Living in a land of fire
Professor Rob Whelan, University of Wollongong
Professor Peter Kanowski, Australian National University
Dr Malcolm Gill, Australian National University
Dr Alan Andersen, CSIRO Tropical Ecosystems Research Centre
prepared for the 2006 Australian State of the Environment Committee, 2006
Every fire is different, because its behaviour and impacts depend on particular weather conditions, topography, fuel loads and distribution, and on suppression activities. The impacts of successive fires on the environment depend on their frequency (or ‘between-fire interval’), intensity, seasonality and type (namely ‘peat’ or ‘above-ground’). Together these characteristics are known as the fire regime. The concept of the fire regime is now recognised as central to understanding the ecological impacts of fire. Understanding the fire regime is also important for defining risks to people and property, and for mitigation and management decisions (Bradstock et al. 2002).
A particular species or ecosystem is likely to respond differently to a regime of high-intensity canopy fires than it would to a regime of low-intensity surface fires, to fires in milder winter or spring conditions than to fires in drier late summer or autumn conditions, or to a series of fires in successive years than to infrequent fires many decades apart. If the fire regime is altered to one outside the bounds under which species and ecosystems evolved, there are likely to be detrimental effects on those species and ecosystems.
Fire regimes have varied over time in any one location on the Australian continent. The wetter, rainforest-clad continent of 45 million years ago dried out as it moved northwards following the break-up of Gondwanaland; vegetation changed to a sclerophyll-dominated flora in many areas, and the extent and frequency of fires increased (White 1998). Periodic climatic oscillations produce drier periods with recurrent large-scale fires, and contraction of the extent of vulnerable plant communities (Bowman 2003, Lindesay 2003).
Indigenous occupation of Australia altered the fire regime, but detailed evidence of specific impacts is not yet conclusive. The extent of Indigenous burning undoubtedly varied from place to place (see, for example, chapters by Bowman, Hill and Liddle in Cary et al. 2003), depending on climate and vegetation; we might expect that Indigenous peoples living on the coast or on rivers influenced fire regimes differently to those living in rainforests, or in regions with little surface water. Climatic variations such as El-Nio would have interacted with anthropogenic fire regimes, as they do now.
European settlement altered prevailing fire regimes, with effects such as increased fire frequency associated with land clearing, decreased fire frequency due to suppression in settled areas and, more recently, changed fire frequencies, intensities and seasons of burning associated with large-scale fuel-reduction burning in forests, woodlands and heathlands. The removal of Indigenous people from their country as a result of European settlement in much of Australia, and subsequent changes to Indigenous peoples’ way of life, has also profoundly altered fire regimes across Australia. For example, the breakdown of traditional burning practices in northern Australia has led to a far higher incidence of large-scale, wildfires late in the dry season (Russell-Smith et al. 2003).
The seasons in which fires typically occur vary across the Australian continent; fires in northern Australia occur during the dry season in ‘winter’ and ‘spring’, while in the southeast and southwest the fire season is in summer and autumn. Fire regimes vary across Australia. Fires are absent or exceptionally infrequent in rainforests, and of low intensity when they do occur unless the forest has been disturbed by cyclones, logging or frost. Fires in wet sclerophyll forests are infrequent but are often of spectacularly high intensity when they do occur; the average interval between any two fires in Mountain Ash (Eucalyptus regnans) forests in Victoria was estimated to be 37–75 years, and that of intense, tree-killing fires to be 75–150 years (McCarthy et al. 1999). Fires in temperate heathlands may typically occur at intervals of between seven and 30 years, whereas fires in tropical savanna woodlands and grasslands may occur every other year on average (see Table 1 for further examples). Without strong human interference, the intervals between fires vary widely in relation to the average. This variation can be important to the persistence of some species such as Leadbeater’s Possum (Mackey et al. 2002).
Contemporary Australian fire science and management use the concept of fire regime as the framework for understanding the complex relationships between fire and the Australian environment (Bradstock et al. 2002, Cary et al. 2003). The framework allows the development of both principles for managing fire to achieve conservation goals and of operational guidelines for ecologically sustainable fire management, informed by appropriate research studies (for example, the Jervis Bay Fire Response Study, or the Kapalga Experiment).