Fire regime
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 and fire histories. Even when considered individually, aspects of the fire regime such as frequency can be more complex than they first appear, and therefore require a little more thought. For example, a given area may have a fire frequency of 4 fires in 10 years, but the actual intervals between fires can vary substantially. The table below shows how the four fires in the 10 year period could occur in consecutive years, or could be spaced at three year intervals. This timing interval between fires can vary enormously, and can have profound implications for biodiversity.
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
|---|---|---|---|---|---|---|---|---|---|
| X | X | X | X | ||||||
| X | X | X | X |
There have been several fire experiments in northern Australia designed to investigate different aspects of the fire regime. Although they do not represent the ‘real world’ they can provide researchers with very useful information about how these ecosystems work. Fire experiments, as with other biological experiments, generally alter one variable while keeping all others constant, and the fire regime applied is normally fixed (applied consistently) for the duration of the experiment.
The following ‘fact sheets’ describe savanna fire experiments, in which aspects of the fire regime were manipulated, and their impact on communities monitored. We have provided summaries of these studies for you at the following links: Munmulary (northern Australia) (pdf document), Kapalga (northern Australia) (pdf document) and Kruger National Park (South Africa) (pdf document).
Following from the Kapalga fire experiment, a new fire experiment has been established at the Territory Wildlife Park. This is designed to test the effect of different aspects of the fire regime, particularly fire frequency, on biodiversity and ecosystem processes. The different fire regimes used is shown in the table below.
| Fire regime on each plot | 2004 | 2005 | 2006 | 2007 | 2008 | 2009 | 2010 |
|---|---|---|---|---|---|---|---|
| June, annually | X | X | X | X | X | X | X |
| June, biennally | X | X | X | ||||
| June, triennially | X | X | X | ||||
| June, every 5 years | X | X | |||||
| October, triennially | X | X | X | ||||
| Unburnt |
You will notice that the experimental plots are burnt at different frequencies (every 1, 2, 3 and 5 years), and at different seasons (early and late dry season – June & October) and consequently the fires will vary in their intensity. The fires would typically be surface fires burning across the ground consuming grass, litter and fallen woody debris.
Fire Types
Surface fires: burn fuels at the ground surface (e.g. shrubs, grasses, fallen branches, litter). In northern Australia these types of fire are the most common.
Surface fire
Crown fires: burn through tree crowns and are invariably ignited by surface fires. These can move very fast. Spotting is important here. Some of the major forest fires in southern and eastern Australia have been crown fires. They seldom occur in northern Australia. Boreal forests are one of the habitats most prone to crown fires. In Australia some of the most severe fires in southern eucalypt forests can become crown fires.
Crown fire
Ground fires: burn subsurface organic fuels by smouldering combustion. They can be ignited by surface fires. Organic soils such as peat found in artic tundra, bogs and swamps are prone to these types of fires. There were very large ground fires in Indonesia in 1997 causing extensive air pollution and haze as far away as northern Australia. Have a look at the following web sites for more information about fires in Indonesia.
Fire Situation in Indonesia (FAO) and Fire Situation in Indonesia (IFFN).
Aspects of the fire regime such as season, frequency, intensity and fire type can vary with topography as well as with vegetation type and climatic zone.
Activity: Fire regimes
How do fire regimes vary across regions?
The difference in fire regimes between areas is illustrated in the following readings.
Compare the fire regimes in Litchfield National Park, Nitmiluk National Park and the Bradshaw Station. Make a note of how the regimes differ and suggest some reasons why this might be. Specifically, what aspects of the fire regime do climate, fuel load (biomass), and chance of ignition affect?
Readings:
Edwards, A., Hauser, P., Anderson, M., McCartney, J., Armstrong, M., Thackway, R., Allan, G., Hempel, C. & Russell-Smith, J. (2001) A tale of two parks: contemporary fire regimes of Litchfield and Nitmiluk National Parks, monsoonal northern Australia. International Journal of Wildland Fire 10: 79-89.
Yates C. & Russell-Smith J. (2003) Fire regimes and vegetation sensitivity analysis: an example from Bradshaw Station, monsoonal northern Australia. International Journal of Wildland Fire 12: 349-358.
Hot topic: Fire field experiments: pros and cons.
There has been much discussion as to the usefulness of, and contributions made by, fire experiments. Field fire experiments have been criticised for being too fixed, not simulating reality, and of being of little direct relevance to managers on the ground. They do however provide researchers with a more controlled way of studying direct effects. Once these have been determined information and implications of burning can be conveyed to managers.
Designing field experiments to investigate the effects of fires requires explicit consideration of the key components of experimental design, i.e. controls, replication, randomisation, and interspersion (Hulbert 1984), in order to avoid problems of scale, variability, replication and control that commonly plague biological experiments. Fire experiments can be very valuable because they allow more variables to be controlled for, can test specifics of fires regimes (e.g. relative importance of season or frequency of burn), and include controls.
The main criticisms of fire experiments relate to replication and plot size. Often, because of funding and logistical constraints, the scale at which fire experiments are undertaken tends to be small (e.g. fire plots of one to several hectares). Fire is a multi-scale process however, and results obtained from small-scale plots may not scale-up and translate to larger fires. This means that we cannot afford to exclude large-scale fires from our observations. The problem with large-scale extreme fire events is that for obvious reasons, they cannot be replicated. There is however increasing recognition now that alternative approaches, including the use of detailed observation, inference, and modelling, and the use of non-classical approaches to statistical analyses, may be used to differentiate between alternative hypotheses.
Finally, there has been some criticism of the relevance of findings to fire managers on the ground because of the possibly unrealistic nature of some of the regimes. The application of a uniform fire regime is quite often not possible or considered desirable (see Russell-Smith et al. 2003).
Activity: Fire field experiments
How can fire experiments assist researchers?
It is crucial to understand how fire experiments can contribute to our understanding of fire effects on biota and landscapes. This means we need to know the benefits and drawbacks of experiments, how they have assisted us, and what problems and issues researchers still need to address. The following two readings will give you a better understanding of fire experiments.
Reading:
Williams, R.J., Woinarski, J.C.Z. & Andersen, A.N. (2003) Fire experiments in northern Australia: contributions to ecological understanding and biodiversity conservation in tropical savannas. International Journal of Wildland Fire 12: 391-402.
Parr, C.L. & Chown, S.L. (2003) Burning issues in conservation: a critique of faunal fire research in southern Africa. Austral Ecology 28: 384-395.
