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

al. 2008). Cues for masting may be temperature based, since temperatures can be more effective in synchronising dispersed
populations than rainfall, and masting may involve plant anticipatory responses that associate weather events with future
conditions that would improve recruitment (Kelly et al. 2008).
In a dense pastures sward in New Zealand potential N. neesiana cleistogene production was 10,300 m -2 if all culms produced the
maximum of 13 seeds (Slay 2001). Potential cleistogene numbers represented about 25% of total potential seed production.
Under drought conditions, Slay (2002a) found more basal cleistogenes per vegetative tiller on autumn-germinated juvenile plants
in December than on culm-bearing tillers of mature plants. In Northern NSW, an average of 7.2 cleistogenes per flowering tiller
was produced, and a dense ungrazed infestation in 1996 produced an estimated 6,100 m -2 (Gardener et al. 1999 2003a); but note
again that this was probably a mast year. Cleistogenes represented an estimated 21.5% and 26.1% of total seed production in
1996 and 1997. Clipping of flowering tillers above the basal node, and above and below the top node, had no effect on the
number of cleistogenes produced on the nodes below the clipping point (Gardener et al. (1999 2003a).
The numbers of panicle seeds produced per tiller on the Northern Tablelands of NSW (38) and Argentina (27 – small sample
size) are similar and the number of cleistogenes per tiller identical (Gardener et al. 1996b). But there is wide variation in plant
density in the pampas, tussocks are much smaller, and the maximum % basal ground cover is less than half the maximum on the
Northern Tablelands. The number of inflorescences m -2 in the Argentinian sites examined varied from 0 to 200, giving a
maximum seed production of c. 8000 seeds m -2 (Gardener et al. 1996b).
Awns of several seeds often twist together while still attached to the plant, forming a tangled mass that usually includes
infloresence branches, etc. (McLaren, Stajsic and Iaconis. 2004; illustrated by Frederick 2002). Seeds that become entangled are
retained on the plant for a longer period than seeds that do not. According to Groves and Whalley (2002 p. 158) the “ecological
implications of [such] retention are obscure”; however potential advantages of retaining seed in the canopy include protection
from predation and decay processes on the ground surface.
The morphology of the seed and its presentation on the plant are evidently defenses against mammalian herbivores. Sheep and
cattle stop eating the plant as soon as the flowering stalks are produced (Grech 2007a).
Dispersal mechanisms
Poaceae in general have very effective dispersal mechanisms: they comprise approximately 4% of world Angiosperm genera but
account for 13% of cosmopolitan genera (Wheeler et al. 1990). The proportion of introduced grasses in a regional flora is usually
much higher than for the flora as a whole, for instance in the Juan Fernández Islands of Chile 81% of the 53 grass species are
adventive compared to 70% of the whole flora (Baeza et al. 2007). Alien grasses account for a large proportion of the grass flora
in many regions, particularly in “livestock-based economies” (Milton 2004 p. 69). Grazing has long been a backbone of the
economy in Victoria, for example, and 62% of the vascular plant genera in that State and 44% of the species are exotic, with
another 10% of the genera having both native and exotic species (data from Ross and Walsh 2003). In comparison, in southern
Africa 15% of the genera and 12% of the species are exotic (Milton 2004). In mainland Spain Poaceae account for a larger
proportion (16 of 106 spp.) of the etablished alien plant flora than all other families except Asteraceae (20 spp.) (Gassó et al.
2009). More species of Poaceae are considered to be environmental weeds in New Zealand than any other plant families, but in
Australia more Asteraceae are environmental weeds (Williams and West 2000).
Stipeae are commonly adventive species (Watson and Dallwitz 2005) and numerous species have dispersed to remote islands and
intercontinentally. According to Tsvelev (1977 p. 7) “it can hardly be doubted” that Stipa capensis, described from and very
common in South Africa, was carried there by the first colonists. Amelichloa brachychaeta was described in 1853 from French
material by Godron “who at that time was unaware of its native home” (Hayward and Druce 1919 p. 226). Hayward and Druce
(1919) recorded seven South American species (N. neesiana, N. poeppigiana (Trin. and Rupr.) Barkworth, N. pubiflora (Trin.
and Rupr.) E. Desv., N. caespitosa Griseb., N. leptothera (Speg.) Torres, Amelichloa caudata and A. brachychaeta) in the
adventive flora of Tweedside, Scotland, on wool refuse heaps or otherwise associated with wool factories. Nine (Connor et al.
1993) or 12 (Edgar et al. 1991) stipoids have been recorded as naturalised in New Zealand including Nassella spp. and
Austrostipa spp. The Argentinian and Uruguayan Nassella manicata (E. Desv.) Barkworth, established in California,was
probably introduced in the 19th or early 20th centuries (Barkworth 1993 2006) and “apparently hitched a ride ... with South
American vaqueros and their livestock looking for greener pastures” (Amme 2003). California also has introduced populations of
N. tenuissima derived from horticultural plantings (Amme 2003). Five of the 25 “Stipa” species recorded in Italy are exotic
species, all from South America: N. neesiana, N. hyalina, N. trichotoma, N. formicarum (Delile) Barkworth and A. caudata
(Moraldo 1986).The two Nassella species (N. neesiana and N. laevissima (Phil.) Barkworth) found on the Juan Fernández
Islands of Chile are both introduced (Baeza et al. 2007). On a world basis, at least 12 Nassella species have been reported
growing outside their native range (Randall 2002, Barkworth 2006, Baeza et al. 2007), c. 10% of the species.
There is general consensus that human activities are the major cause of N. neesiana seed dispersal in Australia (Bedggood and
Moerkerk 2002, Snell et al. 2007). The panicle seed, like that of stipoids in general, has many adaptations that enable it to attach
to a wide range of objects: according to Slay (2002c p. 23), it “attaches to almost everything”.
However records of actual seed dispersal are very limited and the conclusion that anthropic factors account for current
distributions is surmise based on patterns of infestations, seed biology, and general observations of the carriage of seed on
machinery, vehicles and livestock.
Based on evidence of exotic stipoid dispersal to New Zealand and within that country, Connor et al. (1993) suggested that
stipoids with falcate awns may be more highly dipersible than those (such as N. neesiana) with geniculate awns.
Creeping diaspores
The panicle seeds of N. neesiana are classed as creeping diaspores (Davidse 1986, Connor et al. 1993) that are able to move
along the ground and position themselves in microsites favourable for germination (Gardener and Sindel 1998, Sinclair 2002).
Creeping diaspores of grasses generally result in little actual dispersal via ‘creeping’, the adaptations being more important in
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