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: 1x1, 2x2, 4x4 meter spacing of hybrid poplars; 2x2 meter 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 three 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 4 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 three years, canopy closure was achieved in the 1x1 meter spacing treatment, which reduced weed cover.

The management of mine solid waste remains the most important environmental commitment for mine companies. This research project was carried out on the low sulphur waste rock of the Canadian Malartic mine, with the aim of assessing the erosion control effectiveness of different plantation designs on 33% slopes. Since the mine is located in the boreal forest, the purpose of the project is not only to define the design that provides best soil protection, but also to identify which one best promotes the establishment of trees. Fast-growing poplar may prove to be effective in erosion control on the waste rock slopes because of its fine root development in dry soil and its effect on soil cohesion. The plantation was established in May 2013 and was monitored over two growing seasons with the aim of determining the effect of five treatments (planted trees at three different spacings without hydroseeding; planted trees with hydroseeding; and a control without trees or hydroseeding) on soil loss. Soil loss measurements were related to root morphology, canopy development and understory cover. In the centre of the plantations, soil loss occurred mainly during the spring snowmelt while soil deposition occurred during summer rainfall. During the first two years after planting, the combination of planted trees with hydroseeding showed the best erosion mitigation compared to the control because of its greater herbaceous cover and greater root length density which maximised soil protection. No difference in soil erosion rates was found between the tree spacing treatments. However, the 1 × 1 m tree spacing significantly increased root density and tree canopy cover in the second year after planting. Potentially this should be reflected in lower soil loss compared with the other treatments in the third year after planting.