Last modified Thurs 30th January 1997
Australasia 18,000 14C years ago
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18,000 years ago (LGM)
Distribution of sites used towards reconstructing the LGM palaeovegetation distribution for Australia - 18,000-12,000 14C ya
Sea level and ice extent. Shorelines are drawn at the -150m bathymetric contour. Sea level was in fact probably only 120 - 130 m lower, but would roughly correspond to the -150m contour. A large shelf area (the Sahul Shelf) was present between New Guinea and Australasia, and Tasmania was connected to the Australian mainland. A large lake ('Lake Carpentaria') is thought to have existed on the Sunda shelf area at around the LGM, and its extent here is taken from the map in the thesis of van der Kaars (1990). The two islands of New Zealand were also linked together by the lowering of sea level.
Ice caps existed in the uplands of Tasmania and the SE Australian highlands, and there was a large ice sheet covering the western half of New Zealand, flowing down onto the lowlands (Denton & Hughes 1981).
New Guinea. At the LGM, windblown pollen in cores from the adjacent seas (several pollen bearing cores have been obtained to the west of New Guinea) suggests a considerably drier-than-present climate over most of New Guinea (van der Kaars 1990). Although rainforest is suggested as having persisted along most of the northern-western part of New Guinea, in the south and east the forest seems likely to have been replaced by woodland. On the bridging land that linked New Guinea to Australia at the LGM, savanna with some woodland or deciduous forest seems to have been predominant. This general picture is taken mainly from a map in the thesis of van der Kaars (1990), and various discussions with van der Kaars (pers. comm. 1991, 1992). The descriptions of the vegetation on this land bridge differ between different authors; van der Kaars regards it as being an open savanna-woodland or shrubland cover.
From pollen cores taken in the New Guinea highlands, there appears to have been considerable depression of tree lines and altitudinal zones. At present, lowland rainforest extends up to 1000m with the treeline occurring about 3500-3900m (Kershaw 1988). At the LGM, the treeline seems to have been depressed by as much as 1800m (to around 2100m), and marked by an abrupt change from closed montane forest to no-present-analogue shrub-rich grasslands (Hope 1987). At 3000m this gave way to alpine desert, with ice above 3,500m.
Australia Much drier, with extensive desert, especially in the south. Pollen data from this region are mainly from what are at present cooler and moister areas (especially the extreme southeast), with virtually no fossil information from the interior of the Australian continent. Relative to the present, LGM conditions appear to have been considerably drier and cooler throughout Australia. For example, Hope (1987) and Luly (1993) have suggested that Australia's forests were reduced by around 85% at the LGM. From pollen evidence at Lynch's Crater in Queensland (Hope 1987), the tropical rainforest areas that presently exist in northern Australia seem to have been reduced almost to vanishing point at the LGM, with their place being taken by open savanna-woodland (dominated by Eucalyptus and Casuarinaceae) and shrubland vegetation. The amount by which temperatures were lowered in this region is uncertain. There are indications of vigorous plunge-pool activity at a waterfall site on the NE Coast (Kakadu National Park, N. Territory) at around the LGM, suggesting in contrast that there was increased rainfall relative to the present in this particular area (Nott & Price 1994). However, the very large amount of background evidence for aridity from the region suggests that this site is probably not an accurate reflection of climate history.
The pattern of LGM forest loss is repeated elsewhere in the cooler regions of temperate Australia. In map reconstructions, a small area of forest is generally shown as persisting in the presently moister extreme south-west corner of Australia (e.g. Markgraf et al., 1992). Map reconstructions also generally suggest that a thin and broken band of temperate/subtropical forest or woodland persisted along the eastern and south-eastern coast of Australia (Dodson et al. 1988; Dodson pers. comm. 1992); some coastal pollen sites indicate forest and others do not. However, recently compiled evidence (Thom et al. 1994) of widespread though scattered LGM mobility of inland (rather than purely coastal) dunes under eastward blowing winds (dated by radiocarbon and by stratigraphic correlation) has been suggested as indicating that in fact the woody vegetation was at most confined only to localised favourable microsites (e.g. river valleys), and that there would have been a broken cover of grass or scrub along most of the east coast (Thom et al. 1994). The flux of windbourne dust into the Tasman Sea was also much greater at the LGM, suggesting much drier and/or windier conditions than at present (Hesse 1994). A hiatus in soil humate accumulation at the LGM in the Port Stephens area (eastern coast, approx 30S) is also taken to indicate drier conditions at the LGM (Thom et al. 1994). Thus, it may well be that the maps which I have presented here show too much forest cover in eastern and south-eastern Australia at the LGM.
A dramatic expansion of the central Australian arid region is also suggested from other sources of information such as dunes (see below), slope deposits and alluvial fans (Hope 1987, Nanson et al. 1992). There is uncertainty as to how much cooler the conditions actually were at the LGM, as no unequivocal sources of evidence for temperature change have so far been found (Markgraf et al., 1992). Lake level evidence at sites across southern Australia likewise agrees with the picture of aridity everywhere (Harrison & Dodson 1993).
Spread of dune activity. The reconstruction of a greatly expanded central desert region is based on sand dune and dust deposits, and the directions from which they seem to have blown; this information was obtained from Bowler (1976), Hope (1987), and also from McTainish (1989) and Allen (1990). On the basis of palaeo-dune distributions and TL U/Th dating, Nanson et al. (1992) conclude that dune building and mobility spread out from the centre of Australia after the end of the last interglacial, and reached its greatest intensity at the LGM. Dunes appear to have reached into areas that have very moist climates at present, for example N.E. Tasmania (see below) (McTainish 1976, Sarnthein 1982).
Arid steppe in Tasmania. In Tasmania, pollen evidence shows that areas now mainly covered by temperate evergreen forest were a semi-arid steppe, rich in chenopods, during the LGM (Markgraf et al., 1992). This vegetation would have extended across the land bridge that at this time connected Tasmania to the mainland. The occurrence at several lowland sites there are inland dunes stratigraphically dated to the last glacial. This seems to indicate that many areas in Tasmania, and the exposed land bridges across the intervening strait at the LGM, would have been sparsely vegetated (Thom et al. 1994). The uplands of Tasmania seem to have had a mixture of herbaceous and alpine scrub vegetation, and ice and rock desert seem to have prevailed above 1800m (above present sea level). At the LGM, lowland temperatures seem to have been around 6°C lower than today (Colhoun et al. 1982, Markgraf et al. 1992). In contrast to the other indications of aridity, lake level evidence from a single site gives a picture of no major change from the present (Harrison & Dodson 1993).
New estimates of temperature lowering.A new study adds to the general picture that glacial-age temperatures across Australia were much lower than present, in contrast to the earlier published CLIMAP marine plankton evidence from the region, which suggested a relatively minor cooling. The latest evidence comes from a study of amino acid racematization rates (i.e. the rate of 'flipping' of optically unequal molecules) in emu eggshells around the shores of an internally-draining desert lake (Lake Eyre). During the period between 45 and 16 'real' years ago, mean annual temperatures appear to have remained at least 9 degrees Celsius lower than at present (Miller et al. 1997; Nature v.385 p.241-244). This evidence may be taken to indicate the extent to which the glacial-age climate system was interlinked through such factors as atmospheric haze and dust, lower greenhouse gas concentrations, and ice-albedo feedbacks; even a continental interior region relatively isolated from the oceans was strongly affected by such global-scale factors.
Various published maps of LGM vegetation. There have already been several attempts to map the LGM vegetation of the Australian continent based on scattered pollen and sedomentological evidence. Here, the LGM vegetation reconstruction for northern Australia, north of 20S, is derived from maps in the doctoral thesis of W.A. Van der Kaars (University of Amsterdam, 1990), and from maps of G. Hope (pers. comm. 1990). For Australia south of 20S, the maps are derived from those of G. Hope and also from J. Dodson (March 1990), a map published by Dodson et al. (1988), and a map for the whole of Australia published in Markgraf et al. (1992).
The map reconstruction shown here is thus mainly a copy and interpretation of these previously published map interpretations of data, with little subsequent attempt to consult the primary literature on which these pre-existing maps were based (it is assumed that those who produced the maps have themselves made a competant effort on the basis of their own knowledge of the evidence and the region concerned). Note that those who work in this area emphasize that a considerable element of conjecture still remains in interpreting the actual distribution and extent of forest cover at the LGM (Dodson pers. comm. 1991, Hope pers. comm. 1991), although they do all agree that it was certainly much reduced relative to the present. For the land bridge linking New Guinea and northern Australia, we have (tentatively) suggested here a predominantly savanna-type vegetation (this choice has been checked with van der Kaars and with Dodson), adding a woodland belt along the coasts where the rain-bearing winds might have penetrated some way inland. As stated above, we have conservatively included a south-eastern temperate forest area for the Australian mainland.
Humans burning vegetation. There is evidence that humans may have been exerting a strong influence in producing the observed vegetation patterns in Australia since well before the LGM. The earliest Europeans to encounter aborigonal populations of Australia reported that they deliberately and extensively used fire in hunting. A large increase in charcoal flux from savanna regions occurs around 120,000 years ago, perhaps coinciding with the arrival of the first humans in Australia (P. Kershaw, pers. comm. 1992), although this would have been a high sea level phase and the earliest unequivocal archaeological evidence of humans is around 40,000-50,000 years ago (Kershaw et al. 1991). It is thought that the arrival of humans may have changed the overall vegetation of the region by burning, producing a different structure and composition (e.g. greater abundance of eucalypts) in many parts of Australia for the LGM and Holocene. However, we are concerned here with differences from the present, so in this sense the human influence throughout may have been relatively constant for each vegetation type, rather than a producer of the observed changes.
New Zealand. Dry and mainly treeless. The abundant pollen evidence for most of New Zealand, which has already been summarised by McGlone (1988) and McGlone et al. (1993), indicates that much of the area was covered with varying proportions of scrub and grassland. This was probably largely due to aridity; lake basins on North Island had lower water levels or were dry at the LGM, and loess from the dry uplands was deposited over the eastern half of the South Island and the southern half of North Island. There were also extensive sand dunes in the SW of North Island.
From the pollen evidence, there was probably more scrub than grassland on the North Island (mainly towards the west), but with grassland predominant on the non-glaciated areas of the linked South Island. McGlone (1988) suggests that scattered patches of forest would have persisted (consisting mainly of Nothofagus and Libocedrus ), but that these would have been a relatively minor component in a largely treeless landscape in the plains. In hilly areas there may have been quite extensive forest vegetation in parts of North Island. There is also pollen evidence at three sites from the northern peninsula of North Island that a predominantly forested vegetation was present shortly after the LGM, consisting mainly of the lowland conifer forest (McGlone et al. 1993) south as far as 36 S. McGlone et al. (1993) suggest that the whole of the northern peninsula of North Island would have been covered by this forest vegetation. However, Newnham et al. (1993) discuss evidence that in fact there was an organic sedimentation hiatus in pollen-bearing cores around 21,000-17,000 years ago, during which time the forest cover might have been much reduced. Newnham et al. also point out that no wood fossils have been found dating to the LGM period (nor indeed to the whole period between 19,000 and 8,000 y.a.), although abundant fossils have been found for the periods before and after.
The western side of the South Island was covered by an ice sheet that had formed along the New Zealand Alps, spreading down into the lowlands below present sea level. In general, mean annual temperatures over New Zealand seem to have been about 5°C lower than at present (McGlone et al. 1993).
8,000 14C years ago Early Holocene.
Although in several parts of the region, tree lines seem to have taken several thousand years to respond fully to post-glacial warming, they do seem to have reached their present level well before 8,000 years ago (Hope 1987).
Australia. Quite similar to the present. By around 8,000 years ago, the vegetation indicated by most pollen cores was essentially similar to that which existed just before European settlement, though often of a slightly more 'moist climate' type. A map by Dodson et al. (1988) for Australia south of the Tropic of Capricorn at 7,000 years ago shows only slightly more Eucalyptus forest/woodland (less than 5% change in area) than in the 'present-natural' eastern Australia. Likewise, there seems to have been slightly more heathland in southwest Australia than at present. A recent lake pollen diagram from semi-arid SE Australia (Victoria) indicates that conditions were significantly moister than now, with mallee dense scrub instead of the present open grassy woodland (Luly 1994).
For northern Australia Kershaw (pers. comm.) has drawn a sketch map for 9,000 years ago indicating that at least up until that time, a savanna woodland still predominated in what is now the open forest belt across northern Australia. However, there are also some other indications of moister-than-present, more vegetated conditions in northern Australia by around 8,000 years ago. The levels of various lakes in northern Australia (Kiina, Kapalga, Euramoo) indicate conditions similar to or slightly moister than at present (Shulmeister & Lees 1995). Moister-than-present conditions may have existed close to the NW Coast (Kakadu National Park), between 10,000 and 5,000 years ago, as indicated by increased plunge pool activity on a waterfall (Nott & Price 1994). Plant fossil evidence from Melville Island showing expanded monsoon vine forest (prior to 8,000 years ago), and (after 7,500 years ago) of a more vigorous mangrove community in the Fitzroy Estuary, has been taken to suggest slightly moister-than-present conditions during this general period (Shulmeister & Lees 1995). However, Shulmeister & Lees suggest that the latter fossil sources of evidence may be fairly unreliable. They note that in apparent contrast to these sources of evidence, the organic sedimentation rate and pollen sedimentation rate in a core from Groote Eylandt (an island on the west side of the Gulf of Carpenteria) suggests slightly drier-than-present conditions (Shulmeister & Lees 1995).
On the Island of Tasmania, human influence in the form of burning of grasslands is thought to go back to at least 8,500 years ago (Becker 1995).
New Zealand. Forest vegetation seems to have attained its late-Holocene extent well before 8,000 14C years ago. Based on changes in the tree species composition forest communities of northern New Zealand, it appears that before 6,000 years ago the climate may have been slightly moister and less seasonal (Newnham & Lowe 1991), but the differences are not dramatic.
5,000 14C years ago; mid-Holocene.
Australia. Similar to the present. The vegetation map given by Dodson et al. (1988) for southern Australia lumps the period 0 - 5,000 years ago together, indicating that the differences from modern pre-European vegetation at 5,000 years ago were negligable. A recent pollen core by Luly (1993) from the semi-arid southeast adds to the general picture that vegetation conditions were very similar to the present-natural at 5,000 years ago. However, Kershaw et al. (1991) note that there has been an expansion of Eucalypt-dominated forests and woodlands in southeastern Australia since around the mid-Holocene, possibly due to changes in the aborigonal fire regime.
In contrast, for north-eastern Australia, pollen evidence from the Atherton Tableland and lake level evidence from Lake Euramoo indicates that annual rainfall may have been as much as 50% (200-1000mm) higher than present at around 5,000 years ago (Kershaw & Nix 1989), with the summer (rainy season) temperatures perhaps 2-3°C warmer than now. Kershaw (pers. comm. 1990) has supplied me with a sketch map suggesting much moister than present conditions in northern Australia at around 6,000-5,000 years ago, with the open forest belt and the savanna woodland belt extending some hundreds of kilometres further southwards towards the continental interior than at present. He also depicts larger bands of rainforest along the northeast coast. His Holocene vegetation reconstructions appear to be based mainly on two pollen sites in the Gulf of Carpenteria, off the north coast of Australia.
In Tasmania, tree lines were higher than today up until about 6,000 years ago, but perhaps not at 5,000 years ago (Markgraf et al. 1992).
New Guinea. Kershaw (1987) shows a larger-than-present extent of rainforest in New Guinea, extending further south into the woodland belt.
New Zealand. On both the North and South Islands at 5,000 years ago, conifer forest was predominant in the lowlands and Nothofagus in the uplands. Forest clearance began only with the arrival of the Maoris about 1,000 years ago (McGlone et al. 1993)
Names and current addresses of QEN participating experts (named in the text above) who have made direct contributions to this work on Australasia:
J.M. Bowler, University of Melbourne, Australia.
J. Dodson, Department of Geography, University of Western Australia, Perth 6907, Australia.
J.R. Flenley, Department of Geography, Massey University, Palmerston North, New Zealand.
G. Hope, Archaeology and Natural History, Australian National University, GPO Box 4, Canberra 2601, Australia.
W.A. van der Kaars, Department of Geography, Royal Holloway College, Egham, UK & Environmental Studies, Monash University, Clayton, Victoria 3168, Australia.
A.P. Kershaw, Dept Geography & Environmental Science, Monash University, Clayton, Victoria 3168, Australia.