm- l ) Weather Wind speed at 2 m (kmh-I) Temperature C°C) Relative humidity COlo) Fuel moisture content (% over dry weight)

ROS

f

ue I f:actor

= rate

Range

Mean

S.E.

0-1883 0-5520 0-14628

521 1177 2·0 3119

58 117 0·08 313

4-36 19-50 14-48

18 27 27

0·7 0'7 0'7

12-31

18

0·5

~'5

of spread (ms- I )

= Y (% spinifex . . / cover/%

bare ground v fuel moisture

... /

X v

h'

pate mess

- Ytemperature x exp(wind velocity) ' h umiiditty re Ianve

weat h er f:actor -

fuel moisture = % oven dry weight, patchiness = spinifex patchiness ratio, temperature = DC, relative humidity = %, and wind velocity = ms"! As can be seen from Fig. 3, the model over-predicted rate of spread considerably. Actual rate of spread (ROSA) and predicted rate of spread (ROS p ) were related by the linear equation ROS

= O'44(ROS p ) -

249'7

R 2 = 0.84

(2)

where ROSA and ROS p are in mh -1. Griffin & Allan (1984) define fire factor as fire factor

3

= V(fuel factor x

weather factor).

(3)

Fuel factors for each of our experimental fires were calculated and regressed with rates of spread (ms- I ) measured during this study. The following equation and associated statistics resulted. ROS(ms- l )

R

2

= -0'282 + 0'49 (fire factor)

= 0'84,

standard error

df

Sum of squares

Mean square

1 76

6'365 1'209

6'364 0'015

(4)

= 0'126, F = 399'7.

Analysis of variance:

On examining the components of fire factor, we found that the parameter fuel factor did not contribute significantly to the rate of fire spread for the fuels studied here. During analysis, fuel factor failed to meet the equation entry requirements (C t' ratio = 0'92,

N. BURROWS ET AL.

198

p < O'364). Weather factor alone explained 78% of the variation in rate of spread and was a highly significant variable in the equation ('t' ratio = 16'34, P < 0'00005).

Independent variables including wind speed, fuel moisture content, temperature, relative humidity and a ratio of % spinifex cover to % bare ground were analysed with the rate of fire spread using the stepwise multiple linear regression technique (Norusis, 1985). Wind speed data were transformed by squaring prior to regression. The final form of the equation was: ROS = 3'90 (wind/) - 82'08 (FMC) + 43·50 (temp) - 4935'29

+ 5826'36 (cover)

(5)

where ROS = rate of spread (mh "), wind = wind speed at 2 m (kmh"), FMC = fuel moisture content (% of oven dry weight), cover = % spinifex cover/% bare ground, and temp = air temperature (0C). Statistics relevant to the above equation are Step 1 2 3 4

Variable

Coefficient

Wind 2 FMC Cover Temp Constant

3'90 -82'08 5826'36 43'50 4935'29

S.E.

R2

0'17 11'71 1616'10 17·47 1850'66

0'854 0'905 0'915 0'920 0·920

't' 22·09 -7'01 3'60 2·49 2·49

p

<0·00005 0'00005 0'0006 0'0150 0·0094

Analysis of variance d.f.

Sum of squares

Mean square

4 73

90780944'11 7387296·56

22695236'02 101195'84

F = 224'2, P < 0'00005. The patch nature of the spinifex fuels studied here prevented the development of back and flank fires. Instead, the wind-driven head fires burnt long, narrow strips resulting in classic 'finger' shapes. Head fire speed and direction were very sensitive to wind shifts. Occasionally, burning hummocks behind the main head fire would develop into smaller head fires following changes in wind direction. Head fires would commonly fragment and go out under low wind speeds. The minimum wind speed necessary for the spread of fires varied according to the cover of fuel, the distribution of bare patches and with fuel height. Unfortunately, we were unable to define a precise relationship between minimum wind speed necessary for fires to spread and fuel parameters. For the fuel types studied here (which were structurally similar) wind speeds in excess of 12-17 kmh"! (at 2 m) were needed for fires to spread. Below this, combustion could not be sustained except in small patches of dense spinifex cover. This threshold level appeared to be unaffected by fuel moisture content except when fuels were very moist (>30%). Live spinifex was difficult to ignite at moisture contents in excess of 35%. The influence of wind speed alone on rate of spread is shown in Fig. 4. The low rates of spread observed for fires influenced by low wind speeds (12-17 kmh", and shown as the 'threshold zone' in Fig. 4) contributed to the non-linear form of the function. At higher wind speeds, an almost linear relationship exists. Spinifex fires accelerated quickly and reached a 'quasi steady state' within 5-10 min of ignition. The rate of spread fluctuated with changes in wind speed (Fig. 5).

FIRE BEHAVIOUR IN SPINIFEX FUELS

199

y=O·44 (x)-249·7 R l=O·84 5000

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Figure 3. Actualheadfirerate ofspreadvs. that predictedusingthe modelof Griffin& Allan(1984) for spinifexfireson the GibsonDesert Nature Reserve.

5OOOr--------------------, ROS = 3'983 (W 2)- 254·9 R2=O·85 4000

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Figure 4. Relationship betweenheadfirerate ofspread (ROS)and windspeed (W) for spinifexfuels in the Gibson Desert Nature Reserve.

N. BURROWS ET AL.

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100

ignition (mins) Figure 5. Head fire rate of spreadwith time after ignition for three experimental fires in spinifex fuels. Time ofter

Discussion The annular growth habit of spinifex and the distribution of individual plants, gives rise to its patchy nature. In all fuel types, spinifex rings were often fragmented and in some instances, senescing. Large spinifex rings and crescents often coalesced to form continuous patches up to 5 m long. Fuel types 1, 3 and 4 were found to be similar in horizontal structure but different in biomass and mean height of clumps. Biomass and clump height varied according to species and edaphic factors. Clumps of P. schinzii were taller than those of T. basedouni, and T. basedouni growing on deep soils were taller than those growing on the shallower soils ofthe buckshot plains. Nutrient and moisture availability probably contribute to these differences. Rate of fuel accumulation is very slow, probably averaging less than 0'30 tonnes ha- 1 year-lover 30 years. The age of the spinifex studied here is estimated to be 30-40 years, and the average fuel quantity measured for each fuel type varied from 2'8 to 10'2 tonnes ha- 1• Griffin & Allan (1984) provide formulae for calculating fuel quantity based on the cover of spinifex and other plants. Using one of these formulae and mean values of cover presented by Griffin & Allan, a mean fuel quantity of 7·5 tonnes ha- 1 was derived, which is within the range offuel quantities measured here. The rate of fuel accumulation in hummock grasslands will be determined to a large extent by rainfall. Winkworth (1973) measured an average biomass level of 4' 24 tonnes ha -1 from spinifiex plains during drought years and Griffin et al. (1983) reported fuel quantities as high as 10'0 tonnes ha- l in long unburnt spinifex. Casson & Fox (1987) studied the postfire biomass reaccumulation of spinifex in the Pilbara region of Western Australia and found annual accumulation rates of 0·50 tonnes ha- 1• Suijdendorp (1981) reported accumulation rates of O'33-1'03 tonnes ha -1 for T. pungens and found production rates to be closely related to the amount and distribution of rainfall. The size and distribution of spinifex and bare patches vary according to the maturity of the vegetation (Griffin & Allan, 1984). The ratio of variance to mean patch size (used by Griffin & Allan) is a measure of this, with a random distribution of patches producing a variance ratio approaching 1. A ratio in excess of 1 reflects a uniform distribution and one considerably less than 1, a clumped or contagious distribution. While mean spinifex patchiness ratios reported here (0'32-0'56) are within the range of those reported by Griffin & Allan (1984), bare ground patchiness ratios determined here were considerably higher and in excess of 1. This higher variation in the size of bare patches probably explains the high wind speeds necessary for fire spread and the over-estimation of the rate

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of fire spread by Griffin & Allan's model. This is discussed in more detail later . The level of bare ground patchiness measured here may be a reflection of very old and over-mature spinifex or of climatic and edaphic factors resulting in poor site productivity. The fine particle size of spinifex blades (surface area to volume ratio = 140 em -I) and loose arrangement of the particles within a clump make it an optimal fuel. Rothermel (1972) developed relationships between packing ratio and potential reaction velocity (a measure of combustion rate) for fuel particles of various dimensions. For particles of similar dimensions to spinifex leaves, the optimum packing ratio was found to be about 0'01, which is similar to the packing ratios reported here for spinifiex (Table 1). Spinifex leaves are arranged randomly within the clump, unlike most annual grasses where fuel particles are mostly vertical. This random arrangement is likely to facilitate rapid combustion because of more efficient transfer of heat from particle to particle. Spinifex is also very resinous, which contributes to its high flammability. Therefore, the physical and chemical properties of spinifex fuel, together with high exposure to weather (especially wind, solar radiation, extreme heat and dryness) provide near-optimum conditions for fire spread. The main factor limiting fire spread is the patchiness of the fuel complex. Due to the patchy nature of spinifex fuels, strong wind is necessary for the spread of fire. Wind increased the combustion rate, which resulted in longer flames which, when tilted by the wind, ignited adjacent hummocks. Fire spread was primarily by a process of flame contact, where as in continuous fuels, radiation is the primary mechanism of fire spread (Anderson, 1968; Frandsen, 1971; Rothermel, 1972). The importance of wind speed on rate of spread is revealed by the equations presented in Fig. 4. The minimum wind speed for fire spread ranged from 12 to 17 kmh -I, depending on fuel type. These speeds are substantially higher than the speeds reported by Griffin & Allan (1984) of 3'6 kmh"! (1 ms -I). Differences in the frequency distribution of bare patch sizes and in the per cent of bare ground probably explain this. Generally, fuels studied by Griffin & Allan contained a higher proportion of vegetation other than spinifex, resulting in a higher total ground cover of live and dead vegetation. For example, fuel type 3 had a mean ground cover offuel of about 45%, whereas Griffin & Allan reported a mean fuel cover of 55% for a similar vegetation type. Differences in fuel structure between those reported here and those reported by Griffin & Allan may be due to slight variations in measurement techniques or operator biases. Of significance, in terms of affecting fire behaviour, is the difference between mean bare ground patchiness ratio. Griffin & Allan reported an average of 0'29, whereas our value ranged from 1·11 to 1'46, indicating a higher frequency of larger bare patches. During this study, bare patches in excess of 3-4 m were often sufficient to either prevent the spread of a small, developing fire or fragment the head of a large, well developed fire. Fires developed quickly under the influence of high wind speeds which reduced the chances of the head fire being stopped by small bare patches (3-4 m), Providing the head fire had developed sufficient width (> 20-30 m) it would burn around small bare patches and continue to spread. Thus, the probability of a fire spreading was not only a function of fuel and weather conditions, but also of fire size. All else being equal, large fire fronts (> 100 m) were unlikely to be stopped by bare patches encountered in the flammable fuel types. Such fires were extinguished by running out of fuel (such as burning into sparse vegetation) or by a fall in wind speed to below threshold levels. The threshold wind speed for fire spread was lowest for fuel type 1 and highest for fuel type 4 (fuel type 2 did not carry fire under the conditions studied here). While each of the flammable fuel types were similar in horizontal structure (Fig. 2) they varied significantly in fuel quantity and average clump height (Table 1). Both of these factors will affect flame height and hence flame length. Fuels with higher biomass will burn to produce longer flames. Therefore, fire will spread from clump to clump (across bare patches) at lower wind speeds than is necessary for shorter, lighter fuel types with correspondingly shorter flames. When wind speed exceeded the minimum necessary for fire spread, then the relationships between wind speed and spread rate applied equally well to each of the fuel types, 1,3 and 4, even though they varied in fuel quantity.

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From equation (5), the square of wind speed explained most of the variation in fire spread rate. Other factors contributed, such as fuel moisture content, cover of fuel to cover of bare ground ratio and air temperature, but to a lesser degree. When fuel moisture content approached 35% of dry weight, spinifex became difficult to ignite and fire did not spread. At high moisture contents (after substantial rain) live spinifex was green in colour, in contrast to its yellowish appearances at low moisture contents (14-20%). The stepwise multiple linear regression technique used here to examine the importance of fuel factor and weather factor (Griffin & Allan, 1984) on rate of spread, selected only the weather factor. Fuel factor (which is a function of cover, patchiness and fuel moisture content) failed to meet the equation entry requirements. The structural similarity of the flammable fuel types (1,3 and 4) probably explains why fuel factor was not entered into equation (4).

Management implications The most notable feature of the Gibson Desert Nature Reserve is that not all vegetation types are flammable; in fact probably less than 30% of the reserve could be considered as such. The reserve is fragmented by natural fire breaks, and large wildfires are unlikely to represent a major management problem. For the flammable spinifex plains, a patch burn strategy to promote animal habitat is desirable and possible under the appropriate conditions offuel and weather. The most desirable fire regime in terms of fire frequency, seasonality and the shape and distribution of burnt patches is not certain. More habitat information is needed. A preliminary investigation of fire scars obvious on black and white aerial photographs of areas of the Great Sandy Desert (to the north of the Gibson Desert) indicate that most burnt patches were less than 100 ha in area, but some were as large as 1000 ha. Aerial photograhy from as early as 1953 will help to reconstruct Aboriginal burning practices. Strength and duration of wind and lightning pattern are key factors likely to affect the resultant burn pattern. Using aircraft to light fires by spot ignition will be feasible providing most incendaries fall onto a patch or clump offuel. As some 50 to 60% of the spinifex plains are bare ground, then the aircraft may need to fly to an altitude and airspeed configuration which will encourage incendaries to skip on the bare ground and bounce into fuel clumps. There is no need to attempt to ignite landform/vegetation types other than the spinifex plains. These stand out quite readily on Landsat imagery and are clearly visible from an aircraft. Recent satellite imagery is useful for planning aircraft patch burns, to identify areas for burning, for mapping burnt areas and for identifying natural fire breaks. Initially, and until managers have confidence in burning spinifex, aircraft patch burns should be conducted under mild conditions in August to October. Weather records (from Giles) indicate that diurnal wind speed and duration are most likely to be suitable at this time. When the initial burn has been executed and the flammable fuels are fragmented by burnt patches then the timing of subsequent burns may not be so critical from an operational viewpoint. An operational trial using a fixed wing aircraft to burn patches on the spinifex plains was executed in September 1988. Results ofthis trial are being evaluated.

Conclusion Spinifex fuels of the Gibson Desert Nature Reserve are patchy and require wind speeds in excess of 12 km h- 1 before fire will spread. Fires burn with a 'finger'-like shape and are highly responsive to wind shifts. Fire behaviour models for hummock grasslands developed by Griffin & Allan (1984) tended to over-estimate the rate of spread of fire in the spinifex fuels studied here. This is probably due to the higher proportion of bare patches and lower ground cover of fuels measured here and variation in measuring techniques. The spatial discontinuity of flammable fuels in the Gibson Desert Nature Reserve would

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prevent the development of large, intense and devastating wildfires which have occurred in sandy deserts around this reserve. A knowledge offire behaviour as gained by this study will assist in planning appropriate fire management in this reserve. Further biological studies on the most appropriate fire regime to satisfy conservation values are needed. We thank the staff of the Department of CALM Goldfields Region for helping with field logistics, Andrew Burbidge, Graeme Griffin, Steve Hopper and Lachlan McCaw who commented on earlier drafts and Glenda Godfrey and Natalie Allday for word processing.

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