Understanding fuel dynamics and bushfire behaviour

Predicting bushfire behaviour and potential impacts on a property using standard calculations and assumptions about the flammability of a vegetation type is useful, but does not always reflect the variability in seasons, terrain and other landscape features particular to that property that influence fire behaviour and severity.

What about fuel? To give sound advice to the community about bushfire hazards and fire management it is important to understand the dynamics of fuel in the various layers of the forest. What is considered as fuel or combustible material; how much fuel has accumulated; how fuel is arranged; how much moisture does it contain; and how severe does the fire have the potential to be based on these fuel ‘ingredients’ determines the potential risk to property from bushfire.

Case study 30.10.20: Panguna Ridge, Smithfield FNQ

Smoke rising above Panguna Ridge north of Cairns during the recent wildfire event October 2020.
Wildfire Panguna Ridge, Smithfield FNQ October 2020 (day 8) wildfire burning under a NE wind change

Observing the fire behaviour daily on Panguna Ridge over the past week in Smithfield north of Cairns provides a great opportunity to look at the connections between fire behaviour and current fuel conditions in an area classed as ‘Very High Potential Bushfire Risk’ under the Cairns Regional Council Bushfire Overlay Codes/mapping. The fire has been moving south along the ridge for 8 days and has traveled approximately 800 metres in that time, as a slow back-burn.

Bushfire Overlay Code Map showing fire rating for Panguna Ridge north of Cairns as 'Very High Potential Bushfire Risk
Panguna Ridge classed as ‘Very High’ Potential Bushfire Risk (Cairns Regional Council 2016)
Burnt area and unburnt areas divided by walking track less than 1 foot wide.
Fire self-extinguished against walking trail in leaf-litter

With the absence of mid-story fuel or grasses the fire (less than 0.5 metre high) relies on wind moving it through the leaf litter only to self-extinguish each evening and pull-up along walking and mountain bike tracks and rocky screes. The burning logs and hollowed out trees flare up the next day in the heat, fall on to unburnt areas with leaf litter and cause narrow fingers of fire to run up and down the eastern face of the ridge under the south-easterly  wind direction. This pattern has been repeating each day, creating a low-intensity and very patchy fire, leaving considerable areas of unburnt bush and habitat.

Burnt areas on top of Panguna Ridge with leaves scattered over burnt ground that will carry a fire again
Burnt areas supporting leaf-drop that will carry fire into
unburnt areas at top of photo

At two sections of the ridge where the canopy is more open and there are small areas of grass the fire has scorched the mid-story canopy of wattles and eucalypts and caused some amount of leaf-drop. Leaf-drop has the potential for a fire to carry again across areas already burnt to unburnt areas.

In these open areas the fire has managed to jump over the network of tracks due to a combination of leaf-drop and falling burning timber and/or floaters (leaves on fire carried by the wind to unburnt areas) igniting the leaf litter on the other side.  However, the fire has trickled at no more than 200 millimetres high down the western face of the ridge, and with no wind quickly loses momentum and extinguishes against the rainforest vegetation less than 15 metres below.

Bushfire less than 20cm high trickling downhill, extinguishing against rainforest vegetation
Leaf litter fire on the western face of the ridge
self-extinguishing into rainforest below

The considerable depth of accumulated leaf-litter during the driest part of the year, and a slow burn with a longer residence time has implications for igniting stumps and logs and the ability for the fire to trickle around for days despite there being little other fuels present, and receiving small showers of rain.

However, the fire still burning, under current wind direction and conditions and fuels present on the ridge and to the west, is unlikely to burn over a metre high and gain enough momentum to create a large fire front or create enough radiant heat to impact on surrounding properties.  A wind direction change would create a fire moving with the wind with a faster rate of spread (ROS) through the leaf litter, but would a faster moving fire in this fuel type have the same ability to ignite logs and timber to create next day flare-ups and ignite unburnt areas burning for days, or would it simply extinguish once it reaches the first trail or track? 

Landslides – disaster or renewal?

Firecraft were left in awe this week and totally humbled by the stark contrast of the ancient forest greenery against the scar on Thornton Peak from the slump landslide. A major geological event that occurred in the Daintree rainforest in July 2018 after significant rainfall.

Some may consider it an environmental disaster having an impact on our oldest rainforest…… or does it have important ecological function similar to a volcano?? Certainly sparks thoughts of what will grow there and inhabit it in our lifetime before it develops its pre-landslide complexity long into the future again.

Eradication of Rubbervine

I recently had a question posed to me – “If we accept foreign people into the country in every State, why is it a problem letting foreign plants in?”

Rubbervine flowering

I found this question really surprising at first…..then it occurred to me that perhaps the extent of our weed problem and the importance of protecting Australia may be unnoticed.

Weeds or invasive plants are arguably one of our biggest threats to our ecosystems and biodiversity, and also to farming. Prohibiting the introductionof plants and the effort that goes into prevention of spread and eradication of weeds is an annual multi-billion-dollar effort.

Take Rubbervine (Cryptostegia grandiflora) for a North Qld and Cape York example. Introduced in 1875 from Madagascar as an ornamental plant for mining camps due to its adaptation to harsh conditions, lush foliage and pretty flowers, it is now a Weed of National Significance (DAF, 2019). Considered one of the worst weeds in the country it is toxic to cattle, takes over our riparian vegetation along our river systems, and degrades important habitat needed for wildlife survival.

Rubbervine extending from the base of the range covering mature trees

This infestation being managed by Firecraft is on the Desailly Range south of the road to Cooktown. The Rubbervine is estimated at 25m high in places, blanketing and killing native trees and shrubs, and creeping its way rapidly up to the road.

Rubbervine extending upwards over the eastern edge of the Desailly Range

Monsoonal Magic

The Mount Carbine Tableland open forest has been transformed in the past few weeks from a dry parched landscape to a picture of lush new growth after the monsoonal rain. Species such as the perennial Native Rosella (Abelmoschus moschatus subsp. tuberosus) lies dormant in the dry seasonal and then emerges from its tuber during the wet season. The bush-tucker plant only flowers for one day. This paddock near Mount Molloy has been subjected to a hot late-season fire that has removed the weedy understory and now is a canvas of iridescent green new growth with splashes of the Rosella’s watermelon-pink hibiscus flowers.

Is the Great Bowerbird a seasoned fire manager?


This Great Bowerbird (Chlamydera nuchalis) bower scattered with snail shells, gravel and various colourful treasures was admired today by Firecraft Crew in the Mt Garnet area.

Bowerbirds have always been recognised for their construction abilities, pedantic arrangement of bowers and particular adornments they choose to scatter around the cleared area of the bower to attract females.

But is the cleared area only to make the bower standout?? Some researchers believe these species of bowerbird may have adapted to fire prone environments by deliberately clearing a ‘firebreak’ around their bower to protect it from wildfire…..

Read more here:Firecraft Environmental, Great Bowerbird bower…/dn18568-zoologger-fireproof…/

Adaptations to fire

It is a well discussed topic that the echidna is a heterothermic mammal – meaning they can regulate their temperature as an adaptation for survival. They are the oldest mammal – a monotreme, having survived the times. It is believed the echidna can drop its body temperature during a wildfire and this is a mechanism that allows it to survive for several reasons.This one was spotted by Firecraft on a hike recently up Macalister Range behind Panguna Valley.

Echidna, fire, Firecraft EnvironmentalEchidna and fire

Wet season burning for pasture management and weed suppression

Storm burning or wet season burning can be difficult to execute due to the restrictions that often come with being prepared to burn within a short timeframe dictated by weather and on-ground conditions. However, there are many benefits both ecological and economic of burning outside of the typical fire season. Read on to learn more.

A cost-effective tool for weed management

Burning after the first wet season rains or before first storms can be a cost-effective tool for weed management. “Extending” the fire season to include a wet season timing, often promotes positive ecosystem responses such as increasing diversity and broadening the fire footprint over a property to ensure the same areas are not burnt every season thus reducing fire frequency.

Storm burns at Mitchellvale

Firecraft Environmental carried out storm burns for pasture management and weed management at ‘Mitchellvale’ pastoral property north of Mount Molloy on the Mount Carbine Tableland in the last week of November. The objectives were to reduce woody weeds that had emerged from recent rainfall and promote green-pick for stock.

Burning at Mitchellvale

When the opportunity arises to burn just after the first storms to suppress freshly germinated weed seedlings and conditions are favourable the benefits are evident. The balance is having enough hayed-off grass or fuel with 1-2 years growth amongst the weeds that will carry a fire under humid conditions, hot enough to kill the weed seedlings. The anticipated storm or rainfall that follows promotes a flourish of new growth in pasture or native grasses.