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

close to the plant are able to bury themselves in the ground. Both are drought adapted. T. triandra has a C 4 photosynthetic
pathway, while the subdominant native grasses and N. neesiana are C 3 species. N. neesiana can form dense closed swards, with
high tussock densities, as does T. triandra.
One factor that may contribute to the superior competitive abilities of N. neesiana is its early-mid spring growth peak, which
coincides better with periods of high soil moisture than the late-spring-early summer T. triandra. In years when rainfall is
limiting to native grassland growth, and that may be most years, established N. neesiana presumably depletes soil moisture pools
that would otherwise be available for use by native forbs and by the later growing T. triandra. A similar process has been
demonstated in competition between seedlings of Bromus tectorum and native perennial grasses in the USA (Evans and Young
1972). Earlier growth may also enable preemption of any soil nutrient pools that form during autumn and winter. These two
impacts on the physical environment reinforce the benefits for N. neesiana: lower success of the dominant native grass means
more resources for N. neesiana in the next growing season. Cool season native grasses may thereby be advantaged.
N. neesiana reportedly excludes all other species (Kirkpatrick et al. 1995), but any long-lived grass may be able to exclude other
plants from the areas it occupies, i.e. effectively hold its ground under metastable management regimes, unless its competitors
have large advantages. Distel et al. (2008) suggested that dominance of unpalatable grasses under livestock grazing is a stable
vegetation state in central Argentine grasslands, caused by continual grazing pressure against palatable species, low seed banks
of palatable species and low availability of safe germination sites for the palatable species. There, successful establishment of
native perennial grasses requires adequate soil moisture in autumn and winter, and the replacement of dominant unpalatable
casespitose species by palatable species requires their destruction, e.g. by disc ploughing, otherwise they are “impervious to
invasion” (Distel et al. 2008).
Native grassland at any density and cover of the dominant native grass is resistant to N. neesiana invasion because N. neesiana
seed germination requires more sunlight than is present in dense swards and seedling survival requires a soil nutrient pool not
available unless existing vegetation is killed. Similarly, Barger et al. (2003) found that native Trachypogon plumosus Nees
grassland in Brazil, in the absence of soil disturbance and external fertiliser addition, was resistant to invasion of Melinis
minutiflora.
Reducing the impact of N. neesiana on biodiversity in grasslands can be achieved by:
1. Maintaining cover of the dominant grass Themeda triandra which is able to resistant invasion (Lunt and Morgan 2002,
Hocking 2005b).
2. Eliminating disturbance that kills native grassland plants, especially the dominant grasses. (e.g. better off not to spray weeds)
including vehicle traffic
3. Creation of larger buffer zones around native grassland, whether or not weed invaded and managing the weeds within that
zone so as to minimise propagule pressure on the grassland.
3. Burning T. triandra grasslands in late spring-early summer (after most forbs have flowered and fruited and before the main
growing period of T. triandra) so that bare ground is not created at a time when it can best be occupied by N. neesiana seedlings.
Hypotheses
1. Soil disturbance that kills dominant native grasses enables N. neesiana invasion.
2. N. neesiana reduces angiosperm diversity
3. N. neesiana supports a greater abundance and diversity of polyphagous native invertebrate phytophages than Themeda
triandra and other dominant native grasses.
N. neesiana may passively occupy voids created by disturbance.
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