Arboriculture & Urban Forestry 34(3): May 2008 Arboriculture & Urban Forestry 2008. 34(3):163–172. 163 Experimental Tree Planting on U.K. Containment Landfill Sites: Results of 10 Years’ Monitoring Andy Moffat, Kirsten Foot, Fiona Kennedy, Martin Dobson, and Geoff Morgan Abstract. A series of experiments was set up in England in the early 1990s on five containment landfill sites engineered to modern standards to test the relative performance of 14 native and nonnative woodland tree species. This article describes the results of monitoring their survival, growth, and nutrition over a 10-year period. The experiments demonstrated that several species, notably ash, whitebeam, white poplar, and wild cherry, can usually be established on landfill sites with survival rates comparable to other brownfield sites. Despite general site infertility, growth of many tree species (for example, ash, beech, English oak, sycamore, Italian alder, silver maple, white poplar, and whitebeam) was similar to that expected on greenfield sites in the locality of the landfill sites. As well as infertility, soil droughtiness and mammal browsing were identified as limiting tree performance of particular species on some sites. After 10 years, there was no evidence of interaction with landfill containment systems or landfill gas. Key Words. Landfill; nutrition; tree growth; tree species; tree survival. The establishment of some form of vegetation is a vital part of the restoration strategy for landfill sites in many parts of the world (Department of the Environment 1996; California Inte- grated Waste Management Board 1999; Environmental Protec- tion Agency 1999; Office of Superfund Remediation and Tech- nology Innovation 2006). The choice of vegetation is especially important because it affects the appearance of the site and the way it fits into the wider landscape. Early guidance issued by the U.K. Department of the Environment (Department of the Envi- ronment 1986) actively discouraged tree planting on capped landfill sites. Principal concerns have included 1) the perception that tree roots could penetrate through an engineered landfill cap, allowing water ingress or escape of landfill gas; 2) shallow root- ing resulting from the presence of dense soil layers or cap ma- terial may lead to drought and windthrow susceptibility; and 3) woodland establishment might disrupt or compromise landfill pollution control measures if the trees were to blow over. In addition, tree survival and performance were considered to be adversely affected by many landfill site conditions. The U.K. Forestry Commission Research Division (now For- est Research) conducted an extensive desk review of The Poten- tial for Woodland Establishment on Landfill Sites in 1993 (Dob- son and Moffat 1993), which evaluated the likely patterns of tree root growth on landfill sites, the ability of tree roots to penetrate a landfill mineral cap, whether trees on these sites would be at risk from windthrow, and whether trees can actually grow on the comparatively harsh conditions of the landfill environment. The findings of the 1993 study suggested that it was possible to establish trees on modern containment landfills provided the sites were engineered to a standard suitable for effective pollu- tion control, the landfill cap was well compacted (bulk density greater than 1.8 g/cm3), and there was sufficient thickness of soil to prevent the threat of tree root penetration into the cap beneath. However, the findings of the desk study were drawn from research not directly involving the landfill environment, because pre-1986 guidance had effectively prevented tree planting on recent landfill sites. The then Department of the Environment and its successors, now the Department of Communities and Local Government, has funded a program of study since 1993 to establish and monitor field experimental plots on landfills that had been constructed close to the specifications identified by Dobson and Moffat (1993). This article presents the results of the field-based experiments into tree performance monitoring on the five landfill sites, which have now been monitored for 10 years. MATERIALS AND METHODS A screening process was used to evaluate the characteristics of a large number of landfill sites identified by the Environmental Services Association and U.K. Waste Licensing Authorities. To select appropriate sites for the study, essential criteria were the presence of an engineered clay cap, an uncompacted soil cover with a minimum thickness of 1 m (3.3 ft), and a range of soil types and climatic conditions to be represented. The sites chosen were located at Bristol (Yanley), Swindon (Shaw Tip), Skel- mersdale (Pimbo), Hatfield (Beech Farm), and Ely (Grunty Fen). Site locations and characteristics are summarized in Table 1. Reclamation and woodland establishment practices at the sites followed, as closely as possible, the recommendations in The Potential for Woodland Establishment on Landfill Sites (Dobson and Moffat 1993). Most sites had soil or soil-forming materials placed by “loose tipping” (Bending et al. 1999) to reduce the risk of soil compaction, although ripping was used to alleviate com- paction at Beech Farm. All sites except Shaw Tip were provided with some recycled topsoil material, although this was usually of limited thickness and of likely poor quality compared with stan- dard definitions (British Standards Institution 1994). Eight tree species were planted (total 1,152 trees) at each site in individual species plots of 36 trees. Species plots were repli- cated four times in a randomized block experimental design (Pearce 1976). Tree species were chosen from a list recom- mended by Dobson and Moffat (1993) with consideration given ©2008 International Society of Arboriculture
May 2008
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