Wildfire Susceptibility

Wildfire susceptibility is the likelihood that vegetation will ignite and sustain an uncontrolled fire. Rapid spread occurs when dry, connected fuels interact with adverse fire weather and terrain that accelerates flame movement.

Fuel condition and vegetation structure

The amount, dryness and arrangement of combustible vegetation determine whether an ignition remains small or becomes a spreading wildfire.

• Prolonged dryness lowers fuel moisture, making grasses, leaves, twigs and dead wood easier to ignite.
• Continuous grasses, litter or shrubs provide horizontal fuel continuity, allowing flames to move without encountering natural breaks.
• Shrubs and low branches act as ladder fuels, carrying surface fire into tree crowns, where spread can become faster and more intense.
• Accumulated dead biomass and highly flammable vegetation increase available fuel and potential fire intensity.

Weather conditions favouring spread

Hot, dry and windy conditions create the most dangerous combination because they dry fuels and increase the rate at which flames and embers advance.

• High temperatures and low relative humidity accelerate moisture loss from vegetation and litter.
• Strong winds bend flames toward unburnt fuel, supply oxygen and transport burning embers ahead of the fire front.
• Extended droughts and heatwaves create prolonged fire-weather periods during which even relatively moist forests can become combustible.
• Dry thunderstorms can provide lightning ignition while producing little rainfall.

Topography and longer-term pressures

Terrain modifies local wind and heat transfer, while climate and ecosystem change can create persistently favourable conditions for fire.

• Fire generally spreads faster upslope because rising heat preheats and dries vegetation above the flames.
• Steep valleys and mountain passes can channel winds, increasing fire speed and producing abrupt changes in direction.
• Climate change can intensify heat and dryness and lengthen fire-weather seasons in many regions, although effects vary by ecosystem.
• Degradation, invasive grasses and poor fuel management can replace patchy vegetation with more continuous, combustible fuel.

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