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Fire scars for 1999 as recorded by NOAA
satellites. Blue - fires before June 30; Red - fires after June
30. Source: WA DLI
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North Australia has the largest and most frequent fires in the
whole continent. The satellite-based image above shows fire scars
from 1999 with blue indicating fires from before June 30 and red
indicating those fires burning after June 30 in that year.
This part of the country is primed for regular fire with vast
grassy landscapes that flourish during the wet season and then,
over months with very little rain, dry to a tinderbox. The problems
these fires create for the people of the north are considerable,
whether it is in managing cattle stations or protecting
fire-sensitive plants and animals.
These newer fire patterns differ from those of traditional
Aboriginal management. With traditional practices, small patches of
grassland were burned throughout the year as people moved through
the country. In the Top End and northern Kimberley, many fires were
lit in the early to mid-dry season. Smaller and less intense fires
of traditional practices maintain more diverse habitats than large
late fires.
Fire regimes
( by Dick Williams and Garry Cook, ‘Savanna
Burning’, page 15)
Frequent and extensive fires in northern Australia are a
consequence of the region’s monsoonal climate with its marked
summer wet season and long and warm winter dry season. The wet
season generates heavy growth of grasses and other herbs, and the
trees are continually dropping leaf litter throughout the dry
season. This dries out or ‘cures’ during the dry season
into tinder-dry, fine fuels for fires. Dry thunderstorms during the
build-up and early wet season have always produced lightning.
Aboriginal people used fire widely across most of Australia and
continue to do so across much of the north. Thus, for thousands of
years there has been the combination of annual supplies of dry,
fine fuel and ignition sources that can sustain regular, frequent
fires.
European settlement has caused significant change to the fire
patterns of northern Australia in the last century. The challenge
for today’s land managers is to work out the optimum mix of
fire patterns at the landscape scale—hundreds to thousands of
square kilometres—that protect life and property, maintain
the productive potential of the land and conserve biodiversity. To
do that, we need to understand the factors that determine the fire
regimes in the savannas.
What are fire regimes?
The term ‘fire regime’ describes when and how often
an area is burned, as well as the intensity, size and patchiness of
fires (for example, yearly late dry season fires that are intense
and extensive). Fire regimes can have different impacts on fuels,
fodder and biodiversity by changing the composition of plant
species and by altering habitats. Different fire regimes may be
needed for different land management goals, and no single regime is
best for all land management purposes.
How much savanna burns each year?
Hundreds of thousands of square kilometres of northern Australia
are burnt each year (see map above). The area burnt varies with
state or territory; in the northern savannas usually more country
is burnt in the NT than in either northern WA or Queensland.
When do the fires occur?
Fires can occur from March to December, but most areas are burnt
late in the dry season (see map above). Of the nine operational
districts of the Bushfires Council of the NT for example, only one
region east of Darwin has more country burnt in the early dry than
the late dry season. Fires can occur in the wet season, especially
in the early part (November–December) if there is available
fuel and if weather conditions allow fires to spread.
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 The fire history of
north Australia as measured by NOAA satellites from 1997-2004.
Darker areas have been burnt almost every year - light pink aras
have been burnt less frequently. White areas have not had large
fires.
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How often is an area burnt?
In the higher rainfall savannas of the NT, for example the
Darwin–Alligator Rivers region, 50–70% of the landscape
may be burnt every year as shown above. Fires are less frequent in
the semi-arid savannas; in the mid-1990s, nearly 80% of the area
south of Katherine (NT) was either unburnt or burnt only once over
a three-year period.
What determines fire regimes?
Regional fire regimes depend upon the weather, the amount of
fuel and when it is available, ignition chance and type, fire
behaviour, and the capacity of fire to spread through the
landscape. Fire regimes are also the product of people; people are
the most common source of ignition and they can manipulate fire
regimes, by prescribing fire or by suppressing it. People also
affect fire regimes by their influence on fuel loads as a
consequence of land use. For example, the difference in fire
frequency between the semi-arid and wetter regions of the NT may be
partly related to climate, but also to land use.
In the wetter regions, the dominant pasture type is annual
Sorghum , a relively unpalatable grass, whereas in the
semi-arid savannas, the pasture species are more palatable
perennial grasses so that cattle may reduce fuel loads by eating
the grass. Property managers burn less extensively for fear of
losing their feed reserves.
Fire regimes and landscape management
Traditional Aboriginal burning
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Fire management on Aboriginal land today can
involve a mix of traditional knowledge and recent technology
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Indigenous people have traditionally used fire for a variety of
reasons—ease of travel, communication, ‘cleaning’
country, hunting and to maintain food sources. In Kakadu, for
example, they started burning very late in the wet season (March)
as soon as the country began to dry out, and continued for
9–10 months until the monsoon arrived. There was a peak of
activity in June–July, but relatively little in August–
September.
Current fire regimes on Aboriginal lands may not reflect past
traditional practices because people are no longer dispersed
through the country, and because traditional knowledge may not have
been passed on to the current generation. Fires arising from
Aboriginal lands today can cause conflict with neighbouring
landholders if their land management goals are different.
It is important to continue learning about traditional
indigenous fire regimes. Today, fire agencies endeavour to employ
traditional burning practices, where possible, as a means of
establishing a diverse set of fire regimes across the savanna
landscape.
Early dry season burning
Early dry season burning is often prescribed in all land tenures
in the savannas, primarily to reduce fuel. The early dry season
burn results in a mixture of burnt and unburnt country and the risk
of extensive, intense fire in the later part of the dry season is
reduced. Early dry season fire can also be used to stimulate
‘green pick’ in pastoral country.
Late dry season fires
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Intense late dry season fires can be useful in
controlling invasive woody plants
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Most of the area burnt in northern Australia results from late
dry season fires—wildfires rather than planned and controlled
operations. Most late dry season fires are undesirable as they are
difficult to suppress, consume vast tracts of country, and can be
detrimental to plant and animal life. However, intense, late dry
season fires do have some uses. They can be used to kill or reduce
the amount of woody sucker or shrub growth that may have
accumulated in unburnt or heavily grazed savanna country. They can
be used in integrated programs to control exotic woody weeds such
as rubber vine ( Cryptostegia grandiflora ) in
Queensland.
Wet season burning
The opportunity for a burn during the wet season depends on the
chance of a dry break during the monsoon. For wet season burning, a
body of cured grass and fuel needs to be retained through the dry
season and then lit during the wet season after a couple of days
without rain. Burning soon after the first storms, or when storms
are imminent, removes old rank grass just before the new growth of
the next season and so improves the quality of fodder for livestock
or native fauna.
An increasingly important use of wet season burning in the Top
End of the NT is to manage stands of annual sorghum. If annual
sorghum is burnt after germination but before seed set, it can be
eliminated locally because it does not have a persistent seed bank.
This can reduce fuel loads in country with sorghum by 50%. However,
the effectiveness of wet season burning depends very much on
timing. If too early in the wet season, before about 100 mm of rain
has fallen, there may be ungerminated seed left in the soil. If too
late, e.g. after the monsoon has set in, fire may not spread.
Early wet season fire or ‘storm burns’ can also be
used to control the density of Melaleuca shoots in the
grassy wetland country on Cape York and to manipulate the
composition of the grasses in the grasslands. This is an important
tool in the management of habitats for species such as the
golden-shouldered parrot.
Fire regimes—past, present and future
There has been considerable change to the fire regimes in the
savannas over evolutionary, prehistoric, historic and contemporary
times.
Fires undoubtedly became more frequent as the climate began to
dry out during the Tertiary period over 20 million years ago.
Before Aboriginal people arrived in the savannas, fires probably
started at the very end of the dry season when there was dry fuel
and the early storms produced lightning.
Aboriginal people modified the timing of fire by burning earlier
in the dry season. Now, much of northern Australia, particularly
the wetter parts, is dominated by a regime of frequent, extensive
fires in the late dry season (see map at top of page). These
extensive, late dry season fires appear to be more common now than
before European settlement in northern Australia. This regime of
frequent, intense, extensive fires is cause for concern in
virtually all savannas.
In much of the semi-arid country, the dominant land use is
pastoralism for which reserves of grass need to be protected from
fire. Fires have been actively suppressed, which has probably
resulted in a decrease in fire frequency and extent.
A change in fire regimes brings with it a change in impacts of
fire on the landscape. We therefore need to know the ways in which
various regimes impact upon the plants and animals of the savannas,
and how we can manipulate fire regimes so that desired impacts are
maximised and undesirable ones are minimised.
