The effect of oil shale ash and mixtures of wood ash and oil shale ash on the above- and belowground biomass formation of Silver birch and Scots pine seedlings on a cutaway peatland

•Silver birch and Scots pine are suitable for afforestation of cutaway peatlands.•Recycling of wood and oil shale ashes stimulates the biomass formation of trees.•Use of ashes on the peatlands helps to balance the content of nutrients in peat.•The biomass production was greatest on the mixture ash treated plots.•The effect of pure oil shale ash on the growth of trees was smallest.