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

Fire effects on weeds
Moore (1993 p. 351) argued that Themeda grasslands remained “remarkably stable” under regimes of marsupial grazing and
periodic burning, and if otherwise undisturbed were “not invaded by introduced species”. Fire results in temporary increases in
nutrient availability and this fluctuation of resources may enhance invasion by exotic plants (Hobbs 1989). In general burning of
south-eastern Australian native grasslands promotes post-fire colonising plants, most of which are exotic annuals that recruit
from a large soil seed bank (Lunt 1990b, Lunt and Morgan 2002). However Stuwe and Parsons (1977) found a smaller
proportion of the vascular flora consisted of alien species at regularly burnt (railway) sites than in grazed or unmanaged T.
triandra grasslands, while Dodd et al. (2007) found that sites burnt less frequently had higher exotic seed banks, and considered
regular burning to be detrimental to the exotics. Burning of some degraded T. triandra grasslands has been reported to lead to
major increases in cover and density of exotic annuals, which then slowly decline (Lunt 1990b 1990c, Morgan 1998d). Scarlett’s
(1994) view was that regular burning of sites with a long history of grazing and trampling rarely has any major impact on exotic
annuals, an effect he linked to the poor re-establishment of the cryptogam crust.
Fire is often considered to provide advantages in reducing weed populations, but may promote or inhibit particular weed species
or functional groups. Fire is demonstrably an effective tool to greatly reduce the cover of exotic invasive grasses when the
invasives are not fire adapted, and to restore native plant cover (e.g. MacDougall and Turkington 2007). In temperate Australian
grasslands spring fires might reduce or prevent seed set in exotic annual grasses (Stuwe 1986) but late autumn burning promotes
their regeneration along with Romulea rosea (L.) Eckl. (Iridaceae) (Lunt 1990c). Burning can reduce the density of the exotic
grasses Briza maxima, Cynosurus echinatus L., Lolium rigidum Gaudin, L. perenne and Bromus hordeaceus in T. triandra
grasslands but promotes a range of others including Aira and Vulpia spp. (Lunt 1990c, McDougall 1989, Adair 1995).
As with the native plants, seasonality of the fire may determine the effect on particular exotics (see Morgan 1996), while the
impacts on propagule production and seed bank levels are obviously important factors. According to Adair (1995) no clear
evidence was then available that the season of burning has a significant effect on introduced grasses and forbs. Exotics are likely
to be advantaged if they account for a high proportion of the soil seed bank, which is generally the case (Kirkpatrick et al. 1995,
Morgan 1998c, Dodd et al. 2007). Robertson (1985) found similar densities of exotic annuals after autumn and spring burns at
Gellibrand Hill. Stuwe (1994) among others argued that biomass-reduction burning now facilitates the growth of both native and
exotic plants, while Lunt (1990b), based on seed bank studies at Derrimut, argued that burning is likely to promote exotics more,
or at least as much as natives. Dodd et al. (2007) argued that exotic dominance of the seed bank is partly due to removal of fire,
which along with exotic dominance in the surrounding landscape matrix, is likely to result in increased weediness of remnants.
Decisions about the desirability of burning need to be based on an understanding of the contents of the soil seed bank and the
phenology of the above ground cover at each site in order to predict whether a burn would be beneficial for the flora.
Determining an appropriate fire regime even at a single small site can thus becomes prohibitively difficult. Most native
grasslands contain a mix of weeds with different life strategies, so fire cannot be used as a general tool to favour the native
species (Lunt 1990c).
Stuwe (1994) warned that burning of areas with a N. neesiana seed bank should not be undertaken unless follow up herbicide
treatment could be undertaken. However there appear to have been no studies on the specific effects of fire timing and intensity
on N. neesiana populations in native grasslands. No information appears to be available about the comparable fuel loads of
grasslands with and without N. neeesiana, so it is difficult to speculate about whether N. neesiana infestations increase or
decrease the frequency and intensity of fires.
In relation to vascular plants of native grasslands, there is a general consensus that the interaction between fire and grazing is the
major factor involved in the abundance of most native species, including threatened taxa, and that low fire frequency is
detrimental to most species (Scarlett and Parsons 1993). Frequent burning (a fire every 2-5 years) is necessary to maintain
grasslands dominated by T. triandra, but other Victorian grassy ecosystems (E. camaldulensis and E. melliodora grassy
woodlands in the Grampians and Sandplain Grassland in the Mallee) appear not to require burning to maintain their vascular
plant diversity (Lunt 1991). In T. triandra grasslands, long intervals between fires results in loss of forb diversity (Stuwe and
Parsons 1977, Lunt 1990c, Lunt and Morgan 1999). Morgan (1998c) found no correlation between seed bank species richness
and fire history for five grasslands in the Victorian volcanic plains. However fires do not generally lead to significant seedling
recruitment of native plants (Henderson 1999, Lunt and Morgan 2002), possibly because of severe depletion of the seed bank
resulting from prior affects of long-term livestock grazing on seed production (Lunt 1990b 1990c). T. triandra is more sensitive
to grazing shortly after fire (Groves and Whalley 2002).
Effects of fire on animals
Combinations of positive and negative effects of fire also occur with other organisms. Fire can change the abundance, range
(spatial and temporal), fecundity and dietary choice of other organisms by altering the provision of food, shelter and habitat
(Low 2002). Fire directly results in the elimination of many animal populations, although many sedentary species survive
underground.
According to Yen (1995 1999) the effects of fire on temperate Australian grassland invertebrates were unknown, while Driscoll
(1994) noted that they were ‘yet to be investigated’. Studies from other ecosystems tend to be equivocal, with few lessons for
grasslands (Driscoll 1994). Edwards (1994) argued that fire often causes local extinction, but recolonisation from unburnt areas
is usual, so in a situation where an invertebate is dependent on a community that is scarce and highly fragmented, recolonisation
is often impossible. This appears to be the case with Synemon spp. (Edwards 1994) and with xanthorhoine moths (McQuillan
1999), although with organisms like Synemon that spend a major part of their lifecycle underground, the precise timing of the
fire with respect to the above-ground stages would appear to be critical.
Removal of dense cover by fire and the creation of open ground will radically alter temperatures at the ground surface. Lunt
(1995a) quantified temperature differences between the canopy surface in T. triandra grassland, the ground surface temperature
and soil temperatures at 3 cm depth, in areas where the canopy was closed and in open gaps from July to April. The canopy
provided excellent insulation with temperatures on the surface similar to those in the soil. Temperatures in gaps were generally
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