Wildland Fire Operations

Transcription

1 Wildland Fire Operations Removal of Tree Crowns Under Winter Conditions: Case Study Using a Terra Torch Fort Providence, NWT Dave Schroeder and Gary Dakin March 2008 Introduction At a recent Western Canada Prescribed Fire Science Planning 1 meeting, fire management agencies strongly supported continuing and even increasing prescribed burning after several decades of decline in the practice. Prescribed burns have declined, to a large extent, due to the associated risk of an escape fire, especially in the form of an uncontrolled crown fire. A common strategy is to conduct low intensity guard burns around all or some of the treatment perimeter prior to the actual fire in order to reduce the likelihood of initiating an unwanted crown fire. These guard burns are weather dependant, costly, and seldom remove crown fuels. However, a technique demonstrated by Parks Canada (Cornelsen 2005) successfully used ground-based ignition to remove crown fuels during low fire hazard. A terra torch may have application as a technique to remove crown fuel within a forest fuel management plan for community protection. Strategic removal of crown fuels around communities could be a cost effective alternative to harvesting and labour-intensive forest fuel treatments, especially if forests have low commercial value. Using the terra torch while snow is on the ground eliminates potential fire spread and reduces the number of resources needed for the operation. Also, northerners prefer to cut standing burned trees for firewood (standing dry firewood). Therefore, community members may get secondary benefits while achieving fuel management. Two research plots at the NWT Community Fire Protection research site (I5 cabins, and I3 thinning) are planned for burning and an escape risk is present given prevailing winds and desired prescription windows. These plots provided an opportunity to examine the potential for crown fuel removal as a guard building tool while improving the likelihood of successful test burns in the summer. This report describes winter crown removal tests at the Fort Providence, NWT research site. Objectives The objectives were: Demonstrate the efficacy of the terra torch treatment on winter crown (needles and some small branches < 0.5 cm diameter) removal in periods of no fire hazard. Demonstrate the effect of limiting fire to crowns due to snow covered ground. Determine the potential of winter crown removal as a community fuel treatment tool available in the NWT. Reinforce existing control lines to complement Community Fire Project research burns. 1 Meeting organized by Alberta Sustainable Resource Development held on Dec 07, Attendees included representatives from Saskatchewan, Alberta, British Columbia, Northwest Territories, Parks Canada, and the Canadian Forest Service. Western Region 2601 East Mall Vancouver, BC Canada V6T 1Z4 (604) Wildland Fire Operations Research Group 1176 Switzer Drive Hinton, AB Canada T7V 1V3 (780) Eastern Region 580 boul. St-Jean Pointe-Claire, QC Canada H9R 3J9 (514)

2 Methods The Buckmaster terra torch from Astaris Canada Ltd. operates by pumping and igniting gelled fuel from a 205 litre barrel. Fuel trajectory is 7-15 m depending on gel/fuel mix ratio. The terra torch works with any gelled fuel although fuel ignition is affected by temperature. For winter operations, we used gasoline mixed with Flash21 gelling agent. The fuel/flash21 mix ratio was. Two methods were employed to burn the crowns. The first was to fire bursts of 2-5 seconds at the base of the live crown. The second was to spray the entire tree and then ignite the fuel. The torch was fitted into a plastic toboggan and pulled by snowmobile (Figure 1). After initial testing to ensure the torch was operating properly, the operational guard burning was initiated. Pre cut trails were used, the torch was pulled into position, and trees within its range were burned. The torch was then be pulled further down the trail to the next unburned trees. Results and discussion Figure 1. Terra torch on snowmobile toboggan. The test fires did not remove enough of the crown fuels to sufficiently and economically reduce crown fire hazard and did not meet the operational objectives (Table 1). The torch worked as expected and we were able to apply burning gel to individual trees at the crown base and even the tops of most trees (Figures 2 and 3). However, it was not possible to generate enough heat for the crowns to develop self sustaining combustion. The burning gel produced by the torch could reach the tops of trees (up to 13 m) but with a loss of accuracy and wastage of fuel. Eventually most of the needles on a given tree could have been burned; however, the amount of gas and gelling agent required, at an operational scale, would have been excessive (e.g., 75 m control line requires combustion of trees). For example, Test 1 used approximately 40 litres of gas for four green jack pine. During the trial, several understory black spruce trees were also burned with better success than overstory crowns; however, self sustaining ignition was not achieved with those trees either (Figure 4). Reinforcement of the control lines around the research plots control lines was not achieved. We felt that partial combustion might have some desirable effect (i.e., partial crown fuel removal) but the fire killed trees would have dead needles attached next summer not a desirable condition for a prescribed burn perimeter. 2

3 Table 1. Summary of terra torch tests conducted in March Test Date Temp ( C) Mix * Species /tree height Result 1 March 3-25 Jack pine /13m 2 March 3-25 Understory black spruce /2.5m Initial torch test. Crown would not ignite with litres gel applied. Snow removal was not done prior to ignition. Needles were consumed, however the crown would not sustain combustion without continually adding gelled gas. 3 March 3-23 > 4 March 4-23 < 5 March 4-20 < Fire killed jack pine / 15 m Black spruce /3m Black spruce /7m Crown would not ignite with litres gel applied. Snow removal was not done prior to ignition. Needles were consumed, however the crown would not sustain combustion without continually adding gelled gas. Needles were consumed, however the crown would not sustain combustion without continually adding gelled gas. The larger tree required much more fuel and not all needles were consumed. 6 March 4-20 < Fire killed jack pine / 15 m Crown would not ignite with litres gel applied. Snow removal was not done prior to ignition. * Mix ratios varied (by no more than a value of 30) but were not measured accurately. Gasoline to Flash21 gelling agent. Figure 2. Gelled fuel applied to crown base of a 13 m tall jack pine. Note, snow was not shaken out of tree. 3

4 Figure 3. Gelled fuel applied to top of crown. This is the same tree as in Figure 2. Tree height is 13 m. Figure 4. Burning two understory black spruce trees (ignition on right and burned tree on left). Three factors were determined to contribute to the difficulty in achieving crown removal. First, the trees were covered in snow and ice crystals (Figures 2 and 3) and much of these remained despite shaking the stems. However, the amount of snow and ice was not quantified. The second factor was the sparse crowns of the jack pine, and less so of the black spruce. In less than ideal weather conditions, a tree crown does not readily burn in a candle-like form unless there is good fuel continuity. Therefore the needles burned during initial bursts of flaming gelled fuel applied to the base of live crown; however the terra torch flame did not produce enough heat to melt the snow/ice in the crown above and then ignite those upper needles (Figure 5). Additional bursts fired into the tree crowns did not result in their complete combustion. Note, the tops of black spruce are denser compared to the base of their live crowns and to jack pine crowns, but even these did not readily burn. By comparison, Wasylenchuk and Trueman (2003) were able to ignite entire dwarf mistletoe infected jack pine crowns with a zippo lighter (tests were done in winter at -20 C. The dwarf mistletoe creates dense branching and greater fuel continuity compared to an uninfected tree. Finally, cold temperatures negatively affected crown combustion. We were not able to repeat the tests during a range of temperatures, but conversations with Parks Canada staff 2 doing similar work as well as observations by FPInnovations staff indicated that total crown consumption requires less fuel applied by the torch when temperatures rose above freezing. We determined that foliar moisture content (FMC) was not a limiting factor on crown ignition. FMC was measured at 110% for black spruce, 100 % for live jack pine, and 25% for the fire killed pine (red needles still attached). However, ignition attempts were not successful for the fire killed pine (Figure 4). We observed that bark on the fire-killed pine burned more readily than that of the green pine. Cornelsen (2005) observed that trees ignited more readily during the spring dip when FMC can drop to as low as 75%. The spring dip at the Fort Providence site is expected to occur in late June. 2 Randy Fingland, Parks Canada, personal communication, March 7,

5 Figure 5. Ignition on fire killed jack pine. The tree s needles were not consumed. The mix ratio of fuel to gel was important as it was desirable to have the fuel stick to the trees as much as possible. If the mixture was too thin the torch has less range, and much of the fuel would not stick to the canopy. Also, much of it was consumed before it reached the tree. If the mixture was too thick. the range of the torch would also be reduced. We were not able to accurately measure the fuel/gel ratio but were as close to as possible. Unfortunately, the mixing barrel did not have volume marked. In the future, FPInnovations will develop a measuring stick calibrated to the barrel for use to better maintain mix consistency. Use of Flash21 allows the mixture to be thinned or thickened as necessary. Performance of the torch is also a good indicator of appropriate mix ratio. If the mixture is too thin, the fuel ignites as soon as it exits the torch nozzle (Figure 6). Ideally, the fuel stream does not ignite until 1-2 m past the nozzle when the flame becomes visible. Cold weather did not limit use of the torch but operations were less efficient. Some of the hoses and the pump itself had to be warmed in the pick up truck s cab prior to being connected and operated. The cold also presents greater risk of breaking equipment. Additionally, the deep snow on site (approximately 70 cm) meant that trails had to be packed with a snowmobile prior to moving the torch into position. The tests showed more burning fuel fell to the ground than expected (Figure 6), and the snow was effective at stopping the fire. Burning during snow-free periods, but at low hazard (i.e., after a rain fall) might be effective for crown removal and still prevent surface fire spread. Some surface fuels will be burned (a benefit) but only where gelled fuel falls from the crowns. Parks Canada has been doing crown removal in partial snow conditions with some cured grass and fine fuels on the surface. The surface fuels were removed where gelled fuel has been applied, but surface fire has not spread. 3 3 D. Schroeder, personal observations, March 07,

6 Figure 6. Fuel component is too high. Note fuel ignition at torch nozzle tip and burning fuel at base of tree (the fuel does not stick to the canopy). Conclusions Winter crown removal tests near Fort Providence, NWT were unsuccessful because of snow/ice in the canopies, discontinuous crown fuels, and cold temperatures. Research and practices by others indicate that the technique can be successful. Saskatchewan Environment has conducted successful winter crown removal work in cold temperatures in ice free fuels with high levels of crown bulk density. Parks Canada has conducted successful test during warmer temperatures (above freezing) and periods of low foliar moisture content. The principle is sound but the prescription needs to be adjusted to the conditions and the desired result. FPInnovations, the Government of the Northwest Territories, and Alberta Sustainable Resource Development will conduct another trial of this technique at the Fort Providence site under more favourable conditions. Implementation In order to achieve the desired result: Ensure tree crowns are free of snow and ice. Crowns can be burned in cold temperatures (below freezing), but expect to use more fuel. Best results will result for trees with the dense continuous needles necessry to propagate the fire. Torching under story black spruce is possible during cold weather. Use short bursts to sustain and conserve fuel from the torch. This technique is not recommended if only partial crown consumption is achieved as fire killed trees may become a hazard during following fire seasons. Ongoing activities FPInnovations will monitor progress with Parks Canada s operational activities, and implement improvements to the terra torch including an accurate volume measuring technique. The technique will be tested during low hazard conditions and warmer temperatures at the Fort Providence site. 6

7 References Cornelsen, S Terra-torch evaluation. Parks Canada, Riding Mountain National Park. Wasagaming, Manitoba. Unpublished report. Wasylenchuk, S.; Trueman, D A feasibility study on intensive management of dwarf mistletoe in jack pine stands of north-central Saskatchewan, through the use of prescribed fire during the snow covered winter months. Forest Ecosystem Technology program, Forest Investigative Report, Saskatchewan Institute of Applied Science and Technology. Prince Albert, Saskatchewan. Unpublished report. Acknowledgements FPInnovations thanks the Government of the Northwest Territories and Alberta Sustainable Resource Development for support of this project. If you want more information about this study, please call Dave Schroeder at Copyright 2008, FPInnovations. 7