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

of these systems is largely explained by the form and growth patterns of caespitose grasses, which develop by intravaginal
tillering, the emerging tillers remaining erect inside the leaf sheaths. The bunching, erect form makes the plant more susceptible
to ungulate grazers than the contrasting rhizomatous grasses, which have extravaginal tillering and axillary bud production.
Ungulate grazers are able to reduce the reproductive potential of tussock grasses to a much larger extent than rhizomatous forms
by grazing the emerging flower heads (Mack 1989). Another contributing factor was the perenniality of the dominant species,
which “made annual re-stablishment unnecessary” (Evans and Young 1972 p. 231). For example in the intermountain west of the
USA the once dominant Agropyron spicatum (Pursh) Scribn. required above average rainfall to establish, so significant
recruitment events were relatively uncommon.
In Australia the syndrome of decay had the following course. Continuous grazing by hard-hooved livestock preferentially
removed the more palatable and sensitive intertussock herbs and the tall C 4 grass (T. triandra); fire exacerbated these losses; T.
triandra was replaced by cool-season C 3 native grasses (such as Austrodanthonia spp.). Further grazing favoured short coolseason
grasses and eliminated or greatly reduced the remaining palatable forb components, and loss of both these functional
groups led to nutrient enrichment of the soil, particularly with N, which in turn allowed invasion by alien forbs and annual
grasses (e.g. Vulpia spp.) of European origin, etc. (Moore 1973, Mack 1989, Moore 1993, Groves and Whalley 2002, Groves et
al. 2003a). In temperate Australian grasslands, the main trends in plant composition have been from summer to winter-growing
grasses, from perennials to annuals and from native to introduced species (Moore 1973, Stuwe and Parsons 1977, Mack 1989,
Moore 1993, Groves and Whalley 2002).
Intense grazing of T. triandra during its reproductive phase when it is mobilising nutrients from leaves to storage organs may
have been the critical factor in its extensive demise (Dunin 1999). Greater palatibility compared with other native grasses,
trampling damage to surface roots, reduced seed production and seedling establishment were probably additional important
factors (Groves et al. 1973, Chan 1980). Alterations to drainage patterns and soil disturbance have also contributed to losses:
Lunt (1990a) noted that T. triandra occurred at Derrimut Grassland only in well-drained areas that had not been ploughed, as
well as areas subjected to no more than brief periods of heavy grazing. A similar mechanism to that reported by Grice (1993) for
the proliferation of Austrostipa and Aristida spp. in the semi-arid woodlands of western New South Wales may be partly
responsible: grazing did not cause greater mortality of the more desirable and long-lived grasses, rather, the plants avoided by
sheep (Austrostipa and Aristida spp.) produced much more seed in grazed areas and so proliferated, while the more palatable
native pasture grasses, were more fecund when ungrazed. T. triandra decline in inland New South Wales has been blamed on
overstocking and low seed production (Whittet 1969).
Native pasture in turn was developed by addition of fertilisers, and the sowing of exotic grasses and herbaceous legumes (Moore
1973 1993). Naturalisation of Trifolium and Medicago species after accidental introduction and spread was important (Moore
1993 p. 345) probably from the time of first settlement onwards. Addition of nutrients as a feature of grassland ‘improvement’
for agricultural grazing became widespread after 1929 when the Australian Government introduced a superphosphate subsidy
(Mansergh et al. 2006a). The advent of cheap superphosphate coincided with government promotion of introduced C 3 perennial
grasses and legumes (Trifolium spp. particularly T. subterraneum, Medicago and Lotus spp.), varieties of which were bred by
State Departments of Agriculture for use in ‘pasture improvement’ programs, which usually involved cultivation (Groves and
Whalley 2002). Seed of T. subterraneum first became commercially available in the early 1920s and that of T. fragiferum L. in
1938, while cultivars of T. repens were first registered in the mid-1930s (Oram 1990). Use of Trifolium spp. and superphosphate
increased rapidly in some areas in the mid 1930s (Browning 1954) and became commonplace during the 1940s and 1950s
(Mansergh et al. 2006a). These changes raised the P and N status of the land to high, facilitating the invasion of new suites of
weeds. The improved pastures, including a legume component, were able to support high intensity grazing, but required ongoing
fertilisation and periodical resowing, and lifted productivity “in the short term” (Keith 2004 p. 105). Spread of the exotic grasses
was the “desired outcome” sought by agronomists” (Cook and Dias 2006 p. 617) and some of these grasses escaped from the
paddock and started to become major weeds of roadsides and eventually natural areas, including remnants of the natural
grasslands - the weed potential of a species for a natural ecosystem being more or less equivalent to its hardiness, persistence and
productivity values as a new pasture grass.
These changes led to the eventual disappearance of the native grasses, particularly T. triandra, in many areas, although some
Austrodanthonia spp. can re-occupy high-nutrient, grazed sites (Groves and Whalley 2002). Groves et al. (1973, echoed by Chan
1980) thought that the mechanisms causing the loss of T. triandra remained to be properly identified, but listed a number of
probable reasons including greater palatability to livestock than other native grasses, susceptibility of the adventitious roots to
grazing damage at the soil surface, gradual exhaustion of underground reserves due to continuous shoot removal, low seed
production and poor seed seedling establishment, and poor competitive abilities for light and nutrients. Chan (1980)
demonstrated that repeated close (2 cm above ground) mowing at intervals of ≤3 months reduced yields and reproductive fitness
of T. triandra, Austrostipa bigeniculata and Austrodanthonia spp., with the least affected of the native grasses examined being
Bothriochloa macra because of its low habit and prostrate tillers. Similar results were obtained by Nie et al. (2009) on a range of
native grasses cut at 3-5 week intervals to a height of 2, 5 or 10 cm. All species tested had reduced survivorship when cut to 2 cm
height, but plant survival was least with the two C 4 grasses, T. triandra (c. 51%) and B. macra (c. 57%). Cutting to 5cm
increased survivorship to c. 85% with T. triandra and >95% with B. macra. Cutting at 5 and 10 cm enabled T. triandra to
increase its shoot biomass compared to the 2 cm cut far more than the other species. Furthermore, most of the species tested had
little or no response to P fertilisation (Nie et al. 2009) so would be outcompeted by exotic pasture species when superphosphate
was applied.
The ecological and evolutionary circumstances that led to the dominance of summer-growing T. triandra in south-eastern
Australian temperate grasslands prior to European occupation have not been adequately explained (but see Bond et al. 2008).
Ostensibly the species appears to be poorly adapted as a dominant in grasslands that have winter rainfall maxima, spring growing
periods and dry summers. This peculiarity of “a system growing partially out of phase with the rainfall regime” may explain why
sheep and rabbit grazing led to rapid decline of T. triandra in south-eastern Australia and complete disappearance in south-west
Western Australia (Moore 1993 p. 351). T. triandra may have been at a disadvantage compared with spring-growing exotic
grasses because its demands for water are highest during the driest time of the year (Groves 1965, Mack 1989). However swards
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