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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In New Zealand, Hurrell et al. (1994) measured a soil seed bank (to about 10 cm depth) under old established stands <strong>of</strong> 2,600-<br />
35,000 seeds m -2 , with an average density <strong>of</strong> 10,500 seeds m -2 , <strong>of</strong> which approximately 70% were cleistogenes, with few or no<br />
aerial seeds in some samples. Tetrazolium testing showed 58% viability <strong>of</strong> cleistogenes and 86% <strong>of</strong> aerial seeds, with a total<br />
average viability <strong>of</strong> 67%. In pastures at Waipawa, New Zealand, the seed bank was composed almost entirely <strong>of</strong> cleistogenes<br />
(Slay 2002c), with a mean <strong>of</strong> 6437 ±3437 viable cleistogenes and 660 ±1289 viable panicle seeds m -2 (Slay 2001). Bourdôt and<br />
Hurrell (1992) found 4000-18,000 viable seeds m -2 under pasture, 99% in the top 25 mm and 0% below 100 mm, on a silt loam<br />
at Marlborough. Viability (as determined by tetrazolium treatment) <strong>of</strong> seed buried in polyproylene mesh bags for 6 years in this<br />
soil increased with increasing depth <strong>of</strong> burial, 24% <strong>of</strong> seed remaining viable at 25 cm depth, 17% at 5 cm, 5% at 2.5 cm and<br />
0.1% for soil at the surface (Bourdôt and Hurrell 1992). Analysis <strong>of</strong> decay data suggested that an increaing proportion <strong>of</strong> buried<br />
seed survived at increasing depth indefinitely in a state <strong>of</strong> dormancy, whereas no surface seed was viable after 1 year (Bourdôt<br />
and Hurrell 1992). However there are doubts about the reliability <strong>of</strong> tetrazolium tests for determination <strong>of</strong> seed viability (Puhar<br />
and Hocking 1996). Seeds that are hard when pressed between the thumb nail and forefinger are “almost certainly viable”<br />
(Bourdôt 1988).<br />
In pastures in New Zealand in the absence <strong>of</strong> seed input, annual depletion rates <strong>of</strong> the soil seed bank were 38% when “regularly”<br />
mown to prevent panicle seeding, 61% with repeated glyphosate applications, and between 66% and 77% for single to repeated<br />
annual cultivations (Bourdôt and Hurrell 1992).<br />
Smaller seed banks have been described in New England Tablelands pastures by Gardener (1998) who made direct counts <strong>of</strong><br />
loose seeds in the soil to 4 cm depth, with all seeds considered to be <strong>of</strong> panicle origin (Gardener et al. 2003b p. 622). Under<br />
dense infestations 681-11,307 viable seeds m -2 were found. If basal cleistogenes, contained in tiller bases and not loose in the<br />
soil, are included, the estimated total seed numbers were increased by 35.5% (Gardener and Sindel 1998) or 20% (Gardener et<br />
al. 2003b). Within N. neesiana tussocks 44.1% <strong>of</strong> seeds were panicle seeds and 55.9% basal cleistogenes (Gardener et al.<br />
2003b).<br />
In native <strong>grass</strong>lands and <strong>grass</strong>y woodlands in the Melbourne area the viable seed bank appears to be substantially smaller. Seed<br />
banks at Iramoo Wildlife Reserve determined by Bram Mason (unpublished) were up to 7000 m -2 (Robinson 2003 2005).<br />
Beames et al. (2005) assessed panicle seed banks in areas <strong>of</strong> high quality native <strong>grass</strong>land at Laverton and Grey Box woodland at<br />
Melbourne Airport that had been subject to a range <strong>of</strong> different management regimes to control N. neesiana. They separated the<br />
seeds into four categories: filled, unfilled (caryopsis absent), successful germinants (germinating at the time <strong>of</strong> sampling) and<br />
unsuccessful germinants. At Laverton a maximum seed bank (seed <strong>of</strong> all categories) <strong>of</strong> ca. 2200 m -2 was found, however unfilled<br />
and unsucessfully germinating seed accounted for ca. 90% <strong>of</strong> seed. In areas subject to N. neesiana management, seed bank<br />
numbers did not exceed 1000 m -2 , with similar proportions in each seed category, except for filled seed, which comprised a<br />
much smaller proportion in two <strong>of</strong> the managed areas. At Melbourne Airport the maximum seed bank exceeded 7000 m -2. The<br />
unburnt, unsprayed treatement had a significantly smaller seed bank than most <strong>of</strong> the managed areas. Significantly more filled<br />
seed was found outside managed areas, which in most cases had zero filled seed. One major conclusion was that most <strong>of</strong> these<br />
differences were the result <strong>of</strong> reduced seed input due to ongoing herbicide treatment, which possibly reduces the proportion <strong>of</strong><br />
viable seed produced and seed persistence. The study indicates that the seed bank in non-agricultural areas is much less<br />
persistent and easier to deplete than had previously been assumed.<br />
Hocking (2005b) reported even lower levels in infestations subject to herbicidal control in southern Victora: 80% unviable seed. He suggested that the size <strong>of</strong> the seed bank may be widely variable on a<br />
regional basis and between sites managed for agriculture and conservation, and that agents in the soil at some sites may be<br />
destroying a high proportion <strong>of</strong> seed. Hocking (in Iaconis 2006b) reported no differences in the soil seed banks <strong>of</strong> agricultural<br />
and natural areas, but high variability between sites, and >50% <strong>of</strong> the seeds unfilled.<br />
In New England pastures, dramatically better fruiting in wetter years (Gardener et al. 1996a) led to major addition to the seed<br />
bank (e.g. 41.6% <strong>of</strong> seeds incorporated in 1996), but in drier years input was only sufficient to maintain existing seed bank<br />
numbers or inadquate to maintain pre-existing levels (Gardener et al. 2003b).<br />
Rates <strong>of</strong> decline <strong>of</strong> the seed bank determined for New England pastures without input after 3 y, were 4676 to 1323 seeds m -2 in<br />
bare plots and 4585 to 1507 seeds m -2 in vegetated plots, with no significant effect <strong>of</strong> ground cover on decline, and a predicted<br />
decline without input (based on a fitted exponential decay curve) to 10 seeds m -2 after 12.4 y (Gardener et al. 1999, Gardener et<br />
al. 2003b). The seed bank longevity was found to be >6 y (Gardener and Sindel 1998) and half-life 1.3 y (Gardener et al.<br />
2003b). There is anecdotal evidence <strong>of</strong> gemination from continuously bared ground after 6 y (Gardener and Sindel 1998).<br />
Where soil cracking occurs in summer, as in the clays <strong>of</strong> the Victorian basalt plains, seed dropped in late spring and early<br />
summer will certainly move into the soil to greater depths than was sampled in the two <strong>Australia</strong>n studies, which both assumed a<br />
similar depth pr<strong>of</strong>ile <strong>of</strong> the seed bank to that found on a silt loam in New Zealand by Bourdôt and Hurrell (1992). This may be<br />
especially significant since viability <strong>of</strong> seed increases with depth <strong>of</strong> burial (Bourdôt and Hurrell 1992). Grasses that produce<br />
relatively few large seeds “<strong>of</strong>ten emerge from seed located deeper in the soil …where … water is available for a longer time …<br />
and many have high seedling growth rates” (Groves and Whalley 2002).<br />
Germination and seedling recruitment<br />
Seed dormancy<br />
The panicle seed have dormancy - the tight lemma may provide a barrier to water and gas exchange and mechanically constrain<br />
the embryo (Gardener and Sindel 1998) - and appear to have an after-ripening requirement after falling from the plant <strong>of</strong><br />
between 3 and 12 months for maximum germination (Gardener et al. 1999, Gardener et al. 2003b). The overlapping margins <strong>of</strong><br />
the lemma and its toughness make it difficult to break open to expose the caryopsis, a feature also <strong>of</strong> Austrostipa spp. (Barkworth<br />
2006). Puhar (1996) found by staining with tetrazolium chloride that 93.5% <strong>of</strong> N. neesiana seeds collected in the previous<br />
summer were viable, but in laboratory tests less than 2% germinated under a day/night cycle <strong>of</strong> 30/20ºC and 14/10 hr light.<br />
Removal <strong>of</strong> the lemma enabled 100% germination within three days under the same conditions.<br />
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