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Keywords:

  • grain size of the substrate;
  • ordination;
  • reference site;
  • restoration ecology;
  • reutilization of soil;
  • succession

Spoil heaps of surplus rock from hydropower tunnel construction negatively impact alpine landscapes unless restored. Such spoil heaps have been created for more than 100 years, but we still lack knowledge about the relative importance of compensatory mitigation (seeding and fertilization), spoil-heap construction method, local environmental factors, and regional climatic factors for restoration success. We studied the species composition of 19 alpine spoil heaps in Western Norway and their undisturbed surroundings using ordination and statistical modeling. Substrate grain size was the principal factor explaining differences in species composition between spoil heaps and their surroundings. Soil characteristics, that is, organic matter content and pH, and reutilization of topsoil were also important. Seeding and fertilization had negligible effects on restoration success. Slow recovery was observed for total vegetation cover and species richness of vascular plants and lichens while bryophyte cover recovered rapidly. Lower bryophyte cover and bryophyte and vascular plant species richness on older than on younger spoil heaps indicated recent changes in spoil-heap construction practices that favor plant colonization. Our results indicate that spoil-heap design is more important for restoration success than compensatory mitigation. We therefore suggest spoil heaps designed with a fine-grained top substrate preferably from stockpiled local topsoil, with uneven surface topography that mimics natural topographic variation, and recommend discontinuation of seeding and fertilization.