There is no fire without fuel. The amount of
fuel — its height, mass, composition and architecture
— all vary annually and seasonally, and with land use. Fuel
load is an important determinant of fire intensity but, unlike the
weather, it is something the landholder can manipulate.
What burns?
Many termites gather the stems of grasses in
their nests
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Most savanna fuels are fine fuels, less than 6 mm in diameter in
at least one dimension. Grass provides most of the fine fuel
although trees can drop increasing amounts of leaf and twig as the
dry season progresses. The amount of tree leaf litter in the fuel
bed increases with increasing tree density, which in turn rises
with increasing annual rainfall. In higher rainfall areas, tall
annual grasses such as Sorghum create fuel loads that dry
quickly after flowering.
The ribbons of bark and layers of shrubs that create 'ladder'
fuels in the eucalypt forests of southern Australia rarely occur in
northern Australia. Thus the flames in savanna fires generally
remain within 5 m of the ground, and severe 'crown fires' in tree
canopies rarely occur.
Fuel dries out as the dry season progresses. The tall annual
grasses start drying during the late wet season (March–April)
while perennials can last until the early dry season (June).
In the semi-arid savannas where the ground layer is dominated by
perennial grasses, fine grassy fuels may accumulate to levels
similar to those in the wetter savannas. Tall annual grasses are
rare in these savannas; where annuals occur they are usually short
and sparse and often preferentially grazed. They usually have
little bulk, especially in the dry season and, if burned, would
support only patchy low intensity fires.
Termites are abundant on the less fertile soils of the north,
and their total body weight per unit area may be greater than that
of large herbivores. They remove considerable grass stem and litter
during the dry season.
Fuel loads fluctuate through the years and seasons, and can be
modified by grazing. Grass builds up rapidly during the wet season,
but in pastoral lands some may be eaten by stock. In practice,
pasture land may have to be rested, with stock totally removed for
one good wet season to allow enough fuel to accumulate so that fire
can occur.
Fuel cover over the ground needs to be relatively unbroken with
more than 50% ground cover needed to support a continuous fire
front.
How much fuel accumulates?
Fuel loads need to be understood to assess the risk of wildfire
or for planning prescribed fires. The fuel load is the oven-dry
mass of fuel in a given area. Fuel loads can be expressed in
different units— tonnes per hectare, kilograms per hectare,
kilograms or grams per square metre.
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Dry grass provides the bulk of fuel for savanna
fires
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Annual inputs of fine fuel from grass growth and litter fall are
about 2–8 tonnes of dry matter per hectare. Under annual
burning, fuel loads generally remain around these levels in both
higher rainfall and semi-arid savannas. If country remains unburnt,
the fuel loads rise but do not generally increase above the
equivalent of a few years’ grass growth and leaf fall because
the litter breaks down quickly during each wet season.
Various studies have measured how fuel loads vary in different
types of savanna country.
In the wetter regions, fuel loads can reach an equilibrium
(between accumulation and decomposition) of about 10 tonnes per
hectare, 2–3 years after fire.
Perennial grass pastures near Katherine, NT (950 mm rainfall)
had grass fuel levels of 2–4 tonnes per hectare when burnt
every two years and reached 6 tonnes per hectare when protected
from fire for four years.
Leaf litter is also a significant fuel for fire:
it increases as the dry season progresses and with tree density
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In Rockhampton, central Queensland (890 mm rainfall), loads
reached 3 tonnes per hectare in annually burnt savannas, and 6-7
tonnes per hectare after three years without fire. In sparse
woodland with spinifex, where fires are less frequent, fuel loads
can reach 10–20 tonnes per hectare after 5–10 years.
However, in the spinifex-dominated landscapes of the stone country
in the wetter savannas, for example the Arnhem Land Plateau, fuel
loads may reach such levels in less than five years if unburnt.
Where exotic grasses invade and are not eaten, fuel loads can
increase dramatically. With some introduced species such as gamba
grass, fuel loads may be 20 tonnes per hectare, four to five time
the normal fuel load for a savanna.
What is fuel curing?
Curing refers to the 'greenness' of the fuel; it indicates the
moisture content of the fuel, its flammability and thus the
potential rate of fire spread. Curing starts as soon as a grass has
flowered and its stem becomes brittle; this is greatly accelerated
by the sudden onset of the dry season.
Fuel curing can vary greatly across the landscape according to
position in the landscape, the dominant grasses and the season.
Effective fire management over broad areas has to take this into
account. For example, grasses such as annual Sorghum on
sandy soils will cure rapidly and therefore be flammable much
earlier than perennial grasses on cracking clays. In general, by
the end of the dry season, the fine grassy fuels of the savannas
are fully cured.
Satellite imagery can help identify areas in different states of
curing across the landscape. Hence we can identify those areas at
greatest risk from uncontrolled fire, or those that are ready for
prescribed fuel reduction burning and how prescribed fires are
likely to be spread.
One way to ‘view’ such features of the landscape is
by using NDVI (Normalised Difference Vegetation Index), derived
from satellite imagery. Maps of NDVI show areas of different
chlorophyll reflectance, indicating fuel 'greenness', and therefore
how cured the fuel is. Photographic standards can be used to
estimate the degree of fuel curing.
A gamba grass infestation along a roadside near
Darwin, NT.
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Exotic grasses and fire
by Trevor Howard
Gamba grass ( Andropogon gayanus ) and mission grass (
Pennisetum polystachion ) are African perennial species used
for pasture trials at Katherine in the 1940s and ‘50s. Gamba
grass has been successful as a pasture species, but both species
have become serious environmental weeds in the Darwin region and
parts of Cape York when not managed.
On the rural–urban interface of Darwin where development
and disturbance coincide, these grasses grow and spread
prolifically. The establishment of gamba grass on flood plain
margins and wetter Melaleuca uplands is enhanced by soil
disturbance while in Eucalyptus woodlands disturbance is not
essential. Establishment of gamba grass is significantly higher in
Eucalyptus woodland that has recently been burnt.
These introduced grasses produce flammable material up to five
times greater than fuel loads in native grasses (which are
typically 2–8 tonnes per hectare), cure later in the year
(June–July versus April) and maintain a tall, upright
structure. Fire intensities and flame heights are increased and
have severe impacts on the less fire-tolerant native grasses,
shrubs and trees. Recovery from fire by these exotic perennials is
rapid.
Gamba grass has a relatively short-lived seed bank with only 1%
of buried seed remaining viable within a year; thus gamba grass
infestations can be eradicated over a couple of years. Glyphosate
herbicide is effective on gamba grass and mission grass especially
if fire can be used to remove rank growth first.
