Agriculture Reference
In-Depth Information
either in eucalypt-dominated woodlands and open forests or in mixtures such as in
heaths and shrublands, are highly variable (see
Table 2.1
), but typically in the
range of 10-20 Mg m
2
. Equilibrium surface loadings are usually reached within
10 yrs postfire (Walker
1981
; Raison
et al.
1983
). Accumulation rates and equilib-
rium loading of surface litter are positively related to rainfall in open forests (Fox
et al.
1979
; Huston
2003
).
Semi-arid shrublands are dominated by narrow-leaf species (Lamont & Groom
1998
) that form litter beds difficult to ignite (Scarff & Westoby
2006
; Plucinski &
Anderson
2008
). Ignition and fire spread depends on the arrangement of live and
dead foliage in the canopy, and is strongly controlled by wind. Patches of eucalypt
litter and other broadleaf species, when dry, can provide the surface energy after
ignition that sustains propagation of wind-driven crown fires in shrub-dominated
vegetation (McCaw
1997
).
Conditional thresholds in fuel moisture, in combination with wind speed, often
govern crown fire propagation in many vegetation types (Catchpole
2002
;
Plucinski & Anderson
2008
). In dry communities such as mallee shrublands and
woodlands, litter fuels are patchy and clustered around individual plants, which
are linked by grasses and ephemeral herbs that carry surface fires. In wet eucalypt
forests litter is often too wet to burn in most years but dependent on anomalously
dry years.
At the wet end of the MTV spectrum (
1000 mm rainfall), there are relatively
few days of Very High to Extreme fire danger, often less than 10 days per year on
average. The number of days increases with decreasing rainfall so that at inter-
mediate levels of rainfall (600-1000 mm) there are 10-20 days per year and at the
dry end (300-600 mm) severe fire weather conditions, on average, occur more than
30 days per year (Bradstock
2010
).
>
Fire Regime Heterogeneity and Diversity
The link between moisture, fuel types and fire weather creates the potential for
variable flammability ensembles, with different combinations of fire frequency
and fire intensity as predicted by Huston (
2003
). Heterogeneity of fire regimes may
lead to the assembly of communities with functional types representing unique
flammability characteristics. Pausas & Bradstock (
2007
) estimated that across a
precipitation range of 250-450 mm the average fire cycle varied from 30 to 200 yrs
in southeastern shrublands. This variation reflects the coupled influences of fire
weather and key functional types. By contrast, in moister open forest at annual
rainfall levels
600 mm the fire cycle is typically
<
20 yrs (Bradstock
2010
), and in
>
wet areas (
1000 mm per annum) occupied by tall open forests, the fire cycle may
increase to century scales (Gill & Catling
2002
).
At the local scale, quantitative evidence of strongly demarcated fire
regime variation between MTV communities is sparse, perhaps due in part to
lack of appropriate measurement and mapping of fire regimes. Boundaries
between heaths, woodlands and forests appear to have limited effects on
fire spread, but may have significant effects on fire severity (Chafer
et al.
2004
;
>
