Literature review: Impact of Chilean needle grass ... - Weeds Australia
Literature review: Impact of Chilean needle grass ... - Weeds Australia
Literature review: Impact of Chilean needle grass ... - Weeds Australia
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esult in a “short-term flush” <strong>of</strong> N mineralisation in the soil, and suggested that T. triandra is uniquely able to take advantage <strong>of</strong><br />
this resource.<br />
The critical factor for survival and resprouting <strong>of</strong> vascular plants <strong>of</strong> frequently burned <strong>grass</strong>lands is the degree to which buds are<br />
protected from fire damage (Overbeck and Pfadenhauer 2007). In general in temperate south-eastern <strong>Australia</strong>n <strong>grass</strong>lands fire<br />
promotes vigorous resprouting by native perennials, but generally very little perennial seedling recruitment: fire enhances the<br />
vigour and flowering <strong>of</strong> many perennial herbs but results in little change in plant species composition (Lunt and Morgan 2002).<br />
However buring can lead to major increase in the abundance <strong>of</strong> annual exotic species on long-unburnt sites (Lunt 1990c). T.<br />
triandra <strong>grass</strong>land fires are relatively cool, and when occurring in summer and early autumn have little negative effect on the<br />
flora, which survives with maximal underground carbohydrate storage, buried buds and buried seed, although seeds on the soil<br />
surface are destroyed (Lunt and Morgan 2002). The majority <strong>of</strong> intact T. triandra <strong>grass</strong>lands on the Victorian volcanic plains are<br />
burnt at intervals <strong>of</strong> 1-5 years (Morgan 1998d) a frequency recommended by McDougall (1989). Healthy T. triandra swards are<br />
usually maintained with fires at this frequency and no grazing (Lunt and Morgan 2000). The time <strong>of</strong> burning (late summer or<br />
autumn) is probably not critical for the health <strong>of</strong> T. triandra populations, but fire greatly reduces the amount <strong>of</strong> T. triandra seed<br />
produced in the following autumn (McDougall 1989). Reduced fecundity in the short term is compensated for by increased plant<br />
surival and vigour. Morgan (1997) predicted that reducing the fire frequency from 1-2 years to 5 or more years would<br />
substantially alter the dynamics <strong>of</strong> the population if seedling recruitment was reduced and established plants were incapable <strong>of</strong><br />
adjusting.<br />
Most <strong>of</strong> our knowledge relates to the <strong>grass</strong>lands in which T. triandra is dominant and highly productive, systems that naturally<br />
promote fire. Much less is known about fire effects in <strong>grass</strong>lands not dominated by T. triandra. Poa spp. “appear to be able to<br />
maintain their dominance without disturbance by burning or other forms <strong>of</strong> biomass removal” while Austrodanthonia and<br />
Austrostipa <strong>grass</strong>lands accumulate little biomass and are not thought to need regular biomass reduction to maintain plant<br />
diversity (Lunt and Morgan 2002 p. 183). Little seems to be known about fire effects in the Riverine Plains <strong>grass</strong>lands.<br />
Themeda triandra biomass accumulation and senescence dieback<br />
The dependence <strong>of</strong> T. triandra <strong>grass</strong>lands on fire is almost universally recognised, despite the very limited understanding <strong>of</strong> their<br />
evolutionary origin and palaeoecology, particularly in relation to ancient grazing regimes. They appear to be similar to temperate<br />
fire-adapted <strong>grass</strong>lands with C 4 dominants worldwide, where the C 4 species promote their own dominance by building high<br />
levels <strong>of</strong> biomass and thus promoting fire. T. triandra stands that are not burnt, or otherwise biomass-reduced, gradually develop<br />
massive quantities <strong>of</strong> dead leaves and litter, and failure to remove this biomass can cause tiller and plant senescence, attributed to<br />
“self-shading” (Lunt and Morgan 2000). Major T. triandra mortality occurred at Derrimut and Laverton North <strong>grass</strong>lands when<br />
fire frequency exceeded 5 y, and when fire was finally used, plant and tiller densities were much lower than in regularly burnt<br />
<strong>grass</strong>land (Morgan and Lunt 1999, Lunt and Morgan 1999a 1999c). Mass dieback <strong>of</strong> T. triandra resulting from the absence <strong>of</strong><br />
fire or other biomass reduction has been described as “<strong>grass</strong>land collapse” (C. Hocking pers. comm.).<br />
According to Lunt and Morgan (1998a p.70) “the <strong>grass</strong>land becomes increasingly choked up with dead <strong>grass</strong> material, above<br />
which the tussocks form a thin mantle <strong>of</strong> new, green growth”, and eventually living tillers can “no longer poke up through the<br />
dead <strong>grass</strong> to reach the sunlight, causing the tussocks to die”. Supposedly, there is “insufficient light penetrating through the<br />
canopy <strong>of</strong> old foliage to the young tillers to enable them to photosynthesise sufficient energy” (Lunt et al. 1998). T. triandra<br />
<strong>grass</strong>land not burnt for greater than 6 years is now thought to senesce in this way (Morgan and Lunt 1999, Lunt and Morgan<br />
2002), rather than reach a steady state (Lunt and Morgan 2002). Low soil fertility and moisture levels may sometimes prevent<br />
such senescence (Kirkpatrick et al. 1995), so on less productive sites T. triandra senescence requires considerably longer periods<br />
or may never occur. O’Shea (2005 p. 161) stated that on productive sites, the phenomenon requires 10-11 years: “the canopy<br />
collapses upon itself and forms a thick layer <strong>of</strong> dead thatch over the soil surface, allowing only minimal seedling recruitment and<br />
preventing new tiller initiation”. According to Muyt (2005 p. 3), the dense T. triandra thatch “undermines the growth <strong>of</strong> [the<br />
<strong>grass</strong>] itself; plants become increasingly brittle and subject to collapse”. After burning T. triandra usually regains high cover<br />
quickly, returning to pre-fire biomass levels in 2-4 years (Morgan 1994, McDougall and Morgan 2005) and can form a complete<br />
dense canopy in as little as 3 years. Death <strong>of</strong> the dominant <strong>grass</strong> results in a major nutrient pulse in the soil, resulting primarily<br />
from decay <strong>of</strong> T. triandra crowns and roots, and this enables invasion by exotic plants (Wijesuriya 1999, Wijesuriya and<br />
Hocking 1999). Weedy exotics are now generally pervasive in these systems, so what would happen in the absence <strong>of</strong> exotic<br />
seed sources after T. triandra die-<strong>of</strong>f is not clear.<br />
The senescence dieback phenomenon has been widely reported worldwide for other dominant temperate and subtropical C 4<br />
tussock <strong>grass</strong>es adpated to fire (Bond et al. 2008). Although <strong>of</strong>ten described under different rubricks (e.g “detritus accumulation”<br />
– Knapp and Seastedt 1986) the effect is the same: accumulation <strong>of</strong> standing dead litter shades out and kills the shade-intolerant<br />
new tillers (Knapp and Seastedt 1986; Everson et al. 1988, Uys et al. 2004). Fire frequency is thus one <strong>of</strong> the most important<br />
determinants <strong>of</strong> the <strong>grass</strong> species composition in C 4 <strong>grass</strong>lands: “fire-dependent <strong>grass</strong> species, typically members <strong>of</strong> the<br />
Andropogoneae ... are fire-dependent in the sense that they decrease in, or disappear from, a sward in the absence <strong>of</strong> frequent<br />
burning” (Bond et al. 2008 p. 1747). Lunt and Morgan (1999a) reported a 70% decrease in T. triandra tussock density and a<br />
58% decrease in live tiller density in rarely burnt areas. Similar changes are indicated by Uys et al. (2004) who found that the<br />
cover <strong>of</strong> T. triandra in a mesic (735 mm per annum) South African <strong>grass</strong>land decreased from c. 70% when annually burnt to <<br />
5% after fire exclusion for 4 or more years, and the plant effectively disappeared from mesic and montane sites that were<br />
unburnt..<br />
In southern Brazilian mixed C 3 and C 4 tussock <strong>grass</strong>lands dominated by C 4 <strong>grass</strong>es, Overbeck and Pfadenhauer (2007 p. 35)<br />
observed that “without periodic removal <strong>of</strong> biomass ... shading by dead [<strong>grass</strong>] biomass inhibits survival and tillering and higher<br />
humidity under the litter may cause death and decay <strong>of</strong> underground plant parts within a few years”. Post-fire effects that<br />
increase the vigour <strong>of</strong> tussocks have been recorded for a number <strong>of</strong> dominant species in other parts <strong>of</strong> the world, including<br />
increased photosynthetic activity, growth rates and sexual reproduction (Overbeck and Pfadenhauer 2007).<br />
The T. triandra senescence phenomenon may be compared with ‘normal’ <strong>grass</strong> senescence, which occurs in all Poaceae in<br />
response to drought (Norton et al. 2008) and is a mechanism to reduce plant mortality from water stress, by the gradual<br />
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