| Summary: | The flora of Upper Teesdale contains a large number of "relict" species of disjunct geographical distribution. 'Why is this rich assemblage of species present in Teesdale. It was assumed that arctic alpine and other rare species, supposedly intolerant of competition, were able to survive in the face of lowered competition from typically lowland species. This study was designed to test this hypothesis by answering three questions: (i) What are the communities? (ii) What is their productivity (as a general measure of competition)? (iii) What are the reasons for this level of production? Initial analyses on Widdybank Fell indicated the importance of Limes convergens situations, sharp zones of biotic and abiotic transition, as key habitats for many of the rarities. Detailed phytosociological analyses of a grid matrix on Widdybank Fell, produced an optimum classification of vegetation units. These communities contained many rare species and the affinities of these units with continental phytosociologica groups was considered. The distribution of the quadrats of these communities on the grid was compared with abiotic variables. The productivity, in this context used as a general measure of competition, of those communities delimited in the phytosociological analyses and other sites was determined. A single species increment cropping technique was developed to determine the net annual aerial standing crop of the communities considered. A composite picture of community dynamics in terms of vertical biomass distribution, net annual aerial production and mineral flux was obtained. The rare species were found to be most common in the communities studied that were of low net annual aerial production, particularly those of less than 150 gm/m(^2)/per annum. The importance of grazing and climate in maintaining these low levels of production was emphasised by exclosure and cold frame experiments. Chemical analysis indicated the importance of the highly calcareous sugar limestone soils and indicated that high zinc levels appeared to coincide with low production.
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