The Tasmanian legacy of man and fire

Abstract

The vegetation of Tasmania is complex and much of it is in a state of disclimax. At the time of European settlement, the proportion of non-forest open vegetation was 370/0, about 5% of this at high altitudes. In the present interglacial climate, in regions ofhigh rainfall, where rainforest dominance might be expected, approximately 45% carries sedgeland, grassland, shrub communities and wet sclerophyll forest. Similarly, drier areas carry extensive grassland, sedgeland and heath, instead of dry eucalypt forest. This complexity of distribution and disclimax can be attributed to fire disturbance. Fire not only produces a successional mosaic but, through Ecological Drift, causes extinction of communities. This level of displacement appears to demand a timespan of human-induced fire sufficiently long to affect soil fertility. A palaeontological record of the last five glacial cycles has been analysed from the Darwin Crater in western Tasmania and compared with that from the Chatham Rise, New Zealand. These show parallel behaviour in the proportions of forest and non-forest communities in the earlier cycles. However, the Tasmanian record shows a marked divergence during the Last Glacial cycle, with a twofold increase in open vegetation relative to closed forest. Eucalypt forest increases relative to rainforest, and charcoal increases relative to woody vegetation. These changes occur through a variety of climates, including full glacial and optimum interglacial, but are not apparent in the New Zealand core, making it difficult to attribute them to a climatic cause. In the Tasmanian vegetation, they can be explained by an increase in fire frequency, due to human activity. Since their onset occurs in isotopic oxygen stage 4 and continues in the differing climates thereafter, it may be inferred that the 14C dates of the earliest evidence of occupation by the Aborigines are gross underestimates. A date of about 70 000 yrs BP is more likely. It appears that, when using 14C methodology, such underestimation for dates beyond30 000 yrs BP is to be expected in palaeosamples from western Tasmania.