The Dynamics and Distribution of some Plant Species on the Keen of Hamar, Shetland A thesis submitted for the degree of Doctor of Philosophy in the University of Stirling Susanna Kay Department of Biological and Molecular Sciences University of Stirling October 1997 Abstract 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. Acknowledgements I thank my supervisor Professor John Proctor for his patient support throughout the Ph.D and Dr. David Slingsby for his interest and enthusiasm. Dr. Roy Sexton interested me in the Cerastium hairs and gave me much help together with Linton Brown and Nicola Gray. I thank Dr. Tim Benton for the statistical advice, Dr. Stephen Carter for advice on soils and Alan Duncan for laboratory assistance. I am grateful for financial support from NERC and from Scottish Natural Heritage (SNH), and to SNH staff for help, particularly Dr. Andrew Dowse and Paul Harvey. Donald Kennedy from the Scottish Wildlife Trust showed me the Beinn Iadain Arenaria site and Trondur Leivsson kindly collected Cerastium seeds from Streymoy. The Muckle F1ugga Shore Station folk were friendly and welcoming, and I am grateful to Laurence Johnson for many lifts and to Wendy Dickson for photographs and seeds. I thank Jane and Sandy McCaughley for their warm hospitality at Little Hamar during my November visits. Friends and colleagues have been a great help, especially Alice with the maps, Cathy with the numbers, Hal for the hair, and Petra for countless things. CONTENTS Chapter 1 General Introduction Site descriptions 2 Climate 7 Study species 9 Non-ultramafic seed sources 15 Chapter 2 Population dynamics of Cerastium and Arenaria on the Keen and on Nikkavord Introduction 17 Long term changes in plant numbers 17 Changes in plant numbers with climate 21 Methods Recording of plants 26 Seed banks 30 Data analysis 31 Results Population flux and density 31 Cohort survivorship 35 Size of plants 40 Aowering and seed production 45 Seed bank 50 Discussion 52 Chapter 3 Seed germination and dispersal of Arenaria and Cerastium Introduction 64 Methods Source of seeds for investigations under controlled conditions 66 Seed viability 67 Seed germination under controlled conditions 67 Seed germination in the field 68 Data analysis 71 Results Seed viability 71 Seed germination under controlled conditions 72 Seed germination in the field 75 Discussion 79 Chapter 4 Leaf trichomes, glandular secretory material and drought tolerance of Cerastium nigrescens and Cerastium arcticum Introduction 84 Methods Trichome investigations 85 Drought experiment 87 Data analysis 89 Results Trichome form and distribution 90 Trichome counts 90 Cell ultrastructure 98 Analysis of glandular trichome secretory material 100 Drought experiment 100 Discussion 102 Chapter 5 Soil and plant analyses Introduction 107 Methods Vegetation description 108 Soil chemical properties and particle size 108 Plant nickel concentrations 110 Results Vegetation description 111 Soil chemical properties and particle size 111 Plant nickel concentrations 115 Discussion 116 Chapter 6 General Discussion Summary of findings 118 Monitoring recommendations 122 Further research 125 References 127 Appendix 1 Weather records and plant numbers, 1978-1995 134 Appendix 2 Location of Cerastium on Sobul 137 Appendix 3 Morphological races on the Keen 138 Chapter 1. Introduction Ultramafic rocks are igneous or metamorphic rocks which are rich in ferromagnesian minerals. Iron and magnesium are always relatively high and silicon relatively low, but other than that they vary in their chemical and mineralogical composition (Proctor & Woodell 1975). In the past, biologists have often used the term 'serpentine' to describe ultramafic rocks, even though the rocks did not contain minerals of the serpentine group. The term ultramafic is used in this thesis. Many ultramafic rocks have undergone a process of hydrothermal alteration (serpentinisation). The textural changes associated with serpentinisation result in different physical weathering of ultramafic rocks, which may also have an important affect on the soils (Proctor & Nagy 1992), although in temperate areas, at least, ultramafic soils generally reflect the chemical composition of the parent rock. Ultramafic rocks generally have a vegetation which contrasts with that of the surrounding areas. Although, in Britain, the vegetation is not as distinctive as that reported for many other countries, several of the British sites have an open vegetation of low stature and are the habitat for rare taxa (Proctor 1992). Some sites have quite large areas of skeletal soils e.g. the Keen of Hamar, Shetland (the main study site in this thesis), Meikle Kilrannoch, Angus, and the Hallival/Askival area on the island of Rum. There have been several explanations for the low vegetation cover: low soil nutrient concentrations, high soil MglCa quotients, high soil nickel concentrations, adverse physical factors, or a combination of two or more of these factors (Proctor 1992). The Keen of Hamar is one of the more striking ultramafic sites in Britain (Proctor & Woodell 1975), having large expanses (about 22 ha) of poorly vegetated ochre-brown skeletal soils and these open soils are the habitat for many rare plants e.g. Arabis petraea, Arenaria norvegica subsp. norvegica and Cerastium nigrescens, and also unusual races (with no formal taxonomic status) of more common species e.g. Rubus saxatilis, Plantago maritima and Rumex acetosa. Monitoring of some of the rare species from 1978-1993 (Slingsby et af. 1993) showed that there had been fluctuations in their numbers. The main purpose of this thesis was to investigate the population dynamics of two ofthe rare species, Cerastium nigrescens and Arenaria norvegica subsp. norvegica growing on the Keen and on the neighbouring Nikkavord ultramafic outcrop, which has much smaller areas of open soils. A second purpose of the thesis was to investigate some autecological aspects of Cerastium nigrescens in relation to its drought tolerance and the occurrence and possible function of its glandular hairs. Finally a subsidiary purpose was to shed further light on the role of nickel on the Keen in view of an earlier demonstration (Carter et af. 1987 a) of a very local, fully vegetated, nickel-rich area on its northern slopes. The names of the vascular plants follow Stace (1991) and the bryophytes follow Watson (1981). Site Descriptions The Keen of Hamar The Keen of Hamar, Unst, Shetland (National Grid reference HP 645098), is a hill of ultramafic rock rising to 89 m above sea level (Figs. I. I and 1.2). It is made up of dunite with some pyroxenite on the eastern part of the site (Amin 1954). The Keen comprises two parts, the Eastern Keen (a NNR of30 ha) and the Western Keen (a SSSI of 11 ha) (Fig. 1.3) which are divided by a cattle corridor between areas of pasture. The Western Keen had been used for laying out winter feed for stock, prior to being fenced off in 1986, and now grazing is permitted at very low levels (0.4 sheep per ha), but this option is not currently taken up (Harvey 1997). The whole site had been grazed by fann animals in the past, but there has been no grazing on the Eastern Keen since the early 1970's (Harvey 1997). The Keen had been one of the major areas in the Britain for chromite mining in the nineteenth century (Rivington 1953), and the disused quarries are now full of water. 2 Fig. 1.1 Thc Kccn or Hamar , hll\\'lng L'X1CnSI\'C <lrL:a~ or roody vcgcl<lICU ,oil s. N UNST f."l 0" i ,~'" SHETLAND ISLANDS o SkIn Fig. 1.2 The location of the Keen of Hamar, Nikkavord and the Baltasound weather station on Unst. ~ultramafic areas on Unst and Fetlar. 4 N r Eastern Kccn Western Keen I :.n o 200m L- ______-11 Fig. 1.3 Map of the Keen showing the location of skeletal soils. ~ sparsely colonised soils, c=J moderately colonised soils, r\+t~t+1 well colonised soils. Most of the neighbouring ultramafic outcrops on Unst support a sedge-grass heath which corresponds to the Carex jlacca-Carex pulicaris-Festuca sociation of Spence (1970) and the Antennaria dioica­ Carex pulicaris Association of Birse (1982).