Studies on the Vegetation of Shetland: I. The Serpentine Debris Vegetation in Unst

Abstract

Areas (total 809 ha.) of sparsely-colonized debris (of fragments about 2 cm in diameter) occur among closed vegetation on serpentine from sea level to 152 m. Solifluction phenomena are noted and areas of associated and unassociated debris distinguished. Serpentine soil has an acetic-soluble nickel content of 90-312 ppm. The higher values occur on associated debris, where chromium content is high and weathering rapid. Agrostis stolonifera from debris has an acetic-soluble nickel content of 90-118 ppm of dry matter but shows no toxic symptoms. Mean annual windspeed at altitude 249 m is 37 km/hour; fencing experiments on Unst show that, while in certain places grazing has more effect than exposure or other factors in controlling the vegetation, the converse applies on debris and adjoining ground. A probable 4-phase prisere on debris is described by means of a percentage cover analysis, anomalies being explained by reference to erosion and secondary succession. A number of species with disjunctive distributions are confined, as exclusive pioneers, to the most open phases on associated debris where percentages of boreal species and chamaephytes is far higher than in closed climax vegetation. Study of erosion and physiognomy of debris vegetation shows that wind and soil instability re-expose areas formerly under closed vegetation and retard colonization on the debris (debris species being specially adapted to this environment). These factors partly account for the low-level occurrence of this fell-field. Areas of unassociated debris result from wind, soil instability and, perhaps, "turf-scalping". High nickel content only produces infertility, in Unst, together with the 1st 2 factors; rapid weathering, and chromium, may also be involved. This combination of factors results in the most extensive areas of debris, of the associated type. The presence of serpentinophytes is likely but unproved; almost all the exclusive species are serpentinicolous relics. The inferility of the debris is not entirely a "serpentine effect" but this serpentine shares high nickel content with some serpentine areas in Scandinavia, Scotland and Central and North America.