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
You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
considerably less diverse than those <strong>of</strong> herb-rich <strong>grass</strong>y woodlands in the Grampians and Langi Ghiran areas <strong>of</strong> western Victoria<br />
studied by Lunt (1990d).<br />
Areas <strong>of</strong> <strong>grass</strong>lands on sedimentary soils and soils derived from granites, which are generally less fertile, are less productive so<br />
grow much smaller <strong>grass</strong> tussocks and consequently tend to have greater floristic richness (Stuwe 1994).<br />
Productivity<br />
Little attempt appears to have been made to quantify the productivity <strong>of</strong> the temperate native <strong>grass</strong>lands <strong>of</strong> south-eastern<br />
<strong>Australia</strong>, although there has been considerable interest in the amount <strong>of</strong> above-ground biomass accumulated by dominant<br />
<strong>grass</strong>es, particularly T. triandra. In minimally disturbed (ungrazed and unburnt) communities the matrix species (dominant and<br />
subdominant <strong>grass</strong>es) grow large, accumulate abundant litter and suppress the intertitial forbs (Trémont and McIntyre 1994).<br />
Groves (1965) undertook a seminal study <strong>of</strong> a T. triandra <strong>grass</strong>land at St Albans. He determined the above-ground biomass <strong>of</strong> T.<br />
triandra versus all other species and the root biomass for all species combined at 3-6 weekly intervals for 15 months from 1962<br />
to 1964. The <strong>grass</strong>land had been burnt in the summer <strong>of</strong> 1961-62. T. triandra constituted the major component <strong>of</strong> above-ground<br />
biomass througout the period, with a mimimum <strong>of</strong> 62% and a maximum <strong>of</strong> 90%. The standing biomass one year after fire was<br />
about two thirds <strong>of</strong> that two years after the fire, but nevertheless fell to approximately the same level in summer in successive<br />
years. Standing biomass fell rapidly to very low levels in mid summer after a late spring- early summer peaks. Maximum levels<br />
exceeded 3000 kg ha -1 . Much dead biomass was apparently broken down or moved <strong>of</strong>f site. Only a small fraction <strong>of</strong> root biomass<br />
penetrated below 15 cm. The below-ground biomass followed similar trends to that above ground in one summer and not the<br />
other, but consistently peaked in late spring and early summer. The maximum was c. 8000 kg ha -1 . In autumn, rapid growth<br />
resumed, many seedlings appeared and a carpet <strong>of</strong> moss developed. Standing biomass in April was etimated at 1025 kg ha -1 .<br />
During winter, growth was inhibited and many shoots <strong>of</strong> T. triandra died, but Austrodanthonia spp., Dichelachne crinita and the<br />
exotic annual <strong>grass</strong> Rostraria cristata (L.) Tzvelev grew vigourously. In spring rapid growth <strong>of</strong> T. triandra resumed and the<br />
native forbs Eryngium ovinum, Plantago varia and Wahlenbergia stricta grew. Growth ceased when soil moisture fell to the<br />
permanent wiliting point. T. triandra growth rate peaks from October to early December and growth continued into summer if<br />
there was adequate soil moisture, and there was a minor peak in autumn.<br />
Morgan (1998e) calculated annual peak standing biomass for two T. triandra <strong>grass</strong>lands in the Victorian Volcanic Plains: 1300<br />
kg ha -1 in December for a <strong>grass</strong>land at Derrinallum burnt annnually in February, and 2600 kg ha -1 for a <strong>grass</strong>land at Karrabeal,<br />
burnt biennially in summer, two years after burning. Annual dry matter production in improved pastures in the New England<br />
region <strong>of</strong> New South Wales varies from c. 8000 kg ha -1 in drought years to 16,000 kg ha -1 (Davidson 1982). Estimates for net<br />
above-ground primary productivity <strong>of</strong> Flooding Pampa <strong>grass</strong>lands include 5320 kg ha -1 , with green standing crops <strong>of</strong> 1550-2220<br />
kg ha -1 (Soriano et al. 1992).<br />
Dynamics<br />
The dynamics <strong>of</strong> temperate <strong>Australia</strong>n <strong>grass</strong>lands cannot be adequately explained within the classical botanical framework <strong>of</strong><br />
succession, and there is no climax formation (Mott and Groves 1994). If the exotic components are disregarded, the composition<br />
varies little over time, but widely on a patch scale in otherwise uniform areas (Mott and Groves 1994). Morgan (1998e)<br />
investigated patch-scale (0.01 and 1 m 2 ) dynamics <strong>of</strong> exotic and native vascular species in Victorian volcanic plains <strong>grass</strong>lands<br />
and found a 50% increase in cumulative species richness over 4 years, involving high turnover rates and high spatial mobility <strong>of</strong><br />
species, but little variation in mean species richness. Life-form characteristics were the main determinants <strong>of</strong> the patterns <strong>of</strong> plant<br />
movement: annuals and geophytes tended to have higher turnover and mobility while hemicryptophytes <strong>of</strong>ten had low turnover.<br />
The few species with large, persistent seed banks had high turnover, including exotic annual <strong>grass</strong>es. High turnover <strong>of</strong> geophytes<br />
was somewhat illusory, being explained by their frequent dormancy and failure to produce above-ground parts, but this ‘pseudoturnover’<br />
was displayed by c. 30% <strong>of</strong> species (Morgan 1998e). Such pseudo-turnover might be particularly significant with<br />
Orchidaceae, which may remain dormant for several years (Smith et al. 2009). About 40% <strong>of</strong> species had low mobility at the 1<br />
m 2 scale (Morgan 1998e).<br />
‘Dispersal limitation’ at a range <strong>of</strong> scales is a feature <strong>of</strong> many native <strong>grass</strong>land systems (MacDougall andTurkington 2007) and<br />
may in part result from loss <strong>of</strong> native animals that created safe sites for seedlings and dispersed seed, and low fecundity due to<br />
loss <strong>of</strong> native pollinators. Frequent fire has been suggested to be the most important cause <strong>of</strong> the mobility patterns in Victorian<br />
volcanic plains <strong>grass</strong>lands, but climatic variation may be important for some species (Morgan 1998e). Management history and<br />
‘chance’ appear to be more important determinants <strong>of</strong> the status <strong>of</strong> a particular remnant than recurrent major disturbance (Mott<br />
and Groves 1994). The absence <strong>of</strong> a successional climax means that the dynamics <strong>of</strong> <strong>Australia</strong>n temperate <strong>grass</strong>lands are best<br />
described by ‘state and transition’ models, with disturbance and management regimes determining the dynamics <strong>of</strong> the plant<br />
components. These are discussed in detail below.<br />
Areas <strong>of</strong> bare ground at sites with low vascular plant richness are mostly due to exogenous disturbance, while at sites with high<br />
richness, they are more <strong>of</strong>ten the result <strong>of</strong> constrained production (McIntyre 1993) due to near-complete resource utilisation.<br />
Sharp (1997) created 1 m 2 areas <strong>of</strong> bare ground experimentally using glyphosate herbicide in Dry T. triandra and<br />
Austrodanthonia <strong>grass</strong>lands and studied colonisation <strong>of</strong> the gaps for 18 months. Native <strong>grass</strong> cover had not recovered to pretreatment<br />
levels after 18 months, while exotic <strong>grass</strong> cover and richness initially increased, but after 18 months decreased to levels<br />
similar to those prior to treatment. Native and exotic forb richness and cover was increased. Sharp (1997) also experimentally<br />
removed litter in areas dominated by native <strong>grass</strong>es and tested combined treatments <strong>of</strong> gap formation by herbicides, litter<br />
removal/retention, and soil disturbance (scarification to 2 cm depth). When plant litter was retained, native forb richness and<br />
cover were higher than when litter was removed. The opposite response was found for exotic forbs. Soil scarification had no<br />
significant effects on recruitment <strong>of</strong> species.<br />
The dominant native <strong>grass</strong> can reduce species richness in intertussock spaces (McIntyre 1993). This is a common feature <strong>of</strong><br />
temperate <strong>grass</strong>lands around the world: in the absence <strong>of</strong> biomass reduction by fire or grazing, the highly productive dominant<br />
casepitose <strong>grass</strong>es accumulate dead leaves and litter and gradually exclude forbs, “irrespective <strong>of</strong> their habitus” (Overbeck and<br />
Pfadenhauer 2007). Intertussock spaces disappear in T. triandra <strong>grass</strong>lands not subject to regular biomass reduction because the<br />
T. triandra plants accumulate large canopies <strong>of</strong> dead leaves, which reduce and eventually eliminate bare ground and inter-<br />
100