The effect of woodland growth on a containment landfill site in Hertfordshire, UK

Below-ground growth of four tree species (ash (Fraxinus excelsior L.), alder (Alnus spp.), sycamore (Acer pseudoplatanus L.) and Corsican pine (Pinus nigra var. maritima (Ait.) Melville)) was investigated on a 15-year-old containment landfill in Hertfordshire, UK. The site was restored using a variable but predictable thickness of soil-forming material over the mineral cap and this permitted a study of the relationship between soil thickness and indices of tree root growth. Tree root habit was affected by soil thickness, and the likelihood of root penetration into the cap was significantly reduced when soil cover was greater than 1.25 m. Penetration into the cap appeared to occur because bulk density was often lower than threshold values set to meet permeability criteria necessary for pollution control. The study results suggest that previously published guidance on minimum soil thickness over unprotected landfill caps (1.5 m) should be adhered to, and that risk of root penetration into the cap will be acceptable if it is.