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1. Variable growth of Larix leptolepis (Japanese larch) has been observed on restored opencast coal workings in South Wales. This has implications for the restoration of such sites. A study of the relationship of tree growth with minespoil chemical, nutritional, physical and hydrological factors was carried out.
2. Tree growth was positively related to nitrogen and phosphorus foliar concentrations, but negatively to those of magnesium. Seventy-three per cent of the variation in tree growth was explained by variation in foliar chemistry.
3. Soil pH and extractable magnesium were negatively correlated with tree growth, with cation exchange capacity positively related to it.
4. Minespoils had bulk densities that commonly exceeded 1·7 g cm–3 below 0·2 m depth. Stone contents were high and typically 25% by volume.
5. Root systems of trees excavated were characterized by a high root density within 0·3 m of the minespoil surface. Restricted rooting was attributed to high bulk density and the incidence of shallow water tables.
6. Waterlogging during the spring and early summer, and the consequent presence of anaerobic soil conditions during periods of active growth, was found to be detrimental to tree growth.
7. The study suggests that landform design, selection of suitable soil or soil-forming materials, spoil placement technique and appropriate species choice are central to the future success of forestry schemes on restored ground in South Wales.
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Many factors affect the successful re-establishment of vegetation on a mining site after mineral extraction has finished (Bradshaw 1983; Moffat & Buckley 1995). Establishing vegetation on spoils produced from coal mining often presents particularly challenging problems. These include compaction, poor water-holding capacity, infertility, high acidity, salinity and hostile temperatures regimes (Moffat & McNeill 1994; Richards, Moorehead & Laing Ltd 1996).
Significant areas of Britain are still affected by both deep mining and opencast mining operations. For example, in 1988 about 4700 ha and 8500 ha, respectively, were affected by these two types of coal extraction in England alone (Department of the Environment 1991a,b). Large areas of commercial forestry plantations established by the Forestry Commission during the 1930s and 1940s in South Wales have been disturbed by opencast mining, and traditionally these have been replanted once mining has finished. Broad (1979) calculated that the total area of the 18 former opencast sites (OCCS) restored to forestry by 1975 was about 820 ha. We estimate that a further eight sites covering an additional 470 ha were restored to forestry between 1975 and 1988, bringing the total area to just under 1300 ha. The greatest concentration of former opencast sites is located in and around the upper Neath valley in the central northern part of the South Wales coalfield.
Between 1942 and 1980, little effort was made to conserve the original soil cover on the majority of sites in the region operated by British Coal (Grimshaw 1992). Any soil that was salvaged was devoted to agricultural restoration, while re-afforestation was based on minespoils composed of crushed overburden strata. These mixtures of shales, mudstones and sandstones have been used as ‘soil-forming’ materials over most of the area restored to forestry.
The growth of the four principal species planted on these sites, Pinus contorta var. latifolia S. Wats. (lodgepole pine), Pinus nigra var. maritima (Ait.) Melville. (Corsican pine), Pinus sylvestris L. (Scots pine) and Larix leptolepis (Siebold & Zuccarini) Gordon. (Japanese larch), has always been characterized by extreme heterogeneity. Trees on some sites are of acceptable size and habit, but the poorest are small and stunted and it is unlikely that they will ever form a commercial crop.
Research to investigate the causes of poor tree growth on restored ground in the late 1950s took the form of species trials (White 1959). The emphasis moved to studies of mineral nutrition in the 1960s (Mayhead, Broad & Marsh 1974), and in the 1970s towards considering cultivation and landform (Broad 1979). Unfortunately this research relied heavily on simple measurements of tree growth rate rather than on quantitative measurements of minespoil properties themselves. In May 1988, the British Coal Opencast Executive appointed the Forestry Commission Research Agency to perform a detailed study of the site factors affecting tree growth on the restored ground in South Wales.