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

5°C warmer than under the canopy and much larger differences occurred in summer. Such contrasts will substantially affect the
behaviour and activity levels of invertebrates.
Farrow (1999) found no clear differences in the diversity of canopy-living insects in small burnt patches and unburnt areas in
ACT grasslands and suggested that recolonisation occurred within 6 months of fire. Nearly twice as many individual insects
were present in summer in areas burnt 6 months prior to sampling than in unburnt areas, and more than three times as many in
the following spring. Species that benefit from more open ground, like many ants, probably commonly proliferate. Maintenance
or periodic refreshment of plant diversity by fire will benefit a wide range of herbivorous taxa with particular food plants, and
maintenance of the forb components will benefit nectar and pollen feeders. Maintenance or enhancement of animal diversity at
the primary consumer level will flow on to higher tropic levels, benefitting the diverse array of predators and parasitoids.
Greenslade (1994) found litter removal by fire was likely to markedly reduce Collembola diversity for at least two years. Other
detritivores dependant on plant litter would be expected to be similarly disadvantaged.
Any beneficial effects on fire on plant diversity may or may not flow on to invertebrates, depending, in part on whether source
populations of new and recolonising species exist (Tscharntke and Greiler 1995). Driscoll (1994) concluded that some species
may be completely intolerant of fire, while others may require it for survival. Any species that spend part of their lifecycle
underground are probably relatively resistant to fire, while those that do not are likely to be more highly vagile and have an
ability to recolonise from unburnt areas. Sedentary, non-subterranean species are probably most vulnerable to extinction, but
there are unlikely to many of them in fire adapted vegetation. As with vascular plants (Stuwe 1994) and bryophytes (Morgan
2004), the indigenous invertebrate flora of grasslands that have been subject to periodical fire over many thousands of years must
consist largely of fire-adapted species, whether or not they are fire survivors or recolonisers.
Fire regime management in Victoria now incorporates the goal of providing the conditions necessary for the persistence of the
indigenous biota (Mansergh et al. 2006b) but implementation of this idealfor the various biotic elements, including animals, is
difficult and is bound to produce contradictory results because different valuable elements of the biota may be advantaged or
harmed by the same fire cycle. The difficulty for managers is to evaluate and juggle the often contrary needs and tolerances of
the various components in each grassland remnant.
Nutrients and soil factors
Lowland native grassland in Australia is found on relatively fertile, organic-rich, cracking clay soils on substrates of volcanic
rocks (basalt and dolerite), fine-grained sedimentary rocks (including limestone) or recent alluvium (Kirkpatrick et al. 1995). In
the past, temperate grasslands have erroneously been characterised as low in nutrients, based on analysis of soils. Mott and
Groves (1994 p. 376) state that “Australian grassland soils are of very low fertility, particularly of nitrogen and phosphorus, by
comparison with those of grasslands elesewhere”. Sharp (1997) lists low soil nutrient content as one important factor
determining the distribution of native grassland in the ACT. Australian soils have in general been considered to be nutrient
deficient (e.g. Roberts et al. 2006). However, like tropical rainforest, much of the system nutrients are held in the plants.
Above- and below-ground biomass
In temperate herbaceous communities 60-80% of vascular plant biomass is underground (see Reynolds 2006 p. 57). In grasslands
up to 90% is below-ground (Wijesuriya and Hocking 1999) and it is usual for a high proportion of total plant biomass to be
represented by roots and buried crowns. More of the energy captured in photosynthesis in grasslands is directed to below-ground
parts than those above-ground and most of the nutrient circulation occurs below ground (Soriano et al. 1992). In Australian
grasslands most of the biomass is contained in the dominant grasses (Groves 1965). T. triandra- and Poa-dominated
communities are more productive (i.e. produce more biomass) than the Austrodanthonia and Austrostipa grasslands that
predominate in drier areas (Lunt and Morgan 2002). The mean root: shoot ratios in North American grasslands varies from 13 to
2, being higher in cooler climates (Tscharntke and Greiler 1995). Cooler and drier grasslands generally have ratios between 13
and 6, while warmer, more humid grasslands have ratios between 6 and 2 (Soriano et al. 1992). A ratio of 2.6 was calculated for
one Argentinean pampas grassland (Soriano et al. 1992). Rodríguez et al. (1995) measured below-ground biomass down to 10
cm depth in five grazed grassland communities in Spain and found that 61-68% of the total biomass was below ground, of which
49-68% was in crowns and the remainder below 1 cm depth. Groves (1965) found that the root:shoot ratio of a T. triandra
grassland had very high seasonable variability but that root biomass was generally 2 to 4 times that of above-ground parts, i.e.
66-80% of the biomass of the overwhelming dominant grass was below ground. In one Pampas grassland 65% of the
underground biomass was located above 10 cm, 85% above 30 cm and 100% above 70 cm, a similar distribution to most
temperate, subhumid grasslands (Soriano et al. 1992).
Underground net primary productivity in one Pampas grassland was estimated to be about 5000 kg ha -1 yr -1 , just 17% less than
productivity above ground, with a below-ground: above-ground productivity ratio of 0.9 (Soriano et al. 1992).
Soil nutrient levels
The soils of Australian native grasslands usually have low nutrient levels (Table 8), and a high organic matter content with
organically bound N and P (Wijesuriya and Hocking 1999). However most of the nutrients in the system are locked up in the
biomass of the grasses. In the Victorian basalt plains, soils derived from sediments have higher nutrient levels than those derived
from basalt, and soils on stony rises have the highest fertility (Williams 2007).
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