Fire and the NSW bush
Understanding the ecological effects of fire helps us to conserve our rich diversity of plants and animals. The plants and animals in NSW have evolved particular responses that allow them to survive in specific fire regimes. Changing fire regimes and other environmental pressures can exceed their ability to adapt and cause extinction. Natural fire regimes can be altered through overly effective fire suppression, repeated prescribed burning at low intensity in the wrong season and too frequent burning caused by arson, escaped private property burn offs and prescribed burning. It is important to maintain the natural fire regime as much as possible to conserve biodiversity and reduce the hazard of catastrophic bushfires.
Fires are characterised by their intensity, the frequency with which they occur, the season in which they occur, their spatial pattern or extent, and their type. Combined, these attributes describe the fire regime.
Natural areas can have complex fire regimes. Even when considered individually, aspects of the fire regime such as frequency can be more complicated than they first appear. For example, the intervals between fires can vary substantially although the fire frequency may remain the same. Fires may occur in consecutive years then be separated by a long period of no fire or they may be spaced at regular intervals over the same period of time. This inter-fire interval can vary enormously, and can have profound implications for biodiversity.
Fire Intensity
The intensity of a fire is measured by the amount of heat energy released for every metre of fire front (kW/m) and can be gauged by the flame height.
- Low intensity = flame height <0.5m (<50kW/m)
- Moderate intensity = flame height 0.5 – 1.5m (50-500kW/m)
- High intensity = flame height 1.5 – 3.0m (500-2000kW/m)
- Very high intensity = flame height 3.0 – 10m (2000-4000kW/m)
Intensity depends on the type and amount and arrangement of fuel available, terrain and weather conditions (predominantly wind, relative humidity and temperature) which all influence how a fire behaves.
The most obvious effect of fire intensity is the loss of vegetation cover and vegetation scorching. Low intensity fires with a small flame height will only burn low level vegetation (understorey grasses and shrubs) leaving a significant portion of the mid storey shrubs and vegetation canopy unburnt and un-scorched.
High intensity fires on the other hand consume most of the vegetation. Scorching and defoliation of tree crowns is common and shrubs and grasses are completely consumed leaving very little unburnt.
In general, high intensity fires have greater biological impacts and the ecosystem requires a more extensive period of time to fully recover. High intensity fire however, is required by some vegetation communities to rejuvenate ecosystems, create habitat for animals and assist in the regeneration of plants.
Fire Frequency
Fire frequency is the time between consecutive fire events. Fire frequency has the greatest potential to affect the life cycle of plants of all the fire regime characteristics. Individual plants and vegetation communities have varying responses to fire and have evolved their life cycle to fit within the average time between consecutive fires.
The most vulnerable plants are those that can only regenerate from seed, called obligate seeders. Generally obligate seeders are killed by fire and rely on seeds that they store in the soil or in woody capsules on the plant to regenerate. If fires occur too frequently these species may not be able to reach maturity to produce seed and will not persist. Infrequent fires on the other hand will also affect these and other species that rely on fire to stimulate germination of seeds or regeneration. A lack of fire or too infrequent fire will result in senescence where these species grow old and die and their seeds rot in the soil before they have the opportunity to be germinated by fire. The ecological consequences of high frequency fires have been listed as a Key Threatening Process under the Threatened Species Conservation Act 1995.
Fire Season
As weather has a major influence on fire intensity it logically follows that high intensity fires occur in the hotter, drier periods of the year and low intensity fires occur in the cooler and wetter seasons. Many plants and animals have evolved to coincide with or avoid breeding in seasons of high intensity fire. Changes to this aspect of the fire regime e.g. undertaking prescribed burning in winter, can have profound effects on native plants and animals.
More information on how seasons can influence fire behaviour can be found on the Australian Climate Extremes web page maintained by the Bureau of Meteorology. This site describes how climatic events such as El Nino affect the number and severity of bushfires in Australia.
Fire Extent
The extent of a fire is influenced by many factors including topography, the intensity of the fire, the presence of areas of reduced fuel (natural or from prescribed burning) or moist vegetation and the success of containment and suppression efforts. A high intensity fire during the dry months of the year has the capacity to spread faster and further ie: be more extensive, than a low intensity fire in the cooler months of the year. Low intensity fires in the cooler months are also easier to contain and put out and often go out overnight. Nearly all fires have some degree of patchiness where islands of vegetation are left unburnt in a mosaic pattern. These unburnt islands can provide refuge for native animals and allow native plants to recolonise the burnt areas.
Find out more about fire in the NSW environment
- Learn about how factors such as wind, slope and climate affect how a fire behaves.
- See how the plants and animals of NSW have adapted to evolving in a fiery environment.