Underpinnings of fire management for biodiversity conservation in ...

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Fuel, vegetation and habitat
Fuel in a landscape is not often a simple object, such as a matchstick. Fuel for landscape fires can be
dead litter, grass, tree crowns, live plants, micro-organisms, insects, houses, fences, cars or haystacks.
Is it fuel before it burns? If it never burns, can it be considered fuel? What if it rarely burns? In
practice, fuel is used as a shortcut term for material that has the potential to burn – the class of
materials that are combustible.
While fuels are the materials that burn, not all potential fuels burn in all fires. In grasslands,
consumption of much of the fuel present may occur on most occasions, but in many eucalypt forests
and woodlands this is usually not the case. In forests, litter on the forest floor dominates the carriage
of the fire front but grasses or low shrubs can have an important role too (Cheney et al. 1992; Gould
et al. 2007). Also, forest canopies can catch alight and burn, as can peaty substrates and logs on the
forest floor. Thus much of the above-ground vegetative complex (i.e. excluding living tree trunks and
branches) can potentially burn in many fire-prone vegetation types but do not always do so.
What is considered fuel by some observers may be considered habitat for animals or floral biodiversity
by others. This suggests that there may be parallels between the ways in which vegetation can be
measured as habitat and as fuel (Peter Catling, pers. comm., 2002). Newsome and Catling (1979)
developed a Habitat Complexity Score (HCS) to describe the habitat of small ground-dwelling
mammals in south-eastern Australia. This score included the cover of ground herbage, shrub canopy,
tree canopy and ‘logs, rocks and debris’ on the ground. The potential overlap with potential fuel
arrays may be apparent (Table 3.1).
It is important to note that the HCS is for ground-dwelling mammals only and not for birds or
arboreal animals, for example. Hollows are a significant feature of the habitat of arboreal animals and
many birds, and fires appear to both help create and destroy hollows (see Gill and Catling 2002). Tree
hollows can also aid the combustion of trees but do not feature in HCS or fuel classifications. Lateral
cover of foliage, as a function of height above ground in forests, seems to be important to a variety
of birds, some preferring more open conditions, others denser forests (see Gill and Catling 2002).
Habitats are many and varied for the plethora of vertebrate and invertebrate animals. Brown et al.
(1998) use a combination of macro (vegetation) and micro (e.g. soils, burrows, litter and tree trunks)
attributes to define habitat for the fauna of the Australian Alps National Parks in south-eastern
Australia (after Woods 1996). Thus ways of defining habitat depend on the fauna of interest and the
degree of overlap between classifications of habitat and fuel will vary.
For example, fuels may be classified on the basis of vegetation structure (e.g. forest, woodland,
shrubland and grassland), life form within a structural type (e.g. grass, shrub and tree), plant taxa
(e.g. wire grass, bracken, Lantana and Gamba grass), fuel-component type (e.g. peat, bark and litter),
ignition likelihood (availability) or a combination of these. Sandberg et al. (2001), in ‘characterizing
fuels in the 21st century’ in the United States, highlight the ‘kind, quality and abundance’ of fuels as
a means of inferring their ‘physical, chemical, and structural properties’. Gill and Zylstra (2006) discuss
the flammability of fuels in eucalypt forests at different scales.
Forest fuel in southern Australia is now often depicted as an array of materials – rather than just a
monolayer of litter, for example. Sneeuwjagt and Peet (1985) incorporated shrub components in their
Western Australian fire behaviour tables, while McCaw (1991) described the components of forestfuel
complexes being adopted by Australian Forestry authorities. Cheney et al. (1992) found that
height and dead-fuel moisture content of near-surface fuels contributed to a prescribed-fire rate-ofspread
model in regrowth eucalypt forest in New South Wales. They described the fuel array in terms
of surface fuels (e.g. litter), near-surface fuels (e.g. grasses and short shrubs) and elevated fuels (e.g.
taller shrubs, up to 2 m in height). In Victoria, Wilson (1992a, 1992b, 1993) initiated a descriptive
system for forest fuels. This system, designed to assist in predicting the difficulty of suppression,
included litter (t ha-1 ), bark on trees (a rating) and elevated fuel (a rating) in an overall fuel array. The
bark effect on the overall rating was contingent on a threshold litter level of 4 t ha-1 (Wilson 1993).
Essentially, these systems provide a weighting of different fuel components in relation to their effects
Fire and adaptive management Underpinnings of fire management for biodiversity conservation in reserves