We analysed the Northern European dry grassland communities of shallow, skeletal soils over base-rich bedrock (order Alysso-Sedetalia, subclass Sedo-Scleranthenea, class Koelerio-Corynephoretea) and compared them with their Central European counterparts. The study was based on our personal relevés from the southern part of the Baltic island of Öland (Sweden, n = 182) and on relevés from other Northern European regions (n = 65) and Central Europe (n = 127) compiled from the literature. Both cluster analyses and ordinations show a sharp floristic distinction between the Nordic and the Central European communities. We thus propose to establish a separate Nordic alliance Tortello tortuosae-Sedion albi alongside the temperate Alysso-Sedion. The Nordic communities are characterised by a number of taxa that are either endemic or occur widely disjunct from their main distribution ranges. In addition, the cryptogam layer of the Tortello-Sedion communities is generally much better developed than that of the Alysso-Sedion, in terms of both coverage and species richness. Within the Tortello-Sedion, we distinguish two suballiances with a total of six floristically well-defined associations, most of which are newly described: (1) Tortello tortuosae-Sedenion albi as a negatively differentiated suballiance of non-alvar stands, mostly from the Scandinavian mainland (Cladonio sympbycarpae-Sedetum albi, Ditricho flexicaulis-Sedetum acris), and (2) Tortello rigentis-Helianthemenion oelandici comprising the stands of the Nordic alvar areas (Crepido pumilae-Allietum alvarensis, Fulgensio bracteatae-Poetum alpinae, Helianthemo oelandici-Galietum oelandici, Gypsophilo fastigiatae-Globularietum vulgaris). "Alvars", i.e. almost level areas with superficial Ordovician or Silurian limestone that are only sparsely covered by vegetation, are largely restricted to the Baltic islands of Sweden (Öland, Gotland) and to Estonia, and reach their greatest extent in southern Öland. The classification of the four Ölandic communities of the Tortello-Helianthemion is supported by an indirect ordination analysis whose axes clearly represent a gradient in environmental conditions. Soil depth and soil moisture are the most important differentiating factors. The Crepido-Allietum and the Fulgensio-Poetum grow at sites regularly inundated in winter. Whereas the first can be found on slightly deeper soils that are thus heavily affected by frost movement, the second inhabits the most shallow soils of all the communities and also shows the highest coverage of bare rock. The Helianthemo-Galietum and the Gypsophilo-Globularietum grow on more stable, less flooded and slightly deeper soils. The Nordic communities are characterised by extremely high species densities of vascular plants, bryophytes and lichens. With mean total values of 36.0-53.6 species on 4 m ², they exceed their Central European counterparts two- to threefold. With up to 80 species on 4 m ², the Gypsophilo-Globularietum is one of the communities with the highest small-scale species richness documented so far. We discuss how the extraordinary phytodiversity of the Tortello-Sedion communities could have emerged. A large species pool due to a long-standing habitat continuity, high spatio-temporal small-scale heterogeneity, reduced competition, and the small size of the individual plants areprobable causes.