Living in a land of fire

Intergrative commentary
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

Environmental effects of fire

The environmental effects of fire regimes are determined by the life-history and demography of organisms, the landscape context of the fire, the fuel environment, and fire risk reduction and suppression activities.

Life-history and demography

Not all plants respond the same way when burned by a bushfire. Some die, others resprout, and others still appear little affected. Some of these differences are due to the individual species, some are due to the characteristics of the fire, and some may be due to the particular environmental conditions before and after the fire.

Some species (such as some Xanthorrhoea species) flower abundantly soon after fire, while others (such as Eucalyptus delegatensis) may take more than a decade to produce their first seed. Some plant species appear on burnt ground as if from nowhere because they were not obvious before the fire. They have regenerated from the germination of seed buried in the ground, stimulated by heat or smoke or the reduced competition following fire. This phenomenon is particularly evident in the sandy deserts of Australia.

Some woody plants of heaths and forests are killed by fires even if they are only just intense enough to scorch the foliage, let alone defoliate the canopy. Many of these species bear canopy-borne cones or capsules, which open and release their seeds after fire. Such species include the common Banksia ericifolia around Sydney, Banksia ornata in South Australia, Eucalyptus regnans in Victoria and Tasmania, and Callitris species in many parts of the country. Species such as these are of special management interest because they are vulnerable to short intervals between fires, and can be easily driven to local extinction.

Animal species also behave differently in the face of fires. Koalas are an example of a species that is inevitably exposed to fire, but which can survive if fires are of insufficient intensity to kill foliage. More mobile species may find refuge in burrows. Some, like the iconic Frill-necked Lizard of the tropics may have high mortality rates in a high intensity fire, but nevertheless sustain higher populations after fire because of rapid reproduction or migration from adjacent areas (see Corbett et al. 2003), perhaps because habitat quality improves as a result of fire. Indeed, the effects of fires on mammalian habitats may generally be more important for the survival of the species than the direct effects on the animals themselves (Friend and Wayne 2003).

Fires occur at various scales and with variable patchiness (Gill et al. 2003) and different plants and animals may respond to these differently. Such topics are complex and much is to be learned about the effects of patchiness in single fire events and its flow-on effects through fire regimes.

Table 1: Suggested fire intervals for vegetation types in Southeast Queensland
Vegetation type Fire intervals in years
Rainforest Fire exclusion
Wet sclerophyll forest 20–100+
Grassy dry sclerophyll forest and woodlands 3–6
Shrubby dry sclerophyll forest and woodlands 7–25
Coastal heathlands 7–20
Inland (rocky) heathlands 15–50
Paperbark (Melaleuca quinquenervia) woodlands 15–30

Source: Fisk et al (2003, Table 7.5)

Landscape

The landscape context for fire is important at a range of scales, from the broad gradients in fire seasonality and occurrence, such as those from north to south across Australia, to the much finer-scale variation of topography, vegetation and fire histories in particular landscapes (for example, Allan and Southgate 2002).

The character and composition of landscapes—particularly their topography, their ecosystems and the extent to which they are fragmented and modified, and the forms of land use and management—have significant implications for fire occurrence, behaviour and impacts. Conversely, the same fire can have differential impacts on different parts of the landscape. These effects may be benign for biodiversity in conservation reserves, adverse for primary production systems, and problematic for water quality.

Fuel environment

Each of Australia’s thousands of ecosystems has particular fuel characteristics, in terms of quantity, distribution, persistence and flammability. Because fuel and ignition are the significant determinants of fire behaviour that people can modify (the others are weather and topography), fuel management—for example, through fuel reduction burning or physical removal—is a central activity in fire management by both Indigenous and non-Indigenous Australians.

The introduction of exotic plants and animals, and changes in populations of native herbivores, can have major consequences for the fuel environment. For example, when a highly productive grass such as Gamba Grass is introduced in northern Australia, higher fuel loads result. This is in part because previously disconnected or poorly connected fuels become more contiguous if the new species fills a habitat gap; this is also the case with Buffel Grass in the arid and semi-arid zones, and with some Mediterranean-climate grasses (such as Veld Grass) in southern Australia. Conversely, the introduction of domestic livestock and of browsing pests such as rabbits, or the increased populations of native herbivores such as kangaroos associated with particular land management practices, may diminish fuel loads in some environments through grazing pressure.

Fire risk reduction and suppression activities

Fire risk reduction and suppression activities have the potential to change the environment directly, and indirectly through altering fire regimes. Fuel modification is fundamental component of fire risk reduction, as discussed above and by Ellis et al. (2004). Because fuel reduction burning is the only feasible means of fuel reduction on a landscape scale, it is widely used throughout Australia. Understanding the implications of fuel reduction burning for biodiversity, and developing burning regimes that strike the appropriate balance between fire risk reduction and biodiversity conservation, is a major challenge for land managers (see, for example, Luke and McArthur 1978, Whelan 1995, McCarthy et al. 1999, Bradstock et al. 2002, Abbott and Burrows 2003, Andersen et al. 2003, Cary et al. 2003, Esplin et al. 2003, Ellis et al. 2004).

Fire suppression activities may involve backburning, construction and use of fire trails, and the use of chemical retardants. Each of these can have adverse environmental impacts, although these can be minimised with good planning and management (see, for example, Esplin et al. 2003).

In conclusion, the impact of a particular fire event on the environment depends principally on its context within the fire regime under which a particular ecosystem has evolved. Research will help all Australians better understand how to better manage fire to achieve positive environmental outcomes with minimal impacts on life, health, property and other private and community assets.