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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‘abandonment’ <strong>of</strong> foliage. Senescence dieback <strong>of</strong> T. triandra swards after extended periods <strong>of</strong> biomass accumulation might<br />
possibly be the result <strong>of</strong> altered water relations, rather than the postulated ‘self-shading’: the underlying mechanisms require<br />
further investigation. But like other dominant C 4 <strong>grass</strong>es worldwide, accumulation <strong>of</strong> large quantitites <strong>of</strong> dead biomass by T.<br />
triandra is apparently an adapted strategy that enables it to perpetuate its dominance by providing appropriate conditions for<br />
frequent burning (Hocking and Mason 2001). The high C:N ratio <strong>of</strong> C 4 <strong>grass</strong> foliage means the leaves have low nutritional<br />
quality and limited palatability to herbivores (Moore 1993, Moretto and Distel 2002) so a higher proportion die without being<br />
eaten than leaves <strong>of</strong> C 3 <strong>grass</strong>es, and the litter is more resistant to microbial breakdown than that <strong>of</strong> non-C 4 plants (Wedin 1999,<br />
Groves and Whalley 2002) so more <strong>of</strong> it can accumulate.<br />
Senescence dieback <strong>of</strong> T. triandra increases the soil available nutrients, probably as a result <strong>of</strong> increased rates <strong>of</strong> decay <strong>of</strong> both<br />
above and below ground vegetation, due to increased moisture and temperature under the thatch <strong>of</strong> dead leaves, and by reduced<br />
nutrient uptake by living biomass (Hocking and Mason 2001). However utilisation <strong>of</strong> the nutrient pulse by other plants is not<br />
possible until the high cover <strong>of</strong> dead <strong>grass</strong> decays or or is removed by fire or other biomass reduction. When this occurs major<br />
weed growth usually follows (Hocking and Mason 2001).<br />
Effects <strong>of</strong> T. triandra biomass accumulation on other species<br />
Commonly, in the absence <strong>of</strong> <strong>of</strong> regular biomass reduction by fire, grazing or mowing, litter accumulation by exotic or native<br />
perennial <strong>grass</strong>es results in the suppression <strong>of</strong> the smaller intertussock native vascular plant species (McIntyre 1993, Kirkpatrick<br />
et al. 1995, Morgan 1998e). Entire populations <strong>of</strong> perennial forbs can disappear within a short period in the absence <strong>of</strong> fire<br />
(Morgan 1998e). In T. triandra <strong>grass</strong>land unburnt for >5 years the cryptogam crust also degenerates due to litter accumulation,<br />
shading and increased earthworm activity (Scarlett 1994). However bryophyte diversity in areas burnt at 1-2 year intervals is<br />
reduced compared to longer unburnt areas (Morgan 2004).<br />
Periodic biomass reduction is required to maintain the vascular flora, a large proportion <strong>of</strong> which have soil seed banks which<br />
disappear after 1 year (Lunt 1990c 1995a, McIntyre 1993, Stuwe 1994). Morgan (1995b) for example found that 90% <strong>of</strong><br />
Rutidosis leptorhynchoides seed germinated within a few weeks <strong>of</strong> autumn rains. Endangered species that are threatened when<br />
fire frequency in T. triandra <strong>grass</strong>lands is too low include Senecio macrocarpus (Hills and Boekel 1996) and Rutidosis<br />
leptorrhynchoides (Morgan 1995a, Humphries and Webster 2003). Sharp (1997) experimentally confirmed the hypothesis that<br />
litter removal is required to facilitate establishment and reduce suppression <strong>of</strong> the smaller native <strong>grass</strong>es and the low-growing<br />
and small forb components in ACT <strong>grass</strong>lands. Lack <strong>of</strong> fire or some other management regime with similar effects is therefore a<br />
threat to the continued existence <strong>of</strong> the more mesic, T. triandra dominated <strong>grass</strong>lands, both in terms <strong>of</strong> the keystone species (T.<br />
triandra) and most <strong>of</strong> the other plant components. Suppression <strong>of</strong> other native species by the dominant <strong>grass</strong>es is not generally a<br />
problem in <strong>grass</strong>lands on shallow rocky soils and the more xeric inland plains, and fire is not necessary to maintain their<br />
indigenous vascular plant diversity (Kirkpatrick et al. 1995).<br />
Effects <strong>of</strong> fire on Themeda triandra<br />
T. triandra is well adapted to survive fire but mortality nevertheless occurs. Stafford (1991) reported that very few four-year-old<br />
T. triandra plants survived an intense wildfire in Cleland Conservation Park, South <strong>Australia</strong>, and a fire at Organ Pipes National<br />
Park, Victoria, in April 1997 before a severe drought resulted in substantial mortality and a dramatic cover decline (McDougall<br />
and Morgan 2005). However biennial autumn burning at Organ Pipes usually did not inhibit an already established trend <strong>of</strong><br />
increased T. triandra frequency and cover (McDougall and Morgan 2005). Burning generally does not kill T. triandra tussocks<br />
(Henderson 1999) nor the very high proportion <strong>of</strong> other <strong>grass</strong>land plants which are fire-adapted hemicryptophytes, geophytes<br />
etc., i.e. perennial plants with perennating buds protected underground (Morgan 1996, Lunt 1990a 1990c.). T. triandra usually<br />
regains high cover quickly, returning to pre-fire biomass levels in 2-4 years (Morgan 1994). At Evans St., Sunbury, cover<br />
reached 43% after 9 months and was predicted to reach 100% after 2-3 years (Morgan and Rollason 1995). Creation <strong>of</strong> bare<br />
ground along with an ash bed probably enhances seedling establishment for most native species (Stuwe and Parsons 1977), but<br />
these are favourable conditions for most exotic vascular plants as well.<br />
In South African <strong>grass</strong>land Uys et al. (2004) recorded declines <strong>of</strong> T. triandra related to longer fire frequency but continued<br />
persistence at a semi-arid (550 mm per annum) site 26 years post fire.<br />
Fire also enables T. triandra regeneration from seed. In T. triandra establishment experiments, Stafford (1991) found that<br />
burning <strong>of</strong> areas to which a close thatch <strong>of</strong> whole T. triandra culms had been applied resulted in immediate seed germination,<br />
with an estimated seedling density <strong>of</strong> c. 1000 m -2 . Areas thatched in December and burnt 10 months later produced seedlings at<br />
the end <strong>of</strong> October, the most suscessful <strong>of</strong> which produced seed the following February. However fire kills T. triandra seeds that<br />
have not worked their way into the soil (Hocking pers. comm.).<br />
Effects <strong>of</strong> fire on other vascular plants<br />
Particular forb species may be negatively effected by regular burning during a particular season. The late-flowering native pea<br />
Glycine labrobeana (Meisn.) Benth. is very susceptible to regular fires in late spring-early summer, which destroy its flowers<br />
and seeds (Scarlett and Parsons 1993). Cullen spp., also late flowering peas, may be rare in rail reserves for the same reason<br />
(Morgan 1994). Thesium australe, once widespread in native temperate T. triandra <strong>grass</strong>lands, is short-lived and highly<br />
dependent on annual seedling recruitment, is probably eliminated by annual burning (Scarlett and Parsons 1993), apparently<br />
germinates well without fire and after fire, but seems to require open conditions for growth (Scarlett et al. 2003).<br />
As previously noted, Morgan (2004) found that more frequent fires reduced bryophyte diversity in Victorian basalt plains<br />
<strong>grass</strong>lands by loss <strong>of</strong> species, mostly mosses, although none <strong>of</strong> the 150 m 2 quadrats he surveyed had a richer moss and liverwort<br />
flora than the total <strong>of</strong> 990 m 2 surveyed at the frequently burnt Evans St. Sunbury site by Morgan and Rollason (1995).<br />
Presumably the greater exposure and dessication resulting from frequent fire destroys mosses and removes suitable habitat,<br />
including the shade and increased humidity <strong>of</strong> dense cover provided by T. triandra. Slow recovery and recolonisation <strong>of</strong> mosses<br />
post-fire has been widely reported in other ecosystems, and, as for vascular plants, frequent burning <strong>of</strong> native <strong>grass</strong>lands through<br />
ecological time has probably eliminated the most fire-sensitive species long ago (Morgan 2004). Fire damage to soil cryptogam<br />
crusts can also facilitate <strong>grass</strong> invasion (Milton 2004).<br />
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