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

Dispersal of seed in contaminated soil has been recognised as a dispersal mechanism (Muyt 2001, Snell et al. 2007). Spread of
seed along roadsides by graders and other earthmoving equipment was mentioned by Bourdôt and Ryde (1986), but no further
information was provided.
Liebert (1996) observed that slashing of the plant while seeding was a “primary mechanism” for dispersal. Trengrove (1997) also
observed that dispersal along roadsides is caused by slashers operating at the time of seed set. Bruce (2001) found that N.
neesiana was generally more abundant at sites in the ACT that were mown to some extent, that it was never absent from areas
that were entirely mown, and at infested sites where mowing occurred it was generally spreading from mown into unmown
areas, in some cases very obviously along mown walking tracks into native grasslands. In fact, the overall distribution of N.
neesiana in the ACT correlated extremely well with mown areas (Bedggood and Moerkerk 2002). Ens (2002a) suggested seed
carriage on mowing equipment was the most likely explanation for a number of the infestations she examined in the Sydney
area. Sharp (2002) noted continued expansion of infestations along roadsides in the ACT “despite efforts to control spread with
more appropriate management practices”.
Moerkerk (2005a 2005b 2006a 2006b) analysed the plant propagules found in material manually cleaned from vehicles and
machinery used in natural resource mangement activities, particularly weed management, in Victoria. The flora of each vehicle
reflected the flora of the region in which was used. Nearly three times as many species of Poaceae were found in ‘clean-downs’
than the next most abundant family, and grass species accounted for 6 of the 10 most frequently detected contaminants. N.
neesiana was detected on 5 of 106 State government, local government and private contractor vehicles and machines sampled: 4
passenger vehicles and 1 slasher. A four wheel drive utility vehicle used by the Shire of Hume (an area where N. neesiana is
common) cleaned in June 2005, yielded 24 weed species including N. neesiana, from 23% of the 810 g of material removed. A
Hume Shire tractor and slasher cleaned in June 2005 yielded 26 spp., including N. neesiana, from 27% of 178 g of material. N.
neesiana was found in the cabin, engine bay and tray of vehicles and in the wheel guards and chassis of two vehicles. Noxious
weeds were mostly frequently found in the cabins and engine bays of passenger vehicles. 39% of passenger vehicles and 29% of
machinery items were found to be carrying noxious species. Of the vehicle types examined, 4WD utility vehicles had the highest
rates of contamination, while tractors with attached slashers, and graders had the highest rates of contamination (40%) of the
machinery types examined.
Grech (2005b) ‘cleaned-down’ a four-wheel-drive utility vehicle used on a property infested with N. neesiana using brushes,
manual plucking and high pressure water. The latter method was found to be ineffective and failed to dislodge entangled masses
of N. neesiana seeds. Weighing of the removed material indicated in excess of 10,000 N. neesiana seeds, “equivalent in volume
to a medium sized couch cushion”.
These studies identified important potential vectors but failed to determine under what conditions seed become attached and are
deposited in potentially suitable sites, and the actual spatial effectiveness of vehicles as seed vectors. Transport vehicles have
much higher potential for long distance dispersal and are perhaps mainly to blame for inter-regional dispersal,while machinery is
likely of greater importance at a local scale.
Slashing machinery actively disperses seed around the slasher. Detailed investigations have been undertaken of one particular
model of slasher in relation to N. neesiana seed dispersal (Erakovic et al. 2003, Erakovic 2005). Rotating slasher blades caused
upward air movement within the slasher frame, and high outlet velocities occured at the front of the slasher deck where the
cutting process actually occured. At the commonly used slash height of 7 cm with this machine, most of the slashed material was
deposited within the slashed strip. Over 98% of the expelled material (not deposited in the slashed strip) fell within 1 m and the
remainder within 2 m, with more deposited on the left of the slashed strip than on the right (Erakovic 2005). Sigificant amounts
of slashed chaff and seeds were deposited on the top of the slasher unit, and significant amounts lodged in the debris ejection
protector chains (Erakovic et al. 2003). Pronounced accumulation of seeds on the top front of the deck resulted from direct
dislodgement of seed from the plant that never came into contact with the slasher blades (Erakovic 2005).
More importantly, slashing was also found to disperse seed over longer distances if the slasher was not kept clean. Seed adhered
inside the deck of the slasher in crevices and boltholes, and slashed material accumulated around front and rear internal walls,
the blade shaft and in corners (Erakovic 2005). Decontamination and cleaning was time-consuming and tedious, and it was very
difficult and unsafe to clean the underside of slasher decks in the field (Erakovic 2005). The use of counter-rotating twin blades
and other improvements in a new slasher design, plus the development of slasher accessories (covers and shields, flaps to replace
chains) showed great promise of reducing these problems (Erakovic 2005).
A range of other dispersal processes mainly related to trade and commerce have been implicated in N. neesiana spread. Seed has
been spread in hay bales in New Zealand (Slay 2002c) and movement of contaminated fodder has been identified as likely (Muyt
2001). Some infestations in New Zealand have originated from contaminated pasture seed (Connor et al. 1993, Slay 2002a),
sown as recently as 1980 (Slay 2002c). Spanish infestations may have originated in cereals imported from Argentina (Verloove
2005). Introduction in railway traffic may be the origin of a large population at Bédarieux-Nissergues in France (Verloove 2005).
Cultivation can carry whole plants and seed within paddocks (Slay 2002c). Slay (2002a p. 11) noted that a small infestation at
Waipawa, New Zealand, was thought to have been “spread by lawn mower/lawn clippings”.
Like N. neesiana, other Stipeae spp. have been carried internationally in solid ship ballast, e.g. Amelichloa brachychaeta from
Argentina and N. chilensis (Trin. and Rupr.) E. Desv. from Chile to Portland, Oregon, USA (Hitchcock and Chase 1971), the
latter species “once collected” and not established (Barkworth 2006). Solid ballast such as beach sand and rocks began to be
replaced by water ballast in the late 1870s and all large ships now use it (Jones 1991), so this invasion pathway is now probably
largely obsolete.
Pollen
Pollen (the male gametophyte) is also dispersed. Grasses in general are wind pollinated and produce large amounts of pollen, and
the pollen concentration downwind decreases at a rate inversely proportional to the square of the distance from the source
(Connor 1986). Pollen may have an important role in gene flow and in promoting plant invasions (Petit 2004). For plants as a
61