Site types, species composition, species richness and ecological gradients of rich fens in the Oulanka region, north-boreal Finland

Aims: To describe the species composition and richness of a northern boreal rich fen case, to study, how Cajan-derian site types and species gradients (composition, richness) elucidate the vegetation variation, and how meas-ured environmental variables verify the proposed site type sequence. Location: Oulanka region, Koillismaa, NE Finland. Methods: We chose 15 mires having rich fens for sampling. In those mires, 87 1 x 4 m releves with four 0.25 m2 small plots for seven Cajanderian site types were sampled for vascular plants, liverworts, mosses, and lichens and environmental variables. The relevancy of the classification was tested by the grouping of rel-eves within the ordination surface, and a hierarchic classification by exclusive species, preferential species, and differential species was performed for the site types and subtypes by the survey of the full vegetation table. Additional compositional gradients were interpreted by the ordination. Results: Coherent grouping of releves by the site types in the ordination refers to, that the Cajanderian classification is a relevant vegetation classifica-tion of boreal rich fens in N Europe. The site types differed in relation to water-table levels, and the composi-tional survey revealed groups such as rich fens proper (flark and lawn fens, upper lawn fens, combination site types) and seasonally dry rich-fen wetlands. Ordination analysis only weakly revealed separate extreme-rich fen variants of rich-fen site types, and this variation was partly associated with the prominent mire expanse to margin compositional gradient. Also, species richness was mainly associated with that gradient. The stability of the water regime was another main gradient by the ordination. Surface-soil pH and conductivity values were highest in Palustriella falcata seepage fens and curiously in Sphagnum warnstorfii rich fens. Conclusion: It may be possible to revise the phytosociological fen-classification of northern alliances, too, based on the site type interpretation.