[Front Page] [Features] [Departments] [SGAP Home Page] [Subscribe]
The Impact of Fire: An Historical Perspective
Climatic change and the drying out of the Australian environment resulted in the decline of rainforests and the dominance of sclerophyllous vegetation. Fire has been a major component of this process, promoting those plants which could cope with fire at the expense of those which could not. Aboriginal people added to the firing frequency, but maintained low population densities until the last 5,000 years, when fire became an integral component of their economic system. They used fire as a tool to promote and maintain the vegetation associations which were most productive. Once traditional Aboriginal burning regimes ceased following European settlement, the standing fuel increased, resulting in sporadic but more intense fires.
Geological History of Australian Vegetation
To understand the impact of fire on Australian vegetation, it is first necessary to understand the geological history of Australia. Until about 200 million years ago, all of the major continents were held together, locked up in a supercontinent known as Pangaea. This broke into two groups, one in the north, called Laurasia, and one in the south, Gondwana. We know that this supercontinent, Gondwana, consisting of Africa, South America, Australia, Antarctica, and India, existed until around 190 million years ago, when the various components broke up and began to drift in different directions. India drifted northwards, and ran into Asia about 50 million years ago. About the same time, New Zealand separated from Australia.
The post-Gondwana history of Australia is one of relative stability. At it drifted northward into the Pacific, few mountain ranges were built, and those which were created were relatively low. There was not a great deal of tectonic activity, and as a result, Australia eroded and became a relatively flat continent. Where the rainfall was relatively constant, the surface weathered, and the soils became acidic. Laterisation bound up nutrients like phosphorus, impoverishing the soil, and removing some of the trace elements which were essential for many plant species. New landscapes arose only through erosion of old ones. One of the consequences of poor soil fertility was that the vegetation which found itself on the Australian continent diversified, and filled the niches created by poor quality soils. There are more plant species growing on the Hawkesbury sandstone around Sydney than grow in many European countries.
The old Gondwana forests had been dominated by gymnosperms - the conifers, auracaria and podocarps, but just as Gondwana broke up, the angiosperms began to radiate. Because of the large size and climatic diversity which existed within Gondwana, the vegetation associations were varied, and when Australia separated it took with it enough angiosperms to expand and almost fill the entire continent. Nothofagus, the antarctic beech, was one of the early angiosperms which was present on Gondwana when Australia became isolated. Minor families included the Myrtaceae, the grasses, the Xanthorrhoeas, and the chenopods (Pyne, 1991). A few of the genera which were present and which later became important included Eucalyptus, Banksia, Hakea and Melaleuca. A similar vegetation association characterised parts of Antarctica, South America and New Zealand. Where the rainfall was year round and moderately high, rainforests were maintained, but the minor flora adapted and speciated, filling many of the niches in the drier areas.
|"One of the consequences of poor soil fertility was that the vegetation which found itself on the Australian continent diversified...."|
The slow movement to the north and the consequent climatic fluctuations broke up the rainforests into smaller patches. The Australian continent began to dry out. Australian aridity became seasonal, episodic and chronic (ibid:4). It was entrenched in the north as part of the annual wet and dry season. In some places, there would be drought extending over several years. The moisture-loving vegetation became crowded along the coastal fringe, with the Great Dividing Range acting as a barrier to water movement in the atmosphere. To the east of the ranges the annual rainfall was high; to the west it was low. This process continued during the Pleistocene, and the result was that some of the previously minor components of the Australian vegetation became dominant over much of the country. Those plants which could adapt to dry conditions, the scleromorphs, flourished. These were the plants with small tough leathery leaves, which reduced water loss, and allowed the plant to conserve nutrients as well as water.
The Nothofagus and Podocarpus rainforests contracted to the moist cool southeast. Casuarinas replaced auracarias. Grasses replaced ferns in the understorey, forests were replaced by woodlands, dominated by eucalypts and Acacia. It had been the climatic changes, the increasing aridity, which resulted in a massive radiation in the scleromorphs, creating the broad pattern of Australian vegetation much as we know it today.
Aboriginal Use of Fire
By 38,000 years ago the Auracarian rainforests had just about all disappeared, and Casuarina was beginning to be replaced by eucalypts. Around this time, there was a massive increase in the amount of charcoal deposited in lakes and swamps. Pollen cores in several sites across Australia, but most notably Lynches Crater on the Atherton Tableland, clearly show this abrupt change (Kershaw, 1986). The increase in charcoal probably came about because the new sclerophyll vegetation, under dry conditions, provided an abundant source of fuel. Fires to ignite that fuel could occur naturally or could be initiated by Aboriginal people.
Australian vegetation had evolved along with fire. Lightning strikes start fires, and in parts of tropical Australia the vast majority of fires are started this way (Pyne, 1991). Fire results in the rapid recycling of nutrients, an important factor in many of the poor quality soils in Australia. Australian plants respond to fire in different ways. Some have developed the capacity to reproduce vegetatively rather than by seed production. Others, like the Banksias and Hakeas, are fire-dependant, and require fire in order to release their seeds. The fact that the Australian vegetation evolved along with natural fires is generally accepted. What is not so clear is to what extent did anthropogenic fires determine the patterns of vegetation which Europeans found on their arrival in 1788, and to what extent did the cessation of Aboriginal burning influence the subsequent changes in the environment (Dodson, 1992).
Epicormic shoots develop from the stem of a fire-tolerent Eucalyptus species. This is one sort of adaptation for survival in environments which experience fire. Select the thumbnail image or plant name for a higher resolution image (38k).
The large, woody seed pods of Hakea bakeriana contain only two seeds. The pods remain closed on the plant until they are stimulated to open by the death of the plant, usually through fire. Select the thumbnail image or plant name for a higher resolution image (40k).
It was Rhys Jones in 1969 who first proposed the idea of "firestick farming", the implication being that Aboriginal people unintentionally and intentionally modified the environment by the use of fire (Jones, 1969:224). Certainly Aborigines had been observed using fire to burn large tracts of land since the first European settlements, and it was clear that fire was an important tool to Aborigines right across Australia. However, Jones was the one of the first to suggest that this burning was controlled or directed. He saw fire as an important tool in increasing the productivity of the land, by replacing mature forests with open woodlands and grasslands.
Based on knowledge of the use of fire by traditional Aboriginal communities, who see burning the landscape as "cleaning up the country", and to a lesser extent on ethnohistorical accounts, it is clear that fire was used by Aboriginal people primarily while they were hunting (Kimber, 1983; Nicholson, 1981). However, there were other beneficial consequences. Fire would recycle nutrients and promote new growth, which would subsequently attract herbivores. Many plants were favoured by regular low intensity burning. A burning pattern consisting of frequent, low intensity fires removes the woody understorey, and allows many of the grasses, orchids and lilies to flourish. Orchids and lilies often possess underground storage organs or tubers, which were eaten by the Aborigines (Cribb and Cribb, 1975). Studies of the burrawang, Macrozamia communis, in southern NSW showed that fire could increase the productivity of cones 2-3 times, and although the seeds of this plant are poisonous, Aboriginal people had developed the technology to eradicate the toxins by treatment with heat and running water, and Macrozamia seeds were an important food source (Beaton, 1982).
The large fruiting cones of Macrozamia communis break apart when mature to shed their large orange seeds. Select the thumbnail image or plant name for a higher resolution image (40k).
By the time these changes in charcoal deposition were taking place, Aboriginal people were well established in Australia. Their stone tools have been found in rock shelters in Arnhem Land dating back 50,000 years or more (Roberts et al, 1990). They had reached as far south as Perth by 40,000 years ago, and were almost certainly spread right around the coastal margins by this time. Whether the cause of the fires which produced the charcoal was natural or man-made is still a matter of debate.
Some researchers like David Horton from the Australian Institute of Aboriginal and Torres Strait Islander Studies, suggest that "Aboriginal use of fire had little impact on the environment and ... the patterns of distribution of plants and animals which obtained 200 years ago would have been essentially the same whether or not Aborigines had previously been living here." (Horton, 1982). Horton probably underestimates the impact that Aboriginal people have had on Australian environment. Aboriginal burning generally took place at appropriate times of the year, and when weather conditions were right. This ensured that there was a low intensity burn, and therefore little danger of a crown fire developing. Traditional burning patterns maximised the species diversity in any particular area, because burning tended to leave a mosaic of vegetation which had been burned at different times.
This regular firing favoured not only fire-tolerant or fire-resistant plants, but also encouraged those animals which were favoured by more open country. On this basis, it is clear that Aboriginal burning, in many areas at least, did impact on the "natural" ecosystem, producing a range of vegetation associations which would maximise productivity in terms of the food requirements of the Aborigines. Jones goes so far as to say that "through firing over thousands of years, Aboriginal man has managed to extend his natural habitat zone" (Jones, 1969).
Aboriginal burning has been blamed for a variety of environmental changes, not the least of which is the extinction of the Australian megafauna, a diverse range of large animals which populated Pleistocene Australia. Kershaw (1986), among others, has argued that Aboriginal burning may well have modified the vegetation to the extent that the food resources of the megafauna were diminished, and as a consequence the largely herbivorous megafauna became extinct. Indeed, Kershaw is one of a small but growing group of palynologists who suggest that the arrival of Aborigines may have occurred more than 100,000 years ago, fire-stick in hand, eager to burn to the virgin landscape. He suggests that their burning caused the sequences of vegetation changes which he detects through the late Pleistocene. The first to propose such an early arrival for Aborigines was Gurdip Singh from the Australian National University, who found evidence in his pollen cores from Lake George indicating that Aborigines began burning in the lake catchment around 120,000 years ago (Singh and Geisler, 1985).
|"Clearly, Aboriginal people had some impact, but the significance of that impact is far from clear."|
Flannery (1990) believes that the megafauna were hunted to extinction by Aborigines soon after they arrived. He argues that with the rapid extinction of the megafauna, virtually all of which were herbivorous, a great deal of vegetation was left uneaten, increasing the standing crop of fuel. As a consequence, fires became larger and hotter than before, causing the reduction of fire-sensitive plants to the advantage of those which were fire-resistant or indeed fire-dependant. Flannery suggests that Aborigines then began to burn more frequently in order to maintain a high species diversity and to reduce the impact of high intensity fires on medium-sized animals and perhaps some plants. He argues that twentieth century Australian mammal extinctions are largely the result of the cessation of Aboriginal "firestick farming".
Most of these theories implicates Aboriginal use of fire as a component of the changes to both plant and animal communities within Australia during the last 50,000 years. Clearly, Aboriginal people had some impact, but the significance of that impact is far from clear.
It seems likely that the introduction of the intensive use of fire as a tool did indeed follow, but was not directly a consequence of, the extinction of the megafauna. If, as has been suggested, the megafauna remained in some areas until the Holocene, then we should be looking for evidence within the last 10,000 years for changes induced by new Aboriginal burning patterns (Wright, 1986).
The other factor which few of these researchers have considered is the likelihood that Aboriginal population density increased rapidly and dramatically over the last 5-10,000 years (Mulvaney and White, 1987). An increase in Aboriginal burning frequency may have been associated with the introduction or invention of new technologies which allowed Aboriginal people to concentrate on those large resources which were previously so difficult to capture - kangaroos and large wallabies. Fire was initially used to promote and retain the environments which were most suitable for these animals, and fire was subsequently used for maximising the productivity of these areas after the massive Aboriginal population increase which occurred during the late Holocene, probably because of the greater access to this abundant resource.
The stone technology which Aboriginal people had been using with little modification for over 40,000 years diversified and specialised in the last 5,000 years. Spear barbs and tips peaked about 2,000 years ago, and then completely disappeared from the archaeological record in southeastern Australia. They were replaced by technologies associated with the exploitation of smaller animals - shell fish hooks and bone points along the coast for fishing, axes for hunting possums across the woodlands, and adzes for sharpening digging sticks along the banks of the larger rivers where the yams were abundant. The intensive and regular use of fire was an essential component of this late Holocene shift in resource base (Kohen, 1986). The evidence suggests that Aboriginal burning may well have had an impact on Australian vegetation, but that by far the greatest impact has occurred over the last 5,000 years.
European Fire Regimes
When Europeans first settled in Australia in 1788, they found a landscape dominated by eucalypts. Certainly there were some areas of dense vegetation which contained a greater diversity of trees. However, the dominant feature of the landscape was the ubiquitous gum tree. When expeditions began exploring the countryside around Sydney, they encountered a range of vegetation associations very different to those which we see in the National Parks around Sydney today. On soils derived from Hawkesbury sandstone, Wianamatta shale, Tertiary alluvial deposits, and igneous intrusions, they found environments which reminded them of the manicured parks of England, with trees well spaced and a grassy understorey. Peter Cunningham (1827) described the country west of Parramatta and Liverpool as "a fine timbered country, perfectly clear of bush, through which you might, generally speaking, drive a gig in all directions, without any impediment in the shape of rocks, scrubs, or close forest". This confirmed earlier accounts by Governor Phillip, who suggested that the trees were "growing at a distance of some twenty to forty feet from each other, and in general entirely free from brushwood ..." (Phillip, 1789). It is clear that it was primarily Aboriginal burning practices which maintained an open environment dominated by well spaced trees and grass. Once the Aborigines stopped burning, the underbrush returned where none had previously existed. Benson and Howell (1990: 20) suggest that the growth of Bursaria spinosa in the Sydney area in the 1820s may be related to a changed fire regime, the cessation of Aboriginal burning.
|"While Aboriginal people used fire as a tool for increasing the productivity of their environment, Europeans saw fire as a threat."|
While Aboriginal people used fire as a tool for increasing the productivity of their environment, Europeans saw fire as a threat. Without regular low intensity burning, leaf litter accumulates, and crown fires can result, destroying everything in their path. European settlers feared fire, for it could destroy their houses, their crops, and it could destroy them. Yet the environment which was so attractive to them was created by fire. Indeed, it has been suggested that the European settlement of Tasmania followed almost exactly those areas which the Tasmanian Aborigines had regularly burned (Pyne, 1991).
As European settlement spread out from Sydney, traditional Aboriginal burning practices ceased. Once this happened, vegetation associations changes, animals which were once common rapidly declined, and in some cases disappeared altogether. In the more remote areas, this process took longer. In western NSW it happened in the 1840s and 1850s. In parts of Central Australia, the extinctions and declines still continue, although other factors are now involved. However, it can be argued that many of these changes are the result of changed fire regimes. Certainly some of the extinctions of the smaller terrestrial mammals in arid Australia occurred long before the introduction of competitors such as the rabbit and predators like the fox and cat.
The problem for conservationists is whether to burn or not to burn. Regular low intensity burning will promote certain species and vegetation associations at the expense of others. Perhaps the last word should come from Rhys Jones (1969), who recognised the problem almost a quarter of a century ago, when he said:
"What do we want to conserve, the environment as it was in 1788, or do we yearn for an environment without man, as it might have been 30,000 or more years ago? If the former, then we must do what the Aborigines did and burn at regular intervals under controlled conditions".
- Beaton, J.M. 1982. Fire and water: aspects of Australian Aboriginal management of cycads. Archaeology in Oceania 17(1): 51-58.
- Benson, D. and J. Howell. 1990. Taken for granted. The bushland of Sydney and its suburbs. Kangaroo Press in association with the Royal Botanic Gardens, Sydney.
- Cribb, A.B. and J.W. Cribb. 1975. Wild food in Australia. Collins, Sydney.
- Cunningham, P. 1827. Two Years in New South Wales. London.
- Dodson, J. (ed). 1992. The Naive Lands. Prehistory and environmental change in Australia and the Southwest Pacific. Longman Cheshire.
- Horton, D. 1982. The burning question: Aborigines, fire and Australian ecosystems. Mankind 13(3): 237-251.
- Jones, R. Firestick farming. Australian Natural History 16: 224-228.
- Kershaw, P. 1986. The last two glacial-interglacial cycles from Northeastern Australia: implications for climatic change and Aboriginal burning. Nature 322: 47-9.
- Kimber, R. 1983. Black Lightning: Aborigines and fire in Central Australia and the Western Desert. Archaeology in Oceania 18(1): 38-45.
- Kohen, J.L. 1986. Prehistoric settlement in the Western Cumberland Plain: resources, environment and technology. Unpublished PhD thesis, Macquarie University.
- Mulvaney, D.J. and J.P. White (eds). Australians to 1788. Fairfax, Syme and Weldon, Sydney.
- Nicholson, P.H. 1981. Fire and the Australian Aborigine - an enigma. In A.M. Gill, R.H. Groves and I.R. Noble (eds). Fire and the Australian Biota. Australian Academy of Sciences, Canberra.
- Phillip, A. 1789. The voyage of Governor Phillip to Botany Bay. London.
- Pyne, S.J. 1991. Burning Bush. A fire history of Australia. Henry Holt and Company, New York.
- Roberts, R.G., R. Jones and M.A. Smith. 1990. Thermoluminescence dating of a 50,000 year old human occupation site in Northern Australia. Nature 345: 153-6.
- Singh, G. and E. A. Geisler. 1985. Late Cainozoic history of fire, lake levels and climate at Lake George, New South Wales, Australia. Philosophical Transactions, Royal Society of London 311: 379-447.
- Wright, R. 1986. New light on the extinction of the Australian megafauna. Proceedings of the Linnaean Society of NSW 109: 1-9.
This article is a reproduction of a paper presented by Dr Jim Kohen at the SGAP 17th Biennial Seminar, Robert Menzies College, Sydney, 27 September to 1 October 1993. Jim is a lecturer in the School of Biological Sciences at Macquarie University, Sydney.
[Front Page] [Features] [Departments] [SGAP Home Page] [Subscribe]
Australian Plants online - September 1996
The Society for Growing Australian Plants