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Underpinnings of fire management for biodiversity conservation in reserves Fire and adaptive management Intensity (MW/m) 22 Cheney (1994) considered the head-fire intensity at which suppression was likely to fail during a fire in an open forest of stringy-barks that had reached 10 ha in area and in which suppression was being attempted by various means: for a hand-tool crew of seven, the fire intensity was 800 kW m -1 ; for two D6 (large) bulldozers, it was 2000 kW m -1 ; for one D6B air tanker, the intensity was 2500 kW m -1 ; and for ground tankers on a 40 m wide break, the level was 3500 kW m -1 . This sequence can be taken to reflect effective break width under various fire intensities. Note that this is an interpretation of the data and does not reflect the original intent of its use. Hand tool tracks may be 0.5–1 m wide (Gould 2004), dozer tracks 3–5 m wide, aircraft tracks 20 m wide and the ground tanker break 40 m wide (as stated). If this is so, there needs to be a large increase in break width to attain a small increase in the upper threshold intensity for control (Figure 2.3): a doubling of the assumed width from 20 to 40 m led to only a 1 MW m -1 increase in intensity controlled. This is a contrasting situation to that in grasslands, where a small increase in break width gave effective protection against a large increase in fire intensity, according to Wilson (1988). The reason for this difference may be that in grasslands, as intensity increases the increase in flame length (i.e. proportional increase) decreases and remains within reasonable bounds. In highly spot-fire prone forests, as intensity increases spotting distance quickly exceeds any reasonable track width. Thus track width as a method of passive fire control is much less effective in forests than in grasslands. 4 3.5 3 2.5 2 1.5 1 .5 y = 0.6257Ln(x) + 0.9724 0 0 10 20 30 40 50 Width of break (m) Figure 2.3 Intensity of a forest fire stopped by a widening fuel break with increases in suppression forces in a stringy-bark forest (interpreted from data in Cheney 1994). Notice the few data available in this interpretive diagram. This diagram may best be considered as an hypothesis developed using the best information available. It is important to note another contrast with Figure 2.2. In the grassland case, the maximum intensity was assumed to be 30 MW m -1 (Luke and McArthur 1978). In forests, the maximum intensity is more likely to be 100 MW m -1 (Gill and Moore 1990). A 4 m wide break in forest may assist in the control of a 2 MW m -1 fire, but in grassland a similar break may be effective in holding an 11 MW m -1 fire. It should not be assumed that fires of the same intensity in grassland and forest have the same properties, as this is not the case.
Underpinnings of fire management for biodiversity conservation in reserves Summary The previous discussion highlights the factors that need to be taken into account in planning a network of strategic fuel breaks, including tracks. For grasslands, the following need to be considered: • Width (a small increase in width may give a large increase in controllable intensity when spot fires can be discounted) • Maintenance (keeping tracks free of material that will carry a fire) • Proximity of trees (keep tracks away from trees) and other materials potentially able to cause spot fires, such as thistle heads within 100 m or so of breaks • Grass height and therefore potential flame length near breaks (also, other fuel factors that might affect flame bridging of breaks). For forests, the following suggestions need to be considered: • Increasing track width appears to be much less effective than in grasslands because of the spotting potential of forest fuels • Maintenance of tracks – keeping tracks free of combustible material is important because burningout procedures during suppression operations assume a clean break in the fuel • Fuel treatments that reduce spotting potential are most important in the effectiveness of tracks. The situation for fires burning with the wind in other plant communities may be considered to fall between the extremes considered here. For fires burning against the wind, a fuel break can be a potent inhibitor of spread. Track width in grasslands can be a useful variable to consider in the light of possible fire characteristics, while in forests, track width is more likely to be a consideration of vehicular-access requirements, control of backing fires and prescribed burning activities. What is done in the way of track construction, maintenance and vegetation treatment to enhance break effectiveness should be done in the context of the overall aims of management of the area. Thus action should vary according to whether the area is farmed, cropped or used as a water or biodiversity conservation area, for example. Track density: how much track is enough? Suppression operations can benefit from fuel breaks (tracks) in a number of ways: (i) provide passive breaks to fire spread within the limits of fire properties; (ii) provide trafficable access to the fire and water; (iii) provide a place from which fires can be fought and where rapid egress is possible; (iv) define boundaries within which a burnout of fuel between the fire perimeter and the break can be carried out during a suppression operation; and (v) provide a break against which back-burning – lighting an edge so that the fire created burns back to the nearby oncoming fire front – can take place (often a risky operation). Item (iv) is similar to item (v) in removing fuel between a fuel break and the fire, but distinguished by the proximity of the fire front – nearby in (v), but at a distance in (iv). Before we delve into the issue of track density, it is instructive to review what densities of tracks occur in various places. Track densities The densities of a number of track networks in south-eastern Australia are shown in Table 2.1. The table shows that track density varies widely, probably increasing with proximity to built assets and tourist venues, but also affected by land use and terrain. There is a 100-fold increase in density from that of Yathong Nature Reserve in the Western Division of New South Wales, with a figure of 0.1 km km -2 , to the suburban Canberra figure of about 10.5 km km -2 . Fire and adaptive management 23
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