Background: Fire

Fires are a natural occurrence in the Australian landscape and are necessary to maintain the health of many native species and ecosystems. The ACT landscape has evolved with fire, and Aboriginal people have developed a sophisticated understanding and use of fire to manage land and resources and reduce bushfire risk.^[1]

Impact of fire

Bushfires can have devastating impacts on biodiversity, as well as human settlements and the natural resources communities depend on. Such impacts are mainly the result of changes in fire occurrence and severity. Increased human sources of ignition, the suppression of natural fire to protect human life and assets, and prescribed burning practices for the management of fuel loads can change the natural fire regimes required for biodiversity and ecosystem health. In addition, increased periods of drought and higher temperatures have increased the risk of more frequent and severe fires.

Changes to natural fire regimes and the increasing risk of fire occurrence have many potential social, environmental and economic impacts, including:

significant risks to human health through injury and death.
smoke from both bushfires and controlled burns increase air pollution, especially particulate matter and summer smog, which is particularly significant for those with asthma and chronic lung disease.
increased property and infrastructure loss, both in rural locations and Canberra’s suburbs in bushfire-prone areas.
degraded water quality in catchments and water storages, leading to increased treatment costs for water supply.
reduction in streamflow and inflows to water storages due to the increased uptake of water for plant regeneration.
loss of important heritage, particularly Aboriginal heritage sites, historic sites in rural areas, and damage to landscapes.
loss of agricultural buildings and infrastructure, crops and animals, and plantation forests.
severe and widespread fires can result in the loss of many native animals and the ecosystems they depend on for food and habitat.
changes to ecologically appropriate natural fire regimes can have significant impacts on the composition of vegetation communities and the ecosystems they support, and
post-fire rainfall and runoff can degrade water quality affecting aquatic health and biodiversity.

Causes of bushfires

Bushfires are started by natural causes such as lightning, and by human causes. Human-caused ignitions include deliberate acts such as arson and illegal burning-off, as well as through accidents or careless acts. They also include ignitions from power lines, motor vehicles, campfires, motor vehicle accidents and sparks from machinery.

*Namadgi National Park post-bushfire*. Source: OCSE

Prescribed burning

Prescribed burns are mainly used to reduce fuel loads (fuel reduction burns) such as surface vegetation litter, bark and understorey shrubs. The objective is to reduce the risk of bushfires and potential impacts on people, property, infrastructure, ecosystems and environmental assets such as water catchments. Reduced fuel loads may also improve the effectiveness of fire-suppression activities during bushfire events. Fire-sensitive ecosystems and those still recovering from bushfires are generally excluded from fuel reduction burning.

Ecological burns are undertaken to improve the condition of ecosystems and biodiversity. Ecological burns vary in size, frequency and patchiness to meet biodiversity and ecological outcomes. This is important where species and ecosystems depend on fire for regeneration and regrowth. Some fuel reduction burns can also have ecological outcomes, for example where vegetation communities are not exposed to fire for extended periods and are outside of their ecologically appropriate natural fire regimes.

Cultural burns are also undertaken in the ACT. The need for cultural burns is determined by Traditional Custodians and Murumbung Rangers to meet cultural objectives for specific locations. The burns are conducted in consultation with Traditional Custodians.

It is important that fuel reduction burns consider ecosystem and biodiversity requirements to ensure ecologically appropriate burning is undertaken. However, this can be difficult where property and asset protection is the main purpose of fuel reduction.

The delivery of the prescribed burn program in the ACT is always limited by seasonal and climatic conditions. Increased temperatures and reduced rainfall in the ACT are making it more difficult to undertake burning activities, particularly in late autumn and spring. The current climate has not only reduced the period suitable for prescribed burns, but has also increased the likelihood of unpredictable weather occurring that may impact prescribed burns already underway.

In addition to prescribed burning, the ACT Government undertakes a range of fuel reduction activities including slashing and mowing, grazing and the physical removal of fuels.

Increasing bushfire risk in the ACT

The ACT has a high risk of bushfires with large areas of forest in the Namadgi National Park, Tidbinbilla Reserve and the Lower Cotter Catchment. This risk extends to the Canberra urban area which is characterised by a mosaic of suburbs and bordering bushland, grassland and forests of the Canberra Nature Park.

Climate change is increasing the threat of bushfires in the ACT. Higher average temperatures, reductions in rainfall, and increased occurrence of severe events such as heatwaves and storms have led to more days of high risk fire danger conditions and longer and more severe bushfire seasons in the ACT.

Typically, the ACT bushfire season occurs from the beginning of October until the end of March the following year. However, climate conditions in the ACT are leading to extensions of the fire season.

Growth in the ACT’s population and extension of urban areas also increases the risk of fire impacting on people and property. Having more people living close to grassland, nature parks or other areas of vegetation means more houses and people within bushfire-prone areas.

Assessing fire risk

The Fire Danger Index (FDI) assesses the potential severity of bushfire occurrence given the predicted conditions. The FDI reflects the difficulty of suppression should a fire occur, and also the risk to community, property and landscape. FDI is determined by the Bureau of Meteorology based on forecast air temperature, relative humidity and wind speed, along with drought conditions and soil moisture levels. It should be noted that FDI does not reflect the observed conditions on the day, which may differ from those forecasted. The FDI is used to determine the Fire Danger Rating which provides a simplified indication of potential fire consequences (Table 1).

The FDI expresses bushfire potential using a number scale. An FDI of 1 means a very low risk of fire occurring and that it will be easy to control; an FDI of 100 means that fires will have the potential to be so severe that control is virtually impossible. High risk fire danger conditions (severe, extreme or catastrophic) occur when the FDI is greater than 50. Days of high risk are not common in the ACT, usually averaging less than three days a year, and mainly occurring in January.^[2] However, climate change is expected to increase both average and severe FDI in the future.

Table 1: Fire Danger Index scores and Fire Danger Ratings used for fire risk in the ACT.

Data sourced from: Environment, Planning and Sustainable Development Directorate.

It should be noted that Australia’s Fire Danger Rating System (AFDRS) was updated in September 2022. The new rating system uses weather, fuel loads, and fire behaviour in different vegetation types to assess fire danger, whereas previous danger ratings for the FDI were based on weather indices alone. The update includes a simplified and nationally consistent warning system.