19.4.1  Introduction

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Figure 19.11 Mosaic of hummock grassland located about 300 km north-west of Alice Springs in the Tanami Desert of central Australia. Spinifex vegetation consisting of Trioldia pungens, T. bitextura and Acacia spp. is found across the site, with fires having formed distinct vegetation mosaics. The lightest central swathe burnt about a year earlier, leaving little remaining biomass. Darker zones to the right and left of this most recently burnt region were burnt about five and more than seven years earlier, respectively. (Photograph courtesy G. Griffin)

 

All floras, including those of Australia and New Zealand, are dynamic. Global climate change throughout prehistory has selected broadly for different vegetation admixtures and specifically for adaptive traits within individual species. While some genetic modifications to plants are considered highly specific responses to a particular environmental factor such as drought, poor nutrition or extremes of temperature, others probably reflect simultaneous changes in several environmental factors. For example, during the Last Glacial Maximum about 20 000 years ago, vegetation was exposed to drier, colder, windier atmospheres strongly depleted in CO2 (180 to 190 ppm) compared to the present day (c. 360 ppm; Chapter 13). Charcoal deposited during these glacial periods is evidence that woody plants had to survive fires as well as extremes in atmospheric conditions. These complex regimes bring into question claims that vegetation has become adapted to a single adverse factor.

Interspersed with major shifts in climate have been periods of changed fire frequency. Palynological evidence presented in Section 19.4.2 shows that fire has been an ‘ecosystem sculptor’ in the Australian landscape for most of the past 15 million years. On the Canterbury Plain of New Zealand’s South Island, by contrast, the Maori transformed Nothofagus and gymnosperm forests into fire-prone grasslands in only about 700 years through burning and cutting (Ogden et al. 1998). Such different time scales of exposure to fire have implications for the flora we see today throughout Australasia. Species of a number of major Australian plant families (e.g. Proteaceae, Myrtaceae) have genetically encoded characters that confer tolerance to the fire regimes described in Sections 19.2 and 19.3, as do plants from other fire-affected parts of the world. These species have evolved over millions of years when fire and climate change consorted to select Australia’s vegetation types. The gymnosperms that grew across the Canterbury Plain also have sclerophyllous leaves but are readily killed by fire, in contrast to the major Australian families. Several important vegetation types in Australia are also severely affected by fire (Figure 19.11). Alpine flora, rainforest species and chenopods from dry rangelands are suppressed or killed by burning.

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