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

The grass component was more resilient than the forbs because grasses are less palatable, grass leaves continue to elongate from
the base, dormant buds that develop lateral shoots are rapidly induced after destruction of apical meristems, and much of the
meristematic tissue is located in the crown and is protected from grazing damage (Tscharntke and Greiler 1995). In general,
rhizomatous and prostrate grasses are the least affected by bovid livestock (Mack 1989) or are favoured, so that grazed
grasslands tend to become two-layered – a short layer of grazed species and a discontinuous taller stratum often dominated by
caespitose Poaceae and other unpalatable species (Overbeck et al. 2007). In south-eastern Australian native pastures the
‘layering’ in the more heavily grazed grasslands tends to have a temporal rather than spatial dimension, with small short lived
annual grasses such as Vulpia forming a large component of the living biomass in spring.
Most of the historical grazing regimes in Australian temperate grasslands have resulted in major alteration to the grass
components. Throughout the world, selective herbivory of palatable grasses by domestic livestock is a major cause of species
replacement (Mack 1989, Moretto and Distel 1998). T. triandra is regarded as a “good forage” in native pastures, although
“mature plants are neglected” (Chan 1980 p. 22), and Audas (1950) warned of the danger of T. triandra being “eaten out” if
grazed in spring and summer. However T. triandra has a high C:N ratio, which makes it less palatable than many other grasses
(Moretto and Distel 2002). It is lost along with other native perennial grasses when there is continuous heavy grazing by sheep
(Stuwe and Parsons 1977, Chan 1980) and it is more susceptible to elimination when grazing follows fire (Groves and Whalley
2002). Heavy grazing by sheep or cattle causes more damage to T. triandra than that by macropods or horses (Kirkpatrick et al.
1995). In the Australian Alps, cattle “prefer inter-tussock herbs (such as Craspedia spp., Celmisia spp. and Leptorynchos
squamatus subsp. alpinus) but make up bulk in their diet with tussock grasses (Poa spp.)”, so they spend more time in grassland
communities than other vegetation formations (McDougall and Walsh 2007 p.6). Discontinuous bovid grazing however enables a
high proportion of native species to survive (Kirkpatrick et al. 1995). Losses of native grasses can continue even after pastures
become highly degraded by invasion of exotic perennial grasses: data of Badgery et al. (2002) indicated declines of (un-named)
perennial C 4 grasses under sheep grazing at 4.5 DSE ha -1 in areas with up to 50% coverage of Nassella trichotoma, along with
declines of about 30% with grazing plus fertiliser (120 kg N and30 kg P ha -1 y -1 ). Stafford (1991) found that remnant T. triandra
in the East Torrens region of South Australia was generally found at sites with a long history of stock exclusion. However T.
triandra, Poa and Austrodanthonia may survive long after the intertussock herbs have been eliminated (Groves et al. 2003a).
T. triandra is eventually replaced by exotic winter-growing species adapted to trampling and close grazing (Moore 1973, Gott
1983, Moore 1993, Kirkpatrick et al. 1995). At Derrimut, Victoria, N. neesiana presence and density is strongly negatively
correlated with that of the dominant grass (T. triandra) and is probably a long-term result of previous heavy grazing and
ploughing (Lunt and Morgan 2000). Grazing exclusion can lead to the dominance of tall caespitose grasses in the pampas
(Overbeck et al. 2007). Long term grazing results in forb loss also in areas that are both burnt and grazed (Lunt 1990b 1997,
Morgan 1997). Intermittent grazing has less severe effects on species composition and may alter or reverse the changes in the
general degradation syndrome (Groves et al. 2003a). Hadden (1997) suggested that suitably modest regimes were 2DSE ha -1
during summer and autumn in Western Plains Grasslands, and 1DSE ha -1 during winter and summer and possibly parts of
autumn in the drier Northern Plains.
Loss of intertussock plant species diversity also occurs in T. triandra grasslands that are protected from ungulate and rabbit
grazing, and not frequently burnt. In the Western Basalt Plains, Hadden (1998) and Hadden and Westbrooke (1999) found a
significant decline in herb species in plots protected from sheep and rabbit grazing in native pasture but an increase in cryptogam
cover. Decline of herbs was due to T. triandra canopy thickening and closure over 2 years. Increase in cryptogams was attributed
to absence of soil trampling. Native grass cover of ungrazed areas reached c. 80%, compared to 35% in grazed areas, while total
plant biomass reached 3,575 kg ha -1 compared to 718 kg ha -1 in grazed areas. Trémont (1994) found that grazed grasslands in
northern NSW contained greater native plant richness than ungrazed grasslands, probably as a result of suppression of dominant
grasses (Sharp 1997). Hadden (1998) found no significant changes in botanical composition between grazed and ungrazed plots
in a Victorian Northern Plains grassland when grazing was excluded.
Bovid grazing continues to threaten grassland remnants, notably along roadsides used for droving and grazing in drought
conditions, e.g. to Comesperma polygaloides (McIntyre et al. 2004).
Gap creation intensifies with increased grazing pressure and pasture gaps are subject to less root and shoot competition and thus
favour the survival of grass seedlings (Moretto and Distel 1998). In native grassland in Argentina Moretto and Distel (1998)
found that gaps in the vegetation dominated by Nassella clarazii (Ball) Barkworth, characterised by low competitive pressure,
enabled seedling establishment of the unpalatable stipoid grasses Jarava ichu Ruiz and Pav and Nassella tenuissima. However
they did not compare regeneration of the palatable species, N. clarazii, so failed to test their stated hypothesis, that such gaps
favour the unpalatable species. Creation of such gaps by overgrazing of the palatable species was nevertheless suggested as the
mechanism enabling establishment of the undesirable grasses.
Bovid livestock are hard-hooved and weigh from 40 kg (Capra hircus) to nearly 1 tonne (Bos taurus) (Groves 1989) Their
activity thus causes soil compaction, exacerbated with proximity to watering points, which they must visit regularly (Moore
1993). Physical effects include increases in bulk density and bearing capacity and decreases in hydraulic conductivity (a measure
of infiltration) that are directly related to stocking rate (Willatt and Pullar 1983). Hoof pressures of >160 kPa for cattle and 64-
100 kPa for sheep have been calculated for animals standing flat on four feet, and higher pressures result when animals are
moving. Comparable pressures from tractors are 30-150 kPa (Willatt and Pullar 1983). Biological effects of compaction include
significant decreases of arthropods and reduction of earthworm body weight and numbers (Brown 1987). In the early days of
ungulate grazing, the basalt soils at Sunbury, Victoria, were changed from loose to hard by continuous trampling (Gott 1983).
Thus Microseris spp., which preferentially germinates in loose soil (Gott 1983), was affected. Soil compaction makes harder
soils, and many studies have demonstrated that root growth is negatively affected as penetration resistance increases (Willatt and
Pullar 1983).
Disturbance of the soil and damage to the cryptogam crust favours exotic species over native (Stuwe 1994). With high levels of
grazing the crust is often completely eliminated except where protected from trampling (e.g. along fence lines), but with sheep
grazing at or below about 2.2. sheep ha -1 the soil crust is discontinuous and still noticeable (Scarlett 1994).
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