Fire Management – 101.Understanding Fuel.

Bushfire behaviour is influenced by many factors including forest type, fuel quantity, fuel arrangement, fuel dryness, drought index, temperature, Relative Humidity (RH), topography, atmospheric stability, aspect and even slope.

Wind has a dominant effect on the Rate of Spread (ROS), and also bushfire size, shape and direction.

Measuring Fuel Hazard.

Fuel quantity (tonnes/ha) affects fire intensity (heat output – kilowatts/metre), the Rate of Spread, flame height and spotting distance.

Put simply – more fuel produces more heat output and greater fire intensity, which directly affects fire suppression difficulty.

But fuel arrangement is just as important as fuel quantity to the overall fuel hazard rating. Fibrous and ribbon bark, wiregrass and suspended leaves, together with elevated and near-surface scrub fuels, act as ladders which can lead flames into the tree canopy. 

Overall fuel hazard can be significantly reduced by rearranging the elevated fuels by mulching or mowing, even though the total fuel quantity initially remains the same.

Perhaps the best simple illustration is to imagine unsuccessfully trying to ignite a thick phone book with a match but then tearing out all the pages and scrunching them up into a pile and then setting it alight. There is no change in overall fuel quantity but the difference in the arrangement significantly affects the resulting fire behaviour.

Fuel size is an important factor in “flammability” too. Fine fuels include dead plant material, such as leaves, grass, bark and twigs less than 6mm thick, and live plant material thinner than 3mm thick. Typically, they burn quickly at the fire edge and contribute the most to the rate of spread and flame height.

Bark quantity and type have a dominant influence on the overall fuel hazard and therefore fire behaviour. Burning can significantly reduce the long-term back hazard.

Under extreme conditions, fibrous bark like messmate (Eucalyptus obliqua) can produce massive spotting 100-500 m, while ribbon bark from manna gum (E. viminalis) in combination with unstable atmospheric conditions can start spot fires 10-20 km ahead of the fire front.

The leaves on standing trees will not, on their own, support a fire, and fires will not occur in tree crowns unless there is an intense fire burning on the ground beneath.  

Available Fuel vs Total Fuel.

The quantity of fuel available to burn (compared to total fuel quantity) depends largely on moisture content. When moisture content exceeds 20-25% not much will burn, whereas 9-17% is the optimal range for safe fuel reduction burning, but if the moisture content drops below 9%, virtually everything will ignite, and fire behaviour can become extreme.

But here is the important bit, fine fuels like leaves and bark act like a sponge and can absorb moisture from the air when the Relative Humidity (RH) is high, and temperature is cool. Conversely, they can also dry out very quickly.

This means that fuel availability can increase rapidly from nearly zero after high RH, rain, dew, drizzle or fog, to many tonnes per hectare as fuel dries out. This can happen quickly over a few hours on hot and windy days. Rapidly drying fuel is the main reason that bushfires escalate during the late afternoon.

Under the normal drying cycle, the fuel dries from the top of the litter bed and the near surface fuel. Under mild conditions, and in spring, the lower litter layer next to the soil often remains too wet to burn.

Heavy forest fuels like big logs on the ground, or dead standing trees (stags), take much longer to dry out and are more affected by prolonged drought and underlying soil dryness. They have a long residual burning time and don’t extinguish quickly or easily like grass. This adds considerably to the complexity and length of the firefighting effort in forests. Campaign bushfires lasting weeks, or even months, are not uncommon in heavy forest fuels.

Finally, the explosive flammability of eucalyptus oil is well known to Australian forest firefighters. The aromatic oil has an energy content similar to petrol, with about 43MJ/kg once its vaporised, but it has a much higher flashpoint like diesel. Fireballs of explosive gas are thankfully uncommon but exhibit erratic behaviour and are very frightening.

Fuel Accumulation.

The rate of fuel accumulation from leaf and bark fall in forests varies between 4 to 10 tonnes/ha/year. The rate of decomposition also depends on forest type, soils, seasonal rainfall and climate.

Contrary to popular belief, forest fuels do not accumulate indefinitely. An equilibrium, or steady state, is eventually reached where the accumulation rate matches the rate of decomposition.

For dry foothill forests commonly found across Victoria, fuel quantities typically range from 15-40 tonnes/ha after 20-25 years. In wet forests like mountain ash it is typically higher.

Wombat – Fire Effects Study Areas (FESA).

One of the most authoritative and comprehensive long-term research projects in Victoria began in 1984 in the Wombat State forest. The research was led by the late Dr Kevin Tolhust and assessed the ecological impacts of repeated, low-intensity prescribed fire. Short and long-term rotation burning in spring was compared to autumn ignition. Some control plots were not burnt.

The structure and type of near surface and elevated fuels in the understory evolved over time. The long-term patterns of fire frequency, intensity, and season play a role in understory and fuel redevelopment (aka – Fire Regime).

The study found that surface fine fuels had an average steady state level of 16 tonnes/ha, but seasonal variations ranged from 9 to 26 t/ha.

Surface fine fuels reaccumulated to within 90% of the long-unburnt state within four years of either spring or autumn burning at the fire low intensities studied. The impact of prescribed burning on surface fuels is therefore relatively short-lived.

The rate of accumulation of surface fine fuels was not significantly affected by the season of burn or the burn frequency.

The study found that while the surface fine fuel complex may reaccumulate very quickly, the overall fuel hazard levels were likely to continue to slowly increase for 20 years or more due to slower of regrowth of bark and accumulation of elevated fuels.

Over the 14-year period, no plant species were either lost or gained as a result of up to four successive spring fires or three successive autumn fires. However, short-rotation spring burning can favour Austral Bracken (Pteridium esculentum), herbs, geophytes and Poa (Poa sieberiana), and disadvantage Forest Wire-grass.

There were also subtle changes in the forest understorey in the absence of fire.  These changes were only small on a year-to-year basis, but amount to significant changes over a period of a decade or more.

When severe bushfire removes or damages the tree crowns and lets the sunshine reach the ground it can lead to a pulse of understory scrub regrowth, particularly in some foothill forests.

In addition to examining fuel dynamics, the Wombat study also investigated the ecological effects of repeated fire on vegetation, tree growth, invertebrates, mammals, birds, bats, reptiles, along with changes to soil nutrition and organic matter.

But the monitoring of these important FESA plots finished in 1999, and it would be wonderful to see them reinstated to put some more science into the debate.