Novel soil reconstruction leads to successful afforestation of a former asbestos mine in southern Quebec, Canada.

The asbestos mining industry has deeply transformed the landscapes of southern Quebec, leaving behind about 800 million tonnes of tailings and waste rock (overburden) in large piles near former mining towns. Some environmental issues arising from these landscapes (e.g., wind and water erosion) have been successfully addressed by covering the piles by mixing by-products to create technosols, which are then seeded with grasses. Yet, no attempt at afforestation had been made thus far because these environments are thought to impose too many constraints on tree establishment and growth. We developed two experimental plantations at a decommissioned asbestos mine in southern Quebec, on waste rocks and on tailings, to test the performance of eight tree species/clones/provenances planted on two types of technosols. Both were constructed from mixtures of municipal biosolids and deinking sludge and configured in small windrows. Soil texture, apparent bulk density, element composition, including carbon, nitrogen and some metals, as well as soil temperature, water potential and volumetric water content were assessed, together with seedling survival and growth (diameter and height). Leaf specific surface area, nutrients, water use efficiency (δ¹³C) and gas exchange (i.e., net photosynthetic assimilation rate, A_net, and stomatal conductance to water vapour, g_sw) were evaluated for three hybrid poplar clones (Populus spp.) during the third growing season. Hybrid poplar clones had significantly higher survival (87–94%) after three years than planted conifers (10–56%) that were tested, although dry planting conditions could have exerted adverse effects on conifers. The conifer species exhibiting the highest survival rates (46–56%) was white spruce (Picea glauca). Average growth rates of hybrid poplar clones were 45 cm y⁻¹, with some tree heights reaching over 300 cm after three years. Clone DN × M-915508 appears to be more drought-tolerant than other clones being tested. Phosphorus and potassium were the most growth-limiting nutrients. Few differences were apparent between technosols; results suggest that the addition of Class B contaminated soils to the main mixture of by-products has benefits with respect to tree survival and growth. Our study demonstrates a promising technology for reclamation through afforestation of asbestos mines in southern Quebec. Some treatments should be added to limit plant competition within the first few years as a means of augmenting survival and growth, whereas limited technosol volume and accompanying low moisture and nutrient availability could compromise mid- to long-term growth of trees. [ABSTRACT FROM AUTHOR]