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A new concept of classification of petrophytic vegetation, i.e. plant communities on rock outcrops and screes, is proposed for the steppe and forest-steppe belts of the southern Siberian mountains, using the Braun-Blanquet approach and original releves from the Altai and Western Sayan Mts. In these areas with arid continental climate, the species composition of petrophytic vegetation seems to be less differentiated from the other habitats than is usual in Europe or the Far East. The main habitats of petrophytic vegetation include rock crevices, shallow soils on weathered rock outcrops, disturbed screes with herbaceous vegetation, and shrubberies in less disturbed places. In the proposed classification petrophytic vegetation is divided into three phytosociological classes. The vegetation of moderately dry rock crevices is included in the Eurasian class Asplenietea trichomanis (Br.-Bl. in Meier et Br.-Bl. 1934) Oberdorfer 1977 and the alliance Selaginellion sanguinolentae Hilbig 2000. Vegetation of disturbed or strongly drought-stressed rock outcrops and screes is included in the class of central Asian steppes, Cleistogenetea squarrosae Mirkin et al. ex Korotkov et al. 1991. Within this class, vegetation with predominance of succulent plants is assigned to the alliance Sedion hybridiail. nova and xeric rock-crevice vegetation to the alliance Eritrichio pectinati-Selaginellion sanguinolentae all. nova. A distinct type of central Asian petrophytic vegetation is assigned to the class Artemisio santolinifoliae-Berberidetea sibiricae cl. nova, which includes xeric shrubberies on mobile screes and rock outcrops of different lithology. Seven associations, three subassociations and four communities without syntaxonomic rank are documented in an ordered species-by-releve table and briefly described with respect to their phytosociological affinities and ecology. The DCA ordination of the releves was used to demonstrate patterns of floristic differentiation of the higher vegetation units in the Altai and Western Sayan.