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

endozoochory in terms of non-basal stem cleistogenes, a repeller of grazers at the time of panicle seed production, and a ‘sit and
wait’ strategist in terms of basal cleistogenes.
Wind
Carr (1993) listed wind as a dispersal agent, but the panicle seeds have no particular adaptations for wind dispersal. In windless
conditions the seed falls vertically (Slay 2002a). Of the 39% of panicle seed recovered in a wind dispersal experiment, Gardener
et al. (2003a) found none more than 2.8 m from the centre of the source plant, and the majority of seed within 1 m. However
these findings may create a misleading impression about the frequency of wind dispersal and the distance that wind may carry
the seed. Small scale, short duration, high intensity atmospheric turbulence events have a very strong impact on aerial transport,
and strong vertical winds associated with thunderstorms can lift seeds that lack special wind dispersal adaptations (Nathan et al.
2005). Seeds could certainly be blown along flat surfaces such as roads, possibly assisted by vehicle eddy or suction currents
(Barwick 1999). Willy willies (small whirlwinds) that carry large amounts of loose plant debris occur frequently in summer in
Australia, particularly in inland areas. Surprisingly large grass seeds can be lifted to high altitudes by natural processes, e.g. the
spikelets of Paspalum spp. have been obtained by aircraft sampling at altitudes of up to c. 1500 m in Louisiana, USA (Hitchcock
and Chase 1971).
Slay (2001 2002c) noted that panicles and stems can be blown short distances by strong winds, resulting in dispersal of
cleistogenes. Wind dispersal of late maturing seed on ‘secondary panicles’ could presumably occur more readily since the
remainder of the panicle, with only glumes attached, could be more readily lifted and carried.
Water
Extensive distribution along floodways and watercourses has led to the inference that movement of N. neesiana seeds in flowing
water is important (Frederick 2002). According to Bourdôt and Ryde (1986) seeds are “carried along water courses, giving rise
to isolated patches of the plant”. Slay (2002b) stated that running water disperses seed. Hayward (Hayward and Druce 1919)
found plants along the river bank downstream of wool processing factories in Scotland. Cook (1999 p. 91) stated that N.
neesiana is a weed of “flood zones” and Iaconis (2003) stated that flood waters are responsible for dispersal. In urban and periurban
Canberra, seeds have possibly been dispersed widely in the drainage system (Jenny Connolly and Sarah Sharp pers.
comms. 2006). Bedggood and Moerkerk (2002 p. 6) state that “run-off water can carry seed from one property to another”. There
is very little detail to support these claims, which mostly appear to be based on the pattern of distribution of infestations in small
catchment areas. No published information appears to be available on the buoyancy of the seeds, their presence in flood debris,
etc. Preliminary observations indicate that they remain afloat in still water for at least 4 days. The awns quickly become entirely
straight while acquiring extreme flexibility, and the seed becomes ‘sperm-like’, a characteristic that would enhance more rapid
and effective carriage in moving water.
Human activities
The consensus view among most commentators is that human activities, little mediated by domestic animals or physical
environmental factors are the major cause of dispersal in Australia (Snell et al. 2007), but the vector strength and tempo are
unknown. Roadside management activities including slashing, mowing and grading, particularly when the plants are seeding
(Frederick 2002), are generally the inferred causes. Roadsides carry some of the densest Australian infestations (Snell et al.
2007) and infestations in a new area frequently occur first along roads. Transport of seeds in hay has been recorded in New
Zealand (Bourdôt and Ryde 1986) and dispersal in contaminated fodder has been called “a primary mechanism” of dispersal
(Frederick 2002 citing Liebert 1996), although there appear to be no specific incidents of such dispersal on the public record in
Australia.
Seeds are said to “adhere” to machinery “via [the] ... callus” (Gardener and Sindel 1998 p. 77) but there are few points on
vehicles and machines which the sharp end of the callus could penetrate, so such adhesion presumably involves the callus hairs
and a range of leverage options with the callus tip, lemma body and awn. The actual mode of lodgement needs to be far better
described. For instance, penetration of rubber vehicle tyres has not been reported – piercing by the callus does not seem to be a
factor. Various characteristics of the whole seed alone and in aggregates are responsible for attachment.
Linear distribution of infestations along roadsides and vehicle paths is widespread and commonplace (Bruce 2001, Frederick
2002, ?Sharp 2002) but this could have a variety of causes unrelated to direct movement attached to machinery or vehicles. In
particular roadsides are subject to a variety of disturbances such as mowing, soil compaction and pollution that can reduce the
competitive abilities of native vegetation (von der Lippe and Kowarik 2007a). Studies of the seed rain from motor vehicles in
long road tunnels along a German motorway have demonstrated that roadsides are “preferential migration corridors” for invasive
plants and that long distance dispersal (> c. 200 m) is routine (Von der Lippe and Kowarik 2007a). Grass species accounted for
four of the 20 most frequent species in the seed rain (including the two cereal crop species Triticum aestivum, L. and Secale
cereale L., plus Poa annua L., and Lolium perenne L.), but at least one long-awned grass Bromus tectorum L., highly invasive in
the USA, was represented. Half of the 204 species detected were not local natives. Seeds of 42% of the regional roadside flora
were found in samples, 69% of the species sampled in the tunnels were found growing within 100 m of the tunnel entrances and
and 98.5% of all seeds sampled were from species in the regional roadside flora. Approximately one third of the species found
were not present in areas near the tunnel entrances. Non-native species were more often subject to long distance dispersal than
natives. Seeds represented