Tree establishment on post-mining waste soils: species, density, and mixture effects

Abstract

Tree establishment to restore degraded boreal post-mining lands is challenged by low soil productivity, a harsh microclimate, and potentially high contaminant levels. The use of mixed vegetation can facilitate the microclimate but increase competition for soil resources. A statistical accounting of plant–plant interactions and adaptation to multispecies conditions is hard to achieve in field experiments; trials under controlled conditions can distinguish effects of planting density and species interactions in the early stages of plant establishment. A greenhouse trial was established in containers (“mesocosms”) with waste rock or fine tailings from gold mines. Pregerminated (1-week-old) seedlings (Alnus viridis subsp. crispa, Picea glauca, Populus tremuloides, Salix arbusculoides) were planted using a Nelder density gradient design, modified for species combinations. A relative competition effect was estimated as a competitiveness index for each species combination, calculated as a ratio of α coefficients in the Holliday growth equation. The specific leaf area (SLA) was measured to indicate plant water stress adaptation. All species grew better in monoculture on fine tailings, while only P. tremuloides grew better in all mixtures on waste rock. Although net positive effects of density on SLA increment during early growth suggested microclimate improvement on fine tailings, no mixture provided advantages for both species in paired combinations.