Findings and Recommendations

• Tree survival rates: for most species, around 50-85 % of originally planted trees remain from early restoration work

• Exceptional survival: some species, including white poplar, whitebeam and ash, have survived exceptionally well considering the hostile landfill site conditions
• Growth: trees have grown steadily over 10 years, although growth rates differ between species and sites
• Limits: some of the tallest trees (exceeding 6 m in height) show slight signs of limitation to their growth
• Fastest growers: fastest growing species remain poplar, alder, cherry, whitebeam and ash
• Nutrients: all sites show signs of nitrogen deficiency, but few have low phosphorus or potassium
• Fertilisation: applications of nitrogen fertiliser may be required to achieve full canopy closure
• Soil cover: sustainable growth for trees over at least a decade require at least 1 m of soil cover; in droughty areas or in soils with low moisture-holding capacity, deeper soil cover is necessary

The research concluded that 1.5 m of soil or soil-forming material over a mineral cap will ensure that trees can be established on landfills without posing a significant threat to cap integrity for at least 16 years. It is still a little early to assess the full impact of tree rooting on cap integrity or the interaction between conditions within the landfill cap and tree rooting over the lifetime of a tree.