Limestone Heaths in South-West Britain: Their Soils and the Maintenance of their Calcicole-Calcifuge Mixtures

(1) Limestone heath is here defined as an intimate mixture of calcicoles and calcifuges growing on a uniform, shallow calcareous soil less acid than pH 6. The only examples to have survived agricultural destruction are on slopes steeper than about 200. The soils are derived from unglaciated Carboniferous Limestone plus some periglacial loess. The adjacent plateaux are mantled in a deeper, more acid soil (pH about 5) derived entirely from loess. (2) Limestone heath vegetation is transitional in species composition between the Alliance Mesobromion of the Class Festuco-Brometea and the Agrosto-Ulicetum of the Class Nardo-Callunetea. It is maintained by a combination of burning and grazing which constantly rejuvenates the calcifuges, many of which rarely reproduce by seed. The calcifuge component may have survived the erosion of a deep loessal soil from the steeper slopes. There are strong affinities between limestone heath and the oceanic heaths of west Britain which also often occur on loessal soil. (3) Chalk heath differs from limestone heath in being confined to acid soils of pH 5-6 on plateaux and shallow slopes. It is usually absent from steep slopes because the softness, porosity and high rate of dissolution of chalk have permitted erosion of the loess mantle and development of a very high carbonate content in the soils, giving a soil pH which often exceeds pH 7 (the upper limit for calcifuge survival even in high rainfall areas).