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

No emergence was observed in undisturbed vegetated areas (Gardener et al. 1999), however on vegetated ground it is difficult to
determine if recruitment is from new tillers of existing plants or from seed (Gardener et al. 2003b). According to Bourdôt and
Hurrell (1987a), after herbicide treatment seedlings “may establish within the tiller bases of the dead tussock”. Pritchard (2002)
recorded widespread seedling presence after herbicide treatment at Laverton North, Victoria, in April 2001, “often growing from
within dead tussocks”.
Approximately 50% of seedlings were derived from cleistogenes in a Marlborough, New Zealand pasture (Bourdôt and Hurrell
1992). When plants were treated with herbicide giving total kill and preventing seed head production, seedling establishment in
the following autumn and winter was from cleistogenes (Slay 2001). Slay (2002c) reported that cleistogenes in the soil seed bank
germinated on bare soils (resulting from herbicidal control during spring) in autumn, before panicle seed. Earlier germination of
these seeds was considered likely to be due to the much reduced toughness of the cleistogene seed coat.
The soil surface conditions needed for germination do not appear to be adequately known. The necessity of bare ground is
generally recognised as a requirement (Gardener et al. 1996a, Gardener 1998, Slay 2001) and this has been attributed to a light
requirement (Slay 2002a). Awns are shed when the seed has penetrated 10-30 mm into the soil and remain on the soil surface
(Slay 2002a).
In summary, Bedggood and Moerkerk (2002) wrote that seedlings establish best in the open but can also establish in shaded
situations under the canopy of other plants. However the evidence for establishment in shade is weak and uncertain.
The seedling “does not appear ... very vigorous” (Duncan 1993) and “grows quite slowly” (Storrie and Lowien 2003).
Demography, growth, persistence and dominance
N. neesiana tussocks are long-lived and “very hardy” (Storrie and Lowien 2003). 73% of plants survived over 3 years (Gardener
et al. 1999) and individual plants have a longevity of over 20 years (Benson and McDougall 2005).
The physical structural characteristics of infestations, their spatial arrangement, patchiness and dynamics have been poorly
described. Bourdôt and Hurrell (1987a) reported that “pure stands” occupying areas from several hectares to several square
metres were common in pastures near Lake Grassmere in New Zealand. When fruiting, such infestations can look like cereal
crops (Slay 2002c). Slay (2001 p. 11) reported that herbicide treatment that reduces competition from other pasture species can
eventually result in “a dense mat, and total cover”. Kirkpatrick et al. (1995 p. 35) stated that N. neesiana in native grasslands
“generally grows to the exclusion of all other species”.
Bourdôt (1988 p. 1) described its areal pattern in New Zealand as “scattered ... clumps and small patches”. Slay (2002c p. 11)
recorded that plants “are generally found in circular patches” in pasture in New Zealand and that infestations may be 3-10 m in
diameter 5 years after establishment, with individual tussocks 5-12 cm in diameter. Slay (2002c fig. 28) illustrated a pasture with
an array of scattered, irregularly rounded patches in New Zealand. Pritchard (2002) recorded a mean tussock density of 20.5 m -2
(“relatively dense”) at Laverton North Grassland, Victoria, in October 2000, in a stand selected because it was “almost pure”; the
tussocks mainly being “large, mature” and “up to 30 cm high”.
Hurrell and Bourdôt (1988 p. 237) stated that N. neesiana “often does not have a dense and well-defined tussock form when in
association with other grasses”, while the ACT Weeds Working Group (2002) noted that it forms “dense thickets”. Gardener
(1998 p. 4) found that it “can completely overrun pastures resulting in canopy ocover of up to 60%”. Slay (2002c) noted that it
can from “continuous pasture” (in contrast to discrete tussocks). Stewart (1996) recorded cover in two 5 x 5 m quadrats in
Broadmeadows Valley Park, Victoria, which ranged from 50-70% in one quadrat and 5-50% in the other over a 6 month period.
Grech et al. (2005) reported that its canopy cover can exceed 60% in invaded pastures. Gardener et al. (2005) stated that it can
can have high basal cover of 20%. Muyt (2005) estimated foliar cover across 102 quadrats each 25 x 25 m (= 25.5 ha) at
ungrazed and not recently burnt ACT natural grassland (Yarramundi Reach) and found cover in excess of 75% in 2 quadrats, 50-
75% in 1 quadrat, 25-50% in 21 quadrats, 5-25% cover in 30 quadrats, many individuals and up to 5% cover in 24 quadrats, and
3-20 individuals in 9 quadrats. Bourdôt and Hurrell (1989a) assessed cover in 161 paddocks in New Zealand and found that most
had ≤5% cover with plants present mainly as clumps or dense groups. Where cover was >25% the plants were present mainly in
pure stands. Plants had persisted at the probable first introduction point in the area for c. 60 years and in 1988 that infestation
consisted of a pure stand of several hundred m 2 (Bourdôt and Hurrell 1989a). Gardener et al. (2005) described an infested
paddock near Guyra, NSW, as consisting of two communities. N. neesiana was dominant in the slightly better drained areas
while the second community on lower, poorly drained areas was dominated by Festuca arundinacea with little N. neesiana.
Bruce (2001) found it was dominant at 8% of sites investigated in the ACT, subdominant at 13% and common at 22%. Liebert
(1996 p. 8) stated that it can “almost completely displace perennial native grasses” including T. triandra and had “destroyed”
two wetlands at Laverton, Victoria, by excluding all other plants, in less than 10 years. In the Geelong area isolated plants on
roadsides had become monocultures within 3 years (Liebert 1996 citing David Boyle). Slay (2002c) noted that it persisted when
other pasture grasses fail.
McDougall and Morgan (2005) measured the cover and frequency of N. neesiana on native grassland re-establishment areas on
former agricultural land at Organ Pipes National Park from 1989 to 2003. The site was burnt in autumn 1993, 1995 and 1997 and
there was a severe drought from March 1997. From initial values close to zero, both % cover and % frequency varied markedly.
Percentage cover never exceeded c. 5% and was much