Soil and Vegetation of the Keen of Hamar Serpentine, Shetland

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Abstract
(1) The Keen of Hamar, Shetland, is an important lowland British serpentine site which has many plant rarities and extensive areas of sparsely colonized rock debris. (2) The site has been subjected to various forms of human disturbance but these do not account for the extent of the debris. (3) The vegetation was recorded from 205 regularly positioned 1 m .times. 1 m quadrats and was classified by TWINSPAN into nine groups. Groups I, II, IV and V were closed vegetation of pasture and heath types; Groups VI to IX were open debris type vegetation; Group III had intermediate characteristics between the closed and open vegetation groups. (4) Climatic data for the years 1959-83 for a meteorological station near the Keen of Hamar are summarized. Although the climate is generally cool and moist, there are dry spells in summer and these could influence the open debris vegetation. (5) Field descriptions of the soils and thin-section analyses of them have shown first that some soils are derived from serpentine drift material and others from bedrock, and secondly that the drift soils were formerly more widespread on the Keen but are now restricted to the lower slopes. Vegetation groups I, II, IV and V occur on drift-derived soils: VI-IX on bedrock-derived soils; and III on both. The open debris vegetation is maintained because colonization of the debris is retarded. (6) Chemical analyses were made of soils from the Keen of Hamar from each of the 205 quadrats sampled for vegetation and from other Shetland serpentine areas. The soils were usually mildly acid, had relatively high concentrations of major nutrients, and moderately high Mg/Ca quotients and nickel concentrations. No chemical characters were observed which might account for the unusual debris vegetation of the Keen of Hamar. (7) The unusual weathering of the dunite serpentine rock produces shallow coarse-textured skeletal soils. The physical nature of these soils and absence of groundwater are likely to result in frequent drought for plants and this factor is emphasized as being an important likely cause of retarded colonization of debris.