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

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