Regeneration Burning Studies in High Elevation Mixed Species Forests in East Gippsland
Gregory J. McCarthy and Glenn M. Dooley
FOREST SCIENCE CENTRE Eastern Research Centre, Orbost
Department of Sustainability and Environment Victoria
Parks and Forests Report Series 04-3 August 2004
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© State of Victoria, Department of Sustainability and Environment, 2004
Published by the Department of Sustainability and Environment PO Box 500, East Melbourne, Victoria, 3002, Australia
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ISSN 1449-2067 ISBN 1 74152 006 1
The Forest Science Centre was commissioned to undertake this project by Forestry Victoria and the Fire Management Branch, Department of Sustainability and Environment.
General Disclaimer
This publication may be of assistance to you, but the State of Victoria and its employees do not guarantee that the publication is without flaw of any kind, or is wholly appropriate for your particular purposes, and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication.
Cover photographs (All taken by Greg McCarthy): 1. HEMS logging slash showing typical arrangement of different size classes of material. 2. Helicopter with an Aerial Drip Torch lighting a slash burn. 3. Strong convection column resulting from HEMS burn (Clarkeville 1) conducted under weather conditions in mid range of prescriptions.
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SUMMARY A suitable seedbed for the natural regeneration of High Elevation Mixed Species (HEMS) in south-eastern Australia can be achieved operationally by the application of high intensity fire to the logging debris (slash). However high elevations can often mean that it is difficult to achieve the required fire intensity to produce satisfactory seedbeds. This problem of reduced fire intensity, in addition to problems of achieving adequate regeneration generally in HEMS, prompted forest managers to investigate the prescriptions associated with high intensity slash burning on HEMS sites.
While traditional burning methods had involved igniting logging slash by hand, the advent of a helicopter-mounted incendiary device - the Aerial Drip Torch (ADT) - had meant changed options for burn managers. A perception developed that the ADT could apply much more intensive ignition to a given area over a shorter time, and therefore may be able to achieve higher fire intensities under less favourable weather conditions.
As there was only limited knowledge about what contributed to a successful HEMS slash burn, this study undertook a detailed appraisal of fuel, weather, topography and ignition method variables at a total of 16 operational logging coupes over two years. Fire intensity achieved was specifically examined by recording: observable fire behaviour; fuel consumption of various fuel size classes; and soil/subsoil temperatures reached. (The second phase of this project, which examined regeneration results, will be reported separately.)
The results obtained indicated that the current HEMS Slash Burning Prescriptions were generally correct. There were also strong indications that soil moisture and Drought Index variables were particularly important to fire intensity achieved. This lead to two small supplementary studies.
The first supplementary study looked at the application of a simple field test for surface soil moisture content, based on the ability of the soil to yield dust when impacted, with dust formation correlated to surface soil moisture content (and therefore fuel moisture content).The second supplementary study looked at Drought Index trends at HEMS elevations.
The major findings were:
1) Fuel moisture and weather conditions, at the experimental regeneration burns studied, supported fuel moisture and weather condition prescription ranges given in the current HEMS Burning Prescriptions (NRE 1998).
2) The additional prescriptive factors of: • % direct sunlight on the coupe; • % cloud cover; • atmospheric stability; and • a field soil moisture test; could be usefully added to the HEMS Burning Prescriptions to increase the level of confidence in obtaining successful burns.
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3) Burn escapes from spotting were uncommon in the HEMS regeneration burns studied. Additionally, the propagation of spot-fires in uncut forest adjacent to HEMS coupes was inhibited by the presence of a secondary or understorey canopy, which gave profile fuel moisture contents outside the coupe of greater than 20 %.
4) Drought Indices at high elevations varied significantly from those at lower elevations, and, given their important influence on soil and fuel moisture conditions (and hence burning opportunities), Drought Index trends at HEMS elevations could usefully be investigated in further research.
5) The ADT, although an extremely useful tool for burn managers (in terms of speed and safety), is no more successful under marginal weather conditions than hand lighting, in producing a level of fire intensity which gives acceptable fuel consumption and soil heating for silvicultural purposes (i.e. receptive seedbed production).
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CONTENTS
SUMMARY ...... iii
CONTENTS...... v List of Tables and Figures ...... vii
1 INTRODUCTION...... 1
2 METHODS...... 2 2.1 Sample Coupe Details...... 2 2.2 Experimental Design and Analysis...... 4 2.3 Fuels ...... 5 2.3.1 Slash fuel distribution transects ...... 6 2.3.2 Branch and stem fuel consumption - wire tie transects...... 6 2.4 Weather...... 7 2.5 Fuel Moisture and Soil Moisture ...... 7 2.5.1 Moisture content of leaves and soil...... 7 2.5.2 Field soil moisture test...... 8 2.5.3 Seasonal soil moisture trends - Drought Index...... 9 2.5.4 Slash fuel curing time ...... 9 2.6 Topography...... 10 2.7 Lighting Method ...... 10 2.8 Fire Behaviour ...... 10 2.9 Fuel Consumption...... 11 2.10 Soil Heating ...... 11 2.11 Seedbed Condition Following Burning...... 12
3 RESULTS ...... 13 3.1 Fuel Structure on Slash Distribution Transects ...... 13 3.2 Short Term Weather, Fuel Moisture and Soil Moisture...... 16 3.2.1 Field soil moisture test - dust formation moisture contents ...... 17 3.3 Long-term Weather ...... 18 3.3.1 Drought Index ...... 18 3.3.2 Weather availability for HEMS slash burning ...... 19 3.4 Topographic Conditions...... 21 3.5 Ignition Method...... 21 3.6 Fire Behaviour Observations...... 22 3.6.1 Convection column formation...... 22 3.6.2 In-draught and tree sway ...... 25 3.6.3 Spot fire formation ...... 25 3.6.4 Fire behaviour effect on seedbed production ...... 26
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3.7 Fuel Consumption...... 27 3.7.1 Consumption of fuel by size classes ...... 27 3.7.2 Correlation of fuel consumption with other factors ...... 29 3.7.3 Curing time...... 30 3.7.4 Total slash volume and volume consumed...... 31 3.8 Soil Heating ...... 32 3.8.1 Soil temperature correlation with other factors ...... 33 3.8.1.1 Subsoil...... 34 3.8.1.2 Surface soil ...... 34 3.8.2 Soil moisture...... 34 4 DISCUSSION ...... 35 4.1 Effects of Fuel Distribution and Quantity ...... 35 4.2 Effects of Weather ...... 35 4.2.1 Short-term weather...... 35 4.2.2 Long-term weather - Drought Index trends ...... 36 4.2.3 Availability of days for HEMS slash burning...... 37 4.2.4 Effects of weather and fuel moisture on control of slash burns..... 37 4.3 Effect of Topography ...... 37 4.4 Effect of Ignition Methods - Aerial Drip Torch (ADT) vs Hand Lighting ...... 38 4.5 Effects of Weather, Curing Time, Lighting Method and Topography on Fuel Consumption ...... 39 4.6 Effects of Fire Behaviour and Soil Moisture on Soil Heating...... 40 4.7 Integration of Results - Burn Prescriptions...... 42 4.8 Timing of Regeneration Burning ...... 44
5 CONCLUSIONS ...... 44
6 RECOMMENDATIONS...... 45
ACKNOWLEDGEMENTS...... 46
REFERENCES ...... 47
APPENDIX 1 ...... 48 Drought Index and the likely effects of long-term dryness on fire behaviour and fuel consumption...... 48
APPENDIX 2 ...... 49 All data variables on the day of the burn, means and ranges...... 49
APPENDIX 3 ...... 50 Data variable values for all slash burns ...... 50 APPENDIX 4 ...... 51 Weather data from the Combienbar & Gelantipy Automatic Weather Stations (Feb – May 2002)...... 51 MAPS OF HEMS COUPE LOCATIONS...... 53
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List of Tables and Figures
Figure 1. Locality Plan, HEMS regeneration coupes 1998/99 & 1999/00 ...... 3 Table 1. Coupe name, location, year burnt, area, elevation, aspect and average slope, presence of carryover slash, and presence of secondary shrub canopy for all the study coupes...... 4 Table 2. Fuel occurrence and size data from visual assessment transects ...... 13 Figure 2. Occurrence of slash fuel elements in the 1998/99 coupes ...... 14 Figure 3. HEMS logging slash showing typical arrangement of different size classes of material...... 15 Figure 4. Occurrence of slash fuel elements in the 1999/00 coupes ...... 15 Figure 5. HEMS logging slash showing typical arrangement of different size classes of material...... 16 Table 3. Weather, soil moisture and fuel moisture on the day of burning for all coupes...... 16 Table 4. Dust formation moisture contents (% ODW) from HEMS soils under laboratory conditions...... 18 Figure 6. Trends in Keetch Byram Drought Index (KBDI) for two selected HEMS coupes, and also for the Orbost AWS for the period 30 Nov 2000 to 15 Feb 2001...... 19 Figure 7. Trends in Keetch Byram Drought Index (KBDI) for two selected HEMS coupes, and also for the Orbost AWS, for the period 1 Dec 2001 to 4 Mar 2002...... 19 Table 5. Total and most likely days of weather available for HEMS slash burning - Feb to May 2002...... 20 Figure 8 . Helicopter with an Aerial Drip Torch lighting a slash burn...... 21 Table 6. Lighting method, ambient weather, observed fire behaviour, FDI and cloud conditions for the study coupes...... 22 Figure 9. Strong convection column resulting from HEMS burn (Clarkeville 1) conducted under weather conditions in mid range of prescriptions...... 23 Figure 10. The Aerial Drip Torch igniting coupe at Coast Range under weather conditions outside prescriptions under NFS Guideline No. 6...... 24 Figure 11. Low fire intensity resulting from the ADT lighting shown in Figure 10...... 24 Table 7. Percentage of type of seedbed produced on 8 of the 9 HEMS coupes studied in 1998/99 ...... 26 Figure 12. Secondary canopy of wet forest shrubs in uncut forest adjoining HEMS coupe...... 26 Figure 13. End of 10 m wire tie fuel consumption transect showing marker pin and wire loops left from fuel elements consumed in the fire...... 27 Figure 14. Average percentage burnt of the three size classes of slash material for the nine coupes studied in 1998/99...... 28 Figure 15. Average percentage burnt of the three size classes of slash material for the seven coupes studied in 1999/00 ...... 28 Table 8. Single factor correlations of fuel consumption variables with fire behaviour variables...... 29 Figure 16. Slash material volume before burning, after burning, and volume consumed during burning for the 16 HEMS coupes...... 31 Figure 17. Aerial view of typical HEMS fuels showing distribution of slash heaps, and areas of disturbed soil following harvesting ...... 32 Figure 18. Average temperatures recorded using heat sensitive crayons placed in surface soil and subsoil at study coupes 1998/99...... 33 Figure 19. Average temperatures recorded using heat sensitive crayons placed in surface soil and subsoil at study coupes 1999/00 ...... 33 Table 9. Comparison of burning prescriptions in NFS Guideline No. 6, and satisfactory conditions observed during this study...... 42
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HEMS Regeneration Burning Studies 1
1 INTRODUCTION HEMS forests are those mixed species forests which occur at similar elevations to Ash forests ( Eucalyptus regnans , E. delegatensis ). They occur at elevations above about 700 m in Eastern Victoria, and are typified by the occurrence of a mix of species including Eucalyptus obliqua (Messmate), E. cypellocarpa (Mountain Grey Gum), E. viminalis (Manna Gum), E. nitens/denticulata (Shining Gum) and E. fastigata (Cut-tail).
The successful regeneration of High Elevation Mixed Species (HEMS) in south- eastern Australia has been a subject of close scrutiny by forest scientists and managers since the mid 1970s. Once thought to be closely related to the relatively well known regeneration processes in Ash-type eucalypts, HEMS regeneration following harvesting has been found to be somewhat more complex.
Failures of regeneration in HEMS (for reasons which were not obvious) caused Victorian forest managers to commence significant research into HEMS silviculture (e.g. Fagg 1981, Lutze et al , 1999). This study forms part of that ongoing research program.
Burning logging residue to produce a receptive seedbed is a task which requires intensive planning. Despite this planning, burning operations may still have to be cancelled due to the occurrence of unfavourable weather conditions. The weather "window", within which successful burning can be carried out, may be less than two weeks within the late Summer/Autumn in most years.
In some years the available weather window may not occur at all. Circumstances of dry weather and high fire danger (in which fire managers are unlikely, on safety grounds, to give approval to the conduct of prescribed burning) may give way rapidly to cool, wet weather, and make conditions for successful burning at higher elevations virtually impossible.
In the last decade, forest managers faced with both this weather constraint, and also the constraint of declining crew resources available for ground ignition, have quickly adopted the Aerial Drip Torch (ADT). The ADT, a helicopter-slung incendiary device, allows for rapid and safe ignition of large areas, and thus can be very cost-effective for regeneration burning operations (Wilkinson et al , 1995).
Sharing of the helicopter and ADT around a number of forest districts during the narrow burning "window" has led to a further (induced) constraint. On some occasions the restricted availability of the helicopter and ADT has virtually forced forest managers to conduct burns under weather conditions which are not ideal for fire development. Burning under less than ideal weather conditions has often resulted in poor quality seedbeds.
Current prescriptions (2002) for successful prescribed burning in HEMS are based on past experience and limited relevant research into combustion processes (Native Forest Silviculture Guideline No. 6 [Site Preparation], NRE 1998). They are expressed mostly in terms of acceptable fine fuel moisture contents (FFMCs). FFMCs within the logged area are specified to be within limits that would ensure successful consumption of logging slash fuels. FFMCs outside the logged area are specified to be above lower limits for fire spread, and would therefore inhibit the spread of any spot fires that occurred. The prescriptions deal generally with other
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factors such air temperature, relative humidity, drought index and wind speed, although the specified ranges for these were based generally on experience rather than documented research.
The main purpose of this study was to investigate whether the current prescriptions were adequate from the aspects of both fire behaviour and seedbed preparation, particularly given the changed burning methods which had applied since the introduction of the ADT. Specifically, the aims of this study were:
1) To investigate the burning conditions - fuel, weather, topography, soil moisture and ignition method - which occurred during HEMS regeneration burning across a range of coupes (including carryover 1) in East Gippsland.
2) To specifically examine: a) observable fire behaviour; b) fuel consumption of the various sizes of fuel elements; c) soil temperatures reached for both surface soil and subsoil; as indicators of both fire intensity obtained and likely seedbed produced - across this range of conditions.
3) To develop draft burning prescriptions for HEMS which would achieve fire intensity outcomes - as indicated by observable fire behaviour, fuel consumption and soil heating levels - acceptable for silvicultural purposes, and also achieve fire control outcomes adequate for fire protection purposes.
A further aim of this project was to investigate the regeneration outcomes from the range of coupes and prescribed burns sampled. This will be the subject of a later report.
2 METHODS
2.1 Sample Coupe Details HEMS slash burning coupes were studied over two years. During the 1998/99 season a total of 9 coupes were studied. During 1999/2000, a further 7 coupes were studied. The following locality plan shows the location of the coupes studied for both years.
Table 1 summarises the location, harvest year, area, elevation, aspect and average slope for the coupes in this study. It also indicates whether the coupes were carryover coupes - that is, whether the logging slash had over-wintered after harvesting, and was thus more than six months old. The secondary canopy in uncut vegetation surrounding the coupe, as noted in Table 1, consisted of tall closed wet forest shrubs, which substantially reduced the amount of sunlight and wind reaching the surface fuels (see Figure 12). Some coupes had previously logged and burnt areas adjoining.
1 (Carryover - logging slash older than 6 months, generally from the previous logging season - i.e. the slash fuel has over-wintered and some fine fuels have started to decompose)
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Figure 1. Locality Plan, HEMS regeneration coupes 1998/99 & 1999/00
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Table 1. Coupe name, location, year burnt, area, elevation, aspect and average slope, presence of carryover slash, and presence of secondary shrub canopy for all the study coupes. Coupe Latitude Longitude Slash Burn Coupe area Elevation name (deg. min. sec.) (deg. min. sec.) year (ha) (m) Yalmy 37 20 26 148 27 40 1998/99 38.3 820 Students 37 22 43 148 55 43 1998/99 41.2 740 Clarkeville 37 13 18 148 52 43 1998/99 42.5 980 Survey Rd 37 15 26 148 49 14 1998/99 15.5 940 Coast Range 37 13 26 149 00 52 1998/99 20.1 1000 Little Bog 37 10 35 149 04 43 1998/99 39.1 920 Watts Ck 37 19 46 147 56 08 1998/99 40.0 740 Mt Tom 37 18 48 147 56 15 1998/99 28.0 820 GW Hope 37 13 28 147 52 20 1998/99 23.8 900
Mt Tom West 37 18 37 147 55 56 1999/00 23.3 760 Playgrounds 37 12 14 148 47 01 1999/00 49.8 960 Yalmy View 37 19 39 148 28 10 1999/00 16.6 800 Waratah 37 17 05 148 36 06 1999/00 31.7 820 Ellery Old 37 26 30 148 44 57 1999/00 39.8 680 Ellery New 37 26 37 148 44 52 1999/00 17.3 660 Clarkeville 2 37 12 44 148 52 56 1999/00 42.3 980
Table 1. cont'd.
Coupe Aspect Average Presence of carryover Presence of a secondary name slope slash shrub canopy in uncut (deg) forest adjoining coupe Yalmy SW 10 N, W & E sides Students E & W 10 All sides Clarkeville ESE 17 All sides Survey Rd W 5 N, W & S sides Coast Range ESE 5 N, W & E sides Little Bog NNE 7 All sides Watts Ck NNE 15 N &E sides Mt Tom S-SE 17 N & E sides GW Hope NNW 12 All sides Mt Tom West SSW 12 100% carryover N & W sides Playgrounds NNE 5 45% c/over and 55% new All sides Yalmy View SSE 17 S & E sides Waratah SSE 15 100% carryover All sides Ellery Old SW 10 100% carryover N, E & W sides Clarkeville 2 NW/NE 12 E, W & S sides Ellery New NE 12 E, W & S sides
2.2 Experimental Design and Analysis This study was based on measurable and observable inputs, and outputs, for a range of prescribed high intensity burns for regeneration in HEMS.
The inputs were principally:
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- fuel structural conditions - i.e. how fuel structure varied amongst coupes, and also with time (new slash versus old/carryover slash); - seasonal and short term weather conditions, leading to variations in soil moisture and fuel moisture; - topographic conditions - variations in slope, aspect and elevation that may have influenced burning; - lighting method - lighting method used: hand vs ADT.
The outputs were principally: - burn outcome in terms of observable fire behaviour; - consumption of fuel as measured by the fuel consumption transects; - soil temperatures measured by the temperature sensitive crayons; - production of receptive seedbed as surveyed following burning.
The principal use of these outputs was to - establish standards of fire behaviour, fuel consumption and soil temperatures for HEMS prescribed burns which achieved acceptable levels of receptive seedbed; - compare these standards with the current prescriptions, and formulate any additional prescriptions which may be apparent from the data analysis.
Data were analysed using stepwise multiple regression procedures. Where possible, multiple factor models were constructed to explain the variation in dependent variables. Otherwise single factor correlations were derived to explain the most variation in dependent variables.
Correlations between dependent and independent variables were examined both on the basis of a probability value, and also a correlation co-efficient. Some correlations with significant probability values, but with poor correlation co- efficients, were included in the results. This indicated that a relationship between the variables was highly likely, but that there was a large spread in the data around the line of exact agreement. This spread of the data was likely to be due to other independent factors which may not have been sampled.
The analysis was hampered by the fact that it was a relatively small data set. Although a relatively large number of variables was sampled, repetition of sampling was restricted to the 16 coupes included in the study.
2.3 Fuels Fuel structure was assessed using two techniques. The first of these was by visual assessment of the fuel at a number of points on a grid across each coupe. At each sample point the fuels were assessed across a 10 m transect. The intention of this was to get a relatively detailed picture of the structure of the fuels across each coupe, particularly:
- what and how many fuel elements were present; - how these fuel elements varied both horizontally and vertically.
The second fuel assessment technique was to establish a number of fuel consumption transects to get detailed data on a smaller scale of which fuel
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elements, and how much of them, were consumed in the prescribed burn. This second technique required substantial time and effort, and therefore was necessarily restricted in comparison with the first technique. 2.3.1 Slash fuel distribution transects Prior to burning, transects of 10 m length for assessment of slash fuel composition were established at selected locations in the slash in each of the study coupes. The number of these transects varied from 6 to 10 per coupe. They were deliberately positioned to intercept the maximum variation in slash fuel elements, and thus generally ran at right angles to the prevailing direction of the slash accumulation heaps. On each transect the following information was collected using visual assessment:
- numbers of branchlets less than 10 mm diameter terminating in leaf bunches; - numbers of bark clusters of about 50-300 mm thickness; - number of 10-50 mm branchlets; - number of 50-100 mm branches; - number of >100 mm stems (including their diameter class, and whether they were elevated or not); - average height of the slash; - % coverage of ground fuels - % of fine fuels (<10 mm) elevated
These transects were done using visual counts of the fuel elements, and visual estimates of the height and cover values. Their accuracy was less than that of the later wire tie transects, as some fuel elements may have been obscured to the observer. 2.3.2 Branch and stem fuel consumption - wire tie transects All experimental coupes were sampled for consumption by the fire of branch and stem material. This was done by establishing 10 m wire tie fuel consumption transects at selected locations in the slash prior to burning. The number of these transects varied according to the time and staff available to establish them. In the first study year, generally three per coupe were established, and, in the second study year, a maximum of six per coupe were established (Appendix 3). These wire tie transects were positioned to sample maximum size class variation, and, as with the fuel distribution transects, were mostly located at right angles to the prevailing direction of the slash accumulation heaps.
Along these 10 m wire tie transects, all branch and stem material greater than approximately 20 mm thickness was tied with loops of 1-1.5 mm wire (common agricultural "tie wire"). Material less than 20 mm thickness was not tied with wire for two reasons. The first reason was the difficulty and time involved in doing this. The second reason was that there was a reasonable expectation that all material of less than 20 mm thickness would be consumed by the fire.
Following the fire, the transects were measured for fuel consumption. This was done by measuring the original diameter of the wire loops, and then measuring the thickness of any material that remained inside the loop.
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2.4 Weather Air temperature, relative humidity and wind direction and wind speed were collected for each prescribed fire at the study coupes (Table 3 and Appendix 3). Initial weather readings were taken prior to ignition, and then collected at about 30 minute intervals during the burn. From this information an approximate Forest Fire Danger Index (FDI) (McArthur 1975) was calculated.
An estimate was made of atmospheric stability and coded as follows: 1–neutral to unstable atmosphere (clear skies, light breezes, fluffy cloud tops) 0–stable atmosphere (no wind, overcast, inversion/low cloud present)
Where cloud cover was present, an estimate was made of both cloud cover percentage, and also percentage of direct sunlight falling on the fuel.
Seasonal moisture trends were monitored by the keeping of Drought Index data. A lack of detailed rainfall data in close proximity to most of the study coupes restricted the amount to which this input could be measured. As an alternative, a retrospective study of Drought Index trends at a number of representative HEMS sites (where detailed rainfall records were available) was undertaken. This was intended to demonstrate general trends at HEMS locations, rather than any specific variations for the individual coupes.
2.5 Fuel Moisture and Soil Moisture Fuel and soil moisture contents, along with ambient weather, were measured at each coupe on the day of the burn. These data were intended to distinguish any burn outcome differences which may have been principally due to soil and fuel moisture variation. Temperature sensitive crayons were placed within the fuel complex on each coupe to measure the outcome from the combination of fuel structure, fuel moisture and soil moisture. 2.5.1 Moisture content of leaves and soil Leaf and soil samples were taken at each study coupe immediately prior to burning as follows (Appendix 3):
Leaves (inside coupe) Elevated leaves Exposed surface leaves Leaves in contact with the soil under slash ("profile leaves") Leaves (outside coupe) Exposed surface leaves Leaves in contact with the soil ("profile leaves") Soil Surface soil inside the coupe under slash (0-1 cm) Surface soil inside the coupe not under slash (0-1 cm) Sub-surface soil (1 cm to 3 cm)
All soil and leaf samples were weighed moist, oven dried at 105º C to constant weight, and then moisture contents calculated as a percentage of the oven dry weight (%ODW).
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The results presented in Table 3 are an average of all the samples taken. Generally two or three jar samples of each type were taken (approx. 200-250 g in a 500 g sealable jar), but operational constraints limited this to one or none on some occasions. Thus the level of sampling was not adequate to measure the inherent variation in some instances. 2.5.2 Field soil moisture test A "field soil moisture test" was also used at all coupes (Table 3). This involved firstly, kicking an accumulation of surface soil - particularly the small windrow of soil left by the passing of the harvest machinery - to see if this would produce visible dust. The kick was with the toe of a boot, and the force applied was roughly equivalent to that used for kicking a football. Secondly, a visual assessment was also made of surface moisture which may have been present. The intention of this was to gain a rapid measure of the extent to which the surface soil had dried out, and also how extensive this drying was across the coupe.
The field testing of surface soil moisture, and assessment of visible moisture were coded as follows:
Field soil moisture test: Visible surface moisture: 1 - no dust, surface soil visibly damp 0 - no visible moisture 2 - no dust, surface soil just moist 1 - very sparse damp patches 3 - dust just raised 2 - damp patches covering 20% of surface 4 - dust raised from all loose soil 3 - damp patches covering 20-50% of surface 5 - dust raised from both loose and compact soil 4 - damp patches covering 50-90% of surface 5 - all visible surfaces damp
Subsequent laboratory testing of the typical soil moisture contents associated with these levels of dust formation in HEMS soils was carried out. The main reason for this was that the field soil moisture test was the only measure of soil moisture which showed some correlation with fire behaviour, fuel consumption and soil heating outcomes. The variations in moisture content of the few jar samples of soil taken on the day of the burn were too large to produce any useful correlations with these outputs. Five replications of HEMS soils samples, for three typical HEMS soils, were wetted and then dried. As drying progressed, the samples were tested for dust production.
Soil samples of approximately 500-800 g total were wetted using a mist applicator. This applicator was used to simulate the effect of steady rainfall. Wetting was continued until all soil material was wet to touch. The intention was to take the samples to a moisture content near to field capacity (McDonald et. al. 1984). Final moisture contents obtained varied from approximately 45% to 49%. The wetting was carried out until the soil was wet to touch but still retained some structure. Some soil structure was destroyed in both the collection and wetting processes, but these HEMS soils retained enough structure so that there were some aggregates still intact. Texturally they were loams and clay loams (McDonald et. al. 1984), which have inherently good structural qualities.
Once wetted, a sub-sample of approximately 100 g of soil was placed in a foil tray. Five of these sub-samples for each soil type were then placed in an oven, and dried at a constant temperature of 105ºC.
Testing for dust, as drying progressed, was carried out by crushing a 5-10 mm aggregate of soil between thumb and forefinger. This required considerable force
HEMS Regeneration Burning Studies 9 to be applied, particularly as the aggregates dried out. The use of this level of force was consistent with the type of force applied to soils in the field testing - that is, by kicking the soil with about the same force as is required to kick a football.
It quickly became apparent which aggregates would yield dust, with aggregates above the "critical" moisture content simply crumbling to smaller moist aggregates without yielding dust. The rapid drying regime applied meant that the outermost aggregates dried more quickly than the ones in the middle of the sample. This was likely the greatest source of error when determining dust formation moisture contents, as weighing of the whole sample, rather than of individual aggregates, was required for calculation.
Samples were dried to constant weight, which was then taken to be 0% moisture content. This assumption may not have been completely valid, as small amounts of residual moisture may have been held very tightly in some of the smallest inter-particle spaces, even after prolonged drying. However this source of error should not have been significant, as these last residual amounts of moisture were unlikely to be more than 1 or 2%. 2.5.3 Seasonal soil moisture trends - Drought Index Due to the location of the HEMS coupes studied, it was not possible to get accurate Keetch Byram Drought Index, KBDI, (Keetch and Byram, 1968) figures for any of the study locations. As a supplementary study, the Drought Index at some selected HEMS coupes was examined in detail during the summers of 2000/01 and 2001/02. Rainfall figures for these coupes were collected by placing rain gauges at each coupe and measuring these on a weekly basis from November/December through to February/March. Temperature data were derived by extrapolating data from the Combienbar Bureau of Meteorology Automatic Weather Station (Combienbar AWS).Temperatures at the coupes were calculated by adjusting Combienbar AWS figures according to the elevation difference, and the standard atmosphere adiabatic lapse rate.
A progressive Keetch Byram Drought Index for each coupe was then calculated using the temperature and rainfall data. These progressive KBDI figures were then plotted to examine any apparent trends.
The occurrence of potential HEMS slash burning days over the burning season for 2002 was investigated by analysing data from the Gelantipy AWS (elevation 750 m) and Combienbar AWS (elevation 650 m). Average temperature, relative humidity and wind speed data for the period 1200-1600 hrs on each day were derived, and combined with Drought Index and recent rainfall records to derive an average Fire Danger Index (FDI) for the same time period. This period, 1200- 1600 hrs, includes the most likely light up times for slash burning. Days with FDI 3 (or greater) were then selected from mid February to early May. A comparison was made between these potential days, and the actual days on which the Bendoc
Forest District conducted slash burning operations. Results of this analysis are given in Appendix 4. 2.5.4 Slash fuel curing time The time between harvesting and slash burning was defined as the curing time. The average curing time, in days, was recorded for each coupe.
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2.6 Topography The elevation, aspect and average slope were recorded for each study coupe.
2.7 Lighting Method The Aerial Drip Torch (ADT) was known to be able to light large areas very rapidly. However there had been speculation as to whether lighting using the ADT may be giving only a superficial burn result, with only the fine elevated fuels being consumed, and there being little opportunity for substantial soil heating and production of "ash-bed" seedbed. It was suggested that the ADT may have been used under marginal weather conditions to "force" what appeared to be an acceptable burn outcome from the aspect of fire protection (i.e. reduction in the elevated fuels), but which may not have had much silvicultural benefit in terms of seedbed preparation. Thus a further aim of data collection for this study was to investigate if lighting method made any significant difference to fire behaviour, fuel consumption or soil heating.
The main lighting method for each study coupe was recorded. This consisted of either hand lighting using handheld drip torches, or aerial ignition using the ADT. Some coupes lit with the ADT had some supplementary hand lighting, but this was not important to the development of significant convection.
2.8 Fire Behaviour Although fire behaviour is notoriously transient and inherently difficult to accurately record, it was possible, using various ranked criteria, to rate fire behaviour outcomes. The principal measures used were: (a) convection column formation and strength, and (b) the production of in-draught winds and tree sway. The occurrence of spotting was also recorded, but was not seen as a principal indicator of burn outcome. The weather conditions associated with the observed fire behaviour were recorded, particularly air temperature, relative humidity, wind strength, atmospheric stability, and amounts of cloud cover and direct sunlight influencing the fuels.
Fire behaviour was observed visually at all coupes. Also photographs were taken of many of the coupes to indicate the level of fire behaviour, and convection column formation. The specific fire behaviour effects of convection column formation, significant in-draught/tree sway and spotting occurrence, were recorded. These were coded as follows:
Convection column formation: 1 - very weak convection column 2 - weak convection with some in-draught 3 - some convection but restricted to hotter sections of burn (multiple weak columns) 4 - strong convection mostly, but with some weaker areas 5 - very strong convection with plume rise to several thousand metres
In-draught/tree sway: 1 - very weak in-draught, no tree sway 2 - weak in-draught, little tree sway 3 - some in-draught and noticeable tree sway occasionally
HEMS Regeneration Burning Studies 11
4 - strong in-draught, most trees swaying 5 - very strong in-draught, all trees swaying significantly
Spot fire formation: 0 - no spotting 1 - embers falling but not sustaining further combustion 2 - some embers taking but spot fires not sustaining after in-draught ceased 3 - some embers taking and spot fires sustaining post-burn 4 - many embers taking and many problematic spot fires
The presence of any type of fire-whirl - i.e. a rotating column of hot air and flame caused by localised heat-driven instability - was also recorded.
2.9 Fuel Consumption Fuel consumption data allowed for analysis of the consumption of both the individual size classes of material, and also of the total volume consumed. Fuel consumption, and the associated production of ash-type seedbed, were seen as being of fundamental importance in ranking the success of this type of high intensity prescribed burning. Lack of seedbed production by inadequate burning had been responsible for expensive supplementary mechanical site preparation in many HEMS coupes in the past (ERDAG HEMS Regeneration Working Group, 1993). The current prescription (NRE 1998) requires a burn to produce at least 75% of receptive seedbed across a coupe to obviate the need for supplementary mechanical site preparation.
The wire tie transects were used to investigate the consumption of the various size classes of slash material, particularly to show any differences between coupes which had different levels of fire behaviour, or significant differences in soil or fuel moisture contents.
The measurements of the wire transects were also used to calculate estimates of the total volume of slash material. Volume estimates were made for slash material present prior to burning, and slash material left following burning. The difference between these figures was used to estimate average slash fuel consumption. Volume calculation of the wire transects was done using van Wagner's formula (Van Wagner 1968), which was developed for estimating forest residue fuels using a line intersect methodology. The accuracy of the volume estimates obtained was limited by the number of wire transects which were placed in each coupe (Appendix 3). Time and resources only allowed for two or three wire transects in some coupes. Other coupes, where five or six wire transects were completed, produced more reliable volume estimates. 2.10 Soil Heating Soil temperature data were collected to distinguish any apparent variations in temperatures reached between burns of different levels of fire behaviour and also different pre-existing soil moisture conditions. Soil heating has been investigated by a number of researchers in the past (see Discussion), and different temperature levels are known to produce some specific effects on soil as a germination medium.
12 HEMS Regeneration Burning Studies
Soil heating was measured by placing sets (generally 5 or 6 per coupe) of heat sensitive crayons around the coupe (crayons were Tempilstik , made by Tempil USA, 1% accuracy claimed). A surface set of 12 pieces of crayon (50-600ºC in 50ºC steps inclusive) wrapped in foil and attached to a piece of wire, was placed on the soil surface within the slash - but not in contact with any actual fuel. A sub- surface set of 12 pieces of crayon (50-500ºC in 50ºC steps inclusive) similarly attached was placed 2-3 cm under the surface set.
The intention of placing these crayon sets was to attempt to measure the temperature reached at both the soil surface, and also at a depth of 2-3 cm. The placement deliberately tried to avoid direct flame contact (and consequent very high temperatures), and yet at the same time assess the influence of a substantial accumulation of slash on the soil as it burnt in close proximity. These crayons were only able to indicate maximum temperatures reached. They did not give any indication of the duration of maximum temperature exposure of the soil.
Wrapping the crayon pieces in foil, and then attaching the pieces of foil to a length of wire, was found to be the only practical way to prepare them for placement. The results of laboratory oven tests, carried out prior to field placement, showed that the foil wrapping had little influence on the temperature reached.
2.11 Seedbed Condition Following Burning Forest district supervisory staff do an on-site visual inspection following burning, during which the whole coupe area is inspected for consumption of slash fuel by the prescribed burn, and production of receptive seedbed (in terms of the 75% standard above). If it becomes obvious during the initial part of this visual inspection that substantial areas of slash have either not burnt, or burnt poorly, then supplementary mechanical disturbance may be arranged before completing the visual assessment.
This assessment would then be completed as mechanical disturbance was proceeding, with the supervisor checking the remaining area of the coupe for further areas requiring disturbance. The aim of the supervisor in this assessment is to determine if the slash burn had consumed the slash fuels down to mineral earth - that is, that there were no substantial areas where unburnt fuels remained which could prevent the primary root shoot of germinating seed from reaching mineral soil.
Most of the coupes from the first year of the study (1998/99) were surveyed for seedbed condition following burning using one or both of two techniques. The results from these seedbed condition surveys were used to assist in defining fire behaviour levels associated with slash burning in HEMS which produced acceptable levels of receptive seedbed. Seedbed condition following burning was assessed using two techniques:
The first technique used visual assessment plots spaced at 20 m intervals along survey lines established during the initial assessment of slash fuels. A marker pin was placed on each survey point and these were relocated following burning. As recommended by NRE (1998), a circular plot of 2.27 m radius at each survey point was assessed for predominant seedbed condition using the following categories of seedbed:
HEMS Regeneration Burning Studies 13
1. Light burn - Fuels >20 mm have not been reduced to charcoal; duff layer (lower portion of surface fuel bed) has not been removed. 2. Medium burn - fuel 20 mm to 70 mm have been reduced to charcoal; duff layer has been removed. 3. Heavy burn - fuel greater than 70 mm has been reduced to charcoal, reddish oxide powder has been produced, duff layer has been removed. 4. Disturbed soil - surface has been disturbed, with friable mineral soil exposed. 5. Undisturbed - mineral soil still covered by substantial vegetative matter.
Categories 1 and 5 were “unreceptive seedbed” - i.e. where there was some barrier to soil penetration by germinating seedling root systems.
The second technique used the same seedbed classification as in the first, but was conducted as a running tally along the same survey lines that were used for the 20 m interval plots. That is, seedbed type was recorded continuously as the assessors moved along the survey lines. This second technique was applied where there was particular concern over the amount of seedbed produced by a lower intensity burn. This second technique gave a broader estimate of seedbed condition across the coupe. In this regard it was closer to the reconnaissance type of post-burn seedbed assessment carried out operationally by forest district staff.
3 RESULTS
3.1 Fuel Structure on Slash Distribution Transects Table 2 shows the results of the visually-assessed fuel transects (10 m each) conducted during both years of the study. The numbers shown are the averages for each coupe from the total transects undertaken (generally between 6 and 10).
Table 2. Fuel occurrence and size data from visual assessment transects
1998/99 Ground % Elevated Average Branchlets Bark fuel 10-50mm 50-100mm >100mm Coupes fuel fine fuels height (no.) elements fuel fuel fuel coverage % (m) (no.) elements elements elements (no.) (no.) (no.) Yalmy 88 34 1.25 16 5 15 8 7 Coast Range 92 41 0.97 14 8 25 14 9 Little Bog 90 58 1.04 15 10 19 12 7 Students Rd 91 53 0.99 17 7 17 10 8 Survey Rd 91 64 1.20 15 7 24 14 8 Clarkeville Rd 83 42 0.95 16 10 25 13 8 Mt Tom 92 49 1.01 19 4 25 15 6 Watts Ck 93 53 1.09 18 6 23 14 7 GWHope 92 49 1.26 18 5 18 9 9 Table 2 cont’d
1990/00 Ground % Average Branchlets Bark fuel 10-50 mm 50-100 mm >100 mm Coupes fuel Elevated height (no.) elements fuel fuel fuel coverage fine fuel (m) (no.) elements elements elements % (no.) (no.) (no.) Playgrounds old 90 42 0.82 15 3 16 7 4 Playgrounds new 73 68 1.03 16 3 18 8 5 Waratah 83 58 1.05 18 4 15 6 6 Ellery old 77 25 0.83 11 4 15 8 5
14 HEMS Regeneration Burning Studies
Mt Tom West 91 52 1.13 21 7 27 11 5 Yalmy view 88 73 1.10 15 4 15 6 4 Ellery new 78 58 0.98 20 5 24 13 6 Clarkeville 2 75 68 1.30 15 4 26 9 5
The following figures show how the fuel elements were distributed on the 10 m visually assessed transects, undertaken with initial slash fuel assessment. Figure 2 shows the results from nine coupes studied in 1998/99, and Figure 4 shows the results from the seven coupes studied in 1999/00. The levels depicted are the averages from all transects taken on each coupe.
Figure 2. Occurrence of slash fuel elements in the 1998/99 coupes
Yalmy 30 Coast Range Little Bog Students Rd 25 Survey Rd Clarkeville Rd 20 Mt Tom Watts Ck GWHope 15 Number
10
5
0 Branchlets Bark 10-50mm 50-100mm >100mm Fuel element
HEMS Regeneration Burning Studies 15
Figure 3.
HEMS logging slash showing typical arrangement of different size classes of material. Photo is looking along a typical slash heap. Coast Range coupe in Bendoc Forest District.
Figure 4. Occurrence of slash fuel elements in the 1999/00 coupes
Clarkeville 2 30 Ellery New Ellery Old 25 Mt Tom West
20 Playgrounds Waratah 15 Yalmy View Number 10
5
0 Branchlets Bark 10-50 mm 50-100 mm >100 mm Fuel element
16 HEMS Regeneration Burning Studies
Figure 5. HEMS logging slash showing typical arrangement of different size classes of material. The secondary canopy under the adjoining uncut forest can be clearly seen also. HEMS coupe in Bendoc Forest District.
3.2 Short Term Weather, Fuel Moisture and Soil Moisture Table 3 shows the average weather, fuel moisture and soil moisture variables for all experimental coupes.
Table 3. Weather, soil moisture and fuel moisture on the day of burning for all coupes
HEMS slash burn Air Relative Wind Wind % Direct % Field soil Curing coupe temp humidity speed dirn sunlight Cloud moisture test for time (days) (degC) (%) (km/hr) (deg) cover dust formation (1-5 scale) Yalmy Rd 25 59 3.0 90 100 20 5 88 Clarkeville R 20 67 2.0 45 100 10 5 148 Students Rd 22 79 1.7 270 30 70 3 82 Survey Rd 19 82 0.5 270 0 100 2 120 Coast Range 18 80 6.7 135 0 100 2 85 Little Bog 20 70 3.1 320 5 95 2 – Watts Ck 17 66 3.2 270 95 10 4 117 Mt Tom 18 73 4.7 230 30 70 4 14 GWHope 20 72 2.5 360 80 40 4 113
Clarkeville 2 28 44 3.0 315 100 0 5 87 Playgrounds old 18 76 0.9 270 0 100 3 314 Playgrounds new 18 76 0.9 270 0 100 – 90 Waratah 22 78 0.0 320 0 90 3 325 Yalmy view 20 60 1.0 110 70 35 3 179 Ellery old 23 75 0.1 360 100 95 3 300 Ellery new 25 64 0.1 360 100 95 3 129 Mt Tom West 21 60 1.0 45 100 10 5 300 Table 3. cont'd.
HEMS Regeneration Burning Studies 17
HEMS slash burn FMC, FMC, FMC MC MC MC FMC FMC coupe elevated exposed Leaves in surface surface subsoil surface leaves in leaves surface contact soil inside soil inside the leaves contact inside leaves with the the coupe outside coupe outside with the coupe inside soil inside (%ODW) the coupe (%ODW) the coupe soil (%ODW) coupe the coupe (%ODW) (%ODW) outside (%ODW) (%ODW) the coupe (%ODW)
Yalmy Rd 10 10 - 42 23 88 19 -
Clarkeville R - - - 37 25 - - -
Students Rd 13 - 126 59 9 77 22 62
Survey Rd 17 21 - 30 18 63 21 58
Coast Range 20 90 - 125 17 33 - -
Little Bog 17 13 - 15 6 25 - -
Watts Ck 13 16 20 13 2 19 - -
Mt Tom 14 18 38 35 13 24 - -
GWHope 16 16 - 16 8 27 25 35
Clarkeville 2 14 29 179 36 5 27 45 -
Playgrounds old - - - 45 11 56 - -
Playgrounds new ------
Waratah 16 14 68 47 5 37 25 134
Yalmy view 13 46 - 47 14 39 22 57
Ellery old 14 22 23 54 11 46 52 153
Ellery new 13 13 18 25 18 56 52 153
Mt Tom West 15 26 48 37 5 20 21 40
( - no data collected)
These data, in addition to the data on fuels, topography and lighting methods, were used to build various models to explain variations in fire behaviour, fuel consumption and soil heating outcomes. 3.2.1 Field soil moisture test - dust formation moisture contents Table 4 shows the results of laboratory testing for typical HEMS soil moisture contents associated with the production of dust. It shows that dust begins to form at approximately 15-20 % (moisture content as a percentage of oven dry weight [%ODW]), is produced in significant amounts at moisture contents between 10% and 15%, and is produced by virtually all soil aggregates at moisture contents less than 10%.
Table 4. Dust formation moisture contents (% ODW) from HEMS soils under laboratory conditions
HEMS soil sample No. 1 Sub sample Sub sample Sub sample Sub sample Sub sample 1 2 3 4 5
18 HEMS Regeneration Burning Studies
Dust from edges of sample 19.2 17.4 18.4 18.2 19.1
Dust from 30-70% of aggregates 11.4 9.1 10.8 11.8 12.6
Dust from more than 70% of aggregates 5.1 5.1 5.1 6.3 7.2
HEMS soil sample No. 2 Sub sample Sub sample Sub sample Sub sample Sub sample 1 2 3 4 5
Dust from edges of sample 17.1 19.5 17.1 15.7 19.5
Dust from 30-70% of aggregates 10.8 15.1 12.3 10.1 14.1
Dust from more than 70% of aggregates 5.1 7.3 8.6 4.1 7.5
HEMS soil sample No. 3 Sub sample Sub sample Sub sample Sub sample Sub sample 1 2 3 4 5
Dust from edges of sample 21.2 18.2 20.7 20.1 22.1
Dust from 30-70% of aggregates 13.1 12.5 15.1 15.7 15.4
Dust from more than 70% of aggregates 7.3 5.2 7.2 8.1 9.1
3.3 Long-term Weather
3.3.1 Drought Index Figure 6 shows the Keetch Byram Drought Index (KBDI) trends from 30 November 2000 to 15 February 2001 for the two HEMS coupes selected for the supplementary study (Coast Range, Bendoc, and Granite Mountain, Cann River), and also for the Orbost Bureau of Meteorology Automatic Weather Station (AWS).
Figure 7 shows the KBDI trends from 1 December 2001 to 4 March 2002 for a further two HEMS coupes selected for the supplementary study (Black Hole, Cann River and Tennyson, Cann River), and also for the Orbost AWS. The summer of 2001/02 was much cooler and damper than that of the preceding season. It also had the very unusual event of flood-producing rains in early February - an event which zeroed the KBDI at all locations across East Gippsland.
These KBDI figures were collected to investigate both their absolute level and also their comparative level with KBDI figures at the lower elevation station of Orbost for the same period.
The main outcomes were: • the KBDIs for the four HEMS coupes were significantly lower than that for the Orbost AWS for the same period; • there are some rainfall events impacting on the KBDI at the HEMS coupes which do not show up for the Orbost AWS; • the trends clearly show that the KBDI can drop to very low levels (i.e. 10 or less) in early to mid February at the elevation of HEMS coupes.
Figure 6. Trends in Keetch Byram Drought Index (KBDI) for two selected HEMS coupes, and also for the Orbost AWS for the period 30 Nov 2000 to 15 Feb 2001.
HEMS Regeneration Burning Studies 19
120 Coast Range Granite Mtn 100 Orbost AWS
80
Orbost AWS 60
40 Keetch-Byram DroughtKeetch-Byram Index Coast Range 20 Granite Mtn
0 4-Jan-01 1-Feb-01 8-Feb-01 7-Dec-00 11-Jan-01 18-Jan-01 25-Jan-01 30-Nov-00 15-Feb-01 14-Dec-00 21-Dec-00 28-Dec-00 Date
Figure 7. Trends in Keetch Byram Drought Index (KBDI) for two selected HEMS coupes, and also for the Orbost AWS, for the period 1 Dec 2001 to 4 Mar 2002. 80
70 Orbost AWS
60 Black Hole DI Tennyson DI 50 Orbost DI
40
KBDI(mm) 30
Black Hole 20 Tennyson 10
0 5-Jan-02 2-Mar-02 2-Feb-02 9-Feb-02 1-Dec-01 8-Dec-01 12-Jan-02 19-Jan-02 26-Jan-02 16-Feb-02 23-Feb-02 15-Dec-01 22-Dec-01 29-Dec-01 Date
3.3.2 Weather availability for HEMS slash burning
20 HEMS Regeneration Burning Studies
A further supplementary study was carried out to look at the occurrence of the likely number of days in a burning season when effective HEMS slash burning could be undertaken. As a result of the finding that FDI 3 was the threshold of weather at which acceptable HEMS burning (in terms of fire behaviour, fuel consumption, soil heating and seedbed production) could be carried out (Table 3), the occurrence of days with FDI 3 (or greater) at HEMS elevations was investigated.
Appendix 4 gives a complete record of average weather, average FDI and daily Drought Index for the Combienbar AWS and the Gelantipy AWS, for the first four hours of the afternoon (1200-1600) for each day from mid February to early May 2002. The weather and FDI were averaged over this four hour period to indicate whether suitable HEMS slash burning conditions existed on that day or not. Days where a sustained FDI of 3 or greater existed for the whole four hours were selected. Some days of FDI 2 in February and March were also selected if the temperature was above 20 0C and the RH 65% or less.
Combienbar AWS (elevation 650 m) and Gelantipy AWS (elevation 750 m) were used, as they were the two stations closest to the HEMS study coupes in terms of both elevation and distance. Their elevations place them at the lower end of the elevation range for HEMS (Table 1). HEMS coupes at higher elevations than these two stations will be subject to cooler conditions on average. This was a further reason for selecting FDI 3 at either Combienbar or Gelantipy as the weather threshold for successful slash burning.
Table 5 summarises the findings in relation to the occurrence of days suitable for HEMS slash burning during the 2002 burning season, as indicated by the weather records from Combienbar AWS and Gelantipy AWS. It also indicates the more likely days from the less likely days due to the occurrence of fire restrictions and high fuel moisture contents (FMCs).
Table 5. Total and most likely days of weather available for HEMS slash burning - Feb to May 2002
Automatic Elevation (m) Total days of Days not avail in Days possibly Most likely Weather Station FDI 3 Feb due to fire too soon after number of days (or >) restrictions (& rain, or too late (i.e. the 15 Feb to 10 TFB in March) in the year remainder) May 2002 (FMCs too high)
Combienbar 650 21 3 (1) 6 11
Gelantipy 750 20 1 (1) 9 9
This table shows that there are limited days available for slash burning at the elevations of HEMS coupes. Appendix 4 shows that low temperatures, high relative humidities and low Drought Indices at these elevations are the principal weather factors involved in these limitations. February 2002 was cooler and wetter than average in East Gippsland. In an average year there would be more days available in February. However fire restrictions during February restrict HEMS burns being approved.
For comparison, Appendix 4 also shows the actual days on which the Bendoc Forest District conducted HEMS slash burning during 2002. This shows good
HEMS Regeneration Burning Studies 21 agreement between this methodology for selecting burning opportunities and the actual undertaking of slash burning.
3.4 Topographic Conditions Elevation, aspect and average slope for each experimental coupe are shown in Table 1. The only topography variable that had significant correlation with fire behaviour was slope . This relationship between slope and convection column formation, although significant (p = 0.05), was able to explain less than 20% of the variation in the data.
3.5 Ignition Method The two principal ignition methods were: 1) The Aerial Drip Torch (ADT) 2) Hand lighting with hand-held drip torches.
Of the 16 coupes studied, 10 were lit using the ADT (see Figures 8 & 10), 5 were lit by hand, and 1 was lit using a combination of these two methods (Table 6). There were no significant correlations between lighting method and any of the fire behaviour, fuel consumption or soil heating outputs. There was a weak correlation (p> 0.08) with consumption of the 100 mm + size class material, indicating that hand lighting may have been slightly more effective in igniting and consuming the larger fuels.
Figure 8. Helicopter with an Aerial Drip Torch lighting a slash burn
22 HEMS Regeneration Burning Studies
3.6 Fire Behaviour Observations Table 6 is a summary of the lighting method, ambient weather, Fire Danger Index (FDI), observable fire behaviour and cloud conditions for the 16 coupes studied. The complete data are shown in Appendix 3.
Table 6. Lighting method, ambient weather, observed fire behaviour, FDI and cloud conditions for the study coupes
Coupe name Lighting Weather 1 Fire Danger Fire behaviour 2 Cloud method Index (FDI) Yalmy Hand Acceptable 5 Very High intensity Students ADT Marginal 1 Low to Moderate intensity Cloud cover late AM Clarkeville ADT Acceptable 3 High to Very High intensity Survey Rd ADT/Hand Unacceptable 0 Low intensity Full overcast Coast Range ADT Unacceptable 0 Low intensity Full overcast, RH>70% Little Bog ADT Marginal/ 1 Low intensity Full overcast Unacceptable Watts Ck Hand Acceptable 3 High intensity Mt Tom ADT Acceptable 2 Moderate intensity GW Hope ADT Marginal/ 2-3 Moderate to High intensity Cloud early, clear later Acceptable
Mt Tom West ADT Acceptable 4 High to Very High intensity Playgrounds ADT Unacceptable 1 Low intensity Near full overcast Yalmy View Hand Acceptable 3 Moderate to High intensity Waratah Hand Marginal 2-3 Moderate to High intensity Cloudier later Ellery Old ADT Marginal 2-3 Low to Moderate intensity High RH later Ellery New ADT Marginal/ 3 Moderate intensity Acceptable Clarkeville 2 Hand Acceptable 7 High to Very High intensity
1 Weather : Acceptable =well within prescriptions (NRE, 1998, Appendix 1) for temp and RH - mostly FDI 3-10, Marginal = at the lower end of the prescriptions for temp and RH - mostly FDI 1-3, Unacceptable = outside the lower end of the prescriptions for temp and RH - FDI 1 or less. 2 Fire behaviour: Very High intensity = strong convection/tree sway, strong in-draughts, minor fire-whirls, High intensity = moderate to strong convection, some tree sway, moderate in-draught, Moderate intensity = moderate convection, little tree sway or in-draught) Low intensity = weak or no convection, no tree sway, no in-draught) 3.6.1 Convection column formation Convection column formation varied from very strong (e.g. Figure 9) to very weak (e.g. Figures 10 & 11)). This was associated with: variations in FDI from 0 to 7; variations in the % of cloud cover from 0 to 100%; variations in the field soil moisture test from 5 (dust from all surface soil) to 2 (no dust and visible patches of damp soil); and variations in atmospheric stability from stable (0) to neutral to unstable (1) (see Appendix 3).
HEMS Regeneration Burning Studies 23
Figure 9. Strong convection column resulting from HEMS burn (Clarkeville 1) conducted under weather conditions in mid range of prescriptions under Native Forest Silviculture Guideline No. 6 (~FDI 4).
Note clear skies and full direct sunlight on coupe.
The factors which showed significant correlation with the formation of a strong convection column were: (multiple factors) FDI and % cloud cover (single factor) atmospheric stability.
A multiple factor model (Equation 1) using FDI and the % cloud cover was able to explain 88% of the variation in the data A single factor model (Equation 2) using atmospheric stability, was able to explain 53% of the variation in the data.
Conv. column formn = 0.22*FDI - 0.02*% cloud cover + 3.97 (Equation 1) (Model: n = 16, r 2=0.88, p < 0.001)
(Where Conv. column formn = degree of convection column formation; FDI = Fire Danger Index, % cloud cover = percentage of cloud cover on the coupe),
Variable Co-eff. (std error) p value FDI 0.22 0.08 0.01 % cloud cover -0.02 0.004 <0.001
24 HEMS Regeneration Burning Studies
Figure 10. The Aerial Drip Torch igniting coupe at Coast Range under weather conditions outside prescriptions under NFS Guideline No. 6. (Temp 18ºC, RH 80 %, Wind <5 km/hr, FDI 1 or less). Note almost complete cover of flat cloud.
Figure 11. Low fire intensity resulting from the ADT lighting shown in Figure 10. HEMS slash coupe at Coast Range. Note very weak convection.
HEMS Regeneration Burning Studies 25
Conv. column formn = 2.17*Atmos stab + 1.57 (Equation 2) (Model: n = 16, r 2=0.53, p < 0.001)
(Where Conv. column formn = degree of convection column formation; Atmos stab =atmospheric stability level)
Variable Co-eff. (std error) p value Atmos stab 2.17 2.1 0.03
3.6.2 In-draught and tree sway Visible in-draught and tree sway varied from very strong (5) to very weak (1). This was associated with variations in % of direct sunlight from 0-100%, and variations in the field soil moisture test from from 5 (dust from all surface soil) to 2 (no dust and visible patches of damp soil).
The best multiple factor model to explain variations in in-draught and tree sway, explaining 92% of the variation was:
In-draught/tree sway = 0.007*% sunlight + 1.02* soilmoistest - 0.97 (Equation 3) (Model: n = 16, r 2=0.92, p < 0.001)
Variable Co-eff. (std error) p value % sunlight 0.007 0.003 0.04 soilmoistest 1.02 0.13 <0.001
(Where In-draught/treesway = degree of in-draught and tree sway; % sunlight = % of direct sunlight on coupe soilmoistest = field soil moisture test)
In-draught and tree sway were also significantly correlated (p<0.05) with FDI, % cloud cover and FMC of the elevated fuel. However these were single factor correlations, and did not explain more than 30% of the variation in the data. 3.6.3 Spot fire formation Spot fires are unplanned ignitions that result from burning embers, or firebrands, being carried outside the planned prescribed burn perimeter. These burning embers commonly result from the detachment of loose burning bark from standing trees within the coupe. Stringybark species can produce large amounts of ember material and contribute to many short distance spot fires. Gum species can produce large ribbons of bark that may be carried longer distances by convection and wind.
Spot fires occurred on only 2 out of the 16 coupes. They were associated with: - fine fuel moisture contents of the surface fuel outside the coupe of 20% or less, - coupes on the lower end of the elevation range for HEMS, - adjoining vegetation which had no secondary canopy cover.
Most of the uncut forest surrounding the HEMS coupes studied had a secondary (or understorey) canopy – see Figure 12. This gave rise to surface fine fuel moisture contents in adjoining forest of generally 20% or greater (material at 20% ODW will generally not burn), and moisture contents of the leaves in contact with the soil outside the coupe of 50% ODW or greater (Table 3).
26 HEMS Regeneration Burning Studies
Figure 12. Secondary canopy of wet forest shrubs in uncut forest adjoining HEMS coupe. Density of canopy is evident, as is high percentage of live/green material in fuel complex.
3.6.4 Fire behaviour effect on seedbed production
Table 7 shows the type and proportion of seedbed produced on 8 of the 9 coupes studied in 1998/99. It also shows the survey technique used. There were no systematic seedbed surveys of the coupes studied in 1999/2000.
Table 7. Percentage of type of seedbed produced on 8 of the 9 HEMS coupes studied in 1998/99
HEMS coupe Seedbed % Light % Medium % Heavy % UB % UB with % Total survey Burn Burn Burn no slash slash unreceptive technique (Light burn + UB with slash) Yalmy* Plots 5.5 11.4 56.0 26.2 0.9 6.4 Clarkeville* Plots 6.7 16.4 40.4 34.1 2.4 9.1 Watts Ck* Plots 8.6 7.0 33.2 47.5 3.7 12.3 Mt Tom* Plots 11.4 2.9 26.7 54.5 4.5 15.9 GWHope* Plots 2.2 6.8 46.7 36.0 8.3 10.5 Little Bog** Plots 11.3 17.5 8.2 49.1 13.9 25.2 Students ** Plots 16.3 32.5 9.2 35.2 6.8 23.1 Students ** Running tally 13.1 36.2 3.7 25.3 21.8 34.9 Survey ** Plots 15.2 18.6 13.4 41.3 11.5 26.7 Survey ** Running tally 7.9 37.3 5.8 20.8 28.2 36.1 (* mod-very high fire intensity, ** low fire intensity
HEMS Regeneration Burning Studies 27
These results show that the burns with moderate to very high fire intensities produced amounts of total receptive seedbed of 85-90% or greater, whereas the low intensity burns generally produced less than 75% of total receptive seedbed.
Three of the burns with the lowest intensities for this year of the study - Little Bog, Students and Survey - produced seedbed amounts which did not meet the 75% total receptive seedbed required under NFSG No. 6 (to avert supplementary mechanical disturbance). The seedbed survey at the Students Road coupe actually indicated 77% when surveyed using the plot methodology - due to the plots falling into some areas which burnt better - but the broader running tally method showed only 65% of total receptive seedbed.
This type of seedbed survey was not conducted on the other study coupe for this year which had a low fire intensity - Coast Range. Forest District staff judged that the burn result from this coupe was so poor, that the decision to conduct supplementary mechanical disturbance was made soon after the burn. Therefore it was decided that this coupe would be of no further use in terms of studying the effect of the slash burn on regeneration outcomes - hence the decision not to conduct seedbed condition assessment following burning.
3.7 Fuel Consumption
3.7.1 Consumption of fuel by size classes Figures 14 and 15 show the pattern of average consumption of the various size classes of fuel elements as measured by the wire tie transects. They show that there is mostly quite high consumption of both the 20-50 mm and 50-100 mm material in most slash burns (see Figure 13). (Note that 90% or more of the <20 mm thickness material was consumed in virtually all burns, with the exceptions being where some areas of slash were classified as "light burn"[with some of the lower surface fuels unburnt], or where slash failed to ignite.)
Figure 13. End of 10 m wire tie fuel consumption transect showing marker pin and wire loops left from fuel elements consumed in the fire.
28 HEMS Regeneration Burning Studies
Figure 14. Average percentage burnt of the three size classes of slash material for the nine coupes studied in 1998/99
20-50 mm 100 50-100 mm 100+ mm 80
60
40
20 for the 3 size classes size 3 the for
% of volume removed by burning burning by removed volume of % 0 Yalmy Survey MtTom Hope Students Little Bog Clarkeville Great White Watts Creek Coast range HEMSCoupes coupe 1998/99 1998/99
Figure 15. Average percentage burnt of the three size classes of slash material for the seven coupes studied in 1999/00 (Playgrounds coupe is split into old and new slash, as consumption varied significantly between these.)
20-50 mm 100 50-100 mm 100+ mm 80
60
40 for the 3 size classes size 3 the for 20 % of volume consumed by burning burning by consumed volume of % 0 Old New Waratah Ellery Old Clarkville 2 Ellery New Ellery New Yalmy View Playgrounds Playgrounds Mt Tom West HEMSCoupes Coupe 1999/00 99/00
HEMS Regeneration Burning Studies 29
3.7.2 Correlation of fuel consumption with other factors
Fuel consumption was affected by fire behaviour, with the burns showing better convection and more in-draught/tree sway having greater fuel consumption. Fuel consumption of all size classes was significantly correlated with the degree of convection column formation, and in-draught/tree sway (Table 8), but these correlations (all single factor) were unable to explain more than 40% of the variation in the data.
It was observed that the older slash on some of the carryover coupes was not consumed quite as completely, with the specific exception of the Mt Tom West coupe (Figure 15). Here, a high to very high intensity burn (strong convection, strong in-draught/tree sway and, notably, a longer burn-out time) in carryover slash gave some of the highest fuel consumptions measured.
Table 8. Single factor correlations of fuel consumption variables with fire behaviour variables
For dependent variable Consumption 20-50 mm
Correlated variable r2 co-eff. (std error) p value Convection 0.36 6.3 1.9 <0.01 In-draught/tree 0.33 6.2 2.2 0.01 sway
For dependent variable Consumption 50-100 mm
Correlated variable r2 co-eff. (std error) p value Convection 0.30 8.5 3.0 0.01 In-draught/tree 0.35 9.5 3.0 <0.01 sway
For dependent variable Consumption 100+ mm
Correlated variable r2 co-eff. (std error) p value Convection 0.42 7.8 2.2 <0.01 In-draught/tree 0.42 8.3 2.3 <0.01 sway
The other factors measured during burning which correlated with the consumption of the various size classes of slash material were as follows:
20-50 mm fuel - the consumption of this fuel size class was best correlated with the field soil moisture test , indicating that consumption of finer fuels was related to short term moisture variation. There was also a weaker correlation with the FMC of the elevated fuel (p > 0.10).
Consumption 20-50 mm = 9.6*soilmoistest+ 56 (Equation 4)
30 HEMS Regeneration Burning Studies
(Model: n = 16, r 2=0.43, p < 0.001) (Where Consumption 20-50 mm = % of 20-50 mm fuel consumed; soilmoistest =field test for soil moisture)
Variable co-eff. (std error) p value soilmoistest 9.6 2.5 <0.01
50-100 mm fuel - the best explanatory model for fuel consumption in this size class was a two factor model using the field soil moisture test and the FMC of the elevated fuel . This model explained 54% of the variation in the data.
Consumption 50-100 mm = 6.9*soilmoistest – 4.9*FMC_EL +128 (Equation 5) (Model: n = 16, r 2=0.54, p < 0.01) (Where Consumption 50-100 mm = % of 50-100 mm fuel consumed; soilmoistest = field test for soil moisture, FMC_EL = Fuel Moisture Content of elevated fuel)
Variable co-eff. (std error) p value soilmoistest 6.9 5.0 0.18 FMC_EL -4.9 2.2 0.05
100+ mm fuel - again the best correlating factors were the field soil moisture test and the FMC of the elevated fue l, with a two factor model able to explain 72% of the variation in the data.
Consumption 100+ mm = 9.4*soilmoistest - 2.7*FMC_EL + 27 (Equation 6) (Model: n = 16, r 2=0.72, p < 0.001) (Where Consumption >100 mm = % of >100 mm fuel consumed; soilmoistest = field test for soil moisture, FMC_EL = Fuel Moisture Content of elevated fuel)
Variable co-eff. (std error) p value soilmoistest 9.4 2.9 0.001 FMC_EL -2.7 1.3 0.06
The consumption of all three size classes of material showed significant correlation with the average slope across the coupe, but models using slope alone did not explain much of the variation in the data. 3.7.3 Curing time Curing time varied from 82 days to 129 days for recent slash (apart from one coupe at 14 days - Mt Tom), and from 179 days to 325 days for carryover slash (Table 3).
It was expected that there may have been some relationship between curing time and the consumption of the >100 mm size class material, given that this size of material may take many months to dry to levels where it will burn easily (Tolhurst and Cheney 1999). However there was no significant correlation for this in the current data. Additionally, curing time did not correlate significantly with any of the other fuel or fire behaviour variables. The data actually suggest that fuel consumption of older slash was lower compared to newer slash under similar weather conditions. At both Ellery and
HEMS Regeneration Burning Studies 31
Playgrounds, the measured consumption of all size classes of slash material was higher for the recent slash than for the carryover slash (Appendix 3). 3.7.4 Total slash volume and volume consumed Figure 16 shows the volume estimates obtained from using the Van Wagner (1968) line intersect method to calculate volume of forest residue fuels. Results from assessing the < 20 mm material showed that it did not vary appreciably across the range of coupes sampled (see Figures 2 and 3). Therefore it was assumed to be roughly constant, and a standard figure of 50 t/ha (derived from weight measurements of representative samples of the smaller fuel elements) was added to the volume data obtained from the unburnt wire transects.
Figure 16 shows, in tonnes per hectare, the average of fuel volume as measured in the slash accumulation heaps. These figures only apply to these sample points and not to the coupe as a whole, as all HEMS coupes had significant areas of low fuels (or no fuel) where the harvesting machinery had traversed the coupe. GIS analysis of aerial photos of some of the study coupes revealed that these areas of low, or no, fuel could cover as much as 30% of the area of the coupe (Figure 17).
Figure 16. Slash material volume before burning, after burning, and volume consumed during burning for the 16 HEMS coupes. Volumes represent an average of the slash sample points only, and do not apply across the whole coupe.
1400 Unburnt av vol
1200 Burnt av vol
1000 Difference
800
600
400
200 Volume (t/ha - av at sample point) sample at av - (t/ha Volume 0 Yalmy MtTom Average Waratah Plygrnds Students Little Bog Little GWHope Watts Ck Watts Ellery Old Ellery Ellery new Ellery SurveyRd YalmyView Clarkeville 1 Clarkeville 2 Clarkeville Coast Range Coast MtWest Tom HEMS slash burn coupe
The total volume of material consumed did not show any significant correlation with the inputs of fuel, weather, topography or lighting method. There were some weak correlations (p>0.05) with elevation and average slope of the coupe. The general trend of this data is that 200+t/ha of fuel (in total at the sampling points) was consumed where the burn produced fire behaviour giving at least a moderate convection column. Two of the burns with the lowest fire intensity,
32 HEMS Regeneration Burning Studies
Little Bog and Coast Range (Table 4), had total fuel consumption levels of less than 160 t/ha (average at the sample points).
Figure 17. Aerial view of typical HEMS fuels showing distribution of slash heaps, and areas of disturbed soil following harvesting (Playgrounds coupe, Bendoc). The older slash can be seen with a more greyish colour - right hand top side of photo.
3.8 Soil Heating Figures 18 and 19 show the trends in soil temperature crayons for the study coupes. The levels shown are the average maximum temperature reached for all the crayons placed across each coupe.
HEMS Regeneration Burning Studies 33
Figure 18. Average temperatures recorded using heat sensitive crayons placed in surface soil and subsoil at study coupes 1998/99. 700 GWHope 600 Yalmy Students 500 Clarkeville Mt Tom 400 Watts Ck 300 Coast Range Survey 200 Little Bog Temperature (deg C)
100
0 Surface Subsurface (2-3cm) Heat sensitive crayon location
Figure 19. Average temperatures recorded using heat sensitive crayons placed in surface soil and subsoil at study coupes 1999/00. (Note that the sub-surface crayons at Playgrounds Old did not register even 50 0C) 700 Yalmy View 600 Clarkeville 2 Waratah 500 Ellery New Playgrounds old 400 Ellery Old
300 Mt Tom West
200 Temperature (deg C)
100
0 surface subsurface (2-3cm) Heat sensitive crayon location
Similar to the results for fuel consumption, the highest temperatures recorded were associated with the higher intensity burn results (Table 6). The particularly low sub-surface temperatures for Coast Range, Survey, Little Bog and Playgrounds (old), were thought to result from a combination of high soil moisture contents, and marginal weather (RH >75%, cloud cover > 70%) on the day of the burn.
3.8.1 Soil temperature correlation with other factors
34 HEMS Regeneration Burning Studies
3.8.1.1 Subsoil The best two factor model to explain variation in subsoil crayon temperature used FMC of the leaves in contact with the soil outside the coupe and the field soil moisture test . Therefore measuring the FMC of leaves in this location, in conjunction with conducting the field soil moisture test, may be a useful way of predicting what subsoil temperature will be reached. This model was able to explain 85% of the variation in the data.
Sub_cryn = 1.9*LICS_OS + 67.4*soilmoistest - 183.1 (Equation 10) (Model: n = 16, r 2=0.85, p < 0.01) (Where Sub_cryn = average temperature of the subsurface crayon, LICS_OS = average FMC of the leaves in contact with the soil outside the coupe soilmoistest = field test for soil moisture)
Variable Co-eff. (std error) p value LICS_OS 1.5 0.3 0.001 soilmoistest 67.3 17.4 0.01
3.8.1.2 Surface soil The factors best correlated (all single factor correlations) with the temperature reached by the surface soil crayon were the degree of convection column formation and the field soil moisture test , indicating that fire intensity and short term drying effects were most important in determining surface temperatures reached. There was a weaker correlation with the FMC of the elevated leaves.
For dependent variable Surf_cryn:
Variable r2 co-eff. (std error) p value convection 0.39 59.4 18.1 0.01 soilmoistest 0.39 81.8 24.9 0.01 FMC_EL 0.28 -27.3 10.9 0.03
(Where Surf_cryn = average temperature of the surface crayon, FMC_EL = average FMC of the elevated leaves in the slash soilmoistest = field test for soil moisture convection = degree of convection column formation) 3.8.2 Soil moisture The soil moisture data (derived from oven drying the collected samples) showed no significant correlation with any of the fuel, weather or fire behaviour outcomes. It is likely that the level of sampling was too low to reduce the inherent variability of this factor.
The only measure of soil moisture to show any correlation with any of the other data was the field soil moisture test . It is likely that this test was able to rapidly integrate the variation in factors determining soil moisture levels, and present a broadly classified summation of soil moisture status.
HEMS Regeneration Burning Studies 35
4 DISCUSSION
4.1 Effects of Fuel Distribution and Quantity The results of sampling the fuel elements showed that the occurrence of the smaller fuel elements was quite uniform across the range of coupes (Figures 2 & 4). It appeared that, for most HEMS logging operations, the material derived from the crowns produced slash heap profiles which varied little in the occurrence of branchlets and bark clumps. The only significant variation in these elements was their vertical distribution, which varied across the range of coupes, and also with the age of the slash. The older slash on carryover coupes, despite having mostly the same number of these smaller fuel elements, tended to have them less elevated. That is, the carryover coupe slash had a slightly lower mean slash height, and also slightly lower percentage of elevated fine fuels (Table 2) - indicating that there had been more time for both the branchlets to become more flattened on the ground, and also for there to be some leaf fall.
The largest variation in fuel elements was in the larger material, with some of both the visually based and wire tie fuel transects showing the occurrence of more material (and of larger average diameter) in the >100 mm size class on some coupes.
The variation in the fuel element size classes appeared to have little influence on burn outcomes, particularly when compared with the more substantial effects of variations in weather and fuel moisture.
4.2 Effects of Weather
4.2.1 Short-term weather The range of short term weather conditions sampled was sufficient to observe a range of fire behaviour outcomes across the study coupes. This range of conditions produced fire behaviour up to a level which, despite very high fire intensities, did not cause any substantial control problems outside the coupe boundaries. Short term weather conditions which could produce substantial control problems (i.e. temperatures > 28ºC, RHs < 30% and, particularly, wind speeds >15 km/hr, FDI >12) were not sampled, as they did not occur during the study period in association with HEMS slash burning operations. (See 4.3.1 for discussion of HEMS burn escapes, and McCarthy(2003).)
As with fire behaviour generally, the fire behaviour observed at the experimental coupes varied with variations in fuel, weather, topography and ignition method. Weather and fuel moisture contents appeared to be the most important variables for the coupes observed. Warmer temperatures (particularly in conjunction with direct sunlight), lower relative humidities and neutral to unstable atmospheres all combined to give better burning conditions with stronger convection columns.
The formation of a strong convection column was inhibited by higher RHs and stable atmospheres, and particularly where these were combined with low level cloud cover, which also significantly reduced the amount of direct sunlight falling on the slash fuels. Measuring temperature and RH before ignition, along with looking for other stability indicators such as flat layered cloud, will be useful in
36 HEMS Regeneration Burning Studies
determining whether atmospheric conditions are likely to be helpful with convection column formation.
Fuel moisture content, as influenced by both surface soil moisture content and relative humidity, also seemed to be particularly important in determining the formation and strength of a convection column. The results of the measuring FMCs of the elevated fuels (using a Wiltronics Fine Fuel Moisture Meter), and undertaking the field soil moisture test, were useful for predicting likely fire behaviour (when used in conjunction with the weather factors). All coupes where strong convection columns formed had elevated fuel FMCs of less than 13% ODW, and surface soil moisture contents which were low enough for easy dust formation when the soil surface was impacted.
The occurrence of in-draughts and tree sway were affected by mostly the same factors as convection column formation, with amount of direct sunlight combined with the field soil moisture test giving the best prediction. These two predictive factors relate directly to the dryness and warmth of the fine fuels, with dry fuels (<13% ODW) warmed by direct sunlight leading to high to very high fire intensities. 4.2.2 Long-term weather - Drought Index trends While coupe-specific KBDI values could not be obtained, the general trends of KBDI at HEMS locations, as shown in the supplementary study, are useful for indicating the likely influence of long-term soil moisture variations on slash burning conditions. The two burning seasons in which the study was conducted did not include a year where substantial drought prevailed. Fire managers should therefore be aware that severe drought conditions may significantly change the soil moisture regime (and hence it's influence on fuel moisture) at HEMS locations.
The supplementary study of KBDI trends for two HEMS coupes for the 2000/01 season, and the two HEMS coupes for the 2001/02 season, showed that, in most years, KBDI values at HEMS coupe locations may be substantially lower than those at lower elevations. The implications of this for likely fire behaviour are significant, particularly the likelihood of prescribed slash burns going out overnight (see Appendix 1). That is, if KBDI values are less than 20 mm, the chances of fires extinguishing overnight are quite high.
Figure 6 shows that the KBDI values for the two HEMS coupes stayed generally below 20 mm for most of January and early February 2002. Hence there is a relatively high likelihood that any surface fire, either within or outside a HEMS coupe, would self extinguish overnight at this time of the year. This could have a significant influence on whether safe prescribed burning could be both approved and undertaken at this time of the year.
A further significant trend which is apparent from Figures 6 & 7, is that HEMS coupes may receive precipitation events which do not occur at lower elevations. This is evident from the small downward trends in KBDI for the two 2000/01 HEMS coupes (2-3 Dec and 4-8 Jan), and for the two 2001/02 HEMS coupes (25-26 Dec and 13-15 Jan) - trends which do not occur for the lower elevation station of Orbost.
HEMS Regeneration Burning Studies 37
4.2.3 Availability of days for HEMS slash burning The supplementary study of weather availability showed that there may be as few as 9 days (Table 5 & Appendix 4) within a burning season which are completely suitable for HEMS slash burning. This shows that burn managers must use resources wisely, and in a timely way, to take advantage of this narrow "weather window" for successful HEMS slash burning. 4.2.4 Effects of weather and fuel moisture on control of slash burns. Spotfires outside the slash burn area in HEMS coupes were unusual in both the 1998/99 and 1999/00 burning seasons. As previously reported, HEMS coupes often have uncut vegetation adjacent which has a secondary or understorey canopy (Figure 5, Figure 12). This secondary canopy often has a significant shading effect on the surface fuels underneath it, which means that surface and profile fuel moisture contents will often not be below 20%. The mean fuel moisture content (FMC) of the exposed surface leaves outside the coupes in this study was 30%, and the lowest measured was 19%. Profile fuels outside the coupe averaged 87% FMC for this study, with the lowest measured being 37% (Table 3). In addition to this, the surface and elevated fuel hazard levels (McCarthy et. al . 1999) of the fuels below the secondary canopy rarely got above High . They were characterised by moist surface fuels, green grasses and low shrubs with little dead material - fuels which were unlikely to provide any substantial fire behaviour.
In a drier year, these values may have been somewhat lower, but it is still likely that, even if exposed surface FMCs dropped to 10% or below, allowing surface fuels to easily ignite from firebrand material, the combination of much moister profile fuels and the presence of a secondary canopy, would substantially reduce the likelihood of spotfires spreading quickly enough to defeat suppression efforts.
In the drier year, the biggest threat of escape would be caused by strong winds. Even in a moister year it would not be wise to burn if winds exceeded about 15 km/hr (the current prescriptions recommend 10 km/hr), as even a central convection lighting pattern may not be able to generate sufficient upward and inward convective motion to counteract the sideways motion of the ambient wind (Tolhurst and Cheney, 1999).
Spotting caused significant escape problems at a HEMS slash burn conducted as part of fire control operations along the Yalmy Road in February 2003 (McCarthy 2003 unpubl.). Severe short distance spotting necessitated the construction of three consecutive fall-back control lines on the northern side of the coupe. Surface FMCs were 10% ODW, and Profile FMCs were 15% ODW, under a secondary canopy adjacent to the coupe. Weather conditions were: temperature 23ºC, relative humidity 25%, and wind southerly at 15-20 km/hr. The KBDI at Orbost was 105. This was following a prolonged period of hot, dry weather which caused many HEMS areas to have seasonally very dry fuels, and, in particular, dry profile fuels as measured here. Approval for prescribed burning is normally not given under these conditions. 4.3 Effect of Topography The variations in average slope, elevation and aspect across the range of coupes studied were sufficient to sample the main differences found across HEMS coupes generally. No coupes on very steep slopes (>20º) were included in the study. Thus
38 HEMS Regeneration Burning Studies
there were no observations of fire behaviour in combination with very steep terrain.
The principal effects of topography on slash burn outcomes for the study coupes were: - coupes at higher elevations were subject to less favourable weather conditions for burning more often than coupes at lower elevations; - coupes with higher average slopes had slightly higher overall fuel consumption levels than coupes with gentler slopes.
Topographic conditions had substantially less influence on fire behaviour outcomes than did ambient weather conditions. However some of the coupes at higher elevations may have had more influence from cooler, moister conditions associated with the increase in elevation, when compared to coupes at lower elevations on the same day. 4.4 Effect of Ignition Methods - Aerial Drip Torch (ADT) vs Hand Lighting Both principal ignition methods - the ADT and hand lighting - were observed to be capable of producing sufficient fire behaviour to give the high to very high fire intensity required for slash burning. Adverse weather and high fuel moisture contents reduced the ability of either method to produce acceptable fire intensities. The important differences were in the speed of ignition, and also the ignition pattern obtained. The ADT was always substantially faster at igniting, especially across larger coupes. However hand ignition was the only method which could light long continuous lines of fire, and was always more accurate in igniting the most receptive areas of the fuel complex.
Thus, although the ADT is an extremely useful tool for rapid ignition of large coupes, it was still observed to have two significant drawbacks. The first important drawback is that the burning fuel is delivered onto the coupe in a series of spot ignitions, and, despite the best pilot skills, many of these spot ignitions fall onto areas of poor ignitability - e.g. bare ground, tracks, large material etc. In this regard the ADT cannot match a hand-held drip torch for both accuracy of ignition into the most ignitable areas, and also the ability to light a continuous line of fire. Even at maximum fuel delivery rates, the ADT can only ever achieve a series of closely spaced spot ignitions, which will always require a certain time to join up into a line of fire. Therefore it will never be able to achieve, as the hand-held drip torch can, a continuous line of fire which will give the maximum instantaneous forward rate of spread, and thus maximum intensity (Tolhurst and Cheney 1999).
The second drawback of the ADT is its availability. Because the HEMS slash burning weather "window" is so short in many years (Table 5), there can be intense competition between neighbouring Forest Districts for the use of the ADT within a given few days (even though there are two ADTs that operate in Victoria).
This led directly to the situation that occurred on the day that the Bendoc Forest District attempted to burn the Little Bog, Coast Range and Survey Road coupes on 10 March 1999. While the weather in the two days prior had been quite satisfactory for HEMS slash burning, this day had low overcast cloud which severely inhibited any vertical motion in the lower atmosphere. Because Bendoc
HEMS Regeneration Burning Studies 39 had ordered the use of the ADT for this day though, an operational decision had to be made as to whether to use it or not. The pressure of: • the presence of the helicopter; • the presence of the mixing crew; • having local staff already tasked to the coupes to be burnt; and • needing to release the helicopter to an adjoining District as soon as possible; meant that a decision was made to try to burn under what turned out to be very marginal conditions. Despite the warm temperatures (20ºC +), the low overcast and consequent atmospheric stability meant that, while the slash fuels would light, they would not sustain a fire capable of producing a convection column. Consequently, as can be seen from the results, none of these burns were able to achieve satisfactory fire behaviour, fuel consumption or soil heating outcomes.
The level of receptive seedbed preparation achieved by these burns was judged to be unsatisfactory by District staff, and a decision was made to do supplementary mechanical site preparation at both the Little Bog and Coast Range coupes.
Under satisfactory weather conditions however, as was the case for many of the other coupes observed during the study, the ADT was able to produce rapid mass ignition over some very large areas (40 ha +). The additional benefits of the ADT - the need for minimum ground crews, greatly enhanced safety of ground crews (no staggering through slash with lit drip torches), and the ability to light a number of coupes within a day - were all apparent under these circumstances of favourable weather.
Most of the hand lit burns observed were relatively successful. The ability of ground crews with drip torches to light much more accurately (than the ADT) was apparent. Also apparent though, were that these operations required many more resources (both people and equipment), that light up generally took considerably longer, and that crews were exposed to the inherent dangers of the operation.
The main conclusions from observing both lighting methods during this study, and the results obtained, are:
• that the ADT is a very useful tool for ignition of HEMS coupes, but that it needs favourable weather (and fuel) conditions to be used to best advantage; • that the ADT cannot overcome poor weather with additional lighting intensity; and • that hand lighting, despite its drawbacks, can still be an effective method of achieving acceptable fuel consumption and soil heating outcomes in HEMS slash burning. 4.5 Effects of Weather, Curing Time, Lighting Method and Topography on Fuel Consumption The highest levels of fuel consumption were always associated with warmer (>20ºC) and drier (<65% RH) weather conditions on the day of the burn.
From the data collected, it appears that in order to produce at least 75% of receptive seedbed across the coupe, a slash burn needs to consume: >90% of the 0-50 mm size class material >70% of the 50-100 mm size class material, and >20% of the >100 mm size class material
40 HEMS Regeneration Burning Studies
This equates to at least 200 t/ha of fuel being consumed within the slash heaps (Figure 16). This proposition is based on the observation that fuel consumption levels less than these were associated with total receptive seedbed amounts of less than 75% (Table 7). These levels of unreceptive seedbed caused staff to undertake supplementary mechanical site preparation works on some of the coupes studied - the current prescription that the combined logging operation and slash burn should produce at least 75% of receptive seedbed across the coupe (NRE 1998). Two of the Bendoc coupes studied, Little Bog and Coast Range, which did not reach these levels, were subsequently scheduled for supplementary mechanical site preparation.
The age of the fuel elements (i.e. curing time, see King et. al . 1993), although not significantly correlated with fuel consumption generally, did seem to influence the fuel consumption outcomes for adjacent old and new slash burnt under marginal weather conditions. For example, at both Playgrounds and Ellery, the older (carryover) slash was not consumed to the same extent as the newer slash, when weather conditions were just within the lower end of the current prescriptions for RH and FMC. Under these marginal weather conditions, the occurrence of more live/green material within the older slash may have also restricted fire behaviour development, and hence fuel consumption.
In contrast to this, at Mt Tom West (a carryover coupe) when the weather conditions were much warmer and drier, the more weathered slash was consumed at a higher rate than almost any other coupe.
A specific example of the Aerial Drip Torch (ADT) not being able to produce sufficient fire intensity, under marginal weather conditions, to achieve these proposed acceptable fuel consumption levels, was observed at the Little Bog coupe. In full overcast conditions with a stable atmosphere, the ADT was unable, despite the application of an entire load of ignited burn gel, to produce enough fire intensity across this coupe to consume more than 30% of the fuel in the 20-50 mm size classes. It is doubtful whether hand ignition could have achieved substantially better results of fuel consumption under these marginal weather conditions, despite more accurate ignition.
The apparent slight correlation between average slope across the coupe, and consumption of all size classes of slash material, may be an artefact of the current data, as a number of the coupes with lower than average slope were also those where poor weather conditions on the day of the burn gave rise to reduced fire behaviour. If however this is a real effect, it may be explained in terms of more slope giving more effective pre-heating to fuels above and therefore consumption levels increasing with more complete combustion. 4.6 Effects of Fire Behaviour and Soil Moisture on Soil Heating The range of average temperatures, recorded by the temperature-sensitive crayons on the study coupes, are in general agreement with temperatures recorded from various other methods, in past studies. Unlike some past studies, discussed below, this study investigated only maximum temperature reached, and did not measure exposure time to maximum temperatures.
Humphreys and Lambert (1965 ) examined temperatures under slash and log fires of various intensities in the mixed species forest in the Central Tablelands of NSW. Using chromel-alumel thermocouples, these authors found temperatures of
HEMS Regeneration Burning Studies 41 between 350ºC and 900ºC at the surface, reducing to a range of 50ºC to 350ºC at 2.5 cm below the surface. The highest surface temperatures were associated with a thermocouple directly under an accumulation of very heavy material. This type of location was not used in the current study.
Cromer and Vines (1967) reported temperatures of between 120ºC and 350ºC at 2.5 cm depth under burning windrows in Pinus radiata plantations in Gippsland in Victoria, again with the higher temperatures being associated with the heaviest concentrations of windrow fuels. They also used chromel/alumel thermocouples, and investigated residence times in addition to absolute temperatures reached.
Craig (1969) also used thermocouples when measuring temperatures under slash fuels in the Mt Disappointment State Forest (Central Victoria), and reported surface temperatures of between 100ºC and 400ºC. Craig noted that thermocouple sensor location was quite critical to the final temperature measured. He also noted significant decrease in temperature with soil depth, with few sub-surface temperatures exceeding 100ºC in the top 2-5 cm of soil.
Floyd (1966) found broadly similar results when using a copper-constantan thermocouple in Blackbutt ( Eucalyptus pilularis ) slash near Coffs Harbour in NSW. He reported maximum surface temperatures of 400ºC to 500ºC, with temperatures in the upper soil of 100ºC or less.
We conclude that, on the basis of our data and earlier studies, it is likely that, where there are substantial slash fuels, surface temperatures are likely to reach 400ºC to 600ºC in good burning conditions. Outside the slash heaps however, surface soils may not even reach 100ºC, depending on their exposure to the ambient radiant heat. Sub-surface temperatures are likely to be much less than this, although, as found by Humphreys and Lambert, it appeared that temperatures of 200ºC to 300ºC could be produced at 2-3 cm below the soil surface in slash burns where the fire behaviour was strongly developed and the soil was relatively dry. Slash burns with less fire behaviour, and with moister soil particularly, may only result in sub-surface temperatures of 100ºC or less.
As most micro-organisms and many seeds are killed at temperatures between 60ºC and 100ºC (Floyd 1966, Craig 1969), it would seem to be preferable that HEMS slash burns raise sub-surface temperatures to as high a level as possible. Floyd noted that subsoil temperatures of 50ºC to 80ºC may be quite favourable for the germination of competing leguminous species, and that the only way to avoid this effect (with burning as the selected regeneration treatment ) was to produce enough heat so that seeds of these species were killed to below their possible emergence depth. He noted that this required very intense fire, and for a duration of 80 minutes or more.
From the results of this study, and from reviewing the work of previous researchers in this field, it is likely that soil temperatures of:
>400ºC at the soil surface; >100ºC at 2-3 cm below the soil surface; are necessary to produce seedbed which will substantially reduce both micro- organisms and competitive plant material (both seed and shoot) from the immediate environs of the germinating eucalypt seed.
42 HEMS Regeneration Burning Studies
However, depending on the amount, and possible emergent capabilities (vigour and depth) of some competing species (particularly wattle and other pea family members), these temperatures may not be sufficient to remove enough regenerative material to reduce ongoing competition. Additionally, these temperatures may not be reached under lighter accumulations of slash, and on areas with no slash. These sites may receive just enough heat to actively stimulate the regeneration of competing species. 4.7 Integration of Results - Burn Prescriptions The findings from this study are in general agreement with the existing prescriptions for prescribed burning in HEMS coupes contained in Native Forest Silvicultural Guideline No.6 - Site Preparation (NRE 1998). This Guideline, along with the local Prescribed Burning Prescriptions (NRE 2001), have previously formed the basis under which forest managers made decisions about when to burn in HEMS coupes.
Table 9 compares the prescribed burning conditions from the NFS Guideline, and the burning conditions found to be effective in this study ( from a short term fire intensity perspective, but not necessarily from a longer term regeneration perspective ).
Table 9. Comparison of burning prescriptions in NFS Guideline No. 6, and satisfactory conditions observed during this study.
Variable Native Forest Silviculture This study - conditions associated with Guideline No. 6 satisfactory fire intensity for fuel [Site Preparation] consumption & soil heating, and also satisfactory control Air temperature 18ºC (min) to 28ºC (max) >18ºC Relative humidity 60% (max) to 35% (min) <65% Wind speed (at 2 m) 0 km/h (min) to 10 km/h Not specified, but low wind speeds aided (max) combustion for damper fuels. 15 km/hr limit for control (suggested). Fuel moisture content - 6-12% <13% elevated fuel (% ODW) Fuel moisture content - Not specified <13% exposed surface fuel (% ODW) Fuel moisture content - profile Not specified <17% fuel (% ODW) Fuel moisture content - >15% >15% exposed surface fuel outside coupe (% ODW) Fuel moisture content - profile Not specified >20% fuel outside coupe (% ODW) % Direct sunlight Not specified >80% Cloud cover Not specified <20% Atmospheric stability Not specified Neutral to unstable Field soil moisture test Not specified Level 4 - able to raise some dust when small heaps of earth kicked with toe - (across the whole coupe) . No visible patches of surface moisture. Drought Index 20 (min) to 75 (max) Not specified, as generally not available for experimental sites. Higher Drought Indices generally meant more consumption of larger fuels. The additional variables that were recorded in this study - for which conditions are not currently specified in the Native Forest Silviculture Guideline were: • amount of direct sunlight
HEMS Regeneration Burning Studies 43
• percentage of cloud cover • atmospheric stability • the field soil moisture test • FMCs for: exposed surface fuel, profile fuel inside and outside the coupe.
Direct sunlight was observed to always be associated with better combustion of fuels. It is proposed (K.Tolhurst - pers. comm.) that there may be a warming effect of the fine fuels by direct sunlight, which raises their temperature slightly, and thus aids in the ignition and combustion process.
Cloud cover was mostly associated with higher relative humidities. Cloud cover which produced relative humidities greater than 65% gave significantly poorer burn results - hence the recommendation above to conduct burns with less than 20% total cloud cover. Cloud cover may become less important in determining burning outcome if fine fuels are drier.
Atmospheric stability estimates are difficult to make by simple observation, but the presence of significant amounts of flat layered cloud indicated that there was some inhibition to vertical motion present at a number of the poorer burns. There are no particularly straightforward ways for field officers to estimate atmospheric stability, but the presence of clear skies, or scattered clouds with fluffy tops, can generally be indicators of neutral to unstable atmospheres. Burning under stable atmospheric conditions - that is, when there is some inhibition to vertical motion of warm air - presents a significant risk of a poorer burn result if fine elevated fuels are moister than 13% ODW. The best indicators of stability are generally flat layered clouds, and if these are present in the lower atmosphere, it may indicate that it will be difficult to create a strong convection column.
Field soil moisture test consisted of two parts. The first part was the presence or absence of areas of visible surface moisture - particularly those due to recent rain. These moist areas were present on a number of the coupes where burn results were poorer, and should alert burn managers to the presence of high soil moisture contents. The second part of the test involved kicking the small accumulations of soil around the coupe to see whether it was possible to produce some dust from the kicking action impact on the soil. Coupes where some dust could be raised by this action always had better burn results (convection column formation, and generally also fuel consumption and soil temperature reached) than coupes where no dust could be raised.
The results from laboratory testing of HEMS soil moisture contents associated with dust production showed that some dust starts to appear at about 15-20 % ODW, is well established at 10-15% ODW, and is produced from nearly all soil aggregates at less than 10% ODW. This should mean that any fine fuel, in contact with soil that produces dust, should also be at a moisture content of 20% or less (when atmospheric moisture is also low), and therefore be available for burning. Even with shading effects from upper fuels in slash heaps, fine fuels (in contact with the soil) of 20-40% ODW FMC would dry out relatively quickly from the heat from burning fuels above, and become available for burning upon reaching <20% ODW.
High soil moisture contents (above 25% ODW, and not producing dust) will influence both the moisture content of the leaves in contact with the soil, as well as reducing the effects of soil drying by drawing heat energy from the fire to evaporate the soil moisture.
44 HEMS Regeneration Burning Studies
4.8 Timing of Regeneration Burning The set of weather, fuel and soil conditions noted above often do not occur at these elevations after the end of March in most years (Appendix 4). They are also quite unlikely to occur after there has been any substantial Autumn rainfall.
There would appear to be scope for Fire Managers to approve regeneration burning of the average HEMS coupe earlier in the year than other prescribed burns (either regeneration or fuel reduction) in other forest types at lower elevations, given that: • the likelihood of spot fire (fire escape) occurrence is low unless surface FMCs drop below 10% ODW; • the likelihood of a continuous surface fire in adjoining vegetation is substantially reduced due to higher FMCs under the shrub or secondary canopy (mostly 20% ODW or greater), and • the Drought Index is often less than half that at lower elevations.
5 CONCLUSIONS Fuel moisture and weather conditions, recorded at the 16 experimental regeneration burns studied, supported fuel moisture and weather condition prescription ranges given in the current HEMS Burning Prescriptions (Native Forest Silviculture Guideline No. 6 [NRE 1998]),
However this study identified that the additional prescriptive factors of: • % direct sunlight on the coupe • % cloud cover • atmospheric stability • a field soil moisture test, and • additional FMC measurements could be usefully added to the Burning Prescriptions to increase the level of confidence in obtaining successful burning of slash fuels.
Fire intensity, fuel consumption and soil heating can give useful indications of likely receptive seedbed production by slash burning.
Burn escapes from spotting were uncommon in the HEMS regeneration burns studied. Additionally, the propagation of spot-fires in uncut forest adjacent to HEMS coupes was inhibited by the presence of a secondary or understorey canopy which gave profile fuel moisture contents outside the coupe of greater than 20 %.
Drought Indices at high elevations may vary significantly from those at lower elevations, and, given their important influence on soil and fuel moisture conditions, could usefully be investigated in further research.
At HEMS elevations, weather, fuel moisture and soil moisture conditions conducive to acceptable high intensity burning results are limited by lower Drought Indices. Thus, during late summer and autumn, opportunities for successful slash burning may be limited to 9 days or less in most years. The Aerial Drip Torch (ADT) is a very useful tool for ignition of HEMS coupes, but it needs favourable weather (and fuel) conditions to be used to best advantage. The ADT cannot overcome poor weather with additional lighting intensity. Hand lighting, despite its drawbacks, can still be an effective method of achieving
HEMS Regeneration Burning Studies 45 acceptable fuel consumption and soil heating outcomes (and thus receptive seedbeds) in HEMS slash burning.
6 RECOMMENDATIONS The following is a summary of the indicative prescriptions for successful regeneration burning in HEMS coupes that have been derived from the results of this study. It is recommended that they be adopted and applied to HEMS slash burning in the future.
Direct sunlight : >80% Cloud cover : <20 % Atmospheric stability : neutral to unstable (clear skies, breezes, fluffy clouds) Temperature : >18ºC Relative humidity : < 65 % Elevated fuel : < 13 % ODW Exposed surface fuel : < 13 % ODW Fuel at soil surface : < 17% ODW Field soil moisture test : 4 - able to raise some dust when small heap kicked with toe of boot. No visible areas of surface moisture (see 2.5.2). Profile fuel moisture content (outside coupe): >20% ODW (if no secondary canopy, >30% ODW) Wind speed : < 15 km/hr at 2 m height.
Use of these prescriptions should result in the following levels of fuel consumption and soil heating:
Fuel consumption: ~ 90% of the 0-50 mm size class material; ~ 70% of the 50-100 mm size class material; ~ 20% of the >100 mm size class material;
Soil heating: ~400ºC at the soil surface; ~100ºC at 2-3 cm below the soil surface;
These fuel consumption and soil heating outcomes, in addition to the disturbed seedbed already provided by harvesting operations, should be sufficient to provide >75% total receptive seedbed on most HEMS coupes.
46 HEMS Regeneration Burning Studies
ACKNOWLEDGEMENTS The authors would like to thank all the Department of Sustainability and Environment staff who helped collect the data for this project. Particularly they would like to thank District staff as follows:
Bendoc Forest District: Wayne Long, Peter Jamieson, Anthony Reed, Brad Bennett, Amy Ware, Anne Partridge.
Orbost Forest District: Peter Geary, Peter Baker, Peter Jenkins, Michael Turner, Nigel Brennan, Dave Kermond, John Cuthbertson.
Swifts Creek Forest District: Neil Crabtree, Phil Mudge, Tim Crawford, Doug Liston, Ben Rankin, Rod Walker.
Cann River Forest District: Wally Biszko, David Miralles, Kirk Dyson-Holland, Maurice Roberts, Phil Timpano, Kathy Gosby.
• Mark Lutze and Maureen Murray, Forest Science Centre, Orbost, advised on early drafts of the report, and helped with much of the fieldwork. Peter Perry, Dan Terrell and Leoni Warren, Forest Science Centre, Orbost, also helped with much of the fieldwork. • Ian Sebire, Forestry Victoria, Bairnsdale, provided the impetus for this project to be undertaken, and provided encouragement throughout. • Peter Fagg, Forestry Victoria, Silviculture Unit (Melbourne), also provided impetus for the project, and was very helpful with structure and content of the final report. Mike Leonard, Fire Management Branch (Melbourne), provided valuable comment on an earlier draft. • Amanda Howard, Forest Science Centre, Orbost, kindly assisted with data collection (and enjoyed the helicopter ride). • Kevin Tolhurst, Forest Science Centre, Creswick, provided scientific advice on the methodology and data analysis.
HEMS Regeneration Burning Studies 47
REFERENCES Craig, F.G. (1969) Field studies of the effects of fire on soil structure. Chapter 4, PhD Thesis, Forestry Faculty, University of Melbourne.
Cromer, R.N. and Vines, R.G. (1967) Soil temperatures under a burning windrow. Australian Forest Research 2 (2):29-34.
ERDAG HEMS Regeneration Working Group (1993) Report to ERDAG (Eastern Research Development and Action Group), Revised Draft, Dept of Conservation and Natural Resources, Vic. (unpubl.)
Fagg, P.C. (1981) Regeneration of high elevation mixed species eucalypt forests in East Gippsland. Research Report No. 175 , Forests Commission Victoria (unpubl.).
Floyd, A.G. (1966) Effect of fire upon weed seeds in the wet sclerophyll forests of northern New South Wales. Aust. J. Bot. 14:243-56.
Humphreys, F.R. and Lambert, M.J. (1965) Soil temperature profiles under slash and log fires of various intensities. Australian Forest Research Vol I. (4):23-29.
Keetch, J.J. and Byram, G.M. (1968) Drought Index for Forest Fire Control. U.S.D.A. For. Serv. Res. Pap. SE-38. 32pp.
King, M., Burgess, J. & Saveneh, A. (1993) Curing rate of Eucalyptus regnans logging slash. VSP Internal Report No. 23 . Dept of Conservation and Natural Resources, Vic.
Lutze, M., Delbridge, J., Terrell, D. & Warren, L. (1999) Regeneration from seed trees in High Elevation Mixed Species Forest in East Gippsland. Research Report 372 . Department of Natural Resources and Environment, Vic, (unpubl).
McCarthy, G.J. and Tolhurst, K.G. (2000) Fuel Moisture and Fire Behaviour . Presentation to Fire Management Techniques training course. DNRE Victoria (unpubl.)
McCarthy, G.J., Tolhurst, K.G., and Chatto, C. (1999) Overall Fuel Hazard Guide . Research Report 47. Fire Management. Dept. of Natural Resources and Environment, Vic.
McCarthy, G.J. (2003) Bogong South Fire Complex, Yalmy Road/Gelantipy Plateau/Mountain Ck Control Strategy, 30 Jan-28 Feb 2003 (unpubl.).
McDonald, R.C., Isbell, R.F., Speight, J.G., Walker, J. and Hopkins, M.S. (1984) Australian Soil and Land Survey, Field Handbook. Inkata Press. 160 pp.
NRE (1998) Site Preparation. Native Forest Silviculture Guideline No. 6 , authors M. Lutze and P. Geary. Dept. of Natural Resources and Environment, Victoria.
NRE (2001) Prescriptions for the Conduct of Prescribed Burning. Gippsland Region, Dept. of Natural Resources and Environment, Victoria. (unpubl.)
Tolhurst, K.G. and Cheney, N.P. (1999) Synopsis of the Knowledge Used in Prescribed Burning in Victoria. Dept. of Natural Resources and Environment, Victoria. 97 pp.
Van Wagner, C. (1968) The line intercept method in forest fuel sampling. Forest Science 14: 20-26.
Wilkinson, G.R., Chuter, R., Davies, M., Plumpton, B.S., and Knox, R. (1995) Use of the aerial drip torch for fuel reduction burning of slash after partial logging operations. Tasforests 7:11-17.
48 HEMS Regeneration Burning Studies
APPENDIX 1
Drought Index and the likely effects of long-term dryness on fire behaviour and fuel consumption The following Table is reproduced from the current (2003) training material for the NRE Fire Management Techniques course (McCarthy and Tolhurst 2000). It was compiled from experience and observation of many prescribed fires, and also correlation of KBDI values against fire behaviour and fuel consumption outcomes at the Wombat Fire Effects Study Areas.
Long-term soil/fuel moisture effects on fuel consumption and fire behaviour
KBDI or SDI (mm Expected fire behaviour and consumption of bark and elevated fuels moisture deficit equivalent) 0-25 Fires will go out overnight. Soil moist. Profile fuels moist. Scorch minimal. Little consumption of bark and elevated fuel. 25-60 Fires will probably go out overnight, but may sustain if warm conditions persist overnight. Some bark will be consumed, depending on current bark hazard levels. For bark hazard Very High or above, most bark on the bottom 2 m will be consumed and some trees will have bark burnt for a substantial distance up the bole. Most dry/dead elevated fuel less than 2 m will be consumed, but taller greener elevated fuels may not burn.
60-120 Fires will sustain overnight. Most bark will be consumed, depending on current bark hazard. For bark hazard High or above, most bark will be consumed to the lower branches in tree canopy. Most elevated fuel (both dry/dead and live/green) to 2 m will be consumed especially if elevated fuel hazard is High or above. Most heavy fuel, including logs and stumps, will catch alight.
120 + All surface fuels, including larger material will be consumed. All bark, including the bark of fine textured species such as Peppermint, will be consumed both on the bole and in the crowns. All elevated fuels will be consumed, including taller green species such as Blanket Leaf. At SDI or KBDI 140 approx. tree death begins to become apparent on drier sites.
HEMS Regeneration Burning Studies 49
APPENDIX 2
All data variables on the day of the burn, means and ranges All data variables collected on the day of the burn, and subsequently, with their mean, minimum and maximum values, and standard deviations.
Variable Valid n Mean Min. Max. Std. Dev. Weather Air temperature ( 0C) 16 21 17 28 2.9 Relative humidity (%) 16 69 44 82 9.8 Wind speed (km/hr) 16 2.0 0.0 6.7 1.8 Wind direction ( 0) 10 209 45 360 136 % Direct sunlight 16 53 0 100 45.7 % Cloud cover 16 61 0 100 39.4 Atmospheric stability (0, 1) 16 0.6 0 1 0.5 Curing time (days) 16 145 14 325 91 Fuel and Soil Moisture Fuel moisture content (FMC), elevated leaves inside 14 14.7 9.9 19.7 2.4 coupe (%ODW) FMC, exposed surface leaves inside coupe (%ODW) 13 26.1 9.4 90.8 21.6 FMC, leaves in contact with the soil inside the coupe 8 65.4 18.8 179.7 58.3 (%ODW) MC surface soil inside the coupe (%ODW) 16 41.7 13.0 125.2 26.1 MC surface soil outside the coupe (%ODW) 16 11.8 2.2 24.6 6.8 MC subsoil inside the coupe (%ODW) 15 42.6 19.0 87.8 21.2 FMC surface leaves outside the coupe (%ODW) 10 30.5 19.0 52.4 13.6 FMC leaves in contact with the soil outside the 8 86.8 35.5 153.0 50.8 coupe (%ODW) Visible surface moisture inside the coupe (0-4) 16 1.5 0 4 1.2 Field soil moisture test for dust formation 16 3.5 2 5 1.1 (1,2,3,4,5) Lighting Method and Fire Behaviour Lighting method (1, 2) 16 1.3 1 2 0.5 Convection column formation (1-5) 16 3.0 1 5 1.5 Visible in-draught and tree sway (1-5) 16 3.2 1 5 1.3 Occurrence of spotting (0-4) 16 0.3 0.0 3.0 0.8 Fuel Consumption and Soil Temperature Fuel consumption Wire plots 20-50 mm (%) 17 82.7 22.6 100 25.4 Fuel consumption Wire plots 50-100 mm (%) 17 70.0 17.7 100 26.8 Fuel consumption Wire plots >100 mm (%) 17 32.6 10.2 71.9 17.4 Temperature crayon surface ( 0C) 16 470 150 600 136 Temperature crayon sub-surface ( 0C) 16 150 0 333 100
HEMS Regeneration Burning Studies 50 50 HEMS Regeneration Burning Studies Burning Regeneration HEMS
APPENDIX 3
Data variable values for all slash burns
Data variables for atmospheric stability, lighting method, visible soil moisture, and fire behaviour on the day of the burn. Fuel consumption and soil temperatures measured after the burn (where variables were recorded continuously over a time period, or where there were multiple measurements, a single summary value is given).
HEMS slash burn Atmos- Lighting Visible Convection Visible in- Occurrence Number of fuel Fuel consumption Fuel consumption Fuel Temperature Temperature coupe pheric Method surface column draught of spotting consumption Wire plots Wire plots consumption crayon surface crayon Stability (1, 2) moisture formation and tree (0-4) wire plots 20-50 mm (%) 50-100 mm (%) Wire plots (0C) subsurface ( 0C) (0, 1) inside the (1-5) sway >100 mm (%) coupe (1-5) (0-4) Yalmy Rd 1 2 0 5 5 3 3 100 94 84 580 250 Clarkeville R 1 1 0 5 5 0 3 100 82 32 550 150
Students Rd 0 1 1 2 2 0 2 100 95 32 500 150 Survey Rd 0 1 3 1 2 0 1* 51 61 26 330 50 Coast Range 0 1 4 1 1 0 2 88 35 21 370 50 Little Bog 0 1 3 3 3 0 2* 70 45 25 250 50 Watts Ck 1 2 0 4 4 0 2 100 100 57 430 50 Mt Tom 1 1 1 2 3 0 2 100 97 51 550 150 GWHope 1 1 1 4 4 0 1 100 58 51 600 200 Clarkeville 2 1 2 0 5 5 0 6 100 92 53 570 120 Playgrounds old 0 1 2 2 2 0 2* 76 52 28 150 0 Playgrounds new 0 1 2 2 2 0 2* 97 77 32 - - Waratah 0 2 2 1 3 0 6 93 98 59 500 300 Yalmy view 1 2 2 3 3 0 6 87 77 47 500 100 Ellery old 1 1 2 2 2 0 3 68 62 43 400 300 Ellery new 1 1 2 4 4 0 5 90 95 57 600 250 Mt Tom West 1 1 0 5 5 2 6 100 100 64 600 200
( - no data collected) (* some additional fuel consumption plots established but unburnt)
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Combienbar Automatic Weather Station (AWS) 2002 APPENDIXAppendix 4 Combienbar 4 Automatic Weather Station (AWS) 2002 Days for Potential HEMS burning (shown in bold italics ) B - Bendoc actual burn days R - restrictions still in force
Average 1200 -1600 Average 1200 -1600 Average 1200 -1600 Date FDI Temp RH Wind DI Date FDI TempRH Wind DI Date FDI Temp RH Wind DI 15-Feb 1 28 50 NE 10 5.3 *B 20-Mar 0 11 95 N 0 20-Apr 0 15 95 SW 10 16-Feb 0 17 90 SW 5 21-Mar 1 11 90 SW 5 21-Apr 0 13 98 SW 15 17-Feb 1 17 80 NE 3 22-Mar 3 20 50 NE 5 22-Apr 0 16 80 N 0 18-Feb 0 14 97 N 0 23-Mar 4 22 50 E 3 32 23-Apr 0 18 70 NE 3 1 19-Feb 1 23 70 NE 8 8.4 B 24-Mar 5 24 45 SW 3 24-Apr 0 23 55 E 3 2.4 R 20-Feb 3 26 50 N 8 25-Mar 5 25 45 NE 5 25-Apr 2 18 65 NE 3 4.5 21-Feb 2 19 60 SW 6 26-Mar 1 15 95 SW 5 26-Apr 1 13 75 SW 5 22-Feb 2 17 55 SW 7 27-Mar 0 16 90 SW 2 0 27-Apr 0 11 80 SW 5 23-Feb 2 22 70 SW 3 28-Mar 0 15 80 S 1 28-Apr 1 14 70 NE 5 R 24-Feb 3 25 50 NE 5 20 29-Mar 0 14 75 SW 7 29-Apr 1 14 70 NE 5 R 25-Feb 3 25 50 NE 5 30-Mar 1 13 70 SW 5 30-Apr 1 15 60 SW 3 8.3 26-Feb 0 24 65 SW 3 0 31-Mar 1 17 70 SW 3 1-May 1 15 65 N 0 27-Feb 0 11 97 SW 5 1-Apr 2 20 55 E 5 3 2-May 2 17 65 E 3 28-Feb 0 12 85 SW 3 2-Apr 3 22 50 SE 2 (fuel too wet?) 3-May 2 18 60 N 0 10.9 1-Mar 1 15 65 SW 9 3-Apr 4 23 45 W 5 (fuel too wet?) 4-May 2 18 60 N 0 2-Mar 1 15 75 SW 7 1 4-Apr 1 10 75 SW 15 5-May 3 17 50 SW 3 13.1
3-Mar 1 16 75 SW 5 5-Apr 1 14 65 SW 5 6-May 2 17 60 SE 2 Studies Burning Regeneration HEMS 4-Mar 2 20 70 NE 7 6-Apr 1 16 65 SW 3 TL 7-May 4 20 40 WSW 7 15.1 5-Mar 2 24 60 NE 7 (fuel too wet?) 7-Apr 2 15 65 SE 5 11 TL 8-May 3 19 50 N 0 B 6-Mar 4 26 45 NE 2 (fuel too wet?) 8-Apr 2 21 60 SE 2 TL 9-May 3 17 50 NW 7 7-Mar 1 14 90 SW 5 9-Apr 4 25 45 SE 2 12 10-May 1 11 80 SW 10 8-Mar 1 15 75 SW 3 10-Apr 2 19 60 SW 5 11-May 1 11 85 NE 3 9-Mar 2 18 60 NE 7 11-Apr 3 23 55 SW 2 12-May 1 15 55 N 0 B 10-Mar 2 21 55 NE 7 12 12-Apr 4 23 45 N 0 17 13-May 1 10 90 SW 2 11-Mar 4 25 45 NE 5 13-Apr 1 14 90 N 0 14-May 0 10 90 NE 2 12-Mar 1 14 70 SW 5 14-Apr 0 11 95 N 0 15-May 1 16 70 NE 5 21.6 13-Mar 2 18 60 SW 5 15-Apr 0 13 95 N 0 16-May 2 16 60 NW 7 14-Mar 1 15 80 N 0 16-Apr 0 17 80 N 0 17-May 2 10 60 W 5 B 15-Mar 2 21 65 SW 3 19 17-Apr 0 15 95 SW 3 18-May 1 10 65 E 5 16-Mar 1 18 75 SW 3 18-Apr 0 16 80 SW 3 19-May 1 9 80 SW 5 B 17-Mar 2 20 60 SW 5 19-Apr 0 15 90 SW 7 0 20-May 0 9 85 SW 10 0 18-Mar 12 30 30 WNW 12 (Outside prescription at top end? TFB?) B 19-Mar 2 21 65 SW 3 27 * - change early at Combienbar? TL (Fuel too wet? Not enough sunlight?, Too late in year at that elevation?) 51
52 HEMS Regeneration Burning Studies 52 HEMS Regeneration Burning Studies Burning Regeneration HEMS
APPENDIX 4 (cont’d) Gelantipy AWS 2002 Appendix 4 (cont'd) Gelantipy AWS 2002 Days for Potential HEMS burning (shown in bold italics ) B - Bendoc actual burn days R - restrictions still in force Average 1200 -1600 Average 1200 -1600 Average 1200 -1600 Date FDI Temp RH Wind DI Date FDI Temp RH Wind DI Date FDI Temp RH Wind DI 16-Feb 0 16 98 SW 13 B 17-Mar 2 19 70 S 14 14-Apr 0 10 98 SE 10 1 17-Feb 0 16 92 SSW 9 18-Mar 15 27 30 WNW 30 20 15-Apr 0 13 97 NE 15 18-Feb 0 14 98 SE 7 B 19-Mar 1 12 98 S 7 16-Apr 0 15 90 S 10 19-Feb 1 22 75 NE 13 B 20-Mar 3 17 70 S 15 17-Apr 0 15 97 SW 10 R 20-Feb 2 22 60 NNE 20 21-Mar 1 11 85 S 12 18-Apr 0 13 98 SW 10 21-Feb 1 18 60 SSW 15 22-Mar 3 18 50 S 10 19-Apr 0 14 98 SW 15 0 22-Feb 1 16 65 SW 15 23-Mar 5 21 45 ESE 9 20-Apr 0 12 98 WSW20 23-Feb 1 17 76 S 12 B 24-Mar 5 23 45 ESE 10 27 21-Apr 0 12 98 SW 25 24-Feb 1 22 70 S 15 25-Mar 3 21 60 NNE 14 22-Apr 0 15 97 SE 10 25-Feb 1 20 78 W 4 26-Mar 1 12 99 WSW 12 23-Apr 0 18 70 S 10 0 26-Feb 1 20 86 S 13 27-Mar 0 16 95 S 10 24-Apr 0 20 70 NE 12 27-Feb 0 12 98 SW 13 28-Mar 0 17 90 S 13 25-Apr 1 20 55 NW 152.5 28-Feb 0 10 95 S 11 29-Mar 0 14 85 SW 14 26-Apr 1 12 75 S 13 1-Mar 1 13 67 SSW 15 30-Mar 1 11 75 S 12 27-Apr 1 10 80 SW 13 2-Mar 2 17 76 S 15 31-Mar 0 7 97 NW 10 28-Apr 1 12 70 NE 14 3-Mar 1 15 81 S 15 TW 1-Apr 4 20 40 SE 5 5 29-Apr 2 14 65 SE 7 4-Mar 2 20 70 SE 10 TW 2-Apr 3 21 45 SE 7 30-Apr 2 14 65 SE 12 5-Mar 3 23 60 SE 11 6 3-Apr 1 13 70 ENE 15 1-May 1 14 65 SE 7 B 6-Mar 4 24 50 WSW 11 8 4-Apr 1 9 80 SW 14 2-May 2 16 60 S 10 7-Mar 1 12 98 SW 13 5-Apr 1 13 75 S 12 3-May 2 16 60 SE 8 8-Mar 1 14 76 SE 15 6-Apr 1 16 70 SW 12 5 4-May 1 15 65 SE 7 9-Mar 2 19 56 SE 10 7-Apr 0 7 98 N 5 TL 5-May 3 16 50 SW 10 9 B 10-Mar 4 21 48 N 5 TW 8-Apr 3 20 55 NE 7 TL 6-May 3 17 55 W 20 11-Mar 6 24 40 NW 9 9-Apr 5 24 45 NW 12 8 TL 7-May 5 20 45 W 15 10 12-Mar 1 13 80 S 15 10-Apr 2 17 80 NNE 20 8-May 2 18 55 E 7 13-Mar 2 16 75 S 10 11-Apr 3 21 55 NE 5 9-May 2 15 55 NE 12 14-Mar 0 11 97 0 12-Apr 7 23 35 SW 8 10-May 1 12 70 SW 25 14 B 15-Mar 2 16 70 S 14 13-Apr 1 13 98 S 10 11-May 1 10 85 NE 15 16-Mar 1 18 75 S 10 TW Too wet? TL (Fuel too wet? Not enough sunlight?, Too late in year at that elevation?)
53 HEMS Regeneration Burning Studies
Location of HEMS slash burn study coupes Bendoc Forest District 1998/99 and 1999/00 HEMS Regeneration Burning Studies Studies Burning Regeneration HEMS
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54 HEMS Regeneration Burning Studies
Location of HEMS slash burn study coupes Swifts Creek Forest District
55 HEMS Regeneration Burning Studies
Location of HEMS slash burn study coupes Orbost and Cann Forest Districts 1998/99 and 1999/00 HEMS Regeneration Burning Studies Studies Burning Regeneration HEMS
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