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Savanna Explorer > All Regions > Fire > Fuel for Fire

Fuel for Fire

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?

Termites-&-fuel

Many termites gather the stems of grasses in their nests

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.

Grassy-fuel

Dry grass provides the bulk of fuel for savanna fires

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 a significant fuel for fire

Leaf litter is also a significant fuel for fire: it increases as the dry season progresses and with tree density

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.


 
GAMBA2

A gamba grass infestation along a roadside near Darwin, NT.

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.