Patrick Audet

Adopting novel ecosystems as suitable rehabilitation alternatives for former mine sites

The nature and extent of environmental disturbance associated with mining commonly entails completely new and challenging combinations of climate, lithology and landform. Consequently, the outcomes of ecological processes associated with the recovery or restoration of ecosystems cannot be predicted reliably from previously known associations between their physical and biological components. For radically disturbed sites, we propose that it is not practicable to aim for the restoration of historical ecosystems. However, hybrid (reversibly different) or novel (irreversibly different) ecosystems comprising new combinations of physical and biological components, including both native and non-native species, could provide levels of stability and functionality acceptable to all stakeholders and within feasible management regimes. We propose that limiting physical conditions of the landscape can be identified and managed, and that alternative species combinations for introduction to these new landscapes may be considered with cautious optimism.

Novel ecosystems in ecological restoration and rehabilitation: Innovative planning or lowering the bar?

Stemming from a special symposium at the 2012 inaugural meeting of the Society for Ecological Restoration Australasia in Perth, Western Australia, this special issue editorial addresses novel ecosystems in ecological restoration and the inherent challenges of maintaining the highest standards of environmental stewardship and biological conservation in the face of increasing urbanization, agricultural expansion, and industrialization. Echoing others, we (the Guest Editors) view novel ecosystems as offering opportunities for conservation and restoration in the coming years and a pragmatic recognition that it may not always be possible, or desirable, to overcome adverse consequences of environmental degradation to reinstate historical systems. Being mindful of hubris and taking into account difficulties with identification, novel ecosystems may be viewed as a temporary or interim stage on the way towards the evolution of other future ecosystems able to supply a variety of ecosystem services, while attempting to maintain and enhance biodiversity, function and resilience. Here, a concise summary of contributions to the special issue and their significance to the field of restoration ecology is provided noting that authors were tasked to answer whether novel ecosystems are innovative planning or lowering the bar in ecological restoration. Core themes shared by the manuscripts are elucidated leading to guiding principles and, more importantly, an assessment of how and why restoration priorities are changing in the 21st century.

Structural development of vegetation on rehabilitated North Stradbroke Island: Above/belowground feedback may facilitate alternative ecological outcomes

IntroductionThis study depicts broad-scale revegetation patterns following sand mining on North Stradbroke Island, south-eastern Queensland, Australia.MethodsBased on an ecological timeline spanning 4–20 years post-rehabilitation, the structure of these ecosystems (n = 146) was assessed by distinguishing between periods of ‘older’ (pre-1995) and ‘younger’ (post-1995) rehabilitation practices.ResultsThe general rehabilitation outlook appeared promising, whereby an adequate forest composition and suitable levels of native biodiversity (consisting of mixed-eucalypt communities) were achieved across the majority of rehabilitated sites over a relatively short time. Still, older sites (n = 36) appeared to deviate relative to natural analogues as indicated by their lack of under-storey heath and simplified canopy composition now characterised by mono-dominant black sheoak (Allocasuarina littoralis) reaching up to 60% of the total tree density. These changes coincided with lower soil fertility parameters (e.g., total carbon, total nitrogen, and nutrient holding capacity) leading us to believe that altered growth conditions associated with the initial mining disturbance could have facilitated an opportunistic colonisation by this species. Once established, it is suspected that the black sheoak’s above/belowground ecological behaviour (i.e., relating to its leaf-litter allelopathy and potential for soil-nitrogen fixation) further exacerbated its mono-dominant distribution by inhibiting the development of other native species.ConclusionsAlthough rehabilitation techniques on-site have undergone refinements to improve site management, our findings support that putative changes in edaphic conditions in combination with the competitive characteristics of some plant species can facilitate conditions leading to alternative ecological outcomes among rehabilitated ecosystems. Based on these outcomes, future studies would benefit from in depth spatio-temporal analyses to verify these mechanisms at finer investigative scales.

Identification of constraining experimental-design factors in mycorrhizal pot-growth studies

In the objective of testing the design of pot-growth experiments, we conducted two greenhouse studies of a “dwarf” sunflower cultivar and an arbuscular mycorrhizal (AM) fungus to determine how pot size and inoculum distribution affect plant growth and AM symbiosis. As predicted, large-potted plants developed a greater overall biomass and root colonization than small-potted ones which we attributed to the larger “rootable” volume. Furthermore, plants grown in a band of high density inoculum substrate showed a higher prevalence of fungal vesicles (sites of lipid storage) indicating a more advanced level of root colonization compared to those grown in a dispersed inoculum substrate; this likely being due to the higher frequency of interaction between roots and fungal propagules. In a second experiment, large-potted AM plants showed a greater tolerance to water deficit than non-AM control plants; however, this mycorrhizal effect was not detected among small-potted plants. We conclude that careful consideration should be made toward design parameters to limit result biases and ultimately facilitate comparison of findings between studies.

Determining the Impact of the AM-Mycorrhizosphere on “Dwarf” Sunflower Zn Uptake and Soil-Zn Bioavailability

An in vivo compartmental pot greenhouse experiment involving “dwarf” sunflower and an arbuscular mycorrhizal (AM) fungus was designed to assess the contribution of non-AM roots (rhizosphere), AM roots and extraradical hyphae (mycorrhizosphere), or strictly extraradical hyphae (hyphosphere) on plant growth, plant metal uptake, and soil parameters using the micronutrient zinc (Zn) as a typical metal contaminant. We observed that, at high soil-Zn concentrations, the mycorrhizosphere treatments had lower Zn concentrations (especially in shoots and flowers) and a lower incidence of leaf chlorosis than the rhizosphere treatments. These phytoprotective effects are believed to be related to AM-induced biosorption processes that reduce soil metal bioavailability to delay the onset of plant metal toxicity. We also observed that the presence of extraradical hyphae causes a slight alkalinisation of the proximal soil environment whereas roots tended to acidify it, this having significant consequences toward metal bioavailability. Altogether, the AM symbiosis is considered to be a key component of ecosystem function involved in buffering plant growth conditions due to the processes of metal biosorption and hyphal alkalinisation which could contribute in enhancing the soils resiliency.

Assessing arbuscular mycorrhizal plant metal uptake and soil metal bioavailability among ‘dwarf’ sunflowers in a stratified compartmental growth environment

Using the micronutrient zinc (Zn) as a metal contaminant, a stratified compartmental pot greenhouse experiment involving ‘dwarf’ sunflowers and an arbuscular mycorrhizal (AM) fungus was designed to assess the role of AM symbiosis toward plant growth and metal uptake, and to differentiate its impact toward edaphic parameters across different soil strata. Consistent with previous hypotheses, AM plants contained up to 40% lower metal concentrations in their shoots than non-AM plants, particularly at the highest soil Zn levels (200 and 400 mg Zn kg−1 dry soil); this, corresponding with an enhanced growth status among AM plants. Upon assessing the soil Zn concentrations and pH, AM treatments also tended to have higher soil Zn levels and more alkaline conditions compared to non-AM treatments. This was found especially in the topmost soil stratum where AM root colonization was deemed most active as evidenced by a higher frequency of extraradical hyphae, vesicles, and arbuscules. Together, these effects were putatively linked to the AM-induced mechanism of metal biosorption known to modulate soil nutrient bioavailability and even delay the onset of metal toxicity.

Arbuscular mycorrhizal symbiosis and other plant-soil interactions in relation to environmental stress

In this chapter, focused on the arbuscular mycorrhizal (AM) fungi and their mostly mutualistic association with the vast majority of herbaceous plant species, we examine the cellular, molecular, and physiological mechanisms by which the mycorrhizal symbiosis can enhance plant stress tolerance in relation to a number of abiotic environmental stressors, such as macro- and micronutrient deficiency, drought, and metal toxicity. Overall, the primary mechanisms of interaction discussed here include: (1) the enhanced uptake of macro- and micronutrients and water; and (2) the stabilization of the soil architecture via mycorrhizal-enhanced soil aggregation and metal biosorption processes. A key facet of this analysis involves the identification of direct vs. indirect benefits of interactions, and their distinctive impacts toward plant development as well as the proximal growth environment. Accordingly, due to the significant and widespread effects of these direct and indirect processes toward plant physiological and soil ecological function, it is suggested that the mycorrhizal symbiosis should constitute an extrinsic stress tolerance strategy that could complement the inherent resistance mechanisms of plants when subjected to an array of potential stressors, and also buffer the growth environment. For this reason, it is recommended that future studies take into account such multitrophic interactions (e.g., above- and belowground relationships) to better depict physiological and ecological phenomena in relation to environmental stress.

Examining the ecological paradox of the ‘mycorrhizal-metal-hyperaccumulators’

This analysis identifies and attempts to resolve the paradox of combining plant hyperaccumulators and arbuscular mycorrhizal fungi (AMF) for the purpose of post-industrial bioremediation due to the divergence of their respective ecological and evolutionary stress-tolerance behaviors. The identification of a ‘dilemma of resource allocation’ associated with plant resources consumed in intrinsic (e.g. metabolic) vs. extrinsic (e.g. symbiotic) stress-tolerance mechanisms could provide a suitable evolutionary reasoning for the apparent dichotomy existing between the hyperaccumulators and AMF–plant life-history strategies. Ultimately, it is considered that any efforts toward integrating such biotechnology innovations into bioremediation strategies (e.g. ‘mycorrhizal–metal-hyperaccumulators’) should first explicitly consider their inherent environmental and (or) evolutionary contexts to avoid misleading and possibly even unproductive outcomes prior to incorporating these attributes as potential technological solutions.