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Fire types


Fire-Scars-99

Fire scars for 1999 as recorded by NOAA satellites. Blue - fires before June 30; Red - fires after June 30.   Source: WA DLI

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.

Fire-History-smallThe 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.

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

Dusk-Burn

Fire management on Aboriginal land today can involve a mix of traditional knowledge and recent technology

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

Woody-weed-fire

Intense late dry season fires can be useful in controlling invasive woody plants

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.