Soil Factors Affecting the Distribution of Plant Communities on the Cliffs of Craig Breidden, Montgomeryshire

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In certain habitats topographical factors have prevented closed plant communities from developing, and often, where the parent rock is basic in nature, a particularly rich flora is present (Pigott & Walters 1954). Cliffs and steep, rocky outcrops of limestone or basic igneous rock provide such habitats; examples include the Avon Gorge (Bristol), the Great Orme (Denbighshire), Arthur's Seat (Edinburgh), Stanner Rock (Radnorshire) and Craig Breidden (Montgomeryshire). The last is an outcrop of basic igneous rock, which possesses a flora that is unique in the British Isles. This comprises not only species that are rare or restricted, e.g. Potentilla rupestrist, Lychnis viscaria (Viscaria vulgaris), Veronica spicata ssp. hybrida, Hieracium peleterianum (Pilosella peleterana), Sedum forsteranum, Geranium sanguineum, but also assemblages of plants that are unusual, for example calcicoles (Origanum vulgare and Scabiosa columbaria) regenerating and growing in close association with calcifuges (Calluna vulgaris and Erica cinerea). Craig Breidden (Nat. Grid Ref. SJ 2914) lies in the extreme north-eastern part of Montgomeryshire in the angle created by the foothills of the Welsh Massif and the Shropshire Hills. The rock mass is a large laccolith of hypersthene dolerite which was intruded into shales during the Ordovician era. Glaciation has removed the surrounding softer rocks and left the hard dolerite as an oversteepened, craggy outcrop rising to a height of 365 m, with extensive cliffs on the southern, western and northern sides of the hill, and it is on the southern and western cliffs (at an altitude of 150-300 m) that the rare species and assemblages of species are found. Because of its geographic position Craig Breidden experiences an unusual climate, being shadowed by the Welsh hills to the west, but receiving more rain than the neighbouring lowland areas of the Shropshire Plain to the east. Furthermore, cliff habitats give rise to heterogeneous microclimatic conditions which probably contribute to the maintenance of the vegetation mosaic. Whilst the importance of climatic conditions was recognized, the following study was undertaken in order to identify soil factors influencing the distribution of plants on Craig Breidden. Details of the mineral nutrition of one of the rare species, Lychnis viscaria, have been discussed elsewhere (Jarvis & Pigott 1973), and in this account particular attention is given to the mixing of calcicoles and calcifuges. There are a number of accounts of such mixtures, but most of these accounts