| Summary: | The aim of this thesis is to investigate the factors controlling the spread and distribution of bracken (Pteridium aquilinum) in Scotland. Bracken has long been an agricultural problem and, more recently, fears have been raised about the plant's carcinogenic properties. Despite the seriousness of the problem, there has been little quantitative research into factors controlling the plant's vigour, although there are many anecdotal references on the subject. It is the aim of this thesis to; study the climatic, edaphic and biotic characteristics of the bracken zone; establish the statistical relationship between these factors and bracken vigour and to apply the findings to explain how these factors affect bracken in Scotland. The climatic, soil, vegetation and biotic characteristics of four sites in the bracken zone (west, south-west, north-east and south-east) are reviewed first. By comparisons of inter-site factors and bracken vigour at the sites, it is possible to formulate hypotheses on the factors that control bracken vigour. Correlation and regression analyses of individual factors with frond height, density and litter depth are than carried out, followed by a Stepwise Regression Analysis. Finally the findings of the two sections are summarised and the results applied to explain bracken distribution and spread in Scotland. The major conclusions of this thesis can be summarised as follows: Early season temperature strongly affects bracken vigour and largely accounts for the east-west difference in bracken vigour. Vigorous bracken in the west can withstand a greater degree of frosting than the less vigorous bracken in the east. Soil moisture stress, heavy frost and relatively intensive agriculture in the east results in a higher bracken zone (and therefore suboptimal soils and temperatures) in the east than in the west.
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