Underpinnings of fire management for biodiversity conservation in ...

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48 What is actually achieved in the field depends on conditions that prevail during the burning operation. For the improvement of later operations, assessment and evaluation of each burning event in relation to objectives, field conditions and logistics is necessary. Poor weather may hamper operations and heterogeneity in fuel moisture may impede fire spread or prevent it in parts of the burn area. Such factors may lead to a lesser amount being burnt than intended. Mapping of outcomes (Heemstra 2006), in terms of unburnt areas and those burnt with various intensities, would be a useful part of assessment of the success of the event, let alone for ecological reasons. Prescribed burning may be used to create linear buffer strips by dropping incendiary capsules from an aircraft in a line, then relying on the timing and placement of capsules to allow fires to join but then extinguish at night or when they reach natural and artificial barriers. This process can be particularly effective in tropical savannas (Crowley et al. 2003, p. 52). While the protection of economic assets is often a prime objective in prescribed burning, it may be carried out for ecological reasons too (Fire Ecology Working Group 2004). The trend seems to be for more control of fire regimes for ecological purposes (Esplin et al. 2003, p. 115). Assessing the effects of burning on fuel In the previous section, the detailed objectives for prescribed burning events were given. The purpose of this section is to look at questions pertaining to the effectiveness of reducing, or otherwise modifying, fuel as an issue in itself, not in relation to wider questions of, say, threats to biodiversity conservation or to property. Fuel modification can be examined in isolation or in a specific context, such as that of the urban interface (e.g. Bradstock et al. 1998b). Calculation of the risk of unplanned fire and its cost (e.g. economic, environmental and social), may help the manager decide whether or not the cure (fuel reduction) and its cost is worse than the disease (high fuel loads). In practice, monitoring the effectiveness of fuel-modification measures should be undertaken in relation to outcomes (especially social and environmental), not just in relation to fuel loads or scores. If the fuels, such as some dominated by grasses, return to a level unacceptable to managers within one year, then burning or other means of fuel modification are necessary every year; if the effects of this situation are to be avoided. Frequent burning may reduce the chance of shrubby fuels dominating the grasses and changing the fuel load and structure in the longer term. In an international review of prescribed burning emphasising forests, Fernandes and Botelho (2003) concluded that ‘Fuel accumulation rate frequently limits prescribed burning effectiveness to a short post-treatment period (2–4 years)’. However, what happens in particular areas depends on local fire weather, growing conditions, species present and suppression capacity, for example. The most recent work on fires in Australian forests, by Gould et al. (2007, p. 79), emphasised both loads and structure in assessing the effects of fuel reduction by prescribed burning on fire behaviour. These authors suggested that in the Jarrah forests of Western Australia the effects of prescribed burning on future fire behaviour may last for 10–15 years. However, because only the bark on the lower parts of the tree bole are affected by low intensity fires (p. 114), bark characteristics on upper boles will continue to change in a way that favours the development of fire brands and spot fires. Consequently, they argue, ‘There may be a good case for alternating mild spring burns with hotter autumn burns’ (p. 79). In grasslands, thresholds for fire control are unknown, although the discussion of fuel-break width provides some leads (Chapter 2). If a threshold fuel load can be identified, then some indices of effectiveness, or lack of it, are: • The time (years) the fuel load is above the threshold as a proportion of the interval between fires • The cumulative sum of the weight of fuel each year it is above threshold loads • Potential maximum fire intensity. Fire and adaptive management Underpinnings of fire management for biodiversity conservation in reserves
The critical time, or time to reach the threshold, tsf c , is: tsf c = -1/k[log e (1 – W c /W max )] Equation 3.4 Underpinnings of fire management for biodiversity conservation in reserves Thus if, as in Figure 3.1, k = 0.2 and W = 18, and if the critical fuel load, W , equals 8, then max c tsf = 3 years. If burnt at this time, the interval between fires is 3 years. If W >W then tsf become c c max c infinite, then no burning will be recommended based on this evidence (but may be on other evidence). The time above the threshold fuel level, tsf , as a proportion of the between-fire interval, tsf , is c b given by [(tsf tsf )/tsf ]. Consider a grassland in which the fuel load reaches much the same, but b – c b undesirable, level within a year and stays there in successive years. Pre-emptively burning every year precludes any time above the threshold level – the ultimate for ‘protection’. Burning every second year means that fuels are above (and below) what the manager may desire for protection objectives half the time. In a forest, the fuel loads (or scores) may be as in Figure 3.1, with zero fuel left after fire; fuel is able to increase above a certain critical level for years. Perhaps the load and time above the threshold is a useful concept. This is given in part by the following equation in which only positive values are considered: (W tsf – W c ) = W max (1 – e -k.tsf ) – W c Equation 3.5 This is equivalent to drawing a horizontal line through W on graphs such as Figure 3.1, and looking c at values above the line each year to give the sum of these levels in the interval between the threshold year and the year that burning takes place. For example, say the critical fuel load is 8 t ha-1 and this is reached in three years, as in Figure 3.1. Then the sum of fuel loads above the threshold value for a six-year interval between fires is approximately 11 tonne-years, while that for a 10-year interval is approximately 38 tonne-years. The fine-fuel load is just one of the fuel variables that may be of interest from a fire or environmental point of view. Curves, such as those in Figure 3.1, may be drawn for shrubs and bark on trees, logs on the ground, dead material in shrub canopies and crowns of trees and shrubs (see Gould et al. 2007 Appendix 2, for example). The value of the decomposition constant, k, is likely to vary for each component, as are the quasi-equilibrium loads (or scores). The remnant fuel loads (or scores) will vary according to component and to the properties of the last fire (e.g. in the absence of crown scorch all the canopy is remnant). As illustrated in Figure 3.1, there can be fuel left after a prescribed fire or an unplanned fire. After a prescribed fire, a certain amount left covering the soil may be seen as being in line with landmanagement objectives. Tolhurst et al. (1992a) reported that 40% was often left unburnt in their prescribed burning experiments. They found that the percentage of fine fuel actually burnt in areas where the fire passed over was a linear function of the Keetch-Byram drought index (Keetch and Byram 1968). However, this relationship varied widely from autumn to spring; a much higher drought index being necessary in spring to burn the same percentage of the fuel. The indices, above, have been considered in relation to a particular point on the ground rather than to an area. For a burning block, landscape or region, four related indices of effectiveness, or lack of it, are: 1. The total amount of fuel on a unit landscape at any one time, a combination of the amount of fuel accumulation with time since fire and the proportions of landscape with different fuel ages; an average fuel load (or score) may be obtained by assuming equal proportions of land affected and the use of Equation 3.1 for total removal each time 2. The geographical spread of certain fuel quantities in the landscape, or fuel quantities graphed by the area they occupy 3. Proportional area with fuel loads below, or above, the threshold amount 4. Patch sizes of fuels in different amount categories The effect of the spatial arrangement of fuels for fire control is considered in Chapter 7. Fire and adaptive management 49
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