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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arundinacea (Gardener et al. 2005). N. neesiana tillers pr<strong>of</strong>usely when grazed (McLaren, Stajsic and Iaconis. 2004) and<br />
produces large amounts <strong>of</strong> unpalatable flower stalks and very little leaf material during flowering and fruiting (Gardener et al.<br />
1996b, Gardener 1998, Grech et al. 2004, Grech 2007a). Stock avoid eating it in the reproductive stage even at stocking rates <strong>of</strong><br />
300 DSE ha -1 (Grech 2007a). However plants are palatable to livestock during much <strong>of</strong> the year (Grech et al. 2004, Grech<br />
2007a). Slay (2002a) reported observations that ewes grazed N. neesiana in preference to Dactylis glomerata and Lolium<br />
perenne in winter near Napier, New Zealand. It can be reduced by appropriately timed grazing at high intensity, but the stocking<br />
rates and practices required are generally unrealistic for <strong>Australia</strong>n conditions (Grech 2007a).<br />
The impact <strong>of</strong> goats on N. neesiana is unknown, but goats are able to substantially reduce infestations <strong>of</strong> N. trichotoma<br />
(McGregor 2003). Goats and alpacas have been considered as potential control agents in <strong>Australia</strong> (Bedggood and Moerkerk<br />
2002) but no trials appear to have been undertaken and no observations <strong>of</strong> N. neesiana herbivory by these species in <strong>Australia</strong><br />
appear to be on record. Studies <strong>of</strong> biotic resistance (e.g. Parker et al. 2006a) suggest that alpacas would preferentially utilise<br />
<strong>Australia</strong>n native plants or European <strong>grass</strong>es rather than N. neesiana or other South American <strong>grass</strong>es with which they may have<br />
coevolved.<br />
No studies <strong>of</strong> marsupial grazing <strong>of</strong> N. neesiana appear to have been undertaken. It is not recorded whether it is eaten by<br />
kangaroos and whether they prefer it to native <strong>grass</strong>es, as predicted by the biotic resistance hypothesis. Studies <strong>of</strong> the feeding<br />
preferences <strong>of</strong> Eastern Grey Kangaroos by Robertson (1985) indicate that species <strong>of</strong> Austrostipa and other <strong>grass</strong>es with sharp and<br />
long-awned seeds are avoided when their inflorescences are present, and that narrow-leaved species that develop substantial<br />
standing biomass <strong>of</strong> dead foliage that resticts selective foraging for green shoots suffer less damage. Thus macropod grazing <strong>of</strong><br />
N. neesiana is probably similar to that by domestic livestock, in that it is avoided when in flower and fruit and has partial<br />
protection <strong>of</strong> green shoots resulting from retention <strong>of</strong> dead biomass, but will be selectively consumed when it is comparatively<br />
more palatable and accessible.<br />
Mammal grazing selects for prostrate or low-growing ecotypes <strong>of</strong> Stipa (Peterson 1962) and many other perennial <strong>grass</strong>es, but<br />
the low-growing forms <strong>of</strong> many <strong>grass</strong>es are the result <strong>of</strong> phenotypic plasticity (Cox 2004). The so called ‘sward form’ <strong>of</strong> N.<br />
neesiana in <strong>Australia</strong> (Randall Robinson pers. comm.) characterised by a large number <strong>of</strong> semi-prostrate stems and more limited<br />
development <strong>of</strong> an uptright tussock form, appears to occur where mowing is frequent as well as in heavily grazed situations, but<br />
whether the plants resume a more normal form if these pressures are relaxed is unclear.<br />
Grazing <strong>of</strong> various European pasture <strong>grass</strong>es has been found to lead to the development <strong>of</strong> ecotypes characteristed by small size<br />
and increased silica concentrations (Tscharntke and Greiler 1995). The relationships between the silica defences <strong>of</strong> <strong>grass</strong>es and<br />
grazing are as yet poorly understood, but it may be presumed that the altered grazing pressures in the introduced environment<br />
will select for different leaf hairiness and phytolith pr<strong>of</strong>iles characters. The possibility that such selection has altered the<br />
morphology or other characteristics <strong>of</strong> N. neesiana in <strong>Australia</strong> has not been investigated.<br />
Physiology and biochemistry<br />
The dominant native <strong>grass</strong>es in <strong>Australia</strong>n temperate lowland <strong>grass</strong>lands are all C 3 species except for the major dominant<br />
Themeda triandra. Various other C 4 species commonly occur (e.g. Aristida spp., Dichanthium sericeum, Panicum spp.) and<br />
some form dense subdominant populations (e.g. Bothriochloa macra ) but none are dominant over wide areas. N. neesiana is a<br />
C 3 <strong>grass</strong>. C 3 and C 4 plants possess different biochemical pathways <strong>of</strong> photosynthetic carbon acquistion and consequently have<br />
markedly different leaf anatomy, different ratios <strong>of</strong> carbon isotopes in their tissues ( 13 C/ 12 C ratios or δ 13 C) and different<br />
responses to climatic factors (Hattersley 1986). The biochemistry <strong>of</strong> these differences is complicated and will not be explained<br />
here. In the simplest terms, the species that produce acids with four C atoms as the main initial stable product <strong>of</strong> CO 2 fixation are<br />
C 4 species, while those that produce the three-C acid 3-phosphoglyceric acid as the primary product are C 3 species (Salisbury<br />
and Ross 1992). There are three biochemical variants <strong>of</strong> C 4 photosynthesis in <strong>grass</strong>es, that differ in the main enzymes that<br />
catalyse the decaroboxylation <strong>of</strong> the four-C acids, the output products <strong>of</strong> that process and, with some additional compications,<br />
the leaf anatomy that supports the biochemistry (Hattersley 1986). Those that form malate utilise PEP carboxykinase and are<br />
called PCK species, while the two types that form aspartate utilise either NADP malic enzyme or NAD malic enzyme and are<br />
NADP-ME and NAD-ME species respectively (Hattersley 1986). Eleven differing biochemical/structural combinations have<br />
been identified. Andropogonanae have the classic structural NADP-ME type (Hattersley 1986), possessed also by T. triandra.<br />
C 3 <strong>grass</strong>es generally have a lower optimum temperature for photosynthesis, and grow better in cooler environments with less<br />
light than C 4 <strong>grass</strong>es, which <strong>of</strong>ten have temperature optima in the 30-40ºC range and are mostly native to unshaded environments<br />
(Monson 1989, Salisbury and Ross 1992, Sinclair 2002, Jessop et al. 2006), although some have a large ecological range<br />
(Christin et al. 2009). C 3 species are most numerous in areas with a cool, wet spring and become dormant in summer (Sinclair<br />
2002). In contrast C 4 <strong>grass</strong>es grow mostly in summer and are more efficient users <strong>of</strong> water and N, in terms <strong>of</strong> CO 2 assimilation,<br />
in high-light environments (Monson 1989). C 4 species have a competitive advantage when photorespiration costs become<br />
important, and are more efficeint in high temperatures and arid and saline environments (Christin et al. 2009) and much more<br />
efficient at producing biomass in warm environments with high light intensity (Salisbury and Ross 1992). The variations in the<br />
evolved biochemical-structural types <strong>of</strong> C 4 <strong>grass</strong>es likely have other ecophysiological and ecological implications, for instance<br />
differential leaf digestibility (Hattersely 1986).<br />
C 3 <strong>grass</strong>es tend to have shallower root systems than C 4 <strong>grass</strong>es, which require water for growth in summer when it is generally<br />
less available, so widespread replacement <strong>of</strong> C 4 with C 3 species may increase deep drainage to water tables, and contribute to<br />
dryland salinity (Sinclair 2002).<br />
Reactions <strong>of</strong> C 3 and C 4 species to increasing atmospheric CO 2 are complex, with neither group necessarily advantaged (Sinclair<br />
2002). The C 4 species grow more rapidly under elevated CO 2 levels and it is likely that they will expand southwards, but this is<br />
dependant on the N usage <strong>of</strong> C 3 spccies under the changed conditions (Keith 2004).<br />
Little is known <strong>of</strong> the specific biochemistry <strong>of</strong> N. neesiana, however a photosystem IIA protein, A9YFV6, has been described<br />
from N. neesiana chloroplasts (Boeckmann et al. 2003). Yeoh and Watson (1986) included N. neesiana in a study <strong>of</strong> the<br />
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