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

Soil temperatures below 45-55ºC cause little damage to vascular grassland plants and higher temperatures resulting from fire are
generally restricted to the top centimetre of soil (Overbeck and Pfadenhauer 2007).
Fire-suppression can have serious negative consequences for biodiversity in fire-adapted grasslands and may result in their
transformation into shrub or tree dominated systems (Hobbs and Heunneke 1992). The extent to which native grasslands in
south-eastern Australia were subject to fire prior to European settlement remains controversial, as does the role of fire, as
opposed to climatic factors, in the maintenance of grasslands through ecological time (Jones 1999b). Flannery (1994), amongst
others, marshalled substantial evidence that grassy woodlands and grasslands were the dominant vegetation types on the
mainland before 1750, or at least much more widespread than contemporary vegetation suggests, and that the land was regularly
burnt by aborigines. According to Kirkpatrick et al. (1995 p. 25) “all evidence suggests that fire was highly frequent, if not
annual” before European occupation. Moore (1993 p. 351) stated that southern Australian “Themeda grasslands evolved under ...
periodic burning”, a clear exaggeration of the extent of scientific understanding.
Benson and Redpath (1997) examined these views of pre-European fire regimes and found no evidence of large scale annual
aboriginal burning in south eastern Australia as a whole. They identified misinterpretation and unwarranted extrapolation from
the records of early explorers and settlers as the basis for the view that ‘fire-stick farming’ was widespread in south-eastern
Australia. However they appear to be overenthusiastic debunkers: no serious researchers make claims that grassy ecosystems
were present in many of the areas they discuss.
Jones (1999b) argued that the persistence of grassland requires fire at a certain frequency, that can burn without impediment
across the landscape. Pollen in a core from Lake George on the southern tablelands of New South Wales indicates that
communities with significant grass components were present during periods of glaciation. According to Jones (1999b) a major
change to the dominance of Poaceae and Eucalyptus occurred during the late Pleistocene (uncertainly dated to c. 130 kybp), but
Kershaw et al. (2000 p. 501) argued that eucalypt and grass dominated vegetation “with fire as a well-established environmental
component” existed for most of the Quaternary before this time, possibly largely due to regional warming. Pollen of Poaceae is
present throughout a swamp core from Lake Terang in western Victorian covering a period to c. 120 kybp, but increases towards
the Holocene (Jones 1999b). Records from charcoal deposits are complex, but they currently provide no general, strong
indication of an increase in fire during the periods (30-50 kybp) of first human occupation and the extinction of the megafauna
(Johnson 2009). It appears likely therefore that any general increase in vegetation biomass expected with the loss of the
megafauna was mainly of shrubland that was not susceptible to wide scale burning under the prevailing cool, wet conditions
(Johnson 2009). However, grassy steppe vegetation that occurred in areas now in Bass Strait 25 kybp “shows abundance
evidence for fire” and carbon particles reach high levels at Lake George at the start of the Holocene (Hope 1994).
According to Stuwe (1994), natural fires, ignited by lightning were of frequent occurence in the natural temperate grasslands of
continental south-eastern Australia. Enough fuel accumulates in most grasslands to allow an annual fire (Kirkpatrick et al. 1995).
Kirkpatrick (2007) argued that lightning has been relatively rare in Tasmania during the last few thousand years but that it
currently caused fires and must have caused the charcoal deposits present in Tasmanian soils before aboriginal occupation.
Flannery (1994) argued that there are more than enough natural ignition events in most areas to burn the standing crop of fuel
before it can decompose, and that deliberately lit fires would therefore have increased fire frequency but decreased fire intensity.
The extent and temporal variation of natural fires are “largely unknown” though evidence suggests that soil and climate rather
than fire determined treelessness, and lack of recent evidence of tree invasion of grasslands not managed by fire supports this
conclusion (Lunt and Morgan 2002).
Very little is known about the frequency and seasons of aboriginal burning in south-eastern Australian grasslands (Morgan
1994). Aboriginal activities increased the fire frequency (Stuwe 1994), but opinions differ about the magnitude of the increase
and the effects. Benson and Repath (1997) considered that “the extent, frequency and season of their use of fire is largely
unkown”. In “the more developed areas” of Tasmania wide aboriginal use of fire appears to have occurred in the last 10,000
years (Kirkpatrick 2007). Gott (1999) argued that burning of the temperate grasslands was deliberately controlled and timed to
provide the most beneficial effects in terms of food production. Lunt et al. (1998) concluded that early historical records indicate
that indigenous grasslands were frequently burnt. Any plants not adapted to frequent fires must therefore have been eliminated or
confined to fire-free refugia long ago (Stuwe 1994).
Grasslands taken up for livestock grazing were not managed with fire. In contrast, non-agricultural grassland remnants were
frequently burnt in rail reserves (1860s?-1970s, often annually in late spring) and on roadsides (Western Victoria at least 1940s-
1970s), as a cost-effective form of management (particularly to protect against wildfires) and those areas had the highest vascular
plant diversity of all grassland remnants (Morgan 1997, Lunt and Morgan 2002), although some species were “presumably...
eliminated” by the historical management activities (Morgan 1997). Sutton (1916-1917 p. 117) observed that in the Keilor Plains
the “fires of spring ... never quite die down”. Recent fire regimes involve deliberate small scale management fires and occasional
wildfires. Many linear remnants (roadside and rail easements) have been regularly burnt in early summer while conservation
reserves have been subject to irregular (often “erratic”) early autumn burning (Lunt and Morgan 2002 p.180).
The fire intensity (energy release per unit area) in temperate south-eastern Australian grasslands is generally low (in comparison
to tropical grassland fires), because of the small quantitites of fuel and the slow rate of spread of managed fires (Lunt and
Morgan 2002). In T. triandra grasslands above-ground plant biomass is in the range of 7-11 tonnes ha -1 in infrequently burnt
grasslands (7-11 years since fire) to c. 1-4.8 tonnes ha -1 in grasslands burnt 1-2 years previously (Lunt and Morgan 2002). T.
triandra stands rarely produce enough litter in the first year after a fire to enable another burn (McDougall 1989). In Poa and T.
triandra grasslands the dominant grass often accounts for >90% of the biomass (Groves 1965, Lunt and Morgan 2002).
Nutrients in ash are more readily lost in runoff or by wind erosion after fire (Flannery 1994). Fire also directly releases nutrients
to the atmosphere, particularly N, and the levels of available soil N are often affected by repeated burning (MacDougall and
Turkington 2007). An estimated 24 kg ha -1 of NO 2 was lost when tropical grassland at Katherine, Northern Territory was burnt
(Flannery 1994). MacDougall and Turkington (2007) however found no such effect of N loss in savannahs in British Columbia,
consistent with other recent studies, and thought that a combination of fire effects on litter, soil temperature, soil microbes, etc.
could have suppressed N mineralisation. Moore (1993 p 352) stated that fires in T. triandra grassland ungrazed by livestock
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