Facilitate Plant Establishment Research Articles

Lessons from patch dynamics for ecology and restoration: The succulent Karoo, Sandveld example

A series of long-term field and glasshouse experiments were conducted in the semi-arid Sandveld Bioregion of southern Africa, across a wide range of post-mining site and soil conditions, on the two major plant groups, leaf-succulent shrubs (LSS, which establish in open micro-habitats) and non-succulent shrubs (NSS, which establish in sheltered micro-habitats). Seeding experiments applied a sequential ecological framework intended to reduce the impact of impeding natural ecological processes whilst incorporating processes that facilitate plant establishment. Experimental plots were scaled to naturally operating ecological dynamics at the patch scale, and various potentially facilitative types of shelter and soil amelioration, tested separately. Results are reported over three or six year periods and from four or seven field sites. Shelter generally had a greater effect on plant establishment than soil ameliorants, but this could be either positive or negative, and varied for the two plant groups: LSS establishment was inhibited or neutral under all forms of shelter, NSS were facilitated by types of artificial shelter that could be removed or biodegraded without impacting young plants. Both groups were strongly inhibited under transplanted nurse plants. Water absorbing gels improved LSS establishment but other soil ameliorants had no effect, while milled kelp or nutrients improved NSS establishment (measured as a biomass index or plant abundance, respectively). Finally, site had a greater effect than either shelter or soil ameliorants, and could not be related to any soil or other factors measured, supporting recent studies that explicitly quantify the substantial unknown effects of site and year. Specific recommendations are made for comprehensive, seed-based, nucleated restoration designs for the Sandveld and similar bioregions, while the over-arching results are of universal import to dryland restoration and ecology.

Soil amelioration induced by nurse shrubs in coal mines reclaimed to pastures and their synergistic effects with grazing

Native shrubs facilitate the establishment of oak seedlings in the opencast coal mines rehabilitated to pastures in Northern Spain, under a Mediterranean sub-humid climate. We evaluate soil changes as one of the possible facilitative effects of nurse shrubs. We hypothesize that nurse shrubs on mining soils can improve edaphic properties directly and indirectly by reducing the negative effects of trampling and grazing by ungulates. Thus, we assessed the combined effects of nurse shrubs (plots with vs without shrubs) and grazing (plots with vs without a fence for preventing browsing and trampling) upon soil properties in a reclaimed coal mine. Chemical properties such as electrical conductivity, K+, cation exchange capacity, and C/N ratio reached higher values beneath shrubs’ canopies, so did total organic matter, total N, total organic C, total P, available P, and Mg2+ under shrubs but only with grazing. In contrast, pH was higher outside the shrubs; also Ca2+, though only in the absence of grazing. With grazing, Na+ decreased under shrubs. Among physical properties, bulk density increased and porosity decreased only in grazed plots, whereas sand content increased under shrubs in the grazed plots, and clay decreased in such locations. Water holding capacity and available water were the highest in ungrazed areas. Overall, we found that shrubs have a positive direct effect on soil fertility, especially relevant in grazed areas because nurse shrubs and grazing have synergistic effects, and a positive indirect effect on physical properties because they attenuate negative effects of grazing, particularly soil compaction, by reducing livestock and wild ungulates trampling. Therefore, these results demonstrate how nurse shrubs contribute to soil amelioration, helping to facilitate plant establishment in reclaimed mines, which has relevant restoration implications for pasture and forest recovery.

Metabarcoding of Soil Fungal Communities in Rupestrian Grassland Areas Preserved and Degraded by Mining: Implications for Restoration.

Rupestrian grasslands are vegetation complexes of the Cerrado biome (Brazilian savanna), exhibiting simultaneously great biodiversity and important open-pit mining areas. There is a strong demand for the conservation of remaining areas and restoration of degraded. This study evaluated, using next-generation sequencing, the diversity and ecological aspects of soil fungal communities in ferruginous rupestrian grassland areas preserved and degraded by bauxite mining in Brazil. In the preserved and degraded area, respectively, 565 and 478 amplicon sequence variants (ASVs) were detected. Basidiomycota and Ascomycota comprised nearly 72% of the DNA, but Ascomycota showed greater abundance than Basidiomycota in the degraded area (64% and 10%, respectively). In the preserved area, taxa of different hierarchical levels (Agaromycetes, Agaricales, Mortierelaceae, and Mortierella) associated with symbiosis and decomposition were predominant. However, taxa that colonize environments under extreme conditions and pathogens (Dothideomycetes, Pleoporales, Pleosporaceae, and Curvularia) prevailed in the degraded area. The degradation reduced the diversity, and modified the composition of taxa and predominant ecological functions in the community. The lack of fungi that facilitate plant establishment and development in the degraded area suggests the importance of seeking the restoration of this community to ensure the success of the ecological restoration of the environment. The topsoil of preserved area can be a source of inocula of several groups of fungi important for the restoration process but which occur in low abundance or are absent in the degraded area.

Effects of soil mitigation on lawn-dwelling invertebrates following residential development

Abstract Residential areas are the most rapidly expanding land use type in the southeastern USA. Residential development impairs soil functions primarily through compaction and the removal or burial of topsoil and natural vegetation, which reduces water infiltration and retention, root penetration, and plant establishment. Plant stress reduces plant-derived ecosystem services and increases vulnerability to pests, often leading to supplemental management inputs in the form of irrigation, fertilizers, pesticides and labor. Soil-dwelling invertebrates, including detritivores and natural enemies of pests, drive valuable ecosystem functions that facilitate plant establishment and reduce maintenance inputs. Although poorly understood, soil disturbance during residential development likely disturbs these communities and reduces the services provided by soil-dwelling invertebrates. Here, we compare the effects of two soil compaction mitigation techniques, tillage with and without compost incorporation, on invertebrate communities and the services they provide over 2 years following residential development. We focus on the relationships between detritivores and detritus decomposition rates, entomopathogenic nematodes and the activity density of a key turfgrass pest and other arthropod herbivores and predators. We found that soil mitigation had no detectable benefit for epigeal arthropods within 1 year after disturbance, but that compost-amended soils supported greater arthropod richness and predator activity density than unmitigated soils in the second year after disturbance. In contrast, we found reduced insect-parasitic nematode activity associated with compost amendment. All taxa increased in abundance with time after development. These results can inform more sustainable residential development and landscape maintenance practices for more biodiverse and functional urban and residential ecosystems.

Diesel fuel differentially affects hyphal healing in Gigaspora sp. and Rhizophagus irregularis.

Hydrocarbon pollution is an increasing problem affecting soil ecosystems. However, some microorganisms can cope with these pollutants and even facilitate plant establishment and thus phytoremediation. Within soil, arbuscular mycorrhizal fungi (AMF) have developed several strategies to survive and flourish under adverse conditions. Among these is the hyphal healing mechanism (HHM), a process allowing hyphae to re-establish integrity after physical injury. This mechanism differs among species and genera of AMF. However, whether and to what extent hydrocarbon pollution impacts the HHM is unknown. Here, the HHM was monitored in vitro on two AMF strains, Rhizophagus irregularis MUCL 41833 and Gigaspora sp. MUCL 52331, under increasing concentrations of diesel (1, 2, and 5% v:v). The addition of diesel slowed-down the HHM in both fungi. On Gigaspora sp., this effect was limited and most hyphae were able to heal after injury. Conversely, all steps of healing were severely impaired in R. irregularis. That fungus reconnected the injured hyphae at a much lower frequency than the Gigaspora sp., instead investing its energy to link neighboring hyphae or roots, or developing new branches from uninjured hyphae.

Constructed pine log piles facilitate plant establishment in mining drylands

Ecological facilitation, though accepted as a main mechanism of plant community assembly, is just starting to be utilized in restoration programmes. Constructing nurse objects that mimic the effect of natural nurse species can be an option to trigger plant nucleation processes in disturbed stressful ecosystems.We hypothesized that arranged log piles might imitate plant facilitation by alleviating abiotic stress and expanding the regeneration niche of beneficiary species, eventually promoting plant establishment, fitness and diversity. With this aim, within a pilot restoration programme in abandoned mining structures in SE Spain where climatic and edaphic stresses concur, we constructed 133 pine log piles from natural wastes generated by local silvicultural activities. We monitored 51 of them plus their adjacent open areas for 15 months, measuring soil temperature, radiation and humidity. We recorded natural seedling establishment, plant nutritional status and heavy metal accumulation. We further performed a seed sowing experiment to investigate how log piles affect plant taxonomic and functional diversity based on 11 establishment and phytostabilization traits.Pine log piles significantly softened microclimatic conditions and accelerated plant establishment in unfertile and metal-polluted mining substrates, simultaneously capturing water, providing shade and pine seeds. Plant communities that naturally established beneath the piles were 15 times denser and five times taxonomically more diverse than those in open areas, despite being skewed towards pine recruitment. Experimental communities sown under log piles were also 1.4 times functionally more diverse, as theory predicts for relaxed abiotic conditions. Log piles improved seedling nutritional status, in terms of P and K content, at the cost of increased metal accumulation.At the landscape scale, nurse objects triggered plant establishment promoting taxonomic and functional diversity in extremely stressful environments. This study exemplifies how soft restoration tools can be based on mechanisms that are widely accepted in the ecological theory.

Substrate-specialist plants for restoring vegetation in post-mining gypsum substrates

Restoration in degraded gypsum areas is challenging due to physical-chemical soil characteristics that limit plant development. Facilitation mediated by the stress-adapted gypsophytes may be a key process in restoration because it potentially favors the establishment of lower stress-adapted species underneath, promoting plant succession. This study aimed to assess the suitability of the gypsophyte Gypsophila struthium Loefl. for restoring vegetation in gypsum mines, focusing on its role as a pioneer species that may act as an ecosystem engineer and hence as a nurse plant. We conducted a vegetation survey in a gypsum mine in NE Spain, in plots where the original topsoil was reinserted in different years after the mineral extraction to study plant community succession and reorganization through time. Complementary, we set a sowing and planting experiment in a gypsum spoil dump to assess how G. struthium may positively affect soil and micro-environmental conditions and, consequently, may act as a nurse species facilitating plant establishment and development. To test the nurse role of G. struthium, we sowed seeds and planted seedlings of Helianthemum squamatum (L.) Pers. and Stipa lagascae Roem. & Schultz under the canopy of adult individuals of G. struthium, and in the bare soil. We found that G. struthium appeared as a pioneer shrub in the restored gypsum mine and accumulated plant diversity in the local vicinity in mature plant communities. This shrub had a positive effect on seed germination and seedling growth of the test species in the experiment and improved soil and micro-environmental conditions under its canopy. Therefore, we recommend two non-exclusive actions to enhance restoration in post-mining gypsum substrates: (1) to promote the establishment of the pioneer plant G. struthium by improving the physical-chemical and biological soil properties through the re-insertion of the original topsoil that contains propagules of the former community; (2) to apply active restoration actions, using G. struthium as a nurse plant through sowing seeds or planting seedlings of other species of interest under its canopy.

The role of nurse shrubs on the spatial patterning of plant establishment in semi-arid gypsum plant communities

Plants may depend on nurse plants to establish and survive on gypsum environments, which are stressful for plant life. Shrubs may act as nurse plants by ameliorating micro-environmental conditions in their local vicinity. Despite its importance, the role of nurse shrubs on the spatial patterning of plant establishment remains largely unknown in gypsum communities. Our aim was to understand the patterns of plant establishment and to identify key nurse species in facilitating plant establishment and structuring plant diversity at community level in these environments. We set an observational study in two gypsum plant communities of NE Spain to assess the microsites where seedlings germinate. Also, we assessed the role of six dominant shrubs (gypsophytes and gypsovags), in spatially structuring plant diversity by assessing species-area relationships in their local vicinity to identify key nurse species. To assess the potential amelioration of micro-environmental conditions associated to shrubs, we analyzed physical-chemical soil properties under shrub canopies compared to open areas. Most plants were spatially associated with adult shrubs. Gypsophytes accumulated more diversity than gypsovags, despite both showing ameliorated conditions under canopy compared to open areas. In conclusion, gypsophyte shrubs play key roles in the structure of gypsum plant communities by facilitating plant establishment.

Apparent Tolerance of Low Water Availability in Temperate Asian Bamboos1

Abstract Suitable plant water dynamics and the ability to withstand periods of low moisture input facilitate plant establishment in seasonally arid regions. Temperate bamboos are a major constituent of mixed evergreen and deciduous forests throughout temperate East Asia but play only an incidental role in North American forests and are altogether absent in the Pacific Northwest forest. Many bamboo species are classified as mesic or riparian, but none are considered drought tolerant. To assess their ability to withstand low water, we subjected five Asian temperate and one North American temperate bamboo species to three irrigation treatments: 100%, 50%, and 10% replacement of water lost through evapotranspiration. Plants were irrigated every four days over a 31-day period. Plant response to treatments was measured with stomatal conductance, leaf xylem water potentials, and intrinsic water use efficiency (iWUE). Pleioblastus distichus and Pseudosasa japonica showed significant reductions in conductance between high and low irrigation treatments. Sasa palmata had significantly lower stomatal conductance in all treatments. Pleioblastus chino displayed significantly higher iWUE in the mid irrigation treatment and Arunindaria gigantea displayed significantly lower iWUE than P. chino and S. palmata in the low irrigation treatment. The Asian bamboo species examined here tolerate low water availability and readily acclimate to different soil moisture conditions. Index words: Temperate bamboos, irrigation response, stomatal conductance, intrinsic water use efficiency. Species used in this study: Giant Cane [Arundinaria gigantea (Walt.) Muhl.]; Pleioblastus chino (Franchet & Savatier) Makino; Pleioblastus distichus (Mitford) Nakai; Pseudosasa japonica (Makino); Sasa palmata (Bean) Nakai.

Functionality of Root-Associated Bacteria along a Salt Marsh Primary Succession.

Plant-associated bacteria are known for their high functional trait diversity, from which many are likely to play a role in primary and secondary succession, facilitating plant establishment in suboptimal soils conditions. Here we used an undisturbed salt marsh chronosequence that represents over 100 years of soil development to assess how the functional traits of plant associated bacteria respond to soil type, plant species and plant compartment. We isolated and characterized 808 bacterial colonies from the rhizosphere soil and root endosphere of two salt marsh plants, Limonium vulgare and Artemisia maritima, along the chronosequence. From these, a set of 59 strains (with unique BOX-PCR patterns, 16S rRNA sequence and unique to one of the treatments) were further screened for their plant growth promoting traits (siderophore production, IAA production, exoprotease production and biofilm formation), traits associated with bacterial fitness (antibiotic and abiotic stress resistance – pH, osmotic and oxidative stress, and salinity) and metabolic potential. An overall view of functional diversity (multivariate analysis) indicated that the distributional pattern of bacterial functional traits was driven by soil type. Samples from the late succession (Stage 105 year) showed the most restricted distribution, harboring strains with relatively low functionalities, whereas the isolates from intermediate stage (35 year) showed a broad functional profiles. However, strains with high trait performance were largely from stage 65 year. Grouping the traits according to category revealed that the functionality of plant endophytes did not vary along the succession, thus being driven by plant rather than soil type. In opposition, the functionality of rhizosphere isolates responded strongly to variations in soil type as observed for antibiotic resistance (P = 0.014). Specifically, certain Pseudomonas sp. and Serratia sp. strains revealed high resistance against abiotic stress and antibiotics and produce more siderophores, confirming the high plant-growth promoting activity of these two genera. Overall, this study contributes to a better understanding of the functional diversity and adaptation of the microbiome at typical salt marsh plant species across soil types. Specifically, soil type was influential only in the rhizosphere but not on the endosphere, indicating a strong plant-driven effect on the functionality of endophytes.

Bioaugmentaton in Technosols created in abandoned pyritic tailings can contribute to enhance soil C sequestration and plant colonization

Creation of Technosols in combination with phytostabilization may be a sustainable strategy to minimize the environmental and human health hazards derived from mine tailings. Bioaugmentation can facilitate plant establishment and growth for efficient phytostabilization. In order to assess if bioaugmentation can increase soil quality and fertility, decrease metal(loid) mobility and accelerate plant colonization, a one-year field experiment was designed with creation of Technosols in two tailings ponds with different pH (acidic (AT) and neutral (NT)), with addition of marble waste (MaW) and two organic materials (pig manure (PM) and sewage sludge (SS), without or with inoculation of effective microorganisms (EM) at three rates. Results showed that MaW was the main factor responsible for maintaining pH ~7 in AT and ~8 in NT, decreasing salinity, and decreasing the soluble fraction of metals (70–99%). The soluble fraction of As decreased ~45% in AT, related to increases in pH up to neutrality, while increased ~90% in NT with PM due to pH>8 and higher content of organic compounds. The addition of PM and SS significantly increased soil organic C (SOC), nutrient contents and microbial biomass and activity in both tailings, being PM more effective. However, a positive priming effect was observed in NT with SS addition likely due to higher C/N ratio and lack of nutrients. There was a significant effect of EM rate on inorganic C, SOC, N, K and microbial biomass and activity, with higher values as rate increased. Vegetation richness and density directly increased with increasing EM rate. Multivariate analyses showed that the most important properties contributing to increase richness and plant density were microbial biomass and N. Thus, bioaugmentation contributed to soil C sequestration (as organic and inorganic C) and soil fertility, related to high soil microbial biomass and activity, which facilitated an effective colonization of vegetation.

Plant nurse effects rely on combined hydrological and ecological components in a semiarid ecosystem

AbstractPlant establishment in semiarid ecosystems is affected by the limited spatial and temporal availability of resources and adequate microsites provided by nurse plants. There has been little research on plant establishment in these ecosystems that consider both the ecological roles of different plant types and the abiotic properties of their microsites. Such studies could provide important insights about the functioning of semiarid ecosystems. Here, we investigated the links between the patterns of plant establishment and the hydrological and microsite properties of shrubs and grasses in a semiarid ecosystem northeastern Spain. For ecological experiments, we measured the spatial patterns of the establishment of shrubs and grasses in eight 6 × 6 m2 quadrats over 2 years; we also sowed seeds of Salsola vermiculata (a dominant shrub) and Lygeum spartum (the dominant perennial grass) under adult shrubs (S. vermiculata) and grasses (L. spartum) and in bare soil and then examined seedling germination, survival, and growth over 4 years. For hydrological experiments, we analyzed soil water content under the two codominant shrubs (S. vermiculata and Artemisia herba‐alba), the dominant perennial grass (L. spartum), and in bare soil over 18 months; we also measured water infiltration and solar radiation at the same four microsites to identify the hydrological processes responsible for the observed ecohydrological patterns. The three potential nurse plants greatly improved the hydrological and microsite conditions. They increased soil water content after rainfall relative to bare soil. Moreover, S. vermiculata and L. spartum slowed the drying process. However, only S. vermiculata acted as a nurse plant. It improved plant density, diversity, performance, and survival during the whole study period. L. spartum facilitated plant establishment during early stages, but interfered with seedling performance and survival during later stages, probably because of the increasing competition for water with seedlings. A. herba‐alba did not facilitate plant establishment at any stage, most likely because of water scarcity during prolonged dry periods and its allelopathic effects. We conclude that the ecological role of a plant cannot be directly inferred from its hydrological or microsite properties. Long‐term ecohydrological studies are required to understand the role of nurse plants on seedling establishment.

Arbuscular mycorrhizal fungal‐assisted phytoremediation of soil contaminated with persistent organic pollutants: a review

Concern about soil contamination by persistent organic pollutants (POPs) that originate from intensive anthropogenic activities has now become a priority in industrialized countries. Some of these compounds can accumulate in the environment and threaten the development of living organisms because of their acute toxicity, mutagenicity and carcinogeneity. Among remediation methods for polluted soil, arbuscular mycorrhizal fungi (AMF)‐assisted phytoremediation is an attractive approach to plant‐based environmental remediation. Arbuscular mycorrhizal associations are important in the restoration of degraded ecosystems because of the benefits to their symbiotic partners. Arbuscular mycorrhizal fungi are not only able to facilitate plant establishment and survival in POP‐contaminated soil by protecting plants against the phytotoxicity of POPs, but also enhance soil bioremediation by stimulating telluric microbial activity and by improving soil structure. This phytotechnology, however, is still in its infancy and there has been little research on aged‐contaminated soils. Its in situ effectiveness remains to be proved. This review summarizes current knowledge about the effect of POPs on the establishment and functioning of arbuscular mycorrhizal symbiosis and its contribution to the phytoremediation of POP‐polluted soil. It focuses on recent developments that aim to improve the efficiency of AMF‐assisted phytoremediation in the dissipation of POPs and future directions.HighlightsArbuscular mycorrhizal fungi (AMF) provide a means of plant‐based environmental clean‐up. AMF‐assisted phytoremediation is based on the synergistic actions of plants and telluric microorganisms. Research is in progress to improve efficiency of this phytotechnology and management of the biomass produced. Development of integrated approaches that combine land remediation with biomass conversion processes is required.

Alleviation of environmental risks associated with severely contaminated mine tailings using amendments: Modeling of trace element speciation, solubility, and plant accumulation.

Tailings are considered one of the most relevant sources of contamination associated with mining activities. Phytostabilization of mine spoils may need the application of the adequate combination of amendments to facilitate plant establishment and reduce their environmental impact. Two pot experiments were set up to assess the capability of 2 inorganic materials (calcium carbonate and a red mud derivate, ViroBindTM ), alone or in combination with organic amendments, for the stabilization of highly acidic trace element-contaminated mine tailings using Atriplex halimus. The effects of the treatments on tailings and porewater physico-chemical properties and trace-element accumulation by the plants, as well as the processes governing trace elements speciation and solubility in soil solution and their bioavailability were modeled. The application of the amendments increased tailings pH and decreased (>99%) trace elements solubility in porewater, but also changed the speciation of soluble Cd, Cu, and Pb. All the treatments made A. halimus growth in the tailings possible; organic amendments increased plant biomass and nutritional status, and reduced trace-element accumulation in the plants. Tailings amendments modified trace-element speciation in porewater (favoring the formation of chlorides and/or organo-metallic forms) and their solubility and plant uptake, which were found to be mainly governed by tailing/porewater pH, electrical conductivity, and organic carbon content, as well as soluble/available trace-element concentrations. Environ Toxicol Chem 2016;35:2874-2884. © 2016 SETAC.

Gasified Grass and Wood Biochars Facilitate Plant Establishment in Acid Mine Soils.

Heavy metals in exposed mine tailings threaten ecosystems that surround thousands of abandoned mines in the United States. Biochars derived from the pyrolysis or gasification of biomass may serve as a valuable soil amendment to revegetate mine sites. We evaluated the ability of two biochars, produced by gasification of either Kentucky bluegrass seed screenings (KB) or mixed conifer wood (CW), to support the growth of plants in mine spoils from the abandoned Formosa and Almeda Mines in Oregon. To evaluate the potential for plant establishment in mine tailings, wheat was grown in tailings amended with biochar at rates ranging from 0 to 9% (w/w). Both KB and CW biochars promoted plant establishment by increasing soil pH, increasing concentrations of macro- and micronutrients, and decreasing the solubility and plant uptake of heavy metals. Formosa tailings required at least 4% biochar and Almeda soil required at least 2% biochar to promote healthy wheat growth. A complimentary experiment in which mine spoils were leached with simulated precipitation indicated that biochar amendment rates ≥4% were sufficient to neutralize the elution pH and reduce concentrations of potentially toxic elements (Zn, Cu, Ni, Al) to levels near or below concern. These findings support the use of gasified biochar amendments to revegetate acid mine soils.

Environmental factors influencing the structural dynamics of soil microbial communities during assisted phytostabilization of acid-generating mine tailings: A mesocosm experiment

Compost-assisted phytostabilization has recently emerged as a robust alternative for reclamation of metalliferous mine tailings. Previous studies suggest that root-associated microbes may be important for facilitating plant establishment on the tailings, yet little is known about the long-term dynamics of microbial communities during reclamation. A mechanistic understanding of microbial community dynamics in tailings ecosystems undergoing remediation is critical because these dynamics profoundly influence both the biogeochemical weathering of tailings and the sustainability of a plant cover. Here we monitor the dynamics of soil microbial communities (i.e. bacteria, fungi, archaea) during a 12-month mesocosm study that included 4 treatments: 2 unplanted controls (unamended and compost-amended tailings) and 2 compost-amended seeded tailings treatments. Bacterial, fungal and archaeal communities responded distinctively to the revegetation process and concurrent changes in environmental conditions and pore water chemistry. Compost addition significantly increased microbial diversity and had an immediate and relatively long-lasting buffering-effect on pH, allowing plants to germinate and thrive during the early stages of the experiment. However, the compost buffering capacity diminished after six months and acidification took over as the major factor affecting plant survival and microbial community structure. Immediate changes in bacterial communities were observed following plant establishment, whereas fungal communities showed a delayed response that apparently correlated with the pH decline. Fluctuations in cobalt pore water concentrations, in particular, had a significant effect on the structure of all three microbial groups, which may be linked to the role of cobalt in metal detoxification pathways. The present study represents, to our knowledge, the first documentation of the dynamics of the three major microbial groups during revegetation of compost-amended, metalliferous mine tailings.

Effects of Surface Properties of Organic Matters on Cation Adsorption in Solution Phase

Organic substances have been commonly added to mine tailings to improve physical and chemical conditions and facilitate plant establishment. Apart from their nutritional benefits, aggregation and physical improvements may be resulted from organo-mineral interactions between their functional groups and cations of variable charges on mineral surfaces. Organic matters (OM), e.g., woodchips (WC) and sugarcane residue (SR), were initially evaluated for their affinities for Cu2+, Ca2+, and Fe3+ in relation to their surface properties at constant or different pH conditions in aqueous phase, before examining the formation of organo-mineral association in solid phase. Results indicated that WC and SR had different adsorption kinetics for these cations. Their surface negative charges indicated by zeta potentials were positively related to the adsorption capacities. 13C NMR analysis of the OM before and after adsorption revealed that carboxylic and phenolic groups were the major contributors to cation adsorption in solution. SR was found to have more carboxylic groups and thus had higher cation exchange capacity than WC. Therefore, organic materials with high density of functional anion groups on the surfaces may be advantageous when stimulating organo-mineral interactions in mine tailings which are rich in primary and secondary minerals with positively charged surfaces. The organo-mineral association between the OM and base metal mine tailings will be examined in the solid phase in a follow-up experiment.

Effects of ungulate herbivory on aspen, cottonwood, and willow development under forest fuels treatment regimes

Herbivory by domestic and wild ungulates can dramatically affect vegetation structure, composition and dynamics in nearly every terrestrial ecosystem of the world. These effects are of particular concern in forests of western North America, where intensive herbivory by native and domestic ungulates has the potential to substantially reduce or eliminate deciduous, highly palatable species of aspen (Populus tremuloides), cottonwood (Populus trichocarpa), and willow (Salix spp.). In turn, differential herbivory pressure may favor greater establishment of unpalatable conifers that serve as ladder fuels for stand-replacing fires. The resulting high fuel loads often require silvicultural fuels reductions to mitigate fire risk, which in turn may facilitate additional recruitment of deciduous species but also additional herbivory pressure. Potential interactions of ungulate herbivory with episodic disturbances of silviculture, fire, and other land uses are not well documented, but are thought to operate synergistically to affect forest dynamics. We evaluated individual and joint effects of ungulate herbivory and fuels reduction treatments in grand fir (Abies grandis) and Douglas-fir (Psuedotsuga menziezii) forests that dominate large areas of interior western North America. We applied fuels reduction treatments of mechanical thinning and prescribed fire and then evaluated the responses of aspen, cottonwood, and willow species to these treatments (N=3) versus areas of no treatment (N=3), and to exclusion from ungulate herbivory versus areas subjected to extant herbivory by free-ranging cattle (Bos taurus), elk (Cervus elaphus), and mule deer (Odocoileus hemionus). Densities of deciduous species were >4 times higher in response to fuels reduction treatments (84.4individuals/ha) compared to areas of no treatment (19.7individuals/ha). Additionally, when ungulates were excluded from fuels treated sites, the density of cottonwood was >5 times higher (122.5individuals/ha) than fuels treated sites subjected to extant herbivory (24.3individuals/ha). Similarly, densities of Populus spp. and Salix spp. were >3 times higher (211.6individuals/ha) on fuels treated sites excluded from ungulate herbivory versus fuels treated sites subjected to extant herbivory (66.1stems/ha). Deciduous species subjected to extant ungulate herbivory also were significantly lower in height, canopy surface area, and canopy volume than the same species inside the ungulate exclosures. Recruitment and long-term survival of aspen, cottonwood, and willow species in coniferous forests of interior western North America require a combination of episodic disturbances such as silviculture and fire to facilitate deciduous plant recruitment, followed by reductions in grazing pressure by domestic and wild ungulates during the time intervals between episodic disturbances to facilitate plant establishment, growth and survival.

Proglacial succession of biological soil crusts and vascular plants: biotic interactions in the High Arctic

To evaluate the hypothesis that biological soil crusts facilitate the establishment and maintenance of vascular plants during succession, we studied the distribution patterns of crusts and vascular plants along a High Arctic glacier foreland and compared the success of plants growing in and out of crusted substrate. Multivariate analyses determined that distance from the glacier and crust cover were the most important variables, explaining 11% and 9% of the variance in the vegetation data, respectively. Surfaces colonized by biological soil crusts generally supported higher plant densities and showed positive associations with the most dominant, long-lived plant species such as Saxifraga oppositifolia L., Salix arctica Pall., and Dryas integrifolia Vahl. Crusts facilitate plant establishment and growth in early and midsuccession but may compete for available resources further along the chronosequence. This study recognizes subtle direct influences of crust on vegetation density but also draws attention to a much larger overall positive effect on community structure. Succession on this foreland proceeds via a “directional-replacement” model and supports a well-developed community of biological soil crusts and vascular plants with greater species richness, cover, and density compared with other glacier foreland vegetation communities previously investigated on Ellesmere Island, Nunavut.

Beneficial microbial allelopathies in the root zone: the management of soil quality and plant disease with rhizobacteria

By virtue of their physiological adaptability and metabolic versatility, bacteria in plant root zones are a key agent of change in soil agroecosystems. Interactions between plant root systems and rhizobacteria have a profound effect on crop health, yield, and soil quality. Through the selective release of exudates and leachates plants activate and sustain specific rhizobacterial communities in the root zone. In turn, root zone bacteria are able to generate a wide array of secondary metabolites which can have a positive influence on plant growth; enhancing the availability of minerals and nutrients, improving nitrogen fixation ability, decreasing susceptibility to frost damage, improving plant health through the biocontrol of phytopathogens, inducing systemic plant disease resistance, and facilitating plant establishment, growth and development. The benefits from root zone bacterial biodiversity are moot in managed agroecosystems, where community complexity is minimized, and ecosystem stability is often disrupted for the purpose of disease control and yield maximization. The complexity of plant–soil–microbial interactions are so varied, that a complete understanding of all the relationships involved is unlikely to be achieved, even in a production monoculture. Nevertheless, the consequences of beneficial biological interactions that stimulate crop yields and improve plant health can be evaluated relatively simply and a number of general management strategies can be devised accordingly.