Overall, the broad-scale rehabilitation outlook for North Stradbroke Island appears promising, whereby an adequate forest composition consisting of desired native vegetation has been achieved across most sites over a relatively short time. With certain key exceptions (described further below), this wide-spread establishment of stable and mostly self-sustaining ecosystems should represent a successful example of land rehabilitation given the size and scale of the anthropogenic disturbance impact and the inherent challenges of reconstructing coastal landscapes. Still, as anticipated from previous field observations, the trajectories of older sites appear to have diverged from their intended natural analogues. Despite having appropriate canopy development, these sites showed lower structural cover in the mid- and under-storeys corresponding with dramatically lower species density within these forest zones. This would suggest that they developed an open-forest canopy structure but not the desired sclerophyllous under-storey, unlike the development of the younger rehabilitated sites and long-standing undisturbed reference sites (Clifford et al. 1979; Specht and Specht 1989; Westman and Rogers 1977). Furthermore, analyses of canopy composition confirmed the emergence of black sheoak (Allocasuarina littoralis) as a predominant species among many older sites—reaching up to 60% of the total tree species density—instead of the desired mixed eucalypt-dominated communities. Since these structural and compositional differences aligned closely with the different periods of land rehabilitation practices, an intriguing aspect pertains to how conditions within the older rehabilitated environments could have enabled the native black sheoak to colonise and then so fundamentally alter the form of the rehabilitated ecosystems. Given that changes in the vegetation composition generally coincided with differences among various growth parameters, we consider that a combination of abiotic factors (e.g., alterations to the post-disturbance soil conditions) and biotic factors (e.g., ‘pioneer’ life-history strategies) could have facilitated this unintended rehabilitation outcome (Baer et al. 2002; Bonebrake et al. 2011; Siddique et al. 2008). Even though site practices have improved over time and younger sites appear to express mostly compliant ecological development within the range of acceptable outcomes, understanding of these phenomena among older sites could provide necessary insight into the management of potentially divergent ecosystems. Based on these outcomes, future studies would benefit from in-depth spatio-temporal analyses to verify these mechanisms at finer investigative scales.
Altered growth conditions enable opportunistic colonisation
An inevitable consequence of the mineral sands mining process is the disturbance of the soil’s matrix structure due to the pre-mining stripping of the topsoil layers (Smith and Nichols 2011). While this strategy is often used to reintegrate native seed and soil-microbial components into the rehabilitated ecosystems (Rokich et al. 2000), the practice of stripping and stockpiling topsoil renders the reconstructed soils relatively more susceptible to weathering and loss of biotic viability (Abdul-Kareem and McRae 1984; Koch et al. 1996). This should account for the generally lower soil fertility parameters found among the rehabilitated vs. reference sites (e.g., total carbon content, total nitrogen content, soil-nutrient content and holding capacity). Admittedly, topsoil rehabilitation practices among ‘older’ sites were not as thoroughly documented as for younger sites; still, we suspect that more detrimental soil changes were likely to have occurred among the older sites due to putatively longer periods of stockpiling. Another consequence of the mining disturbance is the reversion of the soil’s podzolization status (referring to the proportional depths of the A1, A2, and B horizons) compared to the more distinctly stratified soils found among the undisturbed sites due to the industrial processing of the sandy sub-soils (Appendix 2). These aspects are significant since the natural patterns of vegetation establishment on the dunes of eastern Australia have been closely correlated with the soil’s nutrient bioavailability and associated podzolization status (Thompson 1981
1992). In these coastal environments, the intended mixed-Eucalypt communities typically populate high dune environments having intermediate- to late-stage podzol soils (Westman 1975; Westman and Rogers 1977). By contrast, pioneer and/or facilitator species such as black sheoak often populate the adjacent fore-dunes, which have early-stage podzol soils (Lunt 1998b). Recalling that this latter species was directly seeded as part of the rehabilitation strategy (albeit at different seeding intensities depending on the period of rehabilitation), it is suspected that the loss of soil fertility and its reversion to an early podzol status would have facilitated initial growth conditions conducive to the species’ opportunistic colonisation due to its general ‘ruderal’ adaptations to nutrient-poor conditions (Crowley 1984; Diem et al. 2000). Conversely, such post-disturbance soil conditions could have been disadvantageous for the desired and relatively slower-growing mixed-Eucalypts that are typically only sparsely distributed among the fore-dunes (Westman 1975). For these reasons, we suspect that altered edaphic conditions associated with the management of post-disturbance soils along with the seeding of pioneer species likely contributed the primary first steps leading to the divergence of older rehabilitated sites away from the intended natural analogues—unlike younger sites where soil conditions are believed to have been more carefully managed.
Mono-dominance as a result of leaf litter allelopathy
Further to factors associated with soil disturbance, we also consider that (once established) the black sheoak’s above- and belowground feedback behaviour could have facilitated its mono-dominant distribution. Characteristic of all Casuarinaceae, the foliage of Allocasuarina spp. consists of waxy needle-shaped leaves that produce a dense litter-mat (Crowley 1984). Although true allelopathic compounds have yet to be identified, the thick and slowly decomposing litter layer is considered to suppress seed germination by shading and then inhibit seedling establishment. Consequently, Casuarinaceae-dominated ecosystems tend to form a closed canopy structure with an open under-storey (Lunt 1998a
1998b). Likewise, a key on-site observation in our study indicated that, where the black sheoak was most abundant, the soil surface was predominately blanketed by its ‘needles,’ which ultimately coincided with the substantial decrease in mid-storey and ground-storey species density. Hence, this soil–canopy feedback could be responsible for disrupting the development of under-storey heath while maintaining the pre-dominant black sheoak canopy composition (Facelli and Pickett 1991) which is critical for post-mining sites where the pedogenesis of the reconstructed soils has often been reverted to a ‘younger’ soil status (described above). Accordingly, Peh et al. (2011) have suggested that the lack of exogenous disturbance combined with soil nutrient deficiency and the persistence of soil-surface cover—as reported here—can result in species mono-dominance and/or arrested development. Given that black sheoak are believed to live for about 30 years (probably due to its fire response) within the natural succession of coastal ecosystems (Crowley 1984), a general concern for the older rehabilitated environments is that this feedback cycle could perpetuate an ecological regime shift away from the intended Eucalypt-dominated forests (Lindenmayer et al. 2011). Since some of the younger sites currently demonstrate an increasing proportional abundance of black sheoak (e.g., up to 35% in some locations), time will tell whether these sites (which are otherwise on course to achieve similar compositional and structural levels as the reference sites) could develop toward a similar state of arrested development without any future management intervention. Still, this scenario is unlikely given the company’s current awareness of divergent vs. convergent forest blocks, its pre-emptive modification of seeding rates, and active mitigation of ‘weedy’ species proliferation as part of contemporary rehabilitation practices.
On a related topic, Lunt (1998a) predicted that Casuarinaceae-dominated litter should have implications for any potential fire regime (or lack thereof), particularly due to its composition as a compact ground fuel that may not burn under mild fire conditions. Whereas regular fire disturbances as part of natural succession enable coastal eucalypt forests to sustain their open-woodland structure and ensure the regeneration of under-storey species, the lack of any such fire regime could lead to arrested development (Bradstock et al. 2012; Pyke et al. 2010). While the suppression of fire is a beneficial part of the rehabilitation process to prevent scorching of juvenile species, it likely inadvertently influenced the accumulation of Black Sheoak litter among certain rehabilitated forest blocks and further exacerbated its detrimental effects on the development of mid-storey and ground-storey vegetation therein. Since black sheoak is part of the native coastal landscape, the implementation of selective thinning and carefully managed fire regimes would help to reduce the incidence of mono-dominant stands in favour of the desired native mixed-eucalypts, as suggested by Chaffey and Grant (2000), Smith et al. (2001
2004), and later Shackleford et al. (2013) for the management of undesirable species among similar Australian forest ecosystems.
Mono-dominance as a result of N-fixation
A final (yet more tenuous) mechanism potentially contributing to the emergence of black sheoak mono-dominance relates to the species’ symbiotic (i.e., actinorhizal) enrichment of soil N. Although the rehabilitated sites generally had significantly lower soil fertility than the reference sites, soil N enrichment appeared (circumstantially) to correlate with the black sheoak’s increasing proportional abundance. This outcome reflects upon the rehabilitated sites achieving an increasingly stable and self-sustaining ecological status. However, it is suspected that the enrichment of soil-N pools (especially in the topsoil rather than in lower substrata) could alter the proportional distribution of soil nutrients to the benefit of species better adapted for nutrient scavenging (Diem et al. 2000). While further experimental investigation would be needed to verify these facets on North Stradbroke Island, the predominance of N-fixing species (including Acacia spp. and Allocasuarina spp.) among rehabilitated ecosystems can bring about cumulative changes to edaphic conditions (Lamb 2011; Siddique et al. 2008). This mechanism, among others, could account for initial reports of ‘weedy’ behaviour by these species on other rehabilitated mine sites (Grant and Loneragan 1999
2001). Given that N is often a limiting factor affecting primary productivity among temperate ecosystems, N-fixation has been proposed as a plant-driven feedback mechanism affecting the composition of vegetation assemblages (Knops et al. 2002; Laungani and Knops 2009). Within a wider ecological context, Rout and Callaway (2009) have identified this mechanism as potentially underlining an invasive plant paradox whereby above- and belowground biotic relationships (particularly those involving N-cycling) can facilitate plant invasions—or, in this case, mono-dominance—and all the while increase soil nutrient stocks. Hence, this plant–soil–N feedback mechanism could represent a key ecological driver affecting the development of vegetation assemblages among rehabilitated subtropical ecosystems, as well.