Research Priorities for Conservation of Metallophyte Biodiversity and their Potential for Restoration and Site Remediation

Migration patterns of heavy metals from solid waste stockpile soils by native plants for ecological restoration in arid and semi-arid regions of Northwest China

Ecological remediation with native plants is the main measure to control the pollution of solid waste in Northwest China. However, the heavy metal transport characteristics of these native plants are still unidentified. This study analyzed the distribution of 16 heavy metals in native plants in the desulfurization gypsum yard (DGY), the gangue yard (GY) and the fly ash yard (FAY). The results showed that the soil contained many heavy metals in high concentrations. For instance, As concentrations were comparable to the global soil background values, whereas Cr and Mn concentrations in the area were 2-3 times greater than the global soil background values. The content of heavy metals in the plant root system increased first, then decreased as the distance from the yard increased. Ni, Pb, and Cd migrated well in Artemisia frigida Willd and Artemisia sieversiana Ehrhart ex Willd, with A. sieversiana showing a particularly strong migration in GY. A. sieversiana, on the other hand, was more successful at migrating Cd at DGY and had a similar capability for Mg migration in all three locations. Festuca rubra L was potentially suitable for planting in GY for Ni removal. In conclusion, the migration patterns of different heavy metals were not alike for plants in the three landfills. The results provided a basis for plant selection for ecological restoration in arid and semi-arid regions.

Role of the salt glands of Armeria maritima (halophyte) in removal of lead from tissues

Armeria maritima is a halophyte exhibiting a strong tolerance to heavy metals. It grows on zinc-lead waste heaps. This study aimed to determine the role of salt glands in the removal of lead (Pb) from plants and to trace the path of lead from the shoots to the salt glands on the surface of leaves. Mechanisms allowing high tolerance to lead in A. maritima were also evaluated. These examinations were conducted on a lead-tolerant population and a lead-sensitive plant population. The plants were treated with Pb(NO3)2 and the path of lead was traced from the roots to the leaves. The lead-tolerant population transported twice as much lead as the sensitive population. The action of the salt glands resulted in 40% of the leaf lead content in the lead-tolerant population being expelled onto the surface of the leaves. These features indicate the high phytoremediation capabilities of these halophyte plants. The excretion of multi-ionic solutes by the salt glands results in the appearance of tiny crystals on the surface of the leaves. In this publication, for the first time, an attempt was made to determine what chemical compounds build up these crystals and to determine their crystal structure. Solving this problem was possible through the usage of single-crystal X-ray structural analysis.

The efficient applications of native flora for phytorestoration of mine tailings: a pan-global survey

Mine tailings are the discarded materials resulting from mining processes after minerals have been extracted. They consist of leftover mineral fragments, excavated land masses, and disrupted ecosystems. The uncontrolled handling or discharge of tailings from abandoned mine lands (AMLs) poses a threat to the surrounding environment. Numerous untreated mine tailings have been abandoned globally, necessitating immediate reclamation and restoration efforts. The limited feasibility of conventional reclamation methods, such as cost and acceptability, presents challenges in reclaiming tailings around AMLs. This study focuses on phytorestoration as a sustainable method for treating mine tailings. Phytorestoration utilizes existing native plants on the mine sites while applying advanced principles of environmental biotechnology. These approaches can remediate toxic elements and simultaneously improve soil quality. The current study provides a global overview of phytorestoration methods, emphasizing the specifics of mine tailings and the research on native plant species to enhance restoration ecosystem services.

Using Mediterranean Native Plants for the Phytoremediation of Mining Sites: An Overview of the Past and Present, and Perspectives for the Future

Mining exploitation in the Mediterranean Basin has left evident scars on the environment, and poses serious risks for human health and biodiversity, especially when mine wastes are left abandoned. This review analysed the main issues of metal(loid)s pollution related to mine exploitation in the Mediterranean Basin. Here, a list of Mediterranean native plant species studied for phytoremediation is given and, considering their biological forms, vegetational types, and ecology, we categorised them into halotolerant and hydro/hygrophilous vegetation, annual and perennial meadows, garrigues and maquis, and high maquis and woods. The main conclusions of the review are as follows: (1) plant communities established on mine environments are often rich in endemic taxa which ensure a high biodiversity and landscape value, and can help in the psychophysical health of local inhabitants; (2) political and land management should take greater account of the use of native plants for the remediation of contaminated soils; (3) a multidisciplinary approach that includes, among others, studies on biochemical response to metal(loid)s as well as the application of innovative soil amendments gives better results; (4) phytoextraction applications require a detailed recovery plan that takes into consideration several issues, including the negative influence on biodiversity due to extensive use of monotypic plantations, disposal of harvested hazardous plants, and the risk of phytoextracts entering the food chain; and (5) more studies are necessary to increase knowledge and to detect suitable species-especially halophytic ones-for phytoremediation purposes.

Reflectance spectroscopy as a promising tool for 'sensing' metals in hyperaccumulator plants

MAIN CONCLUSION

The VNIR reflectance spectra of nickel hyperaccumulator plant leaves have spectral variations due to high nickel concentrations and this property could potentially be used for discovery of these plants.  Hyperaccumulator plants accumulate high concentrations of certain metals, including manganese, cobalt, or nickel. Of these metals, the divalent ions of nickel have three absorption bands in the visible to near-infrared region which may cause variations in the spectral reflectance of nickel hyperaccumulator plant leaves, but this has not been investigated previously. In this shortproof-of-concept study, the spectral reflectance of eight different nickel hyperaccumulator plant species leaves were subjected to visible and near-infrared and shortwave infrared (VNIR-SWIR) reflectance spectrum measurements in dehydrated state, and for one species, it was also assessed in hydrated state. Nickel concentrations in the plant leaves were determined with other methods and then correlated to the spectral reflectance data. Spectral variations centred at 1000 ± 150 nm were observed and had R-values varying from 0.46 to 0.96 with nickel concentrations. The extremely high nickel concentrations in nickel hyperaccumulator leaves reshape their spectral reflectance features, and the electronic transition of nickel-ions directly contributes to absorption at ~ 1000 nm. Given that spectral variations are correlated with nickel concentrations it make VNIR-SWIR reflectance spectrometry a potential promising technique for discovery of hyperaccumulator plants, not only in the laboratory or herbarium, but also in the field using drone-based platforms. This is a preliminary study which we hope will instigate further detailed research on this topic to validate the findings and to explore possible applications.

Mercury accumulation potential of aquatic plant species in West Dongting Lake, China

West Dongting Lake is a protected wetland with the potential for high levels of mercury release via wastewater and deposition from industry and agriculture during the last decade. To find out the ability of various plant species to accumulate mercury pollutants from soil and water, nine sites were studied in the downstream direction of the flow of the Yuan and Li Rivers, which are tributaries of the Yellow River flowing into West Dongting Lake, where mercury levels arere high in soil and plant tissues. The total mercury (THg) concentration in wetland soil was 0.078-1.659 mg/kg, which varied along the gradient of water flow along the river. According to canonical correspondence analysis and correlation analysis, there was a positive correlation between the soil THg concentration and the soil moisture in West Dongting Lake. There is high heterogeneity in the spatial distribution of soil THg concentration in West Dongting Lake, which may be related to the spatial heterogeneity of the soil moisture. Some plant species had higher THg concentrations in aboveground tissues (translocation factor >1), but none of these plant species fit the criteria as hyperaccumulators of mercury. And some species of the same ecological type (e.g., emergent, submergent, floating-leaved) exhibited very different strategies for mercury uptake. The concentrations of mercury in these species were lower than in other studies but these had relatively higher translocation factors. To phytoremediate soil mercury in West Dongting Lake, the regular harvest of plants could help remove mercury from soil and plant tissue.

X-ray fluorescence spectroscopy (XRF) for metallome analysis of herbarium specimens

BACKGROUND

"Herbarium X-ray Fluorescence (XRF) Ionomics" is a new quantitative approach for extracting the elemental concentrations from herbarium specimens using handheld XRF devices. These instruments are principally designed for dense sample material of infinite thickness (such as rock or soil powder), and their built-in algorithms and factory calibrations perform poorly on the thin dry plant leaves encountered in herbaria. While empirical calibrations have been used for 'correcting' measured XRF values post hoc, this approach has major shortcomings. As such, a universal independent data analysis pipeline permitting full control and transparency throughout the quantification process is highly desirable. Here we have developed such a pipeline based on Dynamic Analysis as implemented in the GeoPIXE package, employing a Fundamental Parameters approach requiring only a description of the measurement hardware and derivation of the sample areal density, based on a universal standard.

RESULTS

The new pipeline was tested on potassium, calcium, manganese, iron, cobalt, nickel, and zinc concentrations in dry plant leaves. The Dynamic Analysis method can correct for complex X-ray interactions and performs better than both the built-in instrument algorithms and the empirical calibration approach. The new pipeline is also able to identify and quantify elements that are not detected and reported by the device built-in algorithms and provides good estimates of elemental concentrations where empirical calibrations are not straightforward.

CONCLUSIONS

The new pipeline for processing XRF data of herbarium specimens has a greater accuracy and is more robust than the device built-in algorithms and empirical calibrations. It also gives access to all elements detected in the XRF spectrum. The new analysis pipeline has made Herbarium XRF approach even more powerful to study the metallome of existing plant collections.

Seed Bank Conservation and Incipient Seed Development in Orchids Colonizing Mining Wastes: Results of a Field Pilot Experiment

As they represent actual or potential risks to human and environmental safety and health, abandoned mines are a major global problem. The heavy metal-polluted tailings dump of Barraxiutta (Domusnovas, southwestern Sardinia, Italy) is home to a metallicolous population of Epipactis tremolsii (Orchidaceae). A reclamation of the abandoned mine area seems to be approaching, and such an intervention may pose a serious risk for the maintenance of the unique orchid population colonizing the mine wastes. In the present work, the seed packet technique was implemented for the first time to observe orchid seed development in mine wastes. This approach allowed us to explore different seed-based conservation options for the metallicolous orchid population and to gain a deeper grasp of population dynamics and ecology. Four different sowing treatments were set up in the tailing dump and in a near unpolluted site (control site). The field phase of the experiment lasted for 10 months, a period in which the experimental seed bank preservation and incipient seed development were observed and statistically approached. Our findings observed no significant seed loss happening during the experiment, demonstrating the suitability of the seed packet technique to also explore seed bank conservation and development in extreme environmental conditions (i.e., polluted mine wastes). This field method will be a useful tool to further explore the more effective translocation and quasi in situ conservation alternatives for the E. tremolsii metallicolous population. Incipient and site-specific seed development (non-mycorrhizal stage) was observed during the experiment. A plant-soil fungus interaction at the seed level was also observed, the nature of which remains to be ascertained in further studies providing a longer duration for the field phases.

An ecological remediation model combining optimal substrate amelioration and native hyperaccumulator colonization in non-ferrous metal tailings pond

The vegetation deterioration and pollution expansion from non-ferrous metal tailings pond have been found in many countries leading to water soil erosion and human health risk. Conventional ecological remediation technologies of mine tailings such as capping were costly and elusive. This study provided an economic and effective model as an alternative by substrate amelioration and vegetation restoration. A field experiment was carried out on a silver tailings pond in southwest China. Tailings substrate was ameliorated by adding organic matter (decomposed chicken manure, DCM), structural conditioner (polyacrylamide, PAM), water-retaining agent (acrylic acid-bentonite water-retaining agent, AAB), and heavy metal immobilizer (biofuel ash, BFA), which were optimized by laboratory experiment. Native heavy metal hyperaccumulator, Bidens pilosa, was colonized. Vegetation coverage and plant height of Bidens pilosa reached about 80% and over 30 cm respectively after 3 months, and the turbidity of tailings leaching solution decreased by 60%. The practice showed that the proportion of available heavy metals in tailings substrate was significantly lower than that in the soil surrounding mining area. Immobilization didn't have stabilization effect on Cd, Zn, and Pb, and As was only 0.002%, phytoremediation had stabilization effect of Cd, Zn, As, and Pb were 2.5-3.5%, 1-2%, 0.25-0.5%, and 0.25-0.75%. Phytoremediation was more effective significantly in controlling heavy metal pollution risk of tailings than immobilization. These results provided a new ecological remediation OSA-NHC model, meaning a combination of optimal substrate amelioration and native hyperaccumulator colonization, which could achieve vegetation restoration and augment heavy metal pollution control in non-ferrous metal tailings pond.

Re-introduction of an extinct population of Pulsatilla patens using different propagation techniques

The study focuses on the propagation of a rare and endangered plant species (Pulsatilla patens) to re-introduce an extinct population from calamine area in Southern Poland. The plants were propagated from seeds, rhizome cuttings, or regenerated in vitro from shoot tips, hypocotyls with roots or cotyledons of seedlings on Murashige & Skoog (MS) medium supplemented with 0.25 or 0.50 mg L^-1 BAP (Benzylaminopurine) via direct and indirect organogenesis or somatic embryogenesis (SE). The most efficient micropropagation method was with shoot tips as an explant on MS + 0.25 mg L^-1 BAP where 97% of the explants produced multiple shoots, mass SE was observed after transfer on ½ MS with 2% saccharose; 267 (35%) shoots rooted on ½ MS + 2% saccharose were acclimatized to ex vitro conditions. Flow cytometry revealed genome size stability of propagated plantlets. Low genetic differentiation between micropropagated plantlets and initial material was indicated by ISSR (Inter Simple Sequence Repeat) markers. Totally, 132 vigorous plantlets obtained on various pathways were introduced to the field plots in 2020; 30.33% survived the winter, and several reached the generative stage and flowered in the spring 2021. In next season (March/April 2022) the number of introduced plants decreased to 25% while the number of flowering and fruiting shoots in different clumps increased in some plots. This is the first report of successful re-introduction of the endangered P. patens based on micropropagation, rhizome cuttings, and seed germination.

Phylogenomic analyses in Phrymaceae reveal extensive gene tree discordance in relationships among major clades

PREMISE

Phylogenomic datasets using genomes and transcriptomes provide rich opportunities beyond resolving bifurcating phylogenetic relationships. Monkeyflower (Phrymaceae) is a model system for evolutionary ecology. However, it lacks a well-supported phylogeny as a basis for a stable taxonomy and for macroevolutionary comparisons.

METHODS

We sampled 24 genomes and transcriptomes in Phrymaceae and closely related families, including eight newly sequenced transcriptomes. We reconstructed the phylogeny using IQ-TREE and ASTRAL, evaluated gene tree discordance using PhyParts, Quartet Sampling, and a cloudogram, and carried out reticulation analyses using PhyloNet and HyDe. We searched for whole genome duplication (WGD) events using chromosome numbers, synonymous distances, and gene duplication events as evidence.

RESULTS

Most gene trees support the monophyly of Phrymaceae and each of its tribes. Most gene trees also support tribe Mimuleae being sister to Phrymeae + Diplaceae + Leucocarpeae, with extensive gene tree discordance among the latter three. Despite the discordance, the monophyly of Mimulus s.l. is rejected, and no individual reticulation event among the Phrymaceae tribes is well-supported. Reticulation likely occurred among Erythranthe bicolor and closely related species. No ancient WGD was detected in Phrymaceae. Instead, small-scale duplications are among potential drivers of macroevolutionary diversification of Phrymaceae.

CONCLUSIONS

We show that analysis of reticulate evolution is sensitive to taxon sampling and methods used. We also demonstrate that phylogenomic datasets using genomes and transcriptomes present rich opportunities to investigate gene family evolution and genome duplication events involved in lineage diversification and adaptation.

Harnessing an arbuscular mycorrhizal fungus to improve the adaptability of a facultative metallophytic poplar (Populus yunnanensis) to cadmium stress: Physiological and molecular responses

Populus yunnanensis Dode, a facultative metallophytic poplar, exhibits afforestation potential in barren mine tailing areas. However, the interactions and functional roles of arbuscular mycorrhizal fungus (AMF) in P. yunnanensis adaptability to heavy metal stress remain unclear. Physiological and molecular responses of P. yunnanensis plantlets to AMF (Funneliformis mosseae) under cadmium (Cd) stress (50 mg kg^-1) were investigated. Results showed attenuation of Cd phytotoxicity effects on cell organelles upon AMF inoculation, which also reduced the Cd concentration in the poplar leaves, stems, and roots. Under Cd stress, AMF-blocking of metal transporter (e.g., Ca²⁺ channel) activity occurred, decreasing root cell Cd influx by reducing H⁺ efflux. Bioaugmentation of rhizosphere sediments by AMF to stabilize metals with a decreasing DTPA-extractable Cd also occurred. The AMF inoculation promoted Cd conversion into inactive, less phytotoxic forms, and helped to maintain ion homeostasis and relieve nutritional ion (e.g., Ca, Mg) disorders caused by excessive Cd. Leaf enzyme and non-enzyme antioxidant systems were triggered. Root and leaf physiological response patterns differed. The AMF regulated the poplar functional genes, and nine metal-responsive gene clusters were identified. We suggest that AMF is a functional component of P. yunnanensis phenotype extension, contributing to strong adaptability to unfavorable mine tailings conditions.

Plant–Metal Interactions in the Context of Climate Change

Expanding fundamental understanding of the complex and far-reaching impacts of anthropogenic climate change is essential for formulating mitigation strategies. There is abundant evidence of ongoing damage and threat to plant health across both natural and cultivated ecosystems, with potentially immeasurable cost to humanity and the health of the planet. Plant–soil systems are multi-faceted, incorporating key variables that are individually and interactively affected by climatic factors such as rainfall, solar radiation, air temperature, atmospheric CO2, and pollution. This synthesis focuses on climate effects on plant–metal interactions and related plant–soil dynamics. Ecosystems native to metalliferous soils incorporate vegetation well adapted to metal oversupply, yet climate-change is known to induce the oversupply of certain immobile soil metals by altering the chemistry of non-metalliferous soils. The latter is implicated in observed stress in some non-metal-adapted forest trees growing on ‘normal’ non-metalliferous soils. Vegetation native to riverine habitats reliant on flooding is increasingly at risk under drying conditions caused by anthropogenic water removal and climate change that ultimately limit plant access to essential trace-metal nutrients from nutrient poor sandy soils. In agricultural plant systems, it is well known that environmental conditions alter soil chemistries and plant responses to drive plant metal toxicity stress. These aspects are addressed with reference to specific scenarios and studies linking climate to plant–metal interactions, with emphasis on land plants.

Manganese distribution in the Mn-hyperaccumulator Grevillea meisneri from New Caledonia

New Caledonian endemic Mn-hyperaccumulator Grevillea meisneri is useful species for the preparation of ecocatalysts, which contain Mn–Ca oxides that are very difficult to synthesize under laboratory conditions. Mechanisms leading to their formation in the ecocatalysts are unknown. Comparing tissue-level microdistribution of these two elements could provide clues. We studied tissue-level distribution of Mn, Ca, and other elements in different tissues of G. meisneri using micro-X-Ray Fluorescence-spectroscopy (μXRF), and the speciation of Mn by micro-X-ray Absorption Near Edge Structure (µXANES), comparing nursery-grown plants transplanted into the site, and similar-sized plants growing naturally on the site. Mirroring patterns in other Grevillea species, Mn concentrations were highest in leaf epidermal tissues, in cortex and vascular tissues of stems and primary roots, and in phloem and pericycle–endodermis of parent cluster roots. Strong positive Mn/Ca correlations were observed in every tissue of G. meisneri where Mn was the most concentrated. Mn foliar speciation confirmed what was already reported for G. exul, with strong evidence for carboxylate counter-ions. The co-localization of Ca and Mn in the same tissues of G. meisneri might in some way facilitate the formation of mixed Ca–Mn oxides upon preparation of Eco-CaMnOx ecocatalysts from this plant. Grevillea meisneri has been successfully used in rehabilitation of degraded mining sites in New Caledonia, and in supplying biomass for production of ecocatalysts. We showed that transplanted nursery-grown seedlings accumulate as much Mn as do spontaneous plants, and sequester Mn in the same tissues, demonstrating the feasibility of large-scale transplantation programs for generating Mn-rich biomass.

Low-molecular-weight ligands in plants: role in metal homeostasis and hyperaccumulation

Mineral nutrition is one of the key factors determining plant productivity. In plants, metal homeostasis is achieved through the functioning of a complex system governing metal uptake, translocation, distribution, and sequestration, leading to the maintenance of a regulated delivery of micronutrients to metal-requiring processes as well as detoxification of excess or non-essential metals. Low-molecular-weight ligands, such as nicotianamine, histidine, phytochelatins, phytosiderophores, and organic acids, play an important role in metal transport and detoxification in plants. Nicotianamine and histidine are also involved in metal hyperaccumulation, which determines the ability of some plant species to accumulate a large amount of metals in their shoots. In this review we extensively summarize and discuss the current knowledge of the main pathways for the biosynthesis of these ligands, their involvement in metal uptake, radial and long-distance transport, as well as metal influx, isolation and sequestration in plant tissues and cell compartments. It is analyzed how diverse endogenous ligand levels in plants can determine their different tolerance to metal toxic effects. This review focuses on recent advances in understanding the physiological role of these compounds in metal homeostasis, which is an essential task of modern ionomics and plant physiology. It is of key importance in studying the influence of metal deficiency or excess on various physiological processes, which is a prerequisite to the improvement of micronutrient uptake efficiency and crop productivity and to the development of a variety of applications in phytoremediation, phytomining, biofortification, and nutritional crop safety.

Metal bioaccumulation alleviates the negative effects of herbivory on plant growth

Metalliferous soils can selectively shape plant species' physiology towards tolerance of high metal concentrations that are usually toxic to organisms. Some adapted plant species tolerate and accumulate metal in their tissues. These metals can serve as an elemental defence but can also decrease growth. Our investigation explored the capacity of natural metal accumulation in a tropical tree species, Eremanthus erythropappus (Asteraceae) and the effects of such bioaccumulation on plant responses to herbivory. Seedlings of E. erythropappus were grown in a glasshouse on soils that represented a metal concentration gradient (Al, Cu, Fe, Mn and Zn), and then the exposed plants were fed to the herbivores in a natural habitat. The effect of herbivory on plant growth was significantly mediated by foliar metal ion concentrations. The results suggest that herbivory effects on these plants change from negative to positive depending on soil metal concentration. Hence, these results provide quantitative evidence for a previously unsuspected interaction between herbivory and metal bioaccumulation on plant growth.

Health risk assessment of heavy metals in soils and screening of accumulating plants around the Wanshan mercury mine in Northeast Guizhou Province, China

The Wanshan mercury mine, which is an abandoned mine located in northeastern Guizhou Province in Southwest China, has introduced serious Hg pollution to the local ecosystem resulting from previous mining and smelting activities. However, it is not clear to date whether soil pollution has actually improved after treatment by related departments. Therefore, the present study investigates the vegetation community and heavy metal contents of the soil and plants in the Wanshan mercury mining area. The results showed that most of Hg, Cd, As, Cu, and Zn contents in soil samples were higher than those of Soil Environment Quality Risk Control Standard for Soil Contamination of Agricultural Land in China (GB15618-2018). The observed plant species mainly consisted of Compositae, followed by Leguminosae. Unfortunately, this investigation found that heavy metal concentrations in these plants were not extremely high and far below the standard of hyperaccumulator. Despite all this, the maximum values of bioaccumulation factor for Pb, Cd, Hg, As, Cu and Zn were Serissa japonica (Thunb.) Thunb., Rhus chinensis Mill., Potentilla sibbaldii Haller f., Erigeron canadensis L., Clerodendrum bungei var. bungei. and Rhus chinensis Mill., respectively. Regardless of the carcinogenic or noncarcinogenic risk index, the potential risk to urban children is higher. Our results suggest that heavy metal pollution was indeed relieved since their contents in soil significantly decreased in comparison with those reported in other previous studies. This finding provides a reference for the long-term treatment of heavy metal pollution in the local environment and other areas employing analogous environmental protection measures.

Population Genetics of Odontarrhena (Brassicaceae) from Albania: The Effects of Anthropic Habitat Disturbance, Soil, and Altitude on a Ni-Hyperaccumulator Plant Group from a Major Serpentine Hotspot

Albanian taxa and populations of the genus Odontarrhena are most promising candidates for research on metal tolerance and Ni-agromining, but their genetic structure remains unknown. We investigated phylogenetic relationships and genetic differentiation in relation to distribution and ploidy of the taxa, anthropic site disturbance, elevation, soil type, and trace metals at each population site. After performing DNA sequencing of selected accessions, we applied DNA-fingerprinting to analyze the genetic structure of 32 populations from ultramafic and non-ultramafic outcrops across Albania. Low sequence divergence resulted in poorly resolved phylograms, but supported affinity between the two diploid serpentine endemics O. moravensis and O. rigida. Analysis of molecular variance (AMOVA) revealed significant population differentiation, but no isolation by distance. Among-population variation was higher in polyploids than in diploids, in which genetic distances were lower. Genetic admixing at population and individual level occurred especially in the polyploids O. chalcidica, O. decipiens, and O. smolikana. Admixing increased with site disturbance. Outlier loci were higher in serpentine populations but decreased along altitude with lower drought and heat stress. Genetic variability gained by gene flow and hybridization at contact zones with "resident" species of primary ultramafic habitats promoted expansion of the tetraploid O. chalcidica across anthropogenic sites.

Keep and promote biodiversity at polluted sites under phytomanagement

The phytomanagement concept combines a sustainable reduction of pollutant linkages at risk-assessed contaminated sites with the generation of both valuable biomass for the (bio)economy and ecosystem services. One of the potential benefits of phytomanagement is the possibility to increase biodiversity in polluted sites. However, the unique biodiversity present in some polluted sites can be severely impacted by the implementation of phytomanagement practices, even resulting in the local extinction of endemic ecotypes or species of great conservation value. Here, we highlight the importance of promoting measures to minimise the potential adverse impact of phytomanagement on biodiversity at polluted sites, as well as recommend practices to increase biodiversity at phytomanaged sites without compromising its effectiveness in terms of reduction of pollutant linkages and the generation of valuable biomass and ecosystem services.

Metal Mobility in Afforested Sites of an Abandoned Zn-Pb Ore Mining Area

Heaps of waste material constitute a serious environmental problem in regions where the historical exploitation and processing of metal ores has taken place. The presented paper describes the trace metal distribution in selected heaps in the lead-zinc mining area of an abandoned mine in Poland, as well as the soil horizons beneath. The study aims at the estimation of the metal remobilization rate in vertical profiles in the spontaneously afforested area in the context of the potential danger it poses to the local groundwater. Individual samples were taken from profiles dug in heaps found in deciduous and coniferous forests. The bulk density, pH, organic matter and carbonate content, as well as the concentration and chemical forms of metals were analysed. Buffer properties and the mineralogical composition were also determined for the selected samples. The investigation indicates excessive cadmium, zinc and lead concentrations in the analysed heap material and the significant secondary enrichment of former soil horizons. A large percentage of these metals occur in potentially mobile forms. It suggests that, despite the high pH of the heap material and the good buffer properties of soil, cadmium and to a lesser extent, zinc, has migrated downwards to depths of at least several dozen centimetres over a period of about 200 years. This is related to soil acidity, particularly in profiles abundant in organic matter resulting from the encroachment of forest communities, particularly of coniferous forest. Spontaneous afforestation forming the litter cover contribute to the stabilization of the heap material and limiting groundwater pollution. Even though specific remediation measures are not needed in this area, it requires long-term monitoring.

Pioneer trees of Betula pendula at a red gypsum landfill harbour specific structure and composition of root-associated microbial communities

The study of root-associated microbial communities is important to understand the natural processes involved in plant recolonisation at degraded areas. Root associated bacterial and fungal communities of woody species colonising a red gypsum landfill (a metal-enriched environment) were characterised through metabarcoding. Among trees naturally growing on the landfill, Betula pendula is the only tree species in the centre of the area, whereas companion tree species such as Populus nigra, P. tremula and Salix purpurea were present on the edges. The bacterial community was dominated by Proteobacteria (38%), Actinobacteria (35%) and Bacteroidetes (20%) and the most abundant bacterial OTU belonged to the family Streptomycetaceae. The fungal community was dominated by Ascomycota (60%) and Basidiomycota (30%) and the most abundant family was Pyronemataceae. Analysis of similarities, heatmap and hierarchical cluster analysis showed that B. pendula grown in the centre of the landfill harboured a specific microbial community, which was unique and different, not only from other tree species (Populus or Salix spp.), but also from other B. pendula growing at the edges. Our findings on relevant indicator OTUs associated to the birches located in the centre of the landfill (such as Otu00716 Catellatospora sp. (family Micromonosporaceae, phylum Actinobacteria) or Otu4_35502 Russula sp. (family Russulaceae, phylum Basidiomycota)) may have important implications for the successful revegetation of these harsh environments using microbial-based phytostabilisation approaches.

Combining genotype, phenotype, and environmental data to delineate site-adjusted provenance strategies for ecological restoration

Despite the importance of climate-adjusted provenancing to mitigate the effects of environmental change, climatic considerations alone are insufficient when restoring highly degraded sites. Here we propose a comprehensive landscape genomic approach to assist the restoration of moderately disturbed and highly degraded sites. To illustrate it we employ genomic data sets comprising thousands of single nucleotide polymorphisms from two plant species suitable for the restoration of iron-rich Amazonian Savannas. We first use a subset of neutral loci to assess genetic structure and determine the genetic neighbourhood size. We then identify genotype-phenotype-environment associations, map adaptive genetic variation, and predict adaptive genotypes for restoration sites. Whereas local provenances were found optimal to restore a moderately disturbed site, a mixture of genotypes seemed the most promising strategy to recover a highly degraded mining site. We discuss how our results can help define site-adjusted provenancing strategies, and argue that our methods can be more broadly applied to assist other restoration initiatives.

Plant Functional Traits on Tropical Ultramafic Habitats Affected by Fire and Mining: Insights for Reclamation

Biodiversity-rich tropical ultramafic areas are currently being impacted by land clearing and particularly by mine activities. The reclamation of ultramafic degraded areas requires a knowledge of pioneer plant species. The objective of this study is to highlight the functional traits of plants that colonize ultramafic areas after disturbance by fire or mining activities. This information will allow trait-assisted selection of candidate species for reclamation. Fifteen plots were established on ultramafic soils in Sabah (Borneo, Malaysia) disturbed by recurrent fires (FIRE plots) or by soil excavation and quarrying (MINE plots). In each plot, soil samples were collected and plant cover as well as species abundances were estimated. Fifteen functional traits related to revegetation, nutrient improvement, or Ni phytomining were measured in sampled plants. Vegetation of both FIRE and MINE plots was dominated by perennials with lateral spreading capacity (mainly by rhizomes). Plant communities displayed a conservative growth strategy, which is an adaptation to low nutrient availability on ultramafic soils. Plant height was higher in FIRE than in MINE plots, whereas the number of stems per plant was higher in MINE plots. Perennial plants with lateral spreading capacity and a conservative growth strategy would be the first choice for the reclamation of ultramafic degraded areas. Additional notes for increasing nutrient cycling, managing competition, and implementing of Ni-phytomining are also provided.

Visualizing Hotspots and Future Trends in Phytomining Research Through Scientometrics

Phytomining has attracted widespread attention as a technique for harvesting “bio-ore.” This technology has potential applications in the metal and minerals industry for low-grade metal and mineral mining as well as metal recycling from polluted soil. The hotspots and future trends of this technology deserve in-depth exploration. This paper presents a systematic review of the phytomining research area through the scientometrics method based on the citation data collected from the Web of Science Core Collection (WoSCC). The results show that the earliest phytomining-related research was published in 1997. Between 1997 and 2019, 232 publications were published in 109 journals. Plant and Soil, the International Journal of Phytoremediation, and the Journal of Geochemical Exploration were the top three most prolific journals and accounted for 18.1% of these publications. Guillaume Echevarria, J.L. Morel, and Antony Van der Ent were the top three most prolific authors, and their work accounted for 40.1% of these publications. The cluster results of document co-citation analysis revealed that the hotspots in phytomining research area mainly includes “nickel accumulation,” “heavy metal uptake,” “mining site,” “heavy metal,” “hyperaccumulation yield,” “growth effect,” and “alternative method.” Keyword burst detection results find that the hot topics have changed over time from “phytomining” to “agromining”; from “contaminated soil” to “serpentine soil”; and from “mechanism” to “phytomining process” and “commercial phytoextraction.” This study describes the intellectual landscape of research and provides future research directions for phytomining research so that researchers can identify future research topics and partners.

Elemental distribution and chemical speciation of copper and cobalt in three metallophytes from the copper–cobalt belt in Northern Zambia

Metallophytes from the Zambian copper–cobalt belt have a complex Cu–Co coordination chemistry and diverse elemental distribution at the tissue-level. This study reveals different ecophysiological responses in hyper-tolerant plant species growing in metalliferous environments.

Early succession on slag compared to urban soil: A slower recovery

Slag, waste from the steel-making process, contains large amounts of calcium, magnesium, iron and other heavy metals. Because of its composition, high pH and low water retention ability, slag is considered inhospitable to plants. Nevertheless, the spontaneously generated plant communities on slag are surprisingly diverse, but the assembly and structure of such communities are poorly studied. Previous studies suggest reduced rates of succession due to low growth rate and slow accumulation of topsoil. To investigate whether slag communities display similar patterns, we used two former industrial sites on the South Side of Chicago, IL, both with high pH (8-9.2) sand content (80%) and calcium concentration (> 9000 ppm). We removed all vegetation from both slag and non-slag plots to test whether recovery differed over one growing season (4 months). To directly assess plant growth, selected focal species were planted on both sites and harvested. We show that recovery from removal differed at slag and non-slag sites: the recruitment process on slag, measured by percent vegetative cover and number of species in plots, was significantly slower at 6-8 weeks of the manipulation and beyond, suggesting a potential stage-dependent effect of slag on plant growth. Certain slag plots recorded less cover than non-slag plots by >30% at maximum difference. Functional trait analysis found that graminoid and early successional species preferentially colonized slag. Overall, slag plots recovered more slowly from disturbance, suggesting a slow succession process that would hinder natural recovery. However, slag also has the potential to serve as plant refugia, hosting flora of analogous habitats native to the area: one of our industrial sites hosts nearly 80% native species with two species of highest Floristic Quality Index (10). Restoration efforts should be informed by the slow process of natural recovery, while post-industrial sites in urban areas serve as potential native plant refugia.

Physiological responses of soybean (Glycine max (L.) Merrill) cultivars to copper excess

Successive applications of copper fungicides on vines have resulted in increased copper content in vineyard soils over the years. This high copper content has affected the growth of young vines in eradicated vineyards. Thus, the cultivation of annual species for a few years is an alternative to copper phytostabilization, because it would be a good way to decrease copper availability to plants. The aim of this study was to evaluate the physiological responses of different soybean cultivars to copper concentration increase. Four different soybean cultivars were grown under three copper concentrations: 0.5, 20 and 40 μM in nutrient solution. The main outcomes of this study were: i) Cultivar M 6410 IPRO recorded the highest photosynthetic rate when plants were exposed to 40 μM of copper in the nutrient solution; ii) plants in cultivar M 6410 IPRO accumulated large copper concentrations in their roots although did not decrease the root dry mass, possibly due to the higher superoxide dismutase activity; iii) cultivar DM 5958 RSF IPRO recorded drastically reduced photosynthetic rate and dry mass production due to copper excess. We conclude that each cultivar responded differently to the excess of copper, but none of them showed tolerance to it.

A real filed phytoremediation of multi-metals contaminated soils by selected hybrid sweet sorghum with high biomass and high accumulation ability

Heavy metal-polluted soil is obtaining increasing global concerns. The phytoremediation is a promising technology that needs further research. This study was aiming to perform a field survey to assess the restoration and accumulation potential of five hybrid sweet sorghum species with high biomass. Those sweet sorghums were planted in three sites containing different toxic levels of Zn, Pb and Cd with one local commercial sweet sorghum as contrast sample. Plants and soils were sampled for the analysis of heavy metal concentrations. BCF and TF values showed that hybrid sweet sorghum species have higher accumulation ability than local one. Five species of hybrid sweet sorghum planted in all three sites showed no obvious toxicity symptoms, and moreover, their biomass were 12-24 times higher than that of the local one, indicating their high tolerance to heavy metals. Among them, the 9312 and G38 specimens were considered as the best-performing specimens due to their high ability to accumulate multiple metals in their shoots and roots without being affected by excessive metal contents. A reasonable disposed plan for harvested sweet sorghum after phytoremediation was proposed. The harvest sweet sorghums used for industrial ethanol and densified biofuel production could combine soil remediation with creating economic benefit. Consequently, those five hybrid sweet sorghum species, especially 9312 and G38 with high biomass production, metal accumulation ability and high tolerance against metal toxicity might have great potential in phytoremediation field.

Fate and transport of metal trace elements from phosphogypsum piles in Tunisia and their impact on soil bacteria and wild plants

The phosphate industry in Tunisia generates large amounts of phosphogypsum (PG) with more than 10⁷ t per year. Environmental impact of this solid waste was studied. Cd, Ce, La, Nd, Sr and Y were analyzed from soils near PG stockpiles (Sfax and M'dhilla) and sediments from marine discharge (Gabes). Their impacts on the bacterial community structure and wild plants were investigated. Metal trace elements (MTE) concentrations (in mg Kg^-1 DM) were much higher in contaminated soil than in the control (at 12 km from PG stockpiles). Highest concentrations were recorded in top soil and decreased with depth. A low bacterial diversity was shown (impacted by plants more than by MTE). The MTE concentrations in aerial parts (AP) and roots varied according to the plant species and were higher in contaminated sites. Sr, La and Cd in the AP ranged 33.10-657.56, 2.22-11.05 and 0.21-14.20 mg Kg^-1 DM respectively. Plants exhibiting the maximal metal concentrations in AP (in mg Kg^-1 DM) were the following: Zygophylum album for Sr (657.56) >Zygophylum album for Cd (14.20) >Zygophylum album (11.05) for La >Conyza canadensis (1.11) for Ce >Conyza canadensis (0.75) for Nd >Arthrocemum inducum (0.72) for Y. Kochia indica showed the highest bioconcentration factor (1.60) for Cd, while Zygophylum album exhibited the highest translocation factor (6.12) for La. Zygophylum album would be the most suitable candidate for MTE phytoextraction. CAPSULE: Phosphogypsum contaminates soils near stockpiles with metal trace elements including rare earth element and selects wild plants able to be used for phytostabilization and phytomining.

Accumulation of heavy metals in metallophytes from three mining sites (Southern Centre Morocco) and evaluation of their phytoremediation potential

The current study aims to perform a field survey of three abandoned mining sites in the southern centre of Morocco to assess the recent metal pollution in soils and accumulation potential of plant species. Native plants and soils were sampled at several sites in the studied mines and analysed for Cu, Zn, Pb and Cd concentrations. Soils in the investigated sites proved to be deficient in major macronutrients and to contain toxic levels of Cu, Zn, Pb and Cd. Botanical survey of the prospected sites showed the abundance of diverse plant communities (46 species and 19 families), with no obvious toxicity symptoms. Results showed that the concentrations of heavy metals were different in the same plant species and from plant species to another. Eight plants of 46 species namely Hirschfeldia incana (L.) Lagr.-Foss, Citrullus vulgaris (L.) Schradi, Portulaca oleracea L., Stipa capensis Thunb., Lactuca viminea (L.) J.Presl & C.Presl, Forsskaolea tenacissima L., Lycium intricatum Boiss. and Hammada scoparia (Pomel) Iljin were considered as the best-performing specimens due to their high ability to accumulate multiple metals in their shoots and roots without being affected by excessive metal contents. This was confirmed by the transfer factors generally higher than 1. Consequently, these tolerant and native plant species could be used as tools for an effective phytorestoration of metal-contaminated sites.

Mining and biodiversity: key issues and research needs in conservation science

Mining poses serious and highly specific threats to biodiversity. However, mining can also be a means for financing alternative livelihood paths that, over the long-term, may prevent biodiversity loss. Complex and controversial issues associated with mining and biodiversity conservation are often simplified within a narrow frame oriented towards the negative impacts of mining at the site of extraction, rather than posed as a series of challenges for the conservation science community to embrace. Here, we synthesize core issues that, if better understood, may ensure coexistence between mining and conservation agendas. We illustrate how mining impacts biodiversity through diverse pathways and across spatial scales. We argue that traditional, site-based conservation approaches will have limited effect in preventing biodiversity loss against an increasing mining footprint, but opportunities to improve outcomes (e.g. through long-term strategic assessment and planning) do exist. While future mineral supply is uncertain, projections suggest demand will grow for many metals and shift mining operations towards more dispersed and biodiverse areas. Initiating dialogue between mining companies, policy-makers and conservation organizations is urgent, given the suite of international agendas simultaneously requiring more minerals but less biodiversity loss.

Content of Zn, Cd and Pb in purple moor-grass in soils heavily contaminated with heavy metals around a zinc and lead ore tailing landfill

The paper presents concentrations and correlations between Zn, Cd and Pb in the aboveground parts of purple moor-grass (Molinia caerulea L.) in forest soils heavily contaminated with heavy metals around a zinc and lead ore tailing landfill at Mining & Metallurgy Enterprise “Bolesław” SA in Bukowno. Field observations have indicated that purple moor-grass, which occurs as one of the few vascular plants in locations with tailing mud, is probably a species with high adaptability to conditions in contaminated environments. The research was carried out in a network of 20 regular monitoring sites. At these sites, a detailed inventory of purple moor-grass was carried out and samples of the aboveground parts of the plants were collected from the leaves and ears and from the soil at a depth of 0-20 cm. It was found that there was no significant correlation between the concentration of heavy metals in the soils and aboveground parts of the plants in the most heavily contaminated zones. This may indicate the existence of mechanisms limiting uptake of heavy metals by this species, and therefore the need for further research in the context of its suitability for biological regeneration of tailing landfills and phytosanitary protection of adjoining areas.

Evidence-based logic chains demonstrate multiple impacts of trace metals on ecosystem services

Trace metals can have far-reaching ecosystem impacts. In this study, we develop consistent and evidence-based logic chains to demonstrate the wider effects of trace metal contamination on a suite of ecosystem services. They demonstrate knock-on effects from an initial receptor that is sensitive to metal toxicity, along a cascade of impact, to final ecosystem services via alterations to multiple ecosystem processes. We developed logic chains to highlight two aspects of metal toxicity: for impacts of copper pollution in soil ecosystems, and for impacts of mercury in freshwaters. Each link of the chains is supported by published evidence, with an indication of the strength of the supporting science. Copper pollution to soils (134 unique chains) showed a complex network of pathways originating from direct effects on a range of invertebrate and microbial taxa and plants. In contrast, mercury pollution on freshwaters (63 unique chains) shows pathways that broadly follow the food web of this habitat, reflecting the potential for mercury bioaccumulation. Despite different pathways, there is considerable overlap in the final ecosystem services impacted by both of these metals and in both ecosystems. These included reduced human-use impacts (food, fishing), reduced human non-use impacts (amenity value) and positive or negative alterations to climate regulation (impacts on carbon sequestration). Other final ecosystem goods impacted include reduced crop production, animal production, flood regulation, drinking water quality and soil purification. Taking an ecosystem services approach demonstrates that consideration of only the direct effects of metal contamination of soils and water will considerably underestimate the total impacts of these pollutants. Construction of logic chains, evidenced by published literature, allows a robust assessment of potential impacts indicating primary, secondary and tertiary effects.

Does foliage metal accumulation influence plant-insect interactions? A field study of two sympatric tree metallophytes

Gossia (Myrtaceae) is a highly restricted tree genus most speciose in New Caledonia and eastern Australia. The latter group accumulates above-normal foliar manganese (Mn) concentrations, with some individuals exhibiting the rare Mn-hyperaccumulative trait. Whether foliar metals contribute to chemical defence has been addressed via numerous feeding experiments and very few field studies. This investigation exploited specifically different insect activities on the foliage of sympatric Gossia grayi (N.Snow & Guymer) and Gossia shepherdii (F.Muell.) N.Snow & Guymer, endemic to north-eastern Australia, to test for direct and indirect effects of foliar Mn enrichment on plant-insect interactions. Leaf organic and inorganic chemistries, specific weight, surface damage, gall infestation and occupancy were quantified. Discovery that both species are Mn hyperaccumulators augments the world listing by 5-7%. Highly elevated gall-Mn concentrations coupled with negligible gall parasitisation suggested chemical fortification and adaptation by the host insect - a Cecidomyiidae fly. Linear mixed modelling (LMM) showed differences in leaf Mn, phenolics, toughness and surface damage across tree species and leaf age. There was no direct relationship between leaf Mn and insect impact. However, LMM did resolve indirect effects, i.e. between insect impact and certain foliar elements, consistent with nutritional dynamics in a physiologically novel plant system where Mn is vastly overaccumulated.

Check-list of vascular plant communities on ironstone ranges of south-eastern Brazil: dataset for conservation

Background

Ironstone ranges are considered hotspots for higher plants α and β diversity. The lack of studies and the intense degradation of the ironstone ranges, due to mining, motivated us to compile, for the first time, a list of vascular plants collected on iron-rich derived substrates from ancient landscape of south-eastern Brazil. All existing records in the Brazilian Virtual Herbarium of Flora and Fungi for each of the 43 municipalities containing ironstone ranges were downloaded, resulting in 17,954 vouchers identified to the species level. We found 2,933 species belonging to 160 families and 818 genera.

New information

For the first time, we identified 148 species mentioned in endangered flora official lists and 48 narrow endemic species. Collecting efforts must still be supported to properly sample the vegetation since, for 143 sites, less than 10 records/site were found. This dataset will assist with the indication of dozens of plant species whose threat criteria must be urgently assessed to subsidise public policies on the use and conservation of the Brazilian flora.

Phyllanthus rufuschaneyi: a new nickel hyperaccumulator from Sabah (Borneo Island) with potential for tropical agromining

BACKGROUND

Nickel hyperaccumulator plants are of much interest for their evolution and unique ecophysiology, and also for potential applications in agromining-a novel technology that uses plants to extract valuable metals from soil. The majority of nickel hyperaccumulators are known from ultramafic soils in tropical regions (Cuba, New Caledonia and Southeast Asia), and one genus, Phyllanthus (Phyllanthaceae), is globally the most represented taxonomic entity. A number of tropical Phyllanthus-species have the potential to be used as 'metal crops' in agromining operations mainly because of their ease in cultivation and their ability to attain high nickel concentrations and biomass yields.

RESULTS

One of the most promising species globally for agromining, is the here newly described species Phyllanthus rufuschaneyi. This species can be classified in subgenus Gomphidium on account of its staminate nectar disc and pistillate entire style and represents the most western species of this diverse group. The flower structure indicates that this species is probably pollinated by Epicephala moths.

CONCLUSIONS

Phyllanthus rufuschaneyi is an extremely rare taxon in the wild, restricted to Lompoyou Hill near Kinabalu Park in Sabah, Malaysia. Its utilization in agromining will be a mechanism for conservation of the taxon, and highlights the importance of habitat and germplasm preservation if rare species are to be used in novel green technologies.

Paradoxical effects of density on measurement of copper tolerance in Silene paradoxa L

This work investigated if the assessment of tolerance to trace metals can depend on plant density in the experimental design. A non-metallicolous and a metallicolous populations of Silene paradoxa were hydroponically cultivated at increasing density and in both the absence (-Cu conditions) and excess of copper (+Cu conditions). In -Cu conditions, the metallicolous population showed a lower susceptibility to plant density in comparison to the non-metallicolous one, explained by a higher capacity of the metallicolous population to exploit resources. In +Cu conditions, an alleviating effect of increasing density was found in roots. Such effect was present to a greater extent in the non-metallicolous population, thus making the populations equally copper-tolerant at the highest density used. In shoots, an additive effect of increasing plant density to copper toxicity was reported. Its higher intensity in the metallicolous population reverted the copper tolerance relationship at the highest plant densities used. In both populations, a density-induced decrease in root copper accumulation was observed, thus concurring to the reported mitigation in +Cu conditions. Our work revealed the importance of density studies on the optimization of eco-toxicological bioassays and of metal tolerance assessment and it can be considered the first example of an alleviating effect of increasing plant number on copper stress in a metallophyte.

Ultramafic geoecology of South and Southeast Asia

Globally, ultramafic outcrops are renowned for hosting floras with high levels of endemism, including plants with specialised adaptations such as nickel or manganese hyperaccumulation. Soils derived from ultramafic regoliths are generally nutrient-deficient, have major cation imbalances, and have concomitant high concentrations of potentially phytotoxic trace elements, especially nickel. The South and Southeast Asian region has the largest surface occurrences of ultramafic regoliths in the world, but the geoecology of these outcrops is still poorly studied despite severe conservation threats. Due to the paucity of systematic plant collections in many areas and the lack of georeferenced herbarium records and databased information, it is not possible to determine the distribution of species, levels of endemism, and the species most threatened. However, site-specific studies provide insights to the ultramafic geoecology of several locations in South and Southeast Asia. The geoecology of tropical ultramafic regions differs substantially from those in temperate regions in that the vegetation at lower elevations is generally tall forest with relatively low levels of endemism. On ultramafic mountaintops, where the combined forces of edaphic and climatic factors intersect, obligate ultramafic species and hyperendemics often occur. Forest clearing, agricultural development, mining, and climate change-related stressors have contributed to rapid and unprecedented loss of ultramafic-associated habitats in the region. The geoecology of the large ultramafic outcrops of Indonesia's Sulawesi, Obi and Halmahera, and many other smaller outcrops in South and Southeast Asia, remains largely unexplored, and should be prioritised for study and conservation.

Responses of serpentine plants to pine invasion: Vegetation diversity and nickel accumulation in species with contrasting adaptive strategies

Introduction of non-native trees is one of the major threats to ecosystem integrity and biodiversity. Stands of maritime pine (Pinus pinaster Ait.) introduced decades ago represent a threat to the specialized plant communities of serpentine outcrops in Italy. This study investigates the effects of such invasions at the community and species level, based on vegetation sampling in three selected sites with comparable environmental conditions. Pine cover caused a decrease of α-diversity by lowering the species evenness of the community, though species richness was not negatively affected. Compositional changes between the two habitats were significant but not clearly associated with a decrease in taxonomic distinctness in the pine stands. As many as nine indicator species were found in the open vegetation, along with the obligate endemics Odontarrhena bertolonii and Armeria denticulata. Both of them declined in the pine stands. Here, an increase in the phytoavailable nickel fraction was associated with a decrease in total nickel concentration in the soil, via mobilization of the metal caused by lowering of pH induced by the conifer litter. The nickel-hyperaccumulator O. bertolonii was able to maintain high metal concentrations in the shoots despite a decrease in root concentration, resulting in a higher shoot/root ratio in the pine stands (~20). Conversely, shoot/root ratio in the non-accumulator Plantago holosteum was <1 and not affected by the conifer, as well as its abundance in this anthropogenic habitat. Contrasting responses of the two species were likely due to their different sensitivity to modified light and soil conditions, whereas stability of shoot nickel-concentration in O. bertolonii did not support increased predation by natural enemies as one of the causes for its decline under the conifer. Progressive thinning of these stands is advocated to limit soil nickel mobilization and to restore a unique ecosystem with its endemic metallophytes.