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

susceptible to N. neesiana invasion. For example “few weed species have successfully established and persisted” in the
Australian Alps, the number declining with increasing altitude (McDougall and Walsh 2007 p. 44). Excluded formations include
aforementioned wetland formations and maritime grasslands, the Western Slopes Grasslands of NSW including Moore’s (1993)
Xerophytic Midgrass (Southern) and Saltbush Xerophytic Midgrass, Eastern Victorian highlands grasslands (e.g. Lake Omeo),
high altitude grasslands of the Monaro region of NSW, and alpine grasslands and meadows. The grasslands that once existed
around the shores of Lake Omeo (Benambra area) were similar to those of the Southern Tablelands and were dominated by T.
triandra (Lunt et al. 1998) or Austrodanthonia (Kirkpatrick et al. 1995), but show clear affinities with high altitude grasslands
(Carter et al. 2003). According to Kirkpatrick et al. (1995) the Lake Omeo grasslands are probably derived from Eucalyptus
pauciflora Sieber ex Spreng. woodland.
The natural occurrence of temperate grasslands in south-eastern Australia was determined by relatively low rainfall (in the range
of 350-1000 mm mean annual, mainly 500-600 mm), flat to undulating topography, and soils that were poorly drained, heavytextured
and moderately to highly fertile (Mott and Groves 1994, Sharp 1997, Jones 1999b, Lunt and Morgan 2002). The
geological parent materials produce soil with a high clay content (Jones 1999b). Heavy clay or clay loam soils, and a relatively
dry, cold climate “are prerequisites for grassland vegetation worldwide” (Benson and Redpath 1997 p. 307). South-eastern
Australian grasslands generally occur on younger soils that have not been heavily leached or become lateritic or infertile, and
thus tend to occur where the parent rocks have weathered in situ and there is little erosion, and the soils are relatively fertile
(Jones 1999b). Most grassland soils have a high water content but low water availability because the clay minerals bind water,
and there is little pore space (Jones 1999b).
Some temperate Australian grasslands occurred in the drier areas down to c. 250 mm annual rainfall on the arid margins of
chenopod shrublands (Mack 1989) although these are probably better categorised as semi-arid formations (Carter et al. 2003). In
the Australian Alps, grasslands are generally found on deep humus soils in valley bottoms subject to cold air accumulation and
frosts (McDougall andWalsh 2007).
Causes of treelessness
Heavy-textured clay substrates dry out and crack deeply in summer (c. 13% on an areal basis according to Patton 1935) and this
may prevent the establishment of trees and shrubs, as may fire, which occurs ubiquitously (Groves and Whalley 2002). Moore
(1993 p 353) explained treelessness to be the result of “shallow penetration of water in environments with relatively low rainfall
and high evaporation, where tree seedlings would be subjected to intense competition for surface-rooted grasses and other
herbaceous species” and in wetter areas “poor aeration following temporary waterlogging after winter rains”. Patton (1935 p.
175) argued that trees can be established artificially on the Victorian basalt plains by “opening up the ground and ... destroying
the native vegetation”, so concluded that the native vegetation itself prevented the establishment of tree seedlings. Elsewhere
however he argued that heavy textured soils inhibit free entry of water, have slow percolation and bad aeration, that summer
cracking leads to deep drying, and that deep rooting is difficult, so it is the physical characteristics of the soil, accentuated by the
evenness of contour in the plains that determines their treelessness (Patton 1930). Barlow and Ross (2001) argue that multiple,
confounded, variable factors are responsible, with soil charcteristics, particularly drainage, the most important and subsidiary
influences from climate and fire. In a trial in the South American pampas competition from tussock grasses prevented
establishment of half the tree species tested (Aguiar 2005). Kirkpatrick et al. (1995) more or less concurred: tree seedlings are
largely excluded by competition in dense grassland swards, which may use all the available soil moisture, and severe
disturbances such a soil digging are required for trees to establish.
In the Volcanic Plains Grasslands of western Victoria, where ever any other geological formation abuts basalt, trees occur.
Eucalyptus camaldulensis grows, for example, where granodiorite is exposed or eroded out of the basalt (Patton 1930). The
granite of the You Yangs provides another notable example. Soil chemistry, as well as structure can determine treelessness: soils
dominated by sodium, common in northern Victoria, usually lack trees (Jones 1999b).
Distribution of grasslands in the ACT is determined by accumulation of cold air pockets in valley floors, creating conditions that
are too cold to permit growth of trees and shrubs (Chan 1980), but low rainfall, heavy-textured soils and the legacy of aboriginal
fire regimes are also important (Sharp 1997). Very low temperatures associated with nocturnal temperature inversions have been
invoked as the cause of treelessness in subalpine valleys (Moore 1993).
Lowland grasslands
The term “lowland” grasslands, considered to include those formations at altitudes below 1000 m, has been widely used (Lunt
1991, McDougall and Kirkpatrick 1993, Kirkpatrick et al. 1995, Sharp 1994, Lunt et al. 1998). Prior to European occupation
they probably covered c. 2 million ha, of which “perhaps” 10,000 ha survived in “more or less natural” condition by 1992
(Kirkpatrick et al. 1995 p. 8). Classifications of lowland grasslands have varied between authors and government authorities and
have not always coincided. The New South Wales classification of Benson (2004) has four classes covering sub-humid
grasslands, including the Temperate Montane Grasslands, found in the south-east, which include high altitude formations
dominated by Poa spp. Such grasslands, including those on the Monaro Plains of NSW, are generally excluded from the
‘lowland’ category by most authors. The change from lowland to montane and alpine grasslands is gradual and clinal, so 1000 m
is an arbitrary delimination (see discussion in Carter et al. 2003).
Secondary and derived grasslands
‘Secondary grassland’ is derived from other vegetation formations, often in Australia from grassy woodland in which trees have
been cleared and on which livestock are grazed (Moore 1993, Mott and Groves 1994, Groves and Whalley 2002). Temperate
grassy woodlands are also a threatened vegetation type (Benson 2004, Keith 2004, McIntyre and Lavorel 2007) and their grassy
stratum may be indistinguishable, for practical conservation purposes, from that of adjoining natural grassland (Carter et al.
2003, Keith 2004). Indeed Moore (1993 p. 343) considered that most of his Temperate Shortgrass communities, including most
of the Western Basaltic Plains of Victoria, the NSW Riverina and Gippsland Plains, to be “mostly the understoreys of temperate
woodlands modified by clearing and grazing by livestock and rabbits”. The grasslands derived from temperate grassy woodlands
are frequently also very similar floristically to natural grasslands ( Mott and Groves 1994) and can have the same ecological
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