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Keywords:

  • annual invader;
  • fertilizer residues;
  • increased resource availability;
  • Mediterranean climate;
  • old-field succession;
  • recruitment limitation;
  • restoration

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  • 1
    Exotic species invasion onto abandoned farmlands has been linked to an increase in the availability of soil nutrients after cultivation. Avena barbata Pott. (hereafter Avena) is an exotic annual grass that invades old-fields in south-western Australia and persists for decades after abandonment. We hypothesized that the competitive ability of Avena against native woody seedlings is increased on old-fields affected by P-fertilizer residues.
  • 2
    We tested this hypothesis in a pot study by growing single native seedlings in competition with increasing densities of Avena, at old-field and pre-agricultural levels of soil P. Then, we planted the same four species, Acacia acuminata Benth., Allocasuarina campestris (Diels) L.A.S.Johnson, Eucalyptus loxophleba Benth. subsp. loxophleba and Hakea recurva Meisn., into plots within an old-field affected by P-fertilizer residues, and removed Avena from half of the plots. Also, as water availability limits seedling establishment, particularly in the presence of Avena, we compared the establishment of seedlings planted into microcatchments to improve water availability with that of seedlings planted into level sites.
  • 3
    We found that P enrichment did not substantially enhance the ability of Avena to compete with any of the native species that we tested in pots. Avena was the superior competitor against all four native species at both P levels.
  • 4
    In the field, the chances of native seedling establishment were markedly improved by the removal of Avena. Microcatchments improved the survival of E. loxophleba subsp. loxophleba in competition with Avena, but not that of the three other native species. In the absence of Avena, microcatchments made inconsistent or little difference to the survival and biomass of the planted native seedlings.
  • 5
    Synthesis and applications. Reducing soil P will not be sufficient to promote species coexistence on old-fields because Avena remains competitive at low soil P. The combination of the land-use legacies, reduced native seed supply and the introduction of an invasive species that is a superior competitor regardless of P availability, all contribute to the persistence of this invasive species on old-fields in south-western Australia. The restoration of the historic ecosystem will require intense effort and there is a risk that planted communities will be prone to re-invasion.

Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Abandoned farmlands are prone to invasion by exotic species, especially where these species are favoured by the legacies of cultivation (Stylinski & Allen 1999; Kulmatiski, Beard & Stark 2006). In particular, increased soil fertility associated with repeated fertilizer applications can promote the growth of exotic species that in turn prevent the recruitment of native species (e.g. Walker et al. 2004). In theory, exotic species invade in response to an increase in the availability of nutrients that usually limit plant growth (Davis, Grime & Thompson 2000). The practical implication of this idea is that it will be necessary to reduce the availability of soil nutrients to promote the growth of the native species (Daehler 2003; D’Antonio & Chambers 2006). For this to occur, native species must be able to out-compete the invasive exotic species in low-nutrient soils – an assumption that has been challenged recently with the evidence that some invaders also have the ability to compete in low-nutrient soils (Funk & Vitousek 2007).

Where the persistence of an exotic species is contingent on elevated soil fertility, the effort required to shift the community away from the invader-dominated state can be substantial. It can take years of intensive management to restore natural fertility to old-fields (Berendse et al. 1992; Marrs 1993; Walker et al. 2004) and such efforts are usually too costly to implement at the landscape scale (Owen & Marrs 2001; Kulmatiski & Beard 2006; Pywell et al. 2007). Where invader persistence is likely regardless of soil fertility, efforts might instead be directed towards the removal or depletion of the exotic species and supplementing the supply of native species.

Abandoned farmlands (old-fields) in south-western Australia are often dominated by the Mediterranean-Basin annual grass Avena barbata even 45 years after abandonment. This species is commonly associated with disturbed native ecosystems in California (Huenneke et al. 1990; Seabloom et al. 2003), eastern and southern Australia (Prober, Thiele & Lunt 2002; Lenz, Moyle-Croft & Facelli 2003), and other (mostly temperate) regions of the world (Clayton, Harman & Williamson 2006). Its success as an invader can be attributed to its ability to respond rapidly to disturbance, particularly where the availability of soil nutrients has increased (Huenneke et al. 1990; Prober et al. 2002; Seabloom et al. 2003; Lenz &Facelli 2005) and its ability to tolerate drought (Lenz & Facelli 2005). In south-western Australia, where plant growth is limited by the availability of phosphorus (Handreck 1997; Lambers et al. 2008), the invasion of Avena spp. has been linked to soil disturbance and increased soil phosphorus (Hobbs & Atkins 1988; Cale & Hobbs 1991). Old-fields affected by P-fertilizer residues appear particularly prone to invasion by Avena and we questioned whether this might partly explain its persistence so long after abandonment (Standish et al. 2006).

In this study, we tested the prediction that the competitive ability of Avena against native woody seedlings is increased on old-fields affected by P-fertilizer residues. In a pot study, we measured the growth of individual seedlings of four native species in competition with increasing densities of Avena at two P-levels, to mimic plant-available P in pre-agricultural and old-field soils. Plants were exposed to growing conditions typical of growing-season conditions in the field. This study enabled us to determine if the persistence of Avena was contingent on elevated soil P, and therefore, whether reduced soil P would promote the establishment of native woody species on old-fields. Subsequently, we planted the same four species into plots within an old-field affected by P-fertilizer residues, and removed Avena from half of the plots, and compared survival and growth. Given that the availability of water limits seedling establishment (e.g. Yates, Hobbs & Bell 1994), we assumed that Avena would out-compete the seedlings by depleting soil water as it does in other Mediterranean-climate regions (Eliason & Allen 1997; Lenz & Facelli 2005). Microcatchments can improve water availability to seedlings in dry climates (Whisenant, Thurow & Maranz 1995), and thus, we predicted a greater proportion of seedlings would establish in microcatchments than in level sites, particularly in the presence of Avena.

Methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

pot study

We used an additive design to test the competitive effect of Avena on the individual-level responses of four native plant species, which included six density treatments (i.e. 0, 2, 4, 8, 16, 32 Avena plants) to determine the per capita effects (Goldberg & Scheiner 2001). The 32 plant-density treatment was equivalent to the maximum density of Avena measured at the field site. The design also included P addition (P)/no P addition (NP) as a fixed factor and three replicates of each treatment (n = 4 × 6 × 2 × 3 replicates = 144 pots). Pots, each containing one native plant, were laid out in three blocks, according to replicate and stratified by treatments. The pots measured 8 × 8 × 18 cm (depth) and were filled with soil collected from a eucalypt woodland remnant unaffected by cultivation that was adjacent to the field site. Plant available bicarbonate extractable P (Colwell 1965) in the 0–5 cm layer was ~7 mg kg−1 and the pH in CaCl2 was ~5·1. Prior to use, the soil was sterilized at 65°C for 3 h and air-dried. Phosphate, 5·16 mg P kg−1 soil, was added to the pots at the time of planting, as Ca(H2PO4)2 (Sigma-Aldrich, Castle Hill NSW) because this form of P is readily available to plants. This level represents plant available P in old-field soil (Standish et al. 2006), assuming that two-thirds of the P added is unavailable to plants (Bolland, Allen & Barrow 2003). The Ca(H2PO4)2 was mixed into the top 5 cm of soil. Otherwise, we did not alter the soil fertility.

Our study species dominate the eucalypt woodland that we are aiming to restore. Two species, Acacia acuminata Benth. (jam wattle; Mimosaceae) and Hakea recurva Meisn. (djarnokmurd; Proteaceae), have naturally recolonized old-fields in the region, whereas Eucalyptus loxophleba Benth. subsp. loxophleba (York gum; Myrtaceae) and Allocasuarina campestris (Diels) L.A.S. Johnson (sheoak; Casuarinaceae) have not (Standish et al. 2007a). However, E. loxophleba subsp. loxophleba is one of three eucalypts that account for a majority of trees planted on farmland in the region since the 1920s (Smith 2008).

Seedlings were grown from seeds collected by Greening Australia (Northam, Western Australia). Acacia acuminata seeds were immersed in boiling water for 40 s to break dormancy and those that floated were discarded. Seeds were germinated on filter paper in Petri dishes, the E. loxophleba subsp. loxophleba and A. acuminata at 25 °C and the A. campestris and H. preissii at 18 °C. At emergence, the seedlings were transferred to trays of sterilized soil and watered. Avena seedlings were grown from seeds collected at the field site in October 2004. Seeds were de-husked and pricked to break dormancy and then germinated on filter paper in Petri dishes at 15 °C. At emergence, Avena seedlings were planted together with the native seedlings into the soil-filled pots. Avena seedlings were less than 2 cm tall and native seedlings were less than 5 cm tall at the time of planting.

Plants were watered by overhead sprinklers, everyday in the first 4 weeks and then three times a week thereafter to promote competition for water. The low-water regime was designed to approximate the long-term average rainfall for the first 3 months of the growing season at the climate station nearest to the field site (Bureau of Meteorology 2005). Dead plants were replaced in the first 4 weeks. To negate a block effect, the position of the blocks was re-randomized at four and 8 weeks after implementing the low-water regime.

Plants were harvested 12 weeks after implementing the low-water regime, and when treatment effects were apparent (3 July 2005). The temperature in the glasshouse ranged from 7–37 °C during the experiment, which is within the range of temperatures experienced at the field site during the first 3 months of the growing season (Bureau of Meteorology 2005). Pots with dead native plants (n = 30) were discarded. The survivorship of seedlings (died or survived) was not independent of species; the number of dead E. loxophleba subsp. loxophleba (19) was greater than that of the other species (χ2 = 36·9, d.f. = 3, P < 0·005). Plant shoots (i.e. above-ground material) were oven-dried at 70 °C to a constant weight. The roots were washed and oven-dried at 70 °C to a constant weight. A sub-sample of Avena shoot samples from P and NP treatments were sent to CSBP Soil and Plant Analysis Laboratories (Bibra Lake, Perth) to determine foliar P (n = 34 samples). Samples were ground and foliar P was determined by digesting plant material in nitric acid using a Milestone microwave and then measured by ICP-AES (McQuaker, Brown & Kluckner 1979).

For each native species (except E. loxophleba subsp. loxophleba), a one-factor ancova was used to test for the effect of Avena biomass (shoot + root) and P treatments on native plant (shoot + root) biomass, shoot biomass, root biomass and root fraction (root biomass as a fraction of total biomass). The treatment effects were consistent for each of these dependent variables, and thus, we present the results for plant biomass only. The factor was P treatment and the covariate was Avena shoot biomass. Before these analyses, log(x + 1) transformations of both dependent and response variables were used to help linearize the relationships. The influence of each data point on each of the fitted regression lines was estimated using the Cook's distance statistic Di; we interpreted values greater than one as being particularly influential (Bollen & Jackman 1990). We estimated the effect sizes of the dependent variables on the response variable using the omega-squared measure (Hays 1994) for P treatment and the Pearson product moment correlation for Avena biomass. ancova was invalid for E. loxophleba subsp. loxophleba because a linear model did not fit the data for P-treated plants (R2 = 0·11, F = 1·42, d.f. = 1, 11; P = 0·26) and there were too few NP-treated plants to fit any model. A one-factor anova was used to test for the effect of P treatment on Avena shoot [P], shoot biomass and root biomass. The error terms were normally distributed and the variances were homogeneous for each of the ancova and anova models.

field experiment

The field experiment was located on an old-field in the central wheat-growing district of south-western Australia (22·3 ha; 31°20′ S, 117°44′ E). The old-field was cleared of native vegetation in 1930 and then cropped until 1990 when it was abandoned. The residual effects of P-fertilizer, soil compaction, erosion, and reduced organic carbon were evident in the old field 14 years after abandonment (Standish et al. 2006). The experiment received 273 mm of rain, which is less than the mean but within one standard deviation of the annual rainfall (323 ± 85 mm) measured within the same period (i.e. August to July the following year; Bureau of Meteorology 2005).

We used a split-plot design to determine the effect of competition from Avena-dominated grassland on the establishment of four native species planted as seedlings. The splitting factor was the presence/absence of Avena, applied to 6 × 3 m plots and replicated five times (n = 10 plots). A second treatment factor, the presence/absence of microcatchments to improve water availability to seedlings (Whisenant et al. 1995), was applied to sub-plots within each plot (n = 2 sub-plots per plot). Microcatchments, one per seedling, were created by removing soil until the depressions were about 10 cm in depth and 10 cm in width; they accumulated leaf litter to a depth of ~1 cm during the experiment. We planted one block of seedlings of each species into each of the two sub-plots in a 2 × 2 block configuration (n = 4 blocks per sub-plot). Each block contained nine seedlings in a 3 × 3 configuration. Seedlings (< 30 cm tall) were purchased from Westgrow Farm Trees (Quellington Road, Meckering) and planted on 4 August 2004. Each sub-plot received 5 L of water at planting, but only rainfall thereafter. Dead seedlings were replaced with live ones on 3 September 2004 but not thereafter. Plots were fenced to protect them from browsing kangaroos (Macropus spp.) and European rabbits Oryctolagus cuniculus.

The analyses included four replicates rather than five as kangaroos jumped the fence around one plot and ate most of the seedlings. We scored the survival of the seedlings and above-ground biomass of the survivors in the remaining eight plots 1 year after planting (n = 219 survivors of 576 planted). Above-ground biomass was determined after drying to constant weight at 70 °C. We used paired one-tail t-tests to determine the effects of the experimental treatments on seedling survival and biomass, assessed at the plot level to avoid pseudoreplication. Our a priori prediction was that the removal of Avena and the presence of microcatchments would have a positive effect on seedling survival and biomass. To compare survival and biomass among species, we used a one-factor anova followed by Tukey's HSD test to determine the pairwise differences between means. All analyses were done using spss (SPSS Inc. 2007).

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

pot study

Avena had a significant competitive effect on three of the four species tested (Fig. 1; Table 1). The competitive effects were not density-dependent because Avena plants tended to grow to fill the pots (Fig. 1b–d). There was also more variation in the biomass of native plants grown without Avena compared with the biomass of native plants grown in the presence of Avena (Fig. 1a,b,d). Consequently, the fitted regression lines were particularly influenced by the results for the native plants grown without Avena (Fig. 1a,b,d).

image

Figure 1. Response of native species to increasing Avena biomass and P application measured 12 weeks after implementing a low-water regime in the glasshouse. inline image and solid regression line, No P application; inline image and dashed regression line, P applied at a rate of 5·16 mg kg−1 soil. Data points that are particularly influential on the fitted regression lines (i.e. Di values > 1) are outlined. A linear model did not fit the data for P-treated E. loxophleba subsp. loxophleba and there were too few NP-treated plants to fit any model.

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Table 1.  For each native species grown in the pot study, these are the results of a one-factor ancova on the effects of P treatment (no P added/P added) and increasing Avena biomass on seedling biomass. Boldface indicates a statistically significant P value at the critical level of 0·05. NS indicates a non-significant P value at the same critical level
 Mean square values (d.f.)
P value (effect size)
SpeciesP treatmentAvena biomassInteractionResidual
Acacia acuminataNS (1,31)0·12 (1,31)0·001 (1,31)0·002
0·65 (0)< 0·001 (−0·81)0·50 
Allocasuarina campestris0·001 (1,23)0·008 (1,23)NS (1,23)NS
0·02 (0)< 0·001 (−0·84)0·08 
Hakea recurva0·05 (1,31)0·07 (1,31)0·005 (1,31)0·001
0·03 (0)< 0·001 (−0·82)0·04 

Avena significantly reduced the biomass of Acacia acuminata at both P levels (Fig. 1a; Table 1). Avena also had a significant competitive effect on Allocasuarina campestris; this effect was reduced with P enrichment, although the difference between the slopes (effect size) was negligible (Fig. 1b; Table 1). Increasing Avena biomass had no detectable effect on the biomass of P-treated E. loxophleba subsp. loxophleba and it was not possible to determine the competitive effect on NP-treated E. loxophleba subsp. loxophleba as most died (Fig. 1c). Avena had a significant competitive effect on H. recurva (Fig. 1d; Table 1). The significant interaction term for this species indicates that P addition increased the biomass of seedlings in the absence of Avena, but had no detectable effect when Avena was present (Fig. 1d; Table 1).

P accumulation in the shoot tissue was greater for Avena in P treatments than for NP treatments (Fig. 2a; MS = 31·73, F = 107, d.f. = 1,32, P < 0·001). Avena shoot biomass was similar between P and NP treatments (Fig. 2b; MS = 0·007, F = 0·03, d.f. = 1,93, P = 0·88). Avena root biomass was greater for P-treated than for NP-treated plants (Fig. 2c; MS = 2·46, F = 4·26, d.f. = 1,93, P = 0·04).

image

Figure 2. Avena shoot [P] (a), shoot biomass (b) and root biomass (c) for plants grown without and with added P (NP and P, respectively). Values are means ± SE; n = 17–51. *** and * indicate that the differences between the bars are significant at P = 0·001 and 0·05, respectively.

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field experiment

The removal of Avena dramatically improved the survival of native seedlings (mean difference in the number of survivors = 46·25 ± 8·96 (95% CI); t3 = 20·23, P < 0·001; Fig. 3). Microcatchments improved the survival of E. loxophleba subsp. loxophleba in competition with Avena (mean difference in the number of survivors = 4·25 ± 3·35 (95% CI); t3 = 4·98, P = 0·02; Fig. 3c), but not the survival of the other three species (Fig. 3a,b,d). Microcatchments made little difference to the survival of seedlings in plots where Avena had been removed (Fig. 3a–d; P values ≥ 0·05); the mean differences in the number of survivors (± 95% CI) were −3·50 ± 5·88, −0·25 ± 7·57, −0·73 ± 0·98 and 0·50 ± 1·96 for A. acuminata, A. campestris, E. loxophleba subsp. loxophleba and H. recurva, respectively. Overall, the mean survival of E. loxophleba subsp. loxophleba was significantly greater than that of the three other species (F3,12 = 15·81, P < 0·001; mean differences significant at P < 0·01).

image

Figure 3. Survival of native seedlings 1 year after planting into field plots with and without competition from Avena (+Avena and –Avena, respectively). M, seedlings planted into microcatchments; No M, seedlings planted into level sites. Data are mean number of survivors, of nine plants per treatment combination per replicate, ± SE; n = 4 replicates. ** indicates that the difference in survival in the presence of Avena is significant at P = 0·01. From Standish, Cramer & Yates (2008).

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For plots where Avena had been removed, microcatchments made inconsistent or little difference to the mean biomass of the planted seedlings (Fig. 4a–d; P values ≥ 0·05); the differences between the means (± 95% CI) were 5·69 ± 23·24 g, 0·90 ± 7·78 g, −3·81 ± 6·61 g and −5·15 ± 30·63 g for A. acuminata, A. campestris, E. loxophleba subsp. loxophleba and H. recurva, respectively. Overall, the mean biomass of A. acuminata and E. loxophleba subsp. loxophleba was greater than those of A. campestris and H. recurva (Fig. 4; F3,12 = 17·42, P < 0·001; mean differences significant at P < 0·05).

image

Figure 4. Biomass of surviving native seedlings 1 year after planting into field plots with and without competition from Avena (+Avena and –Avena, respectively). M, seedlings planted into microcatchments; No M, seedlings planted into level sites. Data are means ± SE; n = 17–35. Note different scales on y-axes. From Standish, Cramer & Yates (2008).

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Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

The P-enriched conditions of old-fields did not favour Avena in competition with the four native species in our pot study. We found that Avena was a better competitor against native seedlings at both old-field levels of P and pre-agricultural levels of P. The root biomass of Avena was significantly greater at old-field levels of P compared with its root biomass at pre-agricultural levels of P, which would enable it to compete for below-ground resources more effectively. However, Avena was the superior competitor at both P-levels, and therefore, its persistence on old-fields is not likely to be contingent on elevated soil P. This result implies that reducing soil P will not effectively control invasive Avena on abandoned farmland.

This study is one of three recent studies to suggest that mechanisms other than the fluctuating resource hypothesis (Davis et al. 2000) might help to explain invasive species persistence. Funk & Vitousek (2007) showed that invasive species were generally more efficient than native species at using limiting resources on short timescales, based on leaf-level physiological data collected from a variety of plant life forms in Hawaii. The idea that plant traits might explain invasiveness is not new (e.g. Rejmánek & Richardson 1996), but considering them in the context of resource gradients has improved our understanding of invasive species persistence in resource-poor systems. Kulmatiski & Beard (2008) suggest that altered microbial communities may also explain the persistence of invasive species on abandoned farmlands. Their data suggest that invasive species can maintain the altered soil conditions that are induced by cultivation, which in turn, favours their growth.

In our case, it is likely that Avena prevents the establishment of native woody plants by exhausting the surface soil-water before native species have the chance to establish. Avena seeds will germinate at low soil moisture (Lenz & Facelli 2005), whereas the germination of native species is generally reliant on adequate soil moisture (Bell 1999). There is also a tendency for Avena and other exotic species to germinate more rapidly than the native species (Standish et al. 2007a). Thus, the challenge for restoration is to establish native plants by promoting their access to soil water until their roots extend beyond the rooting zone of the shallow-rooted exotic grasses. Microcatchments improved the survival of eucalypt seedlings in competition with Avena in our field experiment, but not the establishment (survival and mean biomass) of the three other native species. Microcatchments may have improved water availability to the planted seedlings, and in the case of the eucalypt seedlings planted into Avena, this improvement was enough to increase survival compared with level sites. Unfortunately, the soils were too compact to insert a soil moisture probe, and thus, we were unable to assess this. We found inconsistent or little effect of microcatchments on seedling establishment in the absence of Avena. In contrast, the removal of Avena clearly benefited seedling establishment.

There are several options for controlling Avena on old-fields: mowing, harvesting or broad-scale herbicide application over successive seasons may eventually exhaust the Avena soil seed bank. Harvesting would also strip the soil of excess P which might be necessary for the establishment of some native species because P-sensitivity – a consequence of adaptations to severely P-impoverished ancient soils – is relatively common (Lambers et al. 2008). The response of A. acuminata seedlings to increasing P in a pot study implied that P-fertilizer residues are likely to retard its growth (Standish et al. 2007b). This may explain some of the variation in the biomass of this species in our field experiment.

Intense ‘crash’ grazing by livestock (Vesk & Dorrough 2006) might also be an option for Avena control, at least initially, except where the presence of poisonous Gastrolobium species prevented the use of old-fields for grazing. However, other Mediterranean annual-type pasture species are likely to become dominant where intense grazing reduces grass cover (Rossiter 1966). Indeed, invasion of other exotic species is possible regardless of the method of Avena removal (Zavaleta, Hobbs & Mooney 2001). We found that Trifolium hirtum All., Vulpia myuros L. and Erodium botrys (Cav.) Bertol. were common where we removed Avena. In contrast, native species were not common, although a native perennial grass (Austrostipa eremophila (Reader) S.W.L. Jacobs and J. Everett) established in one pair of plots as did a single Hakea recurva. A long-term study of old-fields affected by Mediterranean-Basin annual grasses in California suggests that ongoing grass control will be required for the restoration of woody species (Cox & Allen 2008).

The competitive exclusion of native species by Avena is not the only reason for its persistence on some old-fields. A decrease in the native seed supply and an increase in the supply of Avena seeds also contribute to the Avena-dominated state (Standish et al. 2007a). In addition, herbivores browse native seedlings on old-fields, with the exception of E. loxophleba subsp. loxophleba (R.J. Standish, personal observation), and thus further reduce the likelihood of native species establishment. Lastly, it is possible that wildfire suppression is limiting recruitment of native species in this agricultural landscape (Yates et al. 1994).

The ultimate challenge is to restore a native-dominated community that can resist re-invasion by Avena and other exotic species (Buckley 2008). Our research indicates that an integrated approach including the removal of Avena, followed by planting and possibly steps to reduce soil P, will be required to restore native species to old-fields. The likelihood of these efforts resulting in the restoration of invasion resistance is unknown. The formation of intact canopies that might prevent re-invasion (Hobbs & Atkins 1991) is likely to be a lengthy process, and soil disturbance during this process is likely to favour re-invasion (Hobbs & Atkins 1988). Thus, there is a risk that despite intense efforts, restoration will not result in the development of a community resembling the historic ecosystem state. Indeed, restoration practitioners might be well-advised to direct their efforts towards more promising sites rather than attempt to restore these old-fields (Daehler 2003). Yet, old-fields dominated by persistent invasive species are likely to become more common in the future wherever native species are poorly adapted to overcome the legacies of large-scale industrial forms of agriculture (Cramer, Hobbs & Standish 2008). The increasing prominence of these ‘novel ecosystems’ will eventually force us to consider the options for their management which might include the maintenance of certain ecosystem services rather than attempting to restore the historic ecosystem state (Hobbs et al. 2006).

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

This research was funded by an Australian Research Council Discovery Grant awarded to R.J. Hobbs. We thank P. & F. Langford for allowing us access to their property. We thank J. Munroe, M. Toner, R. Ovens, R.A. Standish and especially S. Wild for research assistance. Thanks to D. Merritt for advice on the germination requirements of the native seeds and to R. Black, R. Taplin, M. Craig and M. McCarthy for advice on experimental design and data analyses. J. Morgan, M. McCarthy and two anonymous referees provided helpful comments on the manuscript.

References

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
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