Flora of Australia
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The flora of Australia comprises a vast assemblage of plant species estimated to over 20,000 vascular and 14,000 non-vascular plants, 250,000 species of fungi and over 3,000 lichens. The flora has strong affinities with the flora of Gondwana, and below the family level has a highly endemic angiosperm flora whose diversity was shaped by the effects of continental drift and climate change since the Cretaceous. Prominent features of the Australian flora are adaptations to aridity and fire which include scleromorphy and serotiny. These adaptations are common in species from the large and well-known families Proteaceae (''Banksia''), Myrtaceae (''Eucalyptus'' - gum trees), and Fabaceae (''Acacia'' - wattle).
The arrival of humans around 50,000 years ago〔Rasmussen M, et al. 2011 An Aboriginal Australian genome reveals separate human dispersals into Asia. Science 334, 94–98.〕〔Josephine Flood (2004) Archaeology of the Dreamtime, J.B Publishing, Marleston p. 283 ISBN 1-876622-50-4〕 and settlement by Europeans from 1788, has had a significant impact on the flora. The use of fire-stick farming by Aboriginal people led to significant changes in the distribution of plant species over time, and the large-scale modification or destruction of vegetation for agriculture and urban development since 1788 has altered the composition of most terrestrial ecosystems, leading to the extinction of 61 plant species and endangering over 1000 more.
(詳細はGondwana, which also included South America, Africa, India and Antarctica. Most of the modern Australian flora had their origin in Gondwana during the Cretaceous when Australia was covered in subtropical rainforest. Australian ferns and gymnosperm bear strong resemblance to their Gondwanan ancestors,〔Page, C. N. and Clifford, H. T. 1981. Ecological biogeography of Australian conifers and ferns. In A. Keast ''Ecological Biogeography of Australia''. W. Junk〕 and prominent members of the early Gondwanan angiosperm flora such as the ''Nothofagus'', Myrtaceae and Proteaceae were also present in Australia.〔Dettmann, M. E. and Jarzen, D. M. 1990. The Antarctic/Australian rift valley: Late Cretaceous cradle of Northeastern Australasian relicts? ''Review of Palaeobotany and Palynology'' 65:131-144.〕
Gondwana began to break up 140 million years ago (MYA); 50 MYA during the Eocene Australia separated from Antarctica, and was relatively isolated until the collision of the Indo-Australian Plate with Asia in the Miocene era 5.3 MYA. As Australia drifted, local and global climate change had a significant and lasting effect: a circumpolar oceanic current developed, atmospheric circulation increased as Australia moved away from Antarctica, precipitation fell, there was a slow warming of the continent and arid conditions started to develop.〔Bowler, J. M. 1982. Age, origin and landform expression of aridity in Australia. In W. R. Barker, P. J. M. Greensdale. ''Evolution of the Flora and Fauna of Australia''. Australian Systematic Botany Society ISBN 0-909209-62-6〕 These conditions of geographic isolation and aridity led to the development of a more complex flora. From 25-10 MYA pollen records suggest the rapid radiation of species like ''Eucalyptus'', ''Casuarina'', ''Allocasuarina'', ''Banksia'' and the pea-flowered legumes, and the development of open forest; grasslands started to develop from the Eocene. Collision with the Eurasian Plate also led to additional South-east Asian and cosmopolitan elements entering the flora like the ''Lepidium'' and Chenopodioideae.〔Crisp, M. et al. 2004. Radiation of the Australian flora: what can comparisons of molecular phylogenies across multiple taxa tell us about the evolution of diversity in present-day communities? ''Philosophical Transactions of the Royal Society of London. Series B - Biological Sciences'' 359: 1551-1571〕
The development of aridity and the old and nutrient poor soils of the continent led to some unique adaptations in the Australian flora and evolutionary radiation of genera – like ''Acacia'' and ''Eucalyptus'' – that adapted to those conditions. Hard leaves with a thick outer layer, a condition known as scleromorphy, and C4 and CAM carbon fixation which reduce water loss during photosynthesis are two common adaptations in Australian arid-adapted dicot and monocot species respectively. Rising aridity also increased the frequency of fires in Australia. Fire is thought to have played a role in the development and distribution of fire-adapted species from the Late Pleistocene. An increase in charcoal in sediment around 38,000 years ago coincides with dates for the inhabitation of Australia by the Indigenous Australians and suggests that man-made fires, from practices like fire-stick farming, have played an important role in the establishment and maintenance of sclerophyll forest, especially on the east coast of Australia.〔Singh, G. et al. 1981. Quaternary vegetation and fire history in Australia. In A. M. Gill, R. A. Groves and I. R. Nobel. ''Fire and the Australian Biota.'' Australian Academy of Science, 23-54〕 Adaptations to fire include lignotubers and epicormic buds in ''Eucalyptus'' and ''Banksia'' species that allow fast regeneration following fire. Some genera also exhibit serotiny, the release of seed only in response to heat and/or smoke. ''Xanthorrhoea'' grass trees and some species of orchids only flower after fire.〔Gill, A. M. 1981. Adaptive responses of Australian vascular plant species to fire. In A. M. Gill, R. H. Groves, and I. R. Noble. eds. ''Fire and the Australian Biota''. Australian Academy of Science〕
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