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

Based on a study of a three Asteraceae in New England Tablelands grasslands, a rare native Microseris sp., a rare to common
native Cymbonotus lawsonianus Gaudich. and an abundant exotic Hypochoeris radicata L., McIntyre (1995) argued that many
native grassland forbs must have highly specific ecological requirements for regeneration that are rarely being met under current
mangement regimes, and that many of these requirements might possibly be met by the types of exogenous disturbances often
considered to be harmful management practices. For example, a suite of endangered grassland forbs appear to require soil
disturbance, although the general result of soil disturbance is a decline of rare native species and enhanced cover of exotics
(McIntyre 1995). This apparent paradox may be best resolved by determination of the type and nature of ‘natural’, endogenous
grassland disturbances that have been lost, or now occur in few areas or at a different frequency (e.g. soil disturbance by
marsupial foraging or burrowing), and by more detailed examination of processes and disturbances that occur in a range of
marginal habitats in which rare and uncommon species persist or from which they have disappeared.
Morgan (1998) investigated germination responses of many of the forbs, none of which are promoted by darkness (i.e. seed
burial) (Groves and Whalley 2002).
Robinson (2003) compared the survival of planted seedlings of Podolepis sp. 1 and Bulbine semibarbata in disturbed and
undisturbed soil of a Victorian basalt plains grassland. Much greater survival of seedlings occurred on the disturbed soil. In the
same grassland, Reynolds (2006) compared the establishment of seedlings of five native forb species (Podolepis sp. 1, Ptilotus
spathulatus, Velleia paradoxa, Rutidosis leptorhynchoides and Pycnosurus chrysanthes) after surface sowing of seed and
planting of seedlings on disturbed and undisturbed soil. Plots were treated with mowing, raking, repeated herbicide treatments
over a period of a few months and fire to remove all above ground plant biomass prior to sowing. The disturbed soil was dug
with a spade to a depth of at least 20-30 cm. Six months after sowing the proportion of seed that produced established seedlings
on undisturbed soil did not exceed 2%, while in disturbed soil the proportion that established was 2-65%. Four species grown on
disturbed soil flowered, but all seedlings on undisturbed soil remained very small. Mortality of seedlings after six months was
greater on undisturbed soil (54% survival) than disturbed soil (80%) and significantly different for three of the four species for
which there was adequate data. All species grew significantly taller and wider in disturbed soil. Soil digging created an
abundance of suitable microsites for seed germination, whereas the smooth compacted surface of the undisturbed soil provided
very few. Seedlings of exotic weeds originating in the soil seed bank had to be continuously removed from the disturbed plots
during the period of the study but no data about them was provided. Reynolds concluded that the germination of both native
forbs and exotic weeds would be promoted by such soil disturbance.
Soil disturbance affects the population dynamics of the native forbs in a range of ways that remain poorly understood. Soil
disturbances increase seed incorporation into the soil, which advantages some species and not others. Lunt (1995a) found that
buried seed had significantly greater survival than unburied seed for three of the species he tested. Some species exhibited major
differences in the timing of germination between buried and surface seeds, while others did not.
Particular disturbances or disturbance regimes always favour a subset of native species and disadvantage some species. A
changed disturbance regime is small remnants normally results in the disappearance of some species which are unable to
recolonise because of the now highly fragmented geographical distribution of remnants (Kirkpatrick et al. 1995).
A vegetation assessment protocol that enables the classification of native grasslands on the basis of their botanical signficance
has been devised for use in the Australian Capital Territory (Sharp 2006). A grassland is characterised as native if >50% cover
consists of native species. The botanical significance rating (BSR) depends on the level of vascular plant species richness (very
low, low, moderate, high, very high) and the presence of very common, relatively common, less common and uncommon native
species, which in turn are very tolerant, moderately tolerant, sensitive or highly sensitive to anthropogenic disturbances.
Grasslands with the highest BSR have several to many representatives from each of these native species groups including the
most disturbance-sensitive taxa. The systems has been used for conservation planning in the ACT.
Non-vascular plants and fungi
Bryophyta (mosses), Hepatophyta (liverworts) and Anthocerophyta (hornworts), collectively known as bryophytes (Meagher and
Fuhrer 2003) provide another component of biodiversity in south-eastern temperate grasslands. Bryophytes of Australian native
grasslands are relatively poorly studied, e.g. Meagher and Fuhrer (2003) provided lists of genera for numerous habitats and
vegetation formations, but not for native grasslands. The bryophytes along with lichens (symbiotic associations of algae or
cyanobacteria with fungi) and algae, collectively known as cryptogams, are found on exposed rock surfaces but are more
prevalent on the soil surface, forming what is known as the ‘cryptogam crust’ (Scarlett 1994), or where lichens are largely
lacking, the ‘bryophytic mat’ Morgan 2004). This acts to restrict soil erosion and increase water infiltration (Lunt et al. 1998 p.
11). Many of the lichens fix atmospheric N (Lunt et al. 1998) and biological soil crusts also contribute significantly to C fixation
(Morgan 2004). The lichen crust reportedly becomes apparent after the inter-tussock forbs die back (Kirkpatrick et al. 1995).
Effects of the soil crust on shed seed and seedling germination differ between plant species (Morgan 2004). It can provide “a
foothold for germinating seedlings” (Lunt et al. 1998 p. 11) but commonly restricts seedling establishment by limiting contact of
the radicle with mineral soil (Mack 1989).
Cryptogam diversity in native grasslands has rarely been studied, but where it has, it has it has been found to be highly variable,
e.g. there were at least 32 species at Evans St., Sunbury, but only 3 at Derrimut (Morgan and Rollason 1995) perhaps in part due
to the ‘overdominance’ of T. triandra at the latter site (Lunt 1990a) and its long history of grazing. Soils crusts are poorly
developed also at other previously grazed grasslands – Organ Pipes National Park and Laverton North Grassland Reserve
(Scarlett 1994). Willis (1964) found a total of about 85 species in the Victorian basalt plains flora, markedly less than in nongrassland
areas of the State. Scarlett (1994) found that railway reserve grasslands were dominated by mosses and liverworts,
rather than lichens. Morgan (2004) surveyed six T. triandra grassland remnants on the Victorian volcanic plains and found a
total of 19 mosses and 8 liverworts of which 9 were present only at single sites. The richest site has 10 mosses and 7 liverworts
in 150 m 2 , the poorest had 6 mosses and 3 liverworts in the same area. Again lichens were uncommon.
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