Underpinnings of fire management for biodiversity conservation in ...

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Fuels for prescribed burning
When the focus is placed on fuel loads, there are limits to fire control in forests due to, or correlated
with, fire intensity (Chapter 1). Given that, by definition, fuel is a contributor to intensity, there is
potential through prescribed burning to keep fuel loads down, reduce potential intensity and make
suppression easier. If fine-fuel loads in forests are kept below about 8 tha -1 , then fire intensities on
level ground are kept to 4000 kW m -1 and are therefore just controllable under a Forest Fire Danger
Index of 100 (Gill et al. 1987b). If fine-fuel loads do not rise to 8 t ha -1 , then fuel treatment may be
deemed unnecessary. If weather never becomes extreme, then higher forest-fuel loads might be
tolerated. If ground is not level, then lower limits to maximum fuel load for fire control under extreme
conditions are implied (Bradstock et al. 1998b; Esplin et al. 2003, p. 79).
The quantity quoted as the maximum upper limit of forest-fuel load for fire suppression and
control is based on the fine-fuel load only (Gill et al. 1987b); this calculation may be regarded as
unsophisticated. It takes no account of the spatial variation, nor the probable different effects of fine
fuels present as bark on trees or dead and live material in shrubs. It assumes a level of fire-suppression
capacity too. Thus it provides a worked example, an approach and an aim, rather than a definitive
answer to the problem of fire suppression across all forested landscapes.
Whether fine fuels are grasses or forest litter, they follow a path to a quasi-equilibrium level (Figure
3.1) as they accumulate with time after fire (Walker 1981 for grasses, shrubs and litter). The shape
of these curves is based on a constant input of fuel each year and a decomposition constant. The
equation is of the form:
W tsf = W max (1 – a.e -k.tsf ) Equation 3.1
where W tsf is the fuel accumulated since the last fire in a standard area, such as a hectare; W max is the
quasi-equilibrium maximum fuel load for the equivalent area; a is a constant; k is the decomposition
constant and tsf is the time since fire in years (Olson 1963). Sub-year intervals are ignored here, but
see Mercer et al. (1995) for a consideration of these. W max equals A/k where A is the annual accession
using the same units as for W. A is assumed to be constant in the Olson (1963) model but any
change in A with time could be accommodated in the same equation.
The equation favoured by Gould et al. (2007, p. 26) for components of the fuel array in forests has
the form:
W tsf =(a*tsf)/(b + tsf) Equation 3.2
Or, with altered values of the constants a and b:
1/ W tsf = a + b/tsf Equation 3.3
The form of these graphs is similar to that of Olson (1963), above, and is simpler to fit to data, but it
does not reflect the processes of accession and decomposition, such as in Olson’s one (Equation 3.1).
Fire and adaptive management Underpinnings of fire management for biodiversity conservation in reserves