New Forest Research publication, Woodland Establishment on Landfill Sites: Ten Years of Research
News from Forest Research: July 2008
In the past, guidance on restoring landfill sites in the UK actively discouraged tree establishment because of problems with roots penetrating the clay capping. There was also concern that the trees would not thrive or would easily be uprooted because of shallow soils and infertile conditions. However, as reported in a recent Forest Research publication, Woodland Establishment on Landfill Sites: Ten Years of Research, planting on such sites can be an effective land management approach. The report provides new guidance on planting trees when restoring landfill sites while minimising the risk of compromising the necessary pollution control.
In 1992, Forest Research conducted a review of the potential for trees on landfill sites. This suggested that tree establishment was viable if the landfill cap was of suitable quality to prevent root penetration and if soil was of sufficient depth and quality to sustain tree growth.
To investigate this further, new experiments were established. The first, located at a 15-year-old containment landfill site found that tree root development was affected by soil thickness. Root penetration into the cap appeared to occur when the bulk density was lower than that necessary to achieve permeability threshold values for pollution control. The research showed the likelihood of root penetration into the mineral cap was reduced almost to zero when soil depth was greater than 1.5 metres.
In addition, five experiments were set up on modern containment landfill sites across England to test the relative performance of 14 native and non-native woodland tree species over a 10-year period. The experiments demonstrated that several species, including poplar, alder, cherry, whitebeam and ash, can usually be established on landfill sites with survival rates comparable to other brownfield sites. Despite site infertility, growth of many tree species was similar to that on greenfield sites.
Much more detailed information and guidance is given in the 75-page report, which was published by the Department for Communities and Local Government, and is available online.