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

considerably less diverse than those of herb-rich grassy woodlands in the Grampians and Langi Ghiran areas of western Victoria
studied by Lunt (1990d).
Areas of grasslands on sedimentary soils and soils derived from granites, which are generally less fertile, are less productive so
grow much smaller grass tussocks and consequently tend to have greater floristic richness (Stuwe 1994).
Productivity
Little attempt appears to have been made to quantify the productivity of the temperate native grasslands of south-eastern
Australia, although there has been considerable interest in the amount of above-ground biomass accumulated by dominant
grasses, particularly T. triandra. In minimally disturbed (ungrazed and unburnt) communities the matrix species (dominant and
subdominant grasses) grow large, accumulate abundant litter and suppress the intertitial forbs (Trémont and McIntyre 1994).
Groves (1965) undertook a seminal study of a T. triandra grassland at St Albans. He determined the above-ground biomass of T.
triandra versus all other species and the root biomass for all species combined at 3-6 weekly intervals for 15 months from 1962
to 1964. The grassland had been burnt in the summer of 1961-62. T. triandra constituted the major component of above-ground
biomass througout the period, with a mimimum of 62% and a maximum of 90%. The standing biomass one year after fire was
about two thirds of that two years after the fire, but nevertheless fell to approximately the same level in summer in successive
years. Standing biomass fell rapidly to very low levels in mid summer after a late spring- early summer peaks. Maximum levels
exceeded 3000 kg ha -1 . Much dead biomass was apparently broken down or moved off site. Only a small fraction of root biomass
penetrated below 15 cm. The below-ground biomass followed similar trends to that above ground in one summer and not the
other, but consistently peaked in late spring and early summer. The maximum was c. 8000 kg ha -1 . In autumn, rapid growth
resumed, many seedlings appeared and a carpet of moss developed. Standing biomass in April was etimated at 1025 kg ha -1 .
During winter, growth was inhibited and many shoots of T. triandra died, but Austrodanthonia spp., Dichelachne crinita and the
exotic annual grass Rostraria cristata (L.) Tzvelev grew vigourously. In spring rapid growth of T. triandra resumed and the
native forbs Eryngium ovinum, Plantago varia and Wahlenbergia stricta grew. Growth ceased when soil moisture fell to the
permanent wiliting point. T. triandra growth rate peaks from October to early December and growth continued into summer if
there was adequate soil moisture, and there was a minor peak in autumn.
Morgan (1998e) calculated annual peak standing biomass for two T. triandra grasslands in the Victorian Volcanic Plains: 1300
kg ha -1 in December for a grassland at Derrinallum burnt annnually in February, and 2600 kg ha -1 for a grassland at Karrabeal,
burnt biennially in summer, two years after burning. Annual dry matter production in improved pastures in the New England
region of New South Wales varies from c. 8000 kg ha -1 in drought years to 16,000 kg ha -1 (Davidson 1982). Estimates for net
above-ground primary productivity of Flooding Pampa grasslands include 5320 kg ha -1 , with green standing crops of 1550-2220
kg ha -1 (Soriano et al. 1992).
Dynamics
The dynamics of temperate Australian grasslands cannot be adequately explained within the classical botanical framework of
succession, and there is no climax formation (Mott and Groves 1994). If the exotic components are disregarded, the composition
varies little over time, but widely on a patch scale in otherwise uniform areas (Mott and Groves 1994). Morgan (1998e)
investigated patch-scale (0.01 and 1 m 2 ) dynamics of exotic and native vascular species in Victorian volcanic plains grasslands
and found a 50% increase in cumulative species richness over 4 years, involving high turnover rates and high spatial mobility of
species, but little variation in mean species richness. Life-form characteristics were the main determinants of the patterns of plant
movement: annuals and geophytes tended to have higher turnover and mobility while hemicryptophytes often had low turnover.
The few species with large, persistent seed banks had high turnover, including exotic annual grasses. High turnover of geophytes
was somewhat illusory, being explained by their frequent dormancy and failure to produce above-ground parts, but this ‘pseudoturnover’
was displayed by c. 30% of species (Morgan 1998e). Such pseudo-turnover might be particularly significant with
Orchidaceae, which may remain dormant for several years (Smith et al. 2009). About 40% of species had low mobility at the 1
m 2 scale (Morgan 1998e).
‘Dispersal limitation’ at a range of scales is a feature of many native grassland systems (MacDougall andTurkington 2007) and
may in part result from loss of native animals that created safe sites for seedlings and dispersed seed, and low fecundity due to
loss of native pollinators. Frequent fire has been suggested to be the most important cause of the mobility patterns in Victorian
volcanic plains grasslands, but climatic variation may be important for some species (Morgan 1998e). Management history and
‘chance’ appear to be more important determinants of the status of a particular remnant than recurrent major disturbance (Mott
and Groves 1994). The absence of a successional climax means that the dynamics of Australian temperate grasslands are best
described by ‘state and transition’ models, with disturbance and management regimes determining the dynamics of the plant
components. These are discussed in detail below.
Areas of bare ground at sites with low vascular plant richness are mostly due to exogenous disturbance, while at sites with high
richness, they are more often the result of constrained production (McIntyre 1993) due to near-complete resource utilisation.
Sharp (1997) created 1 m 2 areas of bare ground experimentally using glyphosate herbicide in Dry T. triandra and
Austrodanthonia grasslands and studied colonisation of the gaps for 18 months. Native grass cover had not recovered to pretreatment
levels after 18 months, while exotic grass cover and richness initially increased, but after 18 months decreased to levels
similar to those prior to treatment. Native and exotic forb richness and cover was increased. Sharp (1997) also experimentally
removed litter in areas dominated by native grasses and tested combined treatments of gap formation by herbicides, litter
removal/retention, and soil disturbance (scarification to 2 cm depth). When plant litter was retained, native forb richness and
cover were higher than when litter was removed. The opposite response was found for exotic forbs. Soil scarification had no
significant effects on recruitment of species.
The dominant native grass can reduce species richness in intertussock spaces (McIntyre 1993). This is a common feature of
temperate grasslands around the world: in the absence of biomass reduction by fire or grazing, the highly productive dominant
casepitose grasses accumulate dead leaves and litter and gradually exclude forbs, “irrespective of their habitus” (Overbeck and
Pfadenhauer 2007). Intertussock spaces disappear in T. triandra grasslands not subject to regular biomass reduction because the
T. triandra plants accumulate large canopies of dead leaves, which reduce and eventually eliminate bare ground and inter-
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