Fire Ecology
Eucalypts appeared in the fossil record around 80 million years ago. With them were charcoal fragments, and indication of the long association of fire with Australian flora. Although there have been substantial changes in the vegetation associated with change in climate, the fossil record shows that whenever there was a change to a drier environment with associated sclerophyll vegetation, there is also an increase in the frequency of fire. For example, in the Lake George area of New South Wales, whenever there was a shift from plants favoured by a cool temperate climate to plants which were favoured by a dry arid climate (for example, the eucalypts), there was an increase in charcoal associated with the plant pollen. There was a substantial increase in the amount of charcoal present between 200,000 and 250,000 years before present which is well before Aborigines were considered to be within the area. Since then, the presence of natural ignition from lightning and the extensive burning practices of the Aborigines have influenced the development of our forests and heathlands and resulted in a natural mosaic of plant community ages throughout Australia.
Although wildfire is detrimental to our rural production and the built environment, it should be considered to be as much a part of the natural environmental of our forests and heathland ecosystems as the sun and the rain. Some Australian plants are stimulated to flower by the presence of fire or smoke, while others have seeds which require some heat treatment before they will germinate. There is a suite of specific plants and animals which are associated with each fire regime. Annual burning favours communities of grasses and herbs, which in turn are preferred by kangaroos and larger wallabies. The tall eucalypts of the mountain forests of Victoria and Tasmania have seeds which are so small that they will not survive in the deep litter beds that accumulate beneath them. These species require an intense fire under dry conditions to remove the accumulated organic matter on the forest floor so that the tiny eucalypt seedlings can grow and regenerate the forests. If these forests do not receive a fire within their life span, (which is about 250 to 350 years), they will die out and be replaced by temperate rainforest species. In between these two extremes are other plants which have the capacity to produce abundant shoots from stems or lignotubers after fire and so can withstand frequent and repeated firing at regular intervals.
Similarly, some species of our native fauna are favoured by burning at regular intervals, others prefer intermediate intervals between fire, while yet others require long periods without fire for their optimum development. When considering fire in relation to fauna, it is important not to confuse temporary impacts on individual animals with longer term trends which often have a net beneficial effect. During any fire some animals will perish. However, in the absence of fire, changes in vegetation may cause the decline and disappearance of some species. The interdependence of plants and animals with a particular fire frequency can be quite complex. For example, the small rat kangaroo or woylie of Western Australia has a diet which consists mainly of the underground fruiting bodies of a fungus. The fruiting of the fungus is related to the amount of nitrogen in the soil which is, in turn, related to the vigour of understorey plants which are capable of fixing nitrogen. Thus, the optimum conditions for this animal requires a fire frequency which regenerates the understorey shrubs (in the absence of fire the shrubs die out) and yet provides other areas where the shrubs are intact with shelter from predators.
Introduced animals have had a huge impact on the smaller native fauna whose interaction with fire is complicated further by the grazing of native vegetation by rabbits and the predation by foxes and feral cats. It is considered by many that these influences are having a much greater impact on our fauna than the changed frequency of fire.
