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

of T. triandra are able to more effectively trap rainfall and reduce runoff than swards of C 3 grasses, so soil moisture from spring
rainfall appears to be effectively preserved for later use (Dunin and Reyenga 1978, Dunin 1999, Singh et al. 2003). Grazing may
have resulted to general dessication of the system: reduced rainfall infiltration due to soil compaction, general opening of the
sward and destruction of deep rooted perennials with access to subsurface soil moisture.
Currently most of the pasture legumes and some of the deliberately introduced grasses have poor persistence under livestock
grazing, due in part to the prevailing high variability and extremes of climate, difficulty in managing grazing intensity, and to
their near-monoculture nature (pastures with one grass + one legume, or just a single legume species, with volunteer broadleaf
species) (e.g. Madin 1993) which makes them more prone to pest and disease attack. Continued increased nitrification by
legumes has enabled invasion by nitrophilous species such as thistles, while continued use of superphosphate has resulted in
widespread soil acidification (Moore 1993). These ‘improved’ pastures are unstable, requiring ongoing inputs or renovation
(McIntyre and Lavorel 2007).
McIntyre and Lavorel (2007) presented a unifying conceptual model of the states and the transition processes for the grasslands
of south eastern Australia that exemplifies historical chain of developments (Fig. 5). Transitions from one state to another occur
as a result of specific managment activity (or the lack of it). Invaded grasslands with moderate or high exotic components have
been permanently altered (Mack 1989) and appear to represent new metastable states that do not revert to their former status
(Sharp 1997). Native pasture may revert back to native grassland, but its properties and composition will have been more or less
altered. McIntyre and Lavorel (2007) introduced the concept of “enriched grassland”, to include those areas no longer cultivated
or managed for grazing, but which remain nutrient enriched. These are largely dominated by robust exotic perennial pasture
grasses including Paspalum dilatatum, Dactylis glomerata and Phalaris aquatica, along with exotic rosette-forming herbs such
as Plantago lanceolata and Hypochoeris radicata. Much of the enriched grassland is currently in shelterbelts and other areas of
recent tree and shrub planting (McIntyre and Lavorel 2007) although many former pastures on the edges of urban areas are in
this state, and some disused pastures dominated by Nassella spp. can be included in this category. Little is known about the
floristics, functioning and successional dynamics of enriched grasslands but they appears to be “an alternative stable state
[requiring] a very high level of management to shift” (McIntyre and Lavorel 2007p. 15). Management inputs required include
nutrient depletion, weed control (including the dominant grasses) and reintroduction of native species, particularly forbs. A
similar set of problems is faced in extensification of former pastures to species rich grassland in Europe (Eschen et al. 2007).
Figure 5. State and transition model for temperate grassy woodlands of south-eastern Australia. Each state is characterised by a
level of livestock grazing, soil disturbance and soil fertility, straight arrows indicate mangagement factors associated with
transitions between state and ciruclar arrows indicate management associated with maintenance of the state. Source: McIntyre
and Lavorel 2007, p. 14.
Fertilised and sown pastures are considered to be the most unstable states because of their ongoing requirements for inputs
(McIntyre and Lavorel 2007). Removal of inputs results in transition to enriched grassland or to a native pasture. Transition from
reference grassland to enriched grassland generally occurs accidentally through nutrient enrichment from fertiliser drift or water
movement. Changes in composition and function of areas that have ‘regressed’ to native pasture are probable, but currently
unknown.
There are pronounced differences in scientific understanding of the states and processes. This reflects the historical agronomic
approach to the study of agricultural land, and the use of floristic ecological techniques in natural grasslands. The overall
floristics of sown pastures and of enriched grassland are very poorly known, while there is little data for native forbs in fertilised
112