19.4.3  Fire: an ecosystem sculptor

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Rapid increases in charcoal deposition coinciding with sclerophylly are a conjunction of cause and effect. Many sclerophyllous species resist the devastating effects of fire through features such as epicormic shoots and fire-protected seeds (Sections 19.2 and 19.3). One outcome of fire regimes is therefore strong selection for sclerophylly. However, drought and herbivory in the absence of fire can also select for sclerophylly. Once sclerophylly is an established feature of a vegetation type, characterised in much of Australia by flammable dryland species such as eucalypts, the likelihood of fire (and therefore charcoal deposition) rises (Section 19.1).

Palynological evidence reveals the complexity of fire and climate change as selective joint forces for vegetation mix. For example, grasses at Lake George were abundant 250 000 to 300 000 years ago, when charcoal was uncommon, and returned to prominence 8000 years ago when fire had become common. While grasslands must have been burnt by occasional fires and therefore contributed to charcoal deposition, most charcoal is believed to have come from more intense fires as woodlands established in wetter periods were burnt. Thus, the return of grasses 8000 years ago cannot be easily ascribed to fire regimes — more likely, grasses were a secondary outcome of the shift from tall to low open-eucalypt forests allowing light to penetrate the canopy. Fire, therefore, probably provides a selective pressure for the evolution of characters that conspicuously confer tolerance to burning (e.g. fire-induced flowering, epicormic sprouting) but also alters landscapes through more general shifts in vegetation mosaics (e.g. from rainforest to sclerophyll forest).

While climate change is an overarching selective force in determining vegetation types, fire interacts with climate to affect a local flora profoundly over both short and long time periods (decades to millennia). It is therefore not surprising that fire has been used sporadically by humans to alter the landscape and hence food supply in Australasia over thousands of years. The subtlety of this manipulation is difficult to gauge because charcoal deposits reflect large intense fires but tell little about local ‘cool’ burns. None the less, data such as those from Lynch’s Crater show a rapid rise in charcoal deposition in the past 40 000 years, supporting the assertion that local groups of Aboriginal Australians burnt the vegetation consistently to suppress rainforest species and promote open woodland (Kershaw 1986). Over the past 7000 years, fire-sensitive rainforests of Araucaria have been restricted to locales that are protected from fire, allowing angiosperms to emerge as the dominant rainforest group. Imposed fire regimes are likely to have had the greatest impact in wetter regions of Australia (northeast Queensland and southwest Tasmania) where large natural fires are less common than in dry areas (Clark 1981).

During at least part of the 40 000 years since Aboriginal Australians arrived, local burning practices have altered vegetation types and prevented rainforest and tall open-woodland from encroaching on adjacent grasslands that sustained wildlife. In this way, mosaics representing several vegetation types were generated, maximising biological productivity. Eucalypt-dominated grasslands have not been an inevitable outcome of fire regimes imposed by humans, with charcoal evidence showing that under favourable climatic conditions woodlands of Casuarina spp. coexist with controlled burning (Clark 1981).

Fires lit naturally also generate mosaics and demonstrate graphically the interaction of distinct plant communities and fire regimes. Hummock grasslands in arid central Australia illustrate distinct mosaic patterns according to the period elapsed since the last fire was lit by lightning or human activity (Figure 19.11). At the other extreme, the large standing biomass of rainforest and eucalypt forest in tropical North Queensland also produces mosaics defined by sharp transitions between plant communities (Unwin 1989). These transitional zones generally reflect the extent of previous fires, with each subsequent fire re-establishing boundaries according to fire frequency and intensity.

On the Herberton Highland in Queensland, evidence of the dynamic nature of forest boundaries is seen in the distribution of large, old Eucalyptus grandis trees. Tall trees between 100 and 400 years old are found scattered along the margins of open eucalypt forest and occasionally within the adjacent rainforest. Because it is well documented that E. grandis seedlings can only establish in open grassy woodland after fire and cannot establish under rainforest, the presence of E. grandis tens to hundreds of metres within the young rainforest is evidence that rainforest has encroached on open woodland within the life of these trees.

Boundaries are still undergoing continual change through the influence of fire. As grasses are burnt up to the transitional zone, E. grandis and the dominant species of the open woodland, Eucalyptus intermedia, take hold. Where fires have not burnt to the rainforest margins for decades, rainforest species become established, leaving open woodland species surrounded by dense, moist canopies. The advance of rainforest in much of this region is a result of a temporary decline in frequency of fires.

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Figure 19.14 Range of radiocarbon dates recorded in soil charcoal at eight locations in northeast Queensland in relation to annual rainfall. Vertical bars show the range of time that charcoal was deposited beneath the forest, where it has remained. Charcoal deposited over a long time is found beneath today's rainforests that receive an annual rainfall of less than 2000 mm, indicating frequent presence of eucalypts for more than 20 000 years. Rainforest on these drier sites probably established during recent recolonisation. Wetter sites have supported rainforest for almost 10 000 years. (Based on Ecos, 1997; reproduced with permission of Ecos)

 

Carbon dating of charcoal deposits dug from beneath rainforest on the Windsor Tableland in North Queensland has been used to corroborate findings based on species distribution. Eucalypts burnt in the areas that now support lush rainforest left carbon deposits dating back 13 000 to 26 000 years. Apparently all but the wettest regions of rainforest have at some time been colonised by sclerophyllous species capable of supporting fires (Figure 19.14). Some rainforests in northeast Queensland receive less than 1800 mm annual rainfall and are unlikely to be more than 1400 years old. Similar encroachment of rainforests on eucalypt woodland has been observed in rainforests from northern New South Wales.

Many assemblages of plant species in the Australian landscape are the product of fires burning the vegetation throughout millennia, re-ordering the extent and abundance of species. During this time, climate change (Chapter 13) and human impact (Section 19.4) have altered fire regimes, resulting in long-term shifts in boundaries between plant communities and imposing selection pressure that has given rise to wide-spread, but not universal, fire tolerance in Australian flora.

Fire regimes have altered again since European settlement with the fragmentation of original plant communities as a result of grazing, agriculture and forestry. Fire has been used aggressively in the past 200 years of agriculture as a means of suppressing and eliminating native vegetation, including those species able to withstand cyclical fires caused by natural events like lightning. In contrast, burning has been restricted in native forests to maximise resource yields, causing shifts in vegetation mix and age through low fire frequencies and occasional high fire intensities.

 

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