The Scottish Borders fens : controls on vegetation development and composition

The Scottish Borders fens represent an important group of British fen sites. Despite their small size many of these fens support various rare plant communities and nationally and locally rare plant species as well as a wide range of species characteristic of wetland habitats. This study has demonstrated the importance of management history, site chemical conditions and site size and morphology in the vegetation development of the Scottish Borders fens. 2. Sixty-eight sites were included in a general survey. The vegetation at these sites was systematically recorded. Site features (vegetation rafts, springs, drains), surrounding land-use, gross peat stratigraphy and measurements of pH and electrical conductivity of the fen water were also recorded. A subset of contrasting sites was selected for detailed investigations into site chemical conditions and peat fertility, peat stratigraphy and the development of vegetation rafts. The quadrat data were analysed using a range of multivariate classification procedures. One classification was selected as the basis for the description of Scottish Borders fen plant communities. Twenty-four plant communities and variants are described representing rich-fen, poor-fen, bog, tall herb fen, fen meadow and swamp habitat types. The Scottish Borders fen plant communities were compared to existing comprehensive classifications of British fen vegetation. 4. The impact of marl and peat extraction at each site was investigated using documentary, visual and stratigraphic evidence for disturbance at each site. The status of each site was determined (cutover, partly cutover, un-disturbed) and this was related to the development of the present vegetation. Most of the Scottish Borders fens have been cut for peat or marl to some extent. However at some sites the peat stratigraphic sequences represent un-modified peat development since the late glacial. The efficiency of drains, strength of springs and basin morphology are important factors determining the development of vegetation over former peat and marl cuttings. 5. Site chemical conditions show much variation. The intensity of the surrounding land-use is positively correlated with peat fertility at the edges and water inflows of sites. The main axes of floristic variation correspond to the variation in base-richness of the fen water and fertility. There was no simple relationship between chemical variables and the occurrence of different plant communities. 6. The vegetation has developed as a quaking raft over fluid peat at many sites. Two Sphagnum dominated plant communities are confined to vegetation rafts. Their occurrence is determined by the inundation of the vegetation surface with base-rich water, the depth of the fluid peat beneath the raft and the degree of isolation of the vegetation surface from telluric water input. Where the vegetation has developed as a raft over the entire site the thickest rafts are found in the central areas. The mechanisms of central raft thickening are thought to be influenced by differential fluctuation of the water table relative to the vegetation surface across sites.