Underpinnings of fire management for biodiversity conservation in ...

Litter fuel load (t/ha)
20
18
16
14
12
10
8
6
4
2
Underpinnings of fire management for biodiversity conservation in reserves
0
0 5 10 15 20
Time since fire (years)
No residue
14t/ha residue
Figure 3.1 Fine-litter fuel load as a function of time since fire in E. marginata (Jarrah) forest (parameters with zero remnant fuel
from Walker 1981 using data of G. Peet). The complete removal of fuel by prescribed fire (Equation 3.1 with W max as 18 t ha -1 ,
a as 1 and k as 0.2) is contrasted with the illustrative circumstances in which 14 t ha -1 remains (equation as previously, except a
is 0.222) – see the intercept on the Y-axis.
In Figure 3.1, the classic curve of Olson (1963) is contrasted with the situation in which some fuel
persists, or is generated, immediately following the fire (O’Connell 1987). If large quantities of fuel
remain, then the effectiveness of prescribed burning for fuel reduction would be questioned. For
example, in Figure 3.1, a hypothetical illustration only, an average of 14 t ha -1 remains. This may not
be considered to be a satisfactory result for fuel reduction per se. In practice, k varies considerably:
Raison et al. (1983) found values between 0.11 and 0.31; Walker (1981) found values from 0.2 to
0.43; and Cary and Golding (2002) found k to vary from 0.03 to 1.
The form of the graphs given by Equations 3.1 and 3.2 for fuel measures against time since fire
can be applied to all components of the fuel array in a forest, albeit with different parameters (see
Appendix 2 in Gould et al. 2007). However, some of these components are likely to be unaffected by
prescribed fire and variously affected by different intensities of unplanned fire. For example, Gould et
al. (2007, p. 132) assumed – in developing a firebrand model – that ‘elevated and bark fuel are not
consumed at fire intensities less than 1000 kW m -1 ’. The effectiveness of prescribed burning for the
modification of fuels only, is considered further in the next section.
Effectiveness of prescribed burning for fuel modification
Prescribed burning may be undertaken for ecological or other purposes, but more often it is for the
purpose of fuel reduction and modification (Text Box 3.1). In forests, where the practice has a long
history (Gill and Moore 1997), the aims are stated in various ways, but Moore and Shields (1996) are
more comprehensive than most. They state that the aims of fuel management – hazard reduction
burning – are to:
• Reduce the total weight of fuels to reduce the rate of spread and intensity of a fire
• Reduce height of the fuel bed and therefore flame height
• Remove fire-brand material, principally fibrous bark, and therefore the chance of spot fires.
As a result, fire suppression capability is increased, the impact of fire on forest assets is reduced and
the safety of firefighters increased (Moore and Shields 1996).
An immediate effect of fuel reduction by burning in State Forests of New South Wales was to reduce
fine-fuel weight up to 70% over 30 to 60% of the gross area being treated (Moore and Shields 1996).
For strategic corridors and fuel management over broad areas of forest, burning up to 60% of the
area treated was desired (Moore and Shields 1966). In asset-protection zones in Victoria, the aim for
the percentage of area to be burnt is 90% (McDonald 1999). In strategic fuel-reduced corridor zones
it is 80% and in the broad-area fuel reduced mosaic zone it is 50% (McDonald 1999). In Tasmanian
Buttongrass (Gymnoschoenus sphaerocephalus) the figure is >70% (Marsden-Smedley 1993). Thus not
all the fuel is burnt over a chosen area, even for asset-protection burning.
Fire and adaptive management 47