Effects of rooting and tree growth of selected woodland species on cap integrity in a mineral capped landfill site

The above and below ground growth of three tree species (Alnus glutinosa, P inus nigra var. maritima and Acer pseudoplatanus) was studied on a containm ent landfill site at Waterford, Hertfordshire, UK. Tree root architecture w as studied using soil inspection pits excavated next to 12 trees of each sp ecies and mapped in detail. Tree height was related to soil thickness over the compacted mineral cap. No roots entered the cap where soil thickness wa s 1.3 m, but a few roots, especially of alder, were observed within it when the soil cover was 1.0 m or less. Micromorphological analysis of undisturb ed samples of the mineral cap suggested that roots exploited weaknesses in the cap rather than actively causing penetration into it. Alder roots were more tolerant of anaerobic conditions within the cap than the other species examined. The results confirm that mineral caps should be covered by 1.5 m of soil or soil-forming material if tree establishment is intended over a restored landfill site, unless protected by other parts of a composite capp ing system.