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Managing exotic species in native understorey: Some early results from the central western slopes of nsw

I.J. Cole1, W.S. Semple2 and T.B. Koen1

1NSW Department of Land and Water Conservation, Centre for Natural Resources, PO Box 445, Cowra 2794
2
NSW Department of Land and Water Conservation, PO Box 53, Orange 2800

Abstract

Managing a native grassy understorey where exotics are present poses several problems. It is generally considered that in relatively fertile landscapes there is a periodic need to open up the grassy canopy to encourage diversity by allowing regeneration of the native herbs. When this is attempted however, exotic ruderals, rather than natives, are often the first to exploit the gaps created. This paper outlines the preliminary findings from trials conducted on two high conservation value grassy understoreys (one dominated by Poa sieberana and another Themeda triandra) subject to two different opening up treatments followed by one residual herbicide treatment to control exotic annuals.

Burning reduced seeding of T. triandra but not P. sieberana and increased the number of both native and exotic species. Summer slashing + simazine promoted massive seeding of T. triandra but not P. sieberana. The proportion of native species was higher in plots sprayed with simazine compared to control and burning only treatments. This treatment therefore appears to limit the ability of some exotic ruderals to exploit openings of the grassy canopy whilst allowing some native species opportunity to persist, generally increasing the proportion of native species present over the do nothing option.

This trial is ongoing and treatments were re-applied during early 2001.

Introduction

The condition of grassy understoreys (and grasslands) can range from severely degraded (e.g. with an abundance of exotic and very few native species) to “moribund” (where old native grass tussocks occupy most of the space and prevent other native species from establishing). At the very least, “restoring” these communities to something approaching their condition prior to the introduction of exotic plants and animals by Europeans, means reducing the frequency of exotics and/or increasing frequency of natives.

Shifting the composition of a stand dominated by exotics towards one dominated by natives is likely to be difficult. It may involve re-sowing natives (Stafford 1991, McDougall 1989, Phillips 1999), favouring natives by resting from grazing (eg. Semple and Koen 2000a) and/or removing exotics with herbicides, cultivation or strategic grazing. The risk of promoting exotic species rather than natives is high. Seedlings of native grasses may fail to establish due to weed competition (Semple et al. 1999), though this is less likely to occur on “raw” sites such as new road batters (Cole and Hundy 1999) and dumps of overburden from mining operations (Windsor et al.,1999).

Managing an understorey where exotics are a minor and perhaps increasing component poses different problems. It is generally considered that in relatively fertile landscapes there is a periodic need to open up the grassy canopy to encourage diversity by allowing regeneration of forbs (Stuwe and Parsons 1977). Opening up a moribund groundstorey can be achieved relatively easily with disturbances such as burning, slashing or grazing but there is a risk that exotics, rather than natives, will be the first to exploit the gaps created (Lunt 1990).

Fire, for example, is often suggested as a means of managing native understorey but there is no clear evidence from the literature that fire alone will be effective in promoting native grassland species and suppressing exotic species. In fact the opposite appears to be the case. Disparities between native and exotic species in the soil seed bank and the fast growing ruderal nature of many exotics favours the latter, making them better able to respond to any germination opportunity. Burning can provide this by removing standing vegetation and plant litter, resulting in mass germination of exotic species and few if any natives (McDougall 1989, Lunt 1990). What natives do germinate are more likely to be native colonising species and not those usually associated with the original grassland flora (Lunt and Morgan 1999).

Selective herbicides could be used to suppress exotic annuals, allowing native perennial species greater relative competitive advantage from the disturbance. The seedlings of many exotic species are susceptible to herbicides which mature understorey grasses at least appear to have tolerance (Davies 1997). However, the effect of herbicides on the other understorey components is not well documented and we are not aware of any native vegetation management studies that look at the effects of sequential application of herbicides following disturbance.

Two sites where many of the native species were still present and exotics are not dominant were selected for an assessment of potential restoration techniques involving the combined use of disturbance and herbicide.

Canomodine Grassy White Box Reserve, 420 m a.s.l., between Canowindra and Cargo – white box (Eucalyptus albens) overstorey still present, no shrubs, understorey dominated by Poa sieberiana.

Garra Cemetery Reserve, 510 m a.s.l., between Molong and Manildra – white box overstorey gone, scattered clumps of Acacia leucoclada, understorey dominated by Themeda triandra (though possibly P. sieberiana originally).

The primary aim of the experiment was to increase the frequency and diversity of native species and to reduce the frequency of exotics.

Methods

A number of disturbance or “opening-up” treatments were commenced in 2000 and will be repeated in 2001, each being implemented in small (c. 6 x 5 m) plots with 3 replicates:

  • Burning in early autumn, a relatively safe time
  • Burning in early autumn followed by simazine shortly afterwards
  • Slashing in summer followed by simazine in early autumn
  • Slashings as necessary in spring/summer to reduce seed set
  • Control – do nothing

A follow-up application of a residual herbicide (ie. one that inhibits subsequent germinations), Simagranz 900 (simazine) at 4 kg/ha, was included in some of the treatments to discourage annual exotic species. Its effect on native species - particularly those with underground regenerative organs was not available in the literature.

In spring 2000, frequency and importance scores were used to estimate species abundance. Plots were divided into 6 nested concentric quadrats with cumulative areas of 0.75m2, 1.5m2, 3m2, 6m2, 12m2, and 24m2 at the Garra site and 0.63m2, 1.25m2, 2.5m2, 5m2, 10m2, and 20m2 at the Canomodine site according to the technique outlined in (Morrison et al. 1995). These assessments will be repeated in spring 2001 and 2002.

Statistical analysis of the effects of the treatments at Garra made use of generalised linear models. Count data, such as the total number of species per plot and number of exotics per plot, were analysed using a Poisson error distribution and log link function. The number of native species, expressed as a proportion of the total number of species per plot, was analysed using a binomial error distribution and logit link function. The multivariate ordination technique of correspondence analysis was used to graphically explore the interrelationships between the 15 treatment plots (rows) and 25 species (columns) data matrix in the form of a two-way contingency table.

Results

Though it’s only early days, indications in summer 2001 were that both of the burn treatments at Garra reduced the number of seedheads of T. triandra (and may have killed some plants) whereas the summer slashing (and simazine) promoted massive seeding. None of the treatments, however, appeared to affect seeding of Poa sieberiana, the dominant grass at Canomodine.

Table 1. The effects of different management treatments on numbers of native and exotic species, and ratio of native to total number of species, in two different native grass understoreys in Central West NSW.

The average number of species in each of the treatments in late 2000 is presented in Table 1. Compared to the control, burning alone appeared to increase the number of exotic species and the total number of species at both sites. Herbicide treatments reduced the number of exotic species by about 25%, but had no apparent effects on the presence of some native perennials such as chocolate lily (Dichopogon sp.) and the dominant grasses, and few native species appear to have been eliminated. Herbicide treatments increased the proportion of native species.

The results of the correspondence analysis for the Garra site are shown in Figure 1. The left-hand panel shows an ordination of the replicates of the 5 disturbance treatments. In general, treatment labels close together in Figure 1a are similar in their species response profiles, such as the 3 Burn plots. The Control plots similarly group together, and seem to contrast with the Herbicide treatments. Figure 1b shows the contrast between the native and exotic species – and can be interpreted as, where exotic species were found, native species were generally not located, and vice versa. The two panels of Figure 1 can be superimposed, thereby indicating that Burn plots tended to have greater frequency of exotics, and Herbicide plots tending to be dominated by native species.

Whether or not these effects can be duplicated in 2001 remains to be seen.

Figure 1. Correspondence analysis of an incidence data matrix of 15 treatment plots (rows) by 25 species (columns). See text for description of treatments.

Conclusions

The preliminary results suggest that one-off treatments can have important effects on groundstorey composition. Whether or not they have long-term consequences, eg. as glyphosate can on the presence of Austrodanthonia spp (Semple and Koen 2000b), remains to be seen. The effect of burning on Themeda was not unexpected and reinforces the need to proceed cautiously with this highly variable species.

We wish to acknowledge the cooperation of Cabonne Council and the Central Tablelands Heritage Trust in carrying out the experiments at the Garra cemetery and Canomodine Grassy White Box Reserve.

References

Cole, I.A. and Hundy, B. (1999). Exotic versus native. Revegetation during summer using native grasses: Themeda australis and Chloris truncata. Natural Resource Management 2(2), 22-24.

Davies, P.P.G. (1997). Weed Management in Temperate Native Grasslands and Box Grassy Woodlands in South Australia. Black Hill Flora Centre, Botanic Gardens of Adelaide. South Australia.

Lunt, I.D. (1990). Impact of an autumn fire on a long-grazed Themeda triandra (Kangaroo Grass) grassland: implications for management of invaded, remnant vegetations: Victorian Naturalist, 107 , p. 45-51.

Lunt, I.D. and Morgan, J.W. (1999). Vegetation changes after 10 years of grazing exclusion and intermittent burning in a Themeda triandra (Poaceae) grassland reserve in south-eastern Australia: Australian Journal of Botany, 47, p. 537-552.

McDougall, K.L. (1989). The re-establishment of Themeda triandra (Kangaroo Grass): implications for the restoration of grassland. Arthur Rylah Institute for Environmental Research Technical Report Series No.89.

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Semple, W.S and Koen, T.B. (2000b). Recovery of Austrodanthonia eriantha following one-off disturbances in a humid derived grassland. Australian Rangeland Society Centenary Symposium, 21-24 August 2000, Broken Hill, NSW, pp.168-69.

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Windsor, D.M., Clements, A., Nolan, M.B. and Sandercock, H. (2000). Recreating a eucalypt woodland with a grassy understorey on a gold mine in the Central Tablelands of New South Wales. In: ‘Temperate Eucalypt Woodlands in Australia’ (Eds. R. J. Hobbs and C. J. Yates). Surrey Beatty & Sons: Chipping Norton, NSW, pp. 298-317.

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