|Dengler, J; Löbel, S; Boch, S: Dry grassland communities of shallow, skeletal soils (Sedo-Scleranthenea) in northern Europe., Tuexenia, 26, 159-190 (2006)|
|Key words: Alvar vegetation, Alysso-Sedetalia, Öland (Sweden), Saaremaa (Estonia), syntaxonomy, species richness, Tortello tortuosae-Sedion albi|
We studied the dry grasslands of shallow, skeletal soils (Sedo-Scleranthenea, Koelerio-Corynephore- tea) in northern Europe, based on a combination of new relevés from southern Öland (Sweden, n = 182) and Saaremaa (Estonia, n = 73) as well as a comprehensive evaluation of literature data, of which 65 suit- able relevés were directly included in our analyses. Apart from a few vague indications of acidophytic Sedo-Scleranthenea communities (order Sedo-Scleranthetalia), all data refer to basiphytic communities (Alysso-Sedetalia); our analyses are thus focussed on the latter. The Nordic Alysso-Sedetalia communi- ties proved to be quite different from their temperate counterparts and thus are included in a separate alliance, Tortello tortuosae-Sedion albi, which forms the northern counterpart to the central European Alysso-Sedion. Within the northern alliance, we distinguish two suballiances. The more widespread cen- tral suballiance Tortello tortuosae-Sedenion albi inhabits different types of base-rich substrata in both natural and anthropogenic sites, and is comprised of the Cladonio symphicarpiae-Sedetum albi and the Ditricho flexicaulis-Sedetum acris. The second suballiance Tortello rigentis-Helianthemenion oelandici is restricted to the alvar sites (= treeless limestone plateaus) in Öland, Gotland, Västergötland and Estonia. It is characterised by several endemic taxa and a large number of cryptogams typical of alvar. It is com- prised of four associations, Crepido pumilae-Allietum alvarensis, Fulgensio bracteatae-Poetum alpinae, Helianthemo oelandici-Galietum oelandici and Gypsophilo fastigiatae-Globularietum vulgaris. All six Nordic associations are described in detail with respect to their floristic composition, ecology, distribu- tion and lower-ranked units, and each is represented by a vegetation table. The floristic differences within the Nordic communities are worked out in a synoptic table. Whereas several vegetation scientists have pointed out that vegetation types occurring at the limits of their distribution ranges in northern Europe are generally difficult to classify, our application of the Braun-Blanquet approach, which is based on a priori separated structural types and the general application of the central syntaxon concept, has enabled us to characterise and adequately define all Nordic communities. The Tortello-Sedion asso- ciations are two to three times as species-rich as those of the Alysso-Sedion and are among the most diverse small-scale plant communities ever described. We discuss the reasons for this exceptionally high plant diversity and the peculiar species mixture in the Tortello-Sedion and compare the relationship between Alysso-Sedion and Tortello-Sedion to the situation of other Nordic syntaxa of predominantly temperate vegetation types. Our results further underline the uniqueness of Baltic alvars and their para- mount importance for conservation at the European level.