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

enabling microsite lodgement (Peart 1979, Davidse 1986). The awn provides the hygroscopic torsion mechanism by which the
seed is supposed to drill itself into the soil (Murbach 1900, Davidse 1986). Peart (1979) however argued that horizontal rather
than vertical propulsion by awn torsion is usually of greater importance, even in those species such as N. neesiana with a sharp
callus and long, stout active awns that appear best adapted to actually drill into the soil. Alternate wetting and drying of the awn
twists the column, driving the seed forward, the artista provides a brace for leverage, and the retrorse spines on the corona, and
the hairs on the awn, callus and lemma-body restrict backward movement (Bourdôt and Ryde 1986). The awn of Piptochaetium
avenaceum increases in length by 20% when wet, assisting propulsion of the seed (Murbach 1900) and this effect likely occurs
also with N. neesiana. As with similar species (Murbach 1900), the sharp N. neesiana callus leads the seed into the ground and
the callus hairs hold it in place once ground penetration has started and anchor the seed after germination, countering the
opposing force provided by the radicle. The depth to which a stipoid seed is buried is related to the length of the awn: “species
growing in areas where seeds need to be buried to ensure adequate amounts of soil moisture for germination tend to have a long
callus, a long, narrowly cylindrical floret, and long, persistent awns” (Barkworth and Everett 1986 p. 254).
Awns of awned grasses are able to drill seeds into cracks and crevices in the soil but there is little evidence that penetration of an
unbroken soil crust occurs (Peart 1979, Bourdôt and Ryde 1986, Sinclair 2002) although stipoid seeds are often credited with the
ability to ‘bury themselves’ in the soil by this mechanism (e.g. Whittet 1969 p. 129).
Non-hyroscopic straight awns have another function immediately after seed shedding: to rotate the seed while falling in such a
way that the seed lands vertically on the callus (Peart 1984, Sinclair 2002). This is also appears to happen with N. neesiana
seeds, even though their awns are strongly twice-bent and hygroscopic (personal observations).
The dispersal ability of lone panicle seeds of N. neesiana is lost by a significant proportion of seeds, which aggregate in the
panicle when the awns twist together, forming a tangled mass that usually includes infloresence branches (Connor et al. 1993,
McLaren, Stajsic and Iaconis. 2004; illustrated by Frederick 2002). The aggregation often ultimately falls to the ground as a unit
(Gardener et al. 2003a) or may become attached to a vector enabling seed transport en masse (Slay 2002c). These seed clusters
frequently hold seed in the canopy for a longer period than would otherwise be the case. The adaptive significance and
ecological implications of such seed retention is obscure (Groves and Whalley 2002). However seed held in the panicle over a
longer period will be accessible to a greater variety and intensity of dispersal factors, so may have adaptive advantages in areas
where the potential range of the plant has not been reached. Similar seed aggregrates formed by twisting together of awns occur
in a range of other grass tribes, e.g. the East African Acritochaete (Paniceae), in which the whole mass, or parts of it, may be
dispersed by attachment of the exposed calluses to passing animals (Davidse 1986).
Zoochory
Seed dispersal by animals (zoochory) occurs in more than half of all plant species, most commonly by ingestion (endozoochory)
and external attachment (exozoochory) (Stanton 2006), and also by deliberate animal carriage. N. neesiana seeds can be
dispersed via all of these processes.
Slay (2002a p. 15) recorded a report by C. Lee in New Zealand that unspecified birds “use awn/seed clusters for the building of
nests”. Such deliberate dispersal is probably of little significance, but may enable the plant to cross otherwise impenetrable
barriers. Dispersal of N. neesiana seeds by accidental attachment to birds does not seem to have been reported. Conole (1994)
observed Red-rumped Parrots, Psephotus haematonotus, “wading up to their bellies” in drifts of seed-bearing panicles of
Nassella trichotoma in southern Victoria, and suggested that they were highly likely to be exozoochorous dispersal agent of this
much smaller seeded Nassella species.
Endozoochory is certainly much rarer in birds than in mammals but in general has been very little studied (Whelan et al. 2008).
It may occur via faeces, regurgitated pellets, or secondarily via predator consumption of the bird (Twigg et al. 2009). Conole
(1994) also observed N. trichotoma seed consumption by Red-rumped Parrots and argued that a proportion of the seed ingested
could survive and be dispersed. Twigg et al. (2009) found that seed fed to finches, pigeons and ducks was generally passed in the
faeces within 0.3-5.0 hours, with longer passage times in the larger species, and that very few whole seeds survived the digestion
process. Most of the seeds tested were not grasses, but gut passage reduced the viability of wheat seeds by 33%, and significantly
reduced the viability of millet seeds. Finches and parrots are probably less likely to disperse seeds than pigeons because of their
efficient digestive systems, while generalist omnivorous/herbivorous birds are probably the most likely to dispese viable seed
endozoochorously (Twigg et al. 2009).
Some grasses in the Paniceae, Andropogoneae and Olyreae have evolved presumed elaiosomes (lipid-cotaining diaspore
appendages) that may attract ant seed dispersers (Davidse 1986). The structures, containing stable oils, occur in the rachilla,
pedicel, glume base or lemma. They are difficult to identify on herbarium specimens and no field observations that confirm their
function have been identified (Davidse 1986). In other plant families the eliasome is removed by the ants after carriage of the
diaspore to the nest, and the seed itself may often not be damaged. There appears to be no evidence of such eliasomes in Stipeae.
The panicle seed of N. neesiana is able to attach to a wide range of materials. Seeds attach to the coats of livestock and clothing,
and lodge on machinery (Gardener et al. 1999, Slay 2002a). The hairs and corona of the seed, and the twisting together of awns
of adjacent seeds that come into contact, can enhance the adhesion of seeds to objects which the callus cannot penetrate.
Transport of N. neesiana seeds on livestock has been recorded in New Zealand (Bourdôt and Ryde 1986) and Australia
(Gardener 1998). Grazing of sheep is the probable cause of spread in the Hawkes Bay area of New Zealand (Slay 2002c). Panicle
seeds are carried in the fleece of sheep (Connor et al. 1993) and can remain there for at least 166 days (Gardener and Sindel
1998). Larger grass seeds with long appendages are generally retained for longer periods in long pellage than in short, and
retention time is probably little affected by environmental factors (Stanton 2006). Gardener et al. (2003a) found that 25% of N.
neesiana seed naturally lodged in the wool of sheep remained after 5 months, and that shearing prior to seed production reduced
the lodgement rate. However lodged seed often lost their awns, so would have reduced dispersal, and probably survival ability
when shed from the fleece. A high proportion of lodged seed was subsequently shed (Gardener 1998), but details of natural seed
shedding from fleece are scanty, so sheep may not be particularly effective dispersal agents (Connor et al. 1993).
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