Recreation of lowland heathland on ex-arable land: assessing the limiting processes on two sites with contrasting soil fertility and pH.
European heathlands are of high conservation value but have declined as a result of agricultural intensification. Heathland recreation is currently being undertaken on ex-arable land throughout north-west Europe to reverse these losses, but seed limitation, elevated soil pH and fertility and competition from ruderal species have been shown to limit community re-assembly. We examined the relative importance of these constraints over 9 years (1994-2003) on two ex-arable sites with contrasting soil fertility and pH. Recreation treatments comprised seed addition (including Calluna vulgaris) combined with (i) applications of elemental sulphur (S) at two rates, to reduce the pH to 4.5 and 5.5, respectively; (ii) sowing a nurse crop, to facilitate the establishment of sown species; and (iii) topsoil removal, to deplete nutrients. The importance of sowing time was assessed by (iv) comparing autumn with spring seed addition. By 2003 the pH of the S-addition plots had stabilized close to pH 4.5 and 5.5, but levels of extractable phosphorus (P) had increased dramatically at both sites. Topsoil removal reduced soil P and organic matter but increased pH because of exposure of underlying mineral horizons. These conditions were unsuitable for the establishment of Calluna vulgaris. Applications of S (3-6 t S ha-1) were sufficient to recreate Calluna vulgaris-dominated heathland at the less productive site. Acid grassland developed on all other treatments, including S-amended plots at the more productive site. The establishment of desirable species was not significantly enhanced by sowing seed mixtures with a nurse crop or in the spring, whereas S addition and topsoil removal reduced the abundance of potentially competitive species. Synthesis and applications. The application of S can be used to create conditions suitable for the re-assembly of Calluna vulgaris-dominated heathland on unproductive ex-arable soils in north-west Europe. However, interventionist approaches such as S addition may not be practical for large-scale heathland recreation schemes. Competition with more nutrient-demanding species will limit the establishment of slow-growing heathland species under more fertile conditions, even where the pH has been reduced to appropriate levels. Therefore the recreation of acid grassland by seed addition alone is likely to be a more realistic target for the majority of agricultural sites included in agri-environment schemes in north-west Europe.