Underpinnings of fire management for biodiversity conservation in ...

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Wilson’s work is particularly interesting in its implications. It suggests that by increasing break width
from merely 3 m to 4 m, the intensity of a fire that can just be prevented from crossing the break,
rises from 6.3 MW m-1 to 11.3 MW m-1 – a substantial rise. A fire intensity of 9 MW m-1 could arise
from a fuel load of 5 t ha-1 (0.5 kg m-2 ) and a rate of forward fire spread of 1 m sec-1 (3.6 km hr-1 ), if
the same heat yield as used by Wilson (18,000 kJ kg-1 ) was adopted. Wilson’s model suggests that
with a modest increase in the width of a break, there is a large increase in the intensity of fire that can
be passively controlled. This only applies in grasslands where no spot fire is likely.
The model discussed above is the simpler of the two that Wilson (1988) produced. The other much
more complicated model gives the probability of spanning a break for fires of various intensities
burning in grassland fuels with trees at various distances from the break. This model predicts that
the approximate probability of bridging a 3 m wide break is 29% for a 2 MW m-1 (2,000 kW m-1 )
fire. Trees near a break can markedly reduce the value of a break in stopping fires because of their
propensity to produce lofted fire brands. Thus the effectiveness of a fuel break in forests is markedly
reduced, compared with that in grasslands.
Flame length is harder to measure than break width: flames pulsate so length varies all the time,
thereby making objective measurement difficult. What is actually taken as flame length is probably
the average length of the longer flames, rather than the maximum lengths of flames (e.g. flares). If a
photograph is taken, the variation can be observed. The measurement of flame height was discussed
in detail by Gill et al. (1987a). The same principles apply to length. On the CSIRO Grassland Fire
Spread Meter (CSIRO 1997) there is the comment that ‘flashes of flame may extend to twice [given]
values’, the maximum height (not length) given there being 4.4 m in natural pasture with the very
rapid spread rate of 20 km hr-1 (5.6 m sec-1 ).
Although there is no published data on the matter of flame height or length as a function of grass
height and density, the CSIRO Grassland Fire Spread Meter notes that a shift from natural pasture
(generally more than 50 cm tall) to grazed pasture (generally less than 10 cm tall) will halve, or more
than halve, flame height for fires travelling up to 20 km hr-1 (5.6 m sec-1 ). Flame height is a curvilinear
function of rate of spread of the fire (Cheney and Sullivan 1997, p. 25). Rate of spread drops
approximately 20% for fires in natural pastures, compared to those under normal grazing, but much
more in eaten out pastures (ibid, p. 39). Note that the values on the meter are considered to be a
guide only (CSIRO 1997).
A logical consequence of Wilson’s research is that attention needs to be given to the details of break
construction and maintenance. If the width changes even a little, the consequences can be large. If
the break is poorly maintained so that fuel cover is present to some extent, although slight, then the
effective width of the break is reduced. A steep but relatively narrow grassy batter on the downhill side
of a break can support quite a long flame, which may be well beyond that expected. Debris swept to
the side of the break may be a weak link in terms of the effectiveness of the break when it is alight.
While Wilson’s work remains valid for the conditions under which he worked, the person applying
these findings to other situations may need to adapt them to local conditions. Indeed, Wilson himself
pointed out that ‘In southern Australia, firebreaks [i.e. fuel breaks] even wider than 10 m can be
breached when winds are strong enough to transport surface debris such as smouldering animal
manure’ (Wilson 1988). The composition of the particular grassland and other fuels of concern can
be important. Luke and McArthur (1978, p. 107) note that spot fires can be common up to 100 m
ahead of the flame front in grassland fires, due to carriage of burning seed heads of thistles, Phalaris
(a pasture grass) or wheat, for example. While this suggests that in such circumstances an effective
break would need to be 100 m wide, verge treatments, such as slashing, may reduce the problem.
Even so, Noble’s (1991) case history of an extremely fast and intense grassfire in the Riverina of
southern Australia is a cautionary one – the fire jumped a 54 m wide break. In northern Queensland
woodlands, where grassy fuels predominate but trees are common, it is recommended that fuel
breaks (see Plate 2.4) need to exceed one kilometre in width if they are to be effective in the late dry
season when fires are at their most intense (Crowley et al. 2003).
Fire and adaptive management Underpinnings of fire management for biodiversity conservation in reserves