Literature review: Impact of Chilean needle grass ... - Weeds Australia

Oligocene-Miocene boundary, c. 36 mybp, while in the eastern Murray-Darling basin fire appears to have been present through
much of the Tertiary although markedly increasing in the late Miocene (Kershaw et al. 1994). In the south-east, grassland
became more dominant during the Pleistocene (3 mybp +), and may have been as widely developed as today by the late
Pleistocene (Jones 1999a).
Fossil grass pollen has been identified only to family level (Martin 2004). The fossil record in Australia for the period of greatest
interest from the Miocene through the Pleisocene is “very fragmentory” and often difficult to interpret (Kershaw et al. 1994 p.
299).
The Quaternary period
Grassland has dominated much of Australia during much of the last 2 million years (the Quaternary period), expanding during
long, cooler, relatively dry periods and contracting during warmer, wetter interglacial periods (Kirkpatrick et al. 1995, Benson
and Redpath 1997, Keith 2004). On the western plains of Victoria more substantial rainforest occurred during interglacials in the
mid-late Pleistocene than in the early Pleistocene, the formation expanding from refugia such as the Otway Ranges (Kershaw et
al. 2000). Glacial-interglacial oscillations occurred throughout the Quaternary and selected for taxa tolerant of drier, cooler
climates and repeated climatic change, “linked to disturbances including wildfires prior to and coincident with the arrival of
humans” (McGowran et al. 2000 p 449). Little speciation of all terrestrial taxa is believed to have occurred in the Quaternary –
instead species shifted their distribution in response to the dramatic climate changes (Kershaw et al. 2000), and grasslands as a
whole would have undergone wide geographical shifts in their extent and location (Jones 1999a), and probably in their
composition, since “it is very clear” from the Tertiary fossil record “that taxa, not communities, migrate” (Martin 1994). Hope
(1994) however argued that the repeated, rapid destruction of ecosystems due to climate shifts caused extinctions, range
fragmentation and rapid speciation, e.g. of Acacia spp.
An “extensive grassland steppe vegetation” with Casuarinaceae as the tree component occurred from the late Pliocene-early
Pleistocene (c. 2.5 mybp +) and has “no identified modern analogue” (Kershaw et al. 2000 p. 494). This inland vegetation had
high levels of Poaceae and Asteraceae (Hope 1994) and a major Asteraceae component, with the form taxon name
Tubulifloridites pleistocenicus, does not correspond with any extant Australia daisy and may have been similar to North
American Ambrosia and African Stoebe (Kershaw et al. 1994). Evidence of the dominance of Asteraceae, probably woody, c. 1
mybp has been obtained from a site on the edge of Port Phillip Bay, Victoria, and pollen data from the Pejark Marsh volcano in
western Victoria covering the period 1-0.7 mybp indicates Poaceae and Asteraceae co-dominance with Casuarinaceae as the
main trees (Kershaw et al. 2000). Extensive grasslands existed in north-western Australia and possibly in central Australia in the
early Pleistocene (Kershaw et al. 2000). Late Pleistocene data from north-eastern Queensland indicates a major increase in
Poaceae c. 175 kybp indicates “a major expansion of grassland prior to substantial increases in eucalypts and charcoal" (Kershaw
et al. 2000 p. 500). A number of currently dominant taxa, notably Pooidae (Poa spp.) and T. triandra entered Australia from the
north during the Pleistocene, but the number of recent migrant taxa is small (Jones 1999b).
The Holocene (Recent)
Lower rainfall and temperatures associated with the last ice age, which ended about 10 kybp, probably saw much of southeastern
Australia dominated by grasslands and grassy woodlands (Jones 1999a).The many vegetation histories that have been
established from the fossil record indicate that at the peak of the last ice age, c. 18 kybp, most of south-eastern Australia was
dominated by largely treeless vegetation, a very open, cold, dry steppe of Poaceae and Asteraceae with annuals, perennial
geophytes and shrubs, and that open eucalypt woodlands were widespread across the Bassian plain (Hope 1994). Montane
grasslands, such as those in the Monaro region of New South Wales and the midlands of Tasmania are the best current analogue
for these formations (Hope 1994). The current interglacial is unusally warm compared to 85% of the Quaternary, and the
lowland grassland communities present have therefore formed largely from the species set that survived the cooler, drier glacials
(Hope 1994).
Australian grassland formations
Four main types of grassland occur in Australia: 1. tropical summer rainfall coastal grasslands dominated by Sporobolus and
Xerochloa spp., mainly in the Northern Territory and north-western Queensland, 2. Triodia arid hummock grassland of the
continental interior; 3. Astrebla (Mitchell grasss) grasslands in areas with 200-500 mm average annual rainfall, mainly in
summer, in western Queensland, inland northern New South Wales, the Northern Territory and northern Western Australia; and
4. subhumid grasslands of eastern Australia (Specht 1970, Groves 1979, Mott and Groves 1984, Groves and Whalley 2002,
Benson 2004). The latter has been subdivided into three types: 4a. tropical subhumid grasslands of eastern and northern
Queensland, dominated by Dichanthium and Eulalia and sometimes by Bothriochloa and Heteropogon; 4b. temperate grasslands
of New South Wales, Victoria and South Australia, dominated by Themeda triandra, Poa spp., Austrodanthonia and Austrostipa
and 4c. subalpine tussock grasslands of wet tablelands and montane areas of the south east, dominated by Poa spp. and
Austrodanthonia (Groves 1979, Groves and Whalley 2002).
Moore (1993) described a substantially different set of subhumid grassland types present in south-eastern Australia: Temperate
Tallgrass dominated by Themeda, Poa and Dichelachne, Temperate Shortgrass dominated by Austrodanthonia, Austrostipa and
Enneapogon, Subalpine Sod Tussockgrass dominated by Poa, Themeda and Austrodanthonia, Xerophytic Midgrass (Southern)
dominated by Austrostipa, Chloris and Aristida, and Saltbush Xerophytic Midgrass dominated by Atriplex, Maireana and
Austrostipa. The Temperate Shortgrass communities are largely derived from woodlands and consisted mainly of taller warm
season grasses: in the wetter areas Themeda triandra, Austrostipa bigeniculata (Hughes) S.W.L. Jacobs and J. Everett and Poa
labillardieri Steud. and in the drier areas Austrostipa aristiglumis and Themeda avenacea (F.Muell.) Maiden and Betche.
Temperate Tallgrass is a formation corresponding with disturbed forests and heaths and defined by T. triandra, P. labillardieri
and Dichelachne spp. (Moore 1993).
Small areas of other grassland types occur including maritime grasslands (on beaches, headlands etc.) and grasslands associated
with river margins and freshwater wetlands (reed beds, meadows, cane grass swamps, etc.) (Moore 1993, Carter et al. 2003,
Benson 2004). Only the humid, temperate, non-alpine grasslands are considered here, since the others are thought to be less
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