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