The dynamics and distribution of some plant species on the Keen of Hamar, Shetland
Autecological and demographic studies on Cerastium nigrescens and Arenaria norvegica subsp. norvegica on the Keen of Hamar and Nikkavord, two ultramafic outcrops on Unst, are reported. The fluctuations in numbers of the two species on the Keen showed differences within the site but in general were related to low spring rainfall, and to number of day degrees above 5.6 DC. Individuals of the two species were monitored on the Keen from June 1994 to November 1996. Plants of Cerastium showed Deevey type two curves and mature plants had a half life of 3.8 years. Most of the seeds germinated from July to November. Plants of Arenaria showed a Deevey type one curve with high mortality after flowering in the second year. Many Arenaria seedlings were recorded throughout the spring, summer and autumn. Seed bank measurements ranged from 12 - 13 m-2 for Cerastium and from 24 - 43 m-2 for Arenaria. On Nikkavord, Cerastium plants occurred on wetter areas than the Keen plants but showed similar population dynamics to them. Arenaria plants sampled on Nikkavord showed bigger fluctuations in numbers and flowering frequencies than Keen plants. Cerastium seeds were sown on Sobul, an ultramafic outcrop, about 6 km SW of the Keen, where the species did not occur naturally. There was germination and establishment after two years. Pilot studies on the Keen revealed the importance of soil surface microtopography for the establishment of Cerastium and Arenaria. Keen and Nikkavord Cerastium leaves were more densely glandular pubescent than leaves of Faroese Cerastium arcticum. The glands produced fats, pectins and other polysaccharides and may be part of an adaptation to drought. A nickel-rich fully vegetated area on the northern slopes of the Keen suggested that the lower nickel concentrations in the barest soils are not important in retarding successional processes.