The effect of tree planting density on the relative development of weeds and hybrid poplars on revegetated mine slopes vulnerable to erosion

In reclaimed waste rocks slopes, the soil cover spread for revegetation is prone to erosion. This soil needs to be immediately protected from soil erosion by above and belowground vegetation. The seeding of fast-growing herbaceous species used in agriculture is generally used on waste rock slopes to control erosion, but these compete with planted trees for resources needed for growth. The aim of this study was to assess the ability of an alternative to these herbaceous species (i.e. fast-growing hybrid poplars) to mitigate soil losses in the short-term. However, the growth of poplars could be impaired by competition from weeds’ colonization. Tree planting density was expected to change competition levels among trees and between trees and weeds, so influencing above and belowground vegetation development and hence effectiveness at controlling soil erosion processes. Five treatments were installed in 2013: 1 × 1, 2 × 2, 4 × 4 m spacing of hybrid poplars; 2 × 2 m spacing, with hydroseeding of fast-growing herbaceous plants; and a control plot with no tree planting or hydroseeding. The planting of hybrid poplars did not decrease soil losses compared to the control plot, regardless of the tree planting density. Weed development through natural colonization occurred in all the non-hydroseeded plots and was more effective at soil erosion control in the short-term than planted trees. The selected clone of hybrid poplar coped well with any competition from weeds for water, since 100% of trees survived after 3 years and since the non-hydroseeded plots produced greater length of roots per soil volume (root length density, RLD) for the poplars compared with the weed species. As early as the first year, the hydroseeded plots showed the highest RLD and a complete cover of vegetation which effectively controlled soil erosion compared to the control plots. However, both treatments with increased competition levels (i.e. highest tree planting density and hydroseeding treatment) showed less aboveground tree biomass. On the hydroseeded plots, where interspecific competition with weeds was the highest, a greater increase in root length was seen during the third year after planting. After 3 years, canopy closure was achieved in the 1 × 1 m spacing treatment, which reduced weed cover

Tree-Substrate Water Relations and Root Development in Tree Plantations Used for Mine Tailings Reclamation

Tree water uptake relies on well-developed root systems. However, mine wastes can restrict root growth, in particular metalliferous mill tailings, which consist of the finely crushed ore that remains after valuable metals are removed. Thus, water stress could limit plantation success in reclaimed mine lands. This study evaluates the effect of substrates varying in quality (topsoil, overburden, compost and tailings mixture, and tailings alone) and quantity (50- or 20-cm-thick topsoil layer vs. 1-m2 plantation holes) on root development and water stress exposure of trees planted in low-sulfide mine tailings under boreal conditions. A field experiment was conducted over 2 yr with two tree species: basket willow (Salix viminalis L.) and hybrid poplar (Populus canadensis Moench × Populus maximowiczii A. Henry). Trees developed roots in the tailings underlying the soil treatments despite tailings' low macroporosity. However, almost no root development occurred in tailings underlying a compost and tailings mixture. Because root development and associated water uptake was not limited to the soil, soil volume influenced neither short-term (water potential and instantaneous transpiration) nor long-term (δ13C) water stress exposure in trees. However, trees were larger and had greater total leaf area when grown in thicker topsoil. Despite a volumetric water content that always remained above permanent wilting point in the tailings colonized by tree roots, measured foliar water potentials at midday were lower than drought thresholds reported for both tested tree species

Early recruitment of boreal forest trees in hybrid poplar plantations of different densities on mine waste rock slopes

Mine wastes create harsh recruitment conditions for forest tree seedlings, especially waste rock piles where erodible slopes are prone to drought. Plantations using fast-growing tree species can potentially accelerate the conversion of degraded mine sites into forests through facilitation of tree recruitment, while contributing to the stability of slopes. In this study, hybrid poplars were tested as a means of achieving reclamation objectives by providing shelter for forest tree seedlings on waste rock slopes (3H:1V ratio) in the Canadian southern boreal region. Density effects of young hybrid poplars were assessed on the emergence and survival of early, mid and late successional species, naturally occurring or hand-seeded, and on the understory micro-environmental parameters in plantations of different spacings (1 × 1, 2 × 2, 4 × 4 m and control without planted trees). Results were also compared in 2 × 2-m plantations with and without a hydroseeded herbaceous cover, traditionally used to control erosion in slopes. During the 2nd growing season of the plantations, seedling emergence of naturally established Salicaceae (Populus and Salix) species followed a quadratic pattern along the density gradient, as emergence values were higher under an intermediary density. Nonetheless, decrease in light transmission emerged as a limiting factor of seedling survival for these early-successional, shade-intolerant species by the next summer. Following a spring sowing experiment in the 3rd growing season of the plantations, emergence rates for later-successional Picea glauca and Abies balsamea seedlings increased with hybrid poplar density. During their peak emergence period, in early season, higher soil moisture content was found under denser cover. However, at the end of the third year of the plantations, only A. balsamea showed moderate increase in early recruitment success rates under denser tree cover. In hydroseeded plots, a competitive effect of the herbaceous cover was observed on Salicaceae emergence and A. balsamea survival. These results suggest that planting of young plantations without a hydroseeded cover may offer a more suitable solution in order to quickly provide early recruitment opportunities for later-successional seedlings in waste rock slopes. Despite this, a significant decrease in moisture content recorded during the second half of the 3rd growing season under the 1 × 1-m cover, compared to the 2 × 2-m, likely signalled an increasing competitive effect from hybrid poplars, which may compromise their nursing potential in the longer term. Therefore, further monitoring is imperative for a better understanding of longer-term facilitation and competition interactions between nurse trees and understory seedlings in waste rock slopes, where competition for limited resources, such as water, may be severe

Evaluation of Arsenic Leaching Potential in Gold Mine Tailings Amended with Peat and Mine Drainage Treatment Sludge

Peat and mine drainage treatment sludge can be valorized as amendments on mine sites to stabilize gold mine tailings and reduce the potential leaching of contaminants in pore water. However, the influence of organic amendments on the mobility of metalloids and/or metals in the tailings must be validated, as the leached contaminants may vary according to their type, nature, and origin. The objective of the present study was to evaluate over time the effect of peat- and/or Fe-rich sludge amendments on the mobility of As and metallic cations in the drainage water of tailings potentially producing contaminated neutral drainage. Ten duplicated weathering cell experiments containing tailings alone or amended with peat and/or Fe-rich sludge (5-10% dry weight) were performed and monitored for 112 d. The results showed that as low as 5% peat amendment would promote As mobility in tailings' pore water, with As concentrations exceeding Quebec discharge criteria (>0.2 mg L). In addition, As(III), the most mobile and toxic form, was predominant with 10% peat, whereas organic species were negligible in all cells. The use of peat alone as organic amendment for the stabilization of tailing contaminants could increase the risk of generating As-rich contaminated neutral drainage. Conversely, the mix of only 5% Fe-rich sludge with or without peat decreased As concentrations in leachates by 65 to 80%. Further studies on the use of "peat" or "peat + Fe-rich sludge" as cover or amendment should be conducted with a focus on Fe/As and Ca/As ratios

Effects of spacing and herbaceous hydroseeding on water stress exposure and root development of poplars planted in soil-covered waste rock slopes

Root development is important to ensure tree survival in conditions of water stress. Despite their long-recognized role, little attention has been given to their development on waste rock slopes subject to rapid drainage. This study was conducted in an open-pit gold mine in a boreal forest. Its main objective was to establish a plantation design with a moderate level of competition for water resources on a waste rock slope. A hybrid poplar plantation was established in May 2013 on a soil-covered waste rock slope of 33%. The experimental design included three different poplar spacings: 1 × 1 m, 2 × 2 m without herbaceous seeding, 2 × 2 m with herbaceous seeding and 4 × 4 m. The poplars responded to increased competition resulting from closer spacing and herbaceous seeding by investing less energy in diameter and height growth. Poplar individuals that were subject to high levels of competition were able to acclimatize to water stress conditions by increasing root length density and specific root length and by reducing above-ground biomass. This study indicates that some clones of hybrid poplar showing phenotypic plasticity in the ratio of above- and belowground growth can be adapted for short-term revegetation of mine sites

Effect of soil quality and planting material on the root architecture and the root anchorage of young hybrid poplar plantations on waste rock slopes

On mine waste rock slopes, trees with inadequate root development could be prone to uprooting. The anchorage of trees is mainly determined by the architecture of the root systems that drive their mechanical interactions with the soil. The objective of this study was to evaluate the effects of soil quality and of different planting materials on the architecture and resistance to shear stress of root systems of hybrid poplars four years after they were initially planted. The study was conducted in an open-pit-gold mine. A hybrid poplar plantation was established in 2013 on 33% soil-covered waste rock slopes, using a randomised complete block design, that is, 3 replicated blocks × 3 planting materials × 2 soil qualities. The stability of the hybrid poplars (resistance to uprooting) was evaluated using lateral traction tests. Complete excavations were performed to characterise their coarse root (>4 mm) architecture. Results showed no significant differences between treatments in terms of the maximum resistance force to uprooting, which varied between 7142 and 8989 N. After four growing seasons, no significant effects due to soil quality or planting material were observed in the number of lateral roots, mean root diameter, root biomass, aboveground biomass, and shoot/root ratio

The effects of agronomic herbaceous plants on the soil structure of gold mine tailings and the establishment of boreal forest tree seedlings

In Canada, low-grade ore mines generate large amounts of mineral waste, such as mine tailings. To control erosion of the fine-grained tailings particles as quickly as possible, it is common practice for the mining industry to revegetate the mine tailings with agronomic herbaceous plants. However, it is unclear whether this practice is consequential to the natural establishment of boreal species. The first objective of this study was to evaluate which families of agronomic herbaceous plants (legumes or grasses) result in the most favorable physical and chemical soil properties for the establishment of boreal species. The second objective was to determine the effect of the agronomic herbaceous plants on the growth and foliar nutrient concentration on three indigenous boreal forest seedlings; jack pine (Pinus banksiana Lambert), tamarack (Larix laricina Du Roi), paper birch (Betula papyrifera Marshall), and a willow cultivar (Salix miyabeana Seemen). In 2013, a 1-ha in situ experimental surface of mine tailings was set up on the gold mine site in Malartic, Abitibi-Témiscamingue, Quebec. The experimental site was subdivided into three blocks, each further divided in 5 plots. Each plot was randomly seeded as follows: 100% grass, 100% legumes, a mixture of both, topsoil, and a control (tailings only, no seeding). In the 2015 spring season, thirty seedlings of the three boreal tree species and cuttings of the willow cultivars were planted in each treatment plot. Seedling height and root biomass were measured at the end of the 2016 growing season. Soil sample analyses indicated significant differences for bulk density, wilting point, and organic matter content between the topsoil and the different agronomic herbaceous and control treatments; however, no significant differences were found between the different herbaceous treatments and the control for soil pH, bulk density, wilting point, macroporosity, and organic matter content. The mortality rate of jack pine, tamarack, and paper birch seedlings was higher in the control plots compared to all other treatments. Root biomass and height of the willow cultivar were significantly higher in the legumes compared with topsoil treatment. Among the four pioneer tree seedlings studied, this research indicates that the combination of the willow cultivar with the legumes treatment produces the best seedling growth and survival in the highly abiotic and stressful environments inherent to mine tailings

Approche méthodologique pour intégrer des scénarios de végétation forestière tenant compte des changements climatiques dans la prédiction des méthodes de restauration minière sous climat humide au Québec

La restauration des parcs à résidus générateurs de drainage minier acide (DMA) constitue un défi environnemental important pour l’industrie minière. Au Québec, différentes techniques de contrôle du DMA provenant de parcs à résidus sont utilisées pour empêcher la contamination de l’environnement, parmi lesquelles on retrouve les recouvrements d’ingénierie appelés barrières à l’oxygène (Bussière et al., 2003; Aubertin et al., 2016; Bussière et Guittonny, 2020). Dans le contexte de restauration de sites miniers, l’objectif est de mettre en place un recouvrement permettant de contrôler la contamination et pouvant supporter une végétation pérenne qui s’intègre bien à l’environnement du site (MEND, 2014). Cependant, la végétation peut affecter le bilan hydrique, les propriétés des matériaux des barrières à l’oxygène et, par le fait même, leur performance (MEND, 2014; Guittonny et al., 2018). À ce jour, les caractéristiques à long terme de la végétation forestière qui s’installera sur les parcs à résidus restaurés sont encore mal connues et nécessitent de prendre en compte l’influence des changements climatiques (CC) sur l’évolution des écosystèmes. En effet, les CC conduisent à une redistribution généralisée des espèces à l’échelle planétaire (IPCC, 2014; Bonebrake et al., 2018). Un changement des conditions climatiques pourrait entraîner un changement dans la distribution des espèces d’arbres et dans la composition des forêts. Les conditions climatiques ont déjà changé et affectent déjà la distribution de certaines espèces d’arbres au Canada (Williamson et al., 2009; Price et al., 2013; Berteaux et al., 2014; Ressources naturelles Canada, 2017). Le climat existant dans une région où une espèce vit et se reproduit se trouvera vraisemblablement ailleurs dans l’avenir (McKenney et al., 2007; Berteaux et al., 2014; Sittaro et al., 2017). Le présent rapport vise à proposer une approche méthodologique pour la prise en compte de l’influence de la végétation forestière sur la performance à long terme des barrières à l’oxygène sur base des scénarios de végétation sous CC au Québec. En amont de ce rapport, une revue de littérature sur les changements d’aire de répartition des essences clés de la forêt feuillue et boréale, en réponse aux CC, a été réalisée (Botula et al., 2019). Celle-ci a permis d’identifier la composition de la végétation qui sera présente de manière plausible d’ici 2100 pour deux sites miniers : l’un en activité, la mine Westwood en Abitibi, et l’autre abandonné, le site Lorraine au Témiscamingue. En se basant sur ces travaux, des scénarios de végétation d’ici 2100 seront proposés dans ce rapport pour les deux sites, ainsi que des gammes de valeurs correspondantes pour les paramètres de végétation à intégrer aux modèles numériques hydrogéologiques utilisés pour la prédiction de la performance des barrières à l’oxygène. Les résultats présentés ici pourront aider les partenaires miniers à déterminer si l’évolution de la végétation sur leurs sites suite aux CC affectera la performance à long terme des méthodes de restauration utilisées, et si des mesures de gestion de cette végétation devront être mises en place. Ce rapport est divisé en trois chapitres et se termine par une conclusion et des suggestions : le premier chapitre est une synthèse des connaissances en lien avec les effets des CC sur la distribution des essences clés de la forêt feuillue et boréale au Québec; le deuxième chapitre est une synthèse des connaissances en lien avec l’influence de la végétation forestière sur la performance des barrières à l’oxygène au Québec; le troisième chapitre est une analyse qualitative de l’influence des CC sur les paramètres de la végétation utilisés pour évaluer la performance des barrières à l’oxygène avec application à deux sites miniers : Westwood et Lorraine

Le Marais des Paluns

Les zones humides méditerranéennes sont au cœur de multiples enjeux environnementaux. Les zones humides de la Palun connectées à l’étang de Berre sont une zone spéciale de conservation située à l’aval d’un bassin versant anthropisé. Une contamination diffuse et cumulative dans le temps des sédiments y a été identifiée. C’est dans ce contexte qu’un travail a été mené sur un procédé de traitement des eaux du bassin versant de la Palun pour favoriser la résilience du marais récepteur face à la pollution chronique. Les possibilités de réalisation d’une zone humide artificielle en amont du marais, dédiée à l’épuration des eaux, ont été réfléchies en lien avec les usages et les attentes parfois divergentes des acteurs du territoire. La réparation des atteintes environnementales dans le contexte socio-économique contraint de l’étang de Berre a été interrogée à partir des savoirs issus de l’écologie de la restauration et de la sociologie.Mediterranean wetlands are subject to many environmental issues. The Palun wetlands in connection with the industrialized Berre lagoon are a special area of conservation located downstream of an anthropised watershed and where diffuse and cumulative contaminations of the sediments were detected. A work was then carried out on a water treatment process in the Palun watershed to promote the resilience of the receiving marsh. The possibilities of creating a constructed wetland upstream to treat all waters have been reflected in connection with the potential divergent uses and expectations of the territory actors. Repairing environmental damages in the particular socio-economic context of the Berre lagoon was considered from the knowledge of restoration ecology and sociology

Valorisation d'un compost de boues urbaines en garrigue pour le reboisement : comportement des jeunes arbres d'une plantation et modifications de la dynamique de la vegetation naturelle apres amendement

Michele Tremolieres Thierry Gauquelin Jean-Michel CarnusSince sewage sludge dumping has been banned in the EEC, there is renewed interest in studying new ways of recycling it. Mediterranean areas are prone to recurrent fires and violent precipitation events that favour erosion and soil impoverishment. Calcareous degraded soils are often colonized by stable Quercus coccifera garrigues that are adapted to low resource availability, and consequently tree installation is delayed. There, organic amendment could increase soil fertility level, modify the vegetation dynamics and the dominance relationships, and could improve succession and resilience processes, as well as reforestation. Two experiments were settled in Provence (South-eastern France) to study the effect of co-composted sewage sludge and green wastes on the dynamics of the garrigue ecosystem. First, we superficially spread 0, 50 and 100 Mg.ha-1of fresh compost in the natural garrigue biotope, and observed the consequences on ecosystem functioning. Soil, vegetation, and litter microorganisms compartments were monitored during the two years following amendment (2002-2004). Secondly, three rates of fresh compost (0, 20, and 40 kg.m-2) were incorporated into the soil at the stem of planted seedlings. The tree density was 400 seedlings.ha-1, and so 8 and 16 Mg.ha-1 of fresh compost were globally spread. The evolution of soil properties and the development of trees were followed respectively for three years (2001-2003) and two years (2002-2003). Amendment durably (2 years) and efficiently improved soil fertility. However, the soil was greatly enriched in P and Zn, which could lead to serious water pollution problems and to toxic effects on microorganisms. These two elements limit sewage-sludge compost use in natural areas, and repeated amendment at similar rates are not considerable. Compost amendment in the plantation globally improved reforestation by enhancing seedling growth and nutrition, and particularly by increasing their survival under drought. In addition, compost spreading generated fertile patches in garrigue biotope, in which succession processes could locally be enhanced and tree installation later favoured, especially Pinus halepensis'. Moreover, functional diversity is increased by compost spreading, as seeders are improved, which could increase the global resilience of ecosystem after fire. These beneficial effects of compost rather occurred for the intermediate rates. Moreover, the maximal rates should not be used because the fungal colonization of Quercus coccifera litter was depleted on D100, and because P and Zn pollution risks increased with amendment rate. However, the calcareous nature of the experimental soil and the high compost maturity limited water exportations and the plant contamination by potentially toxic elements. Compost superficial spreading rather increased plant sensitivity to drought, while it decreased when compost and soil were mixed. Finally, the compost spreading in garrigue had multiple effect (fertilization, crushing and mulching) and highlighted plant specific strategies in nutrient management and productivity. This work showed the relative importance of species on ecosystem functioning, identified key species for element cycling, and pointed out the complementarity resource use that exist in such natural ecosystem.Depuis que la Communauté européenne a interdit la mise en décharge des déchets non ultimes, la recherche de nouveaux débouchés pour les boues d'épuration devient cruciale. En région méditerranéenne, les feux récurrents et les précipitations parfois violentes favorisent l'érosion et induisent l'appauvrissement des sols. Les sols calcaires dégradés sont souvent colonisés par des formations végétales stables à Quercus coccifera, adaptées à un faible niveau de ressources, ce qui ralentit l'installation naturelle des arbres. Dans de tels milieux, un amendement organique pourrait améliorer la fertilité du sol, modifier la dynamique et les relations de dominance de la végétation, et favoriser les processus de succession et de résilience, ainsi que la reforestation. Deux expérimentations ont été menées en Provence pour étudier les effets d'apports de compost de boues d'épuration urbaines et de déchets verts sur la dynamique d'un écosystème de garrigue. Premièrement, 0, 50, et 100 t.ha-1 de compost brut ont été épandues en surface dans le biotope naturel de garrigue, et les conséquences sur le fonctionnement de l'écosystème ont été étudiées. Les compartiments sol, végétation et microorganismes de la litière ont été suivis pendant deux ans après amendement (2002-2004). Deuxièmement, nous avons incorporé au sol trois doses de compost brut (0, 20, et 40 kg.ha-1) au pied de jeunes plants d'arbres. L'évolution des propriétés édaphiques et du développement des arbres a été suivie respectivement pendant trois ans (2001-2003) et deux ans (2002-2003). L'amendement a durablement (2 ans) et efficacement amélioré la fertilité du sol. Cependant, le sol a été fortement enrichi en P et Zn, ce qui pourrait entraîner des problèmes sérieux de pollution des eaux et de toxicité sur les microorganismes. Ces deux éléments limitent l'utilisation de compost de boues dans les milieux naturels, et des amendements répétés aux doses étudiées sont inenvisageables. Dans la plantation, l'amendement a globalement favorisé le processus de reforestation en améliorant la nutrition et la croissance des plants, et surtout en augmentant leur survie en période de sécheresse. En outre, l'apport de compost en surface a généré l'apparition d'îlots fertiles dans le biotope garrigue, au sein desquels les processus de succession végétale pourraient être localement optimisés, ce qui pourrait favoriser à terme la colonisation par la strate arborée, notamment par Pinus halepensis. Par ailleurs, l'amendement augmente la diversité fonctionnelle en garrigue du fait de son effet bénéfique sur les espèces semencières, ce qui pourrait améliorer la résilience de l'écosystème après incendie. Ces effets bénéfiques du compost se produisent surtout pour les doses intermédiaires. L'utilisation de compost aux doses maximales testées devrait être abandonnée, du fait d'une dépréciation de la colonisation fongique de la litière de Quercus coccifera observée pour 100t.ha-1, et du fait d'une augmentation des risques de pollution par le P et le Zn proportionnellement au taux d'apport. Cependant, la nature calcaire du sol expérimental et la maturité élevée du compost ont limité les phénomènes d'exportation vers les eaux et la contamination des plantes par les éléments potentiellement toxiques. L'apport de compost en surface a plutôt augmenté la sensibilité des plantes à la sécheresse, alors que celle-ci a diminué lorsque le compost a été apporté en mélange au sol. Enfin, l'apport de compost en garrigue a eu des effets multiples (fertilisation, écrasement, dépôt d'une couche fertile) et a permis de mettre en évidence les stratégies spécifiques des plantes dans la gestion des nutriments et de la productivité. Ce travail montre l'importance relative des espèces végétales sur le fonctionnement de l'écosystème, identifie les espèces clef pour le recyclage des éléments, et illustre la complémentarité d'utilisation des ressources qui existe au sein de l'écosystème naturel de garrigue