[EN] A phytosociological study was made on the ephemeral limestone grassland of the "orensano-sanabriense" chorological sector. Diversity and developement optimus were found in the grasslands growing in the bordering areas between Galicia and El Bierzo (NW Spain). The floristic composition of this comunities was correlated to succesional factors and soil characteristics. A new phytosociological subassociation: Sax?frago tridactylites-Hornungietum petraeae Izco 1974 subas, galietosum muralis nova, was described to show the affinities with southern mesomediterranean areas.

[ES] Se hace un estudio fitosociol?gico de los pastizales ef?meros calc?colas orensano-sanabrienses, cuyo ?ptimo de diversidad y desarrollo se localiza en el ?rea lim?trofe entre Galicia y El Bierzo (NW Espa?a). La composici?n flor?stica de estas comunidades se correlaciona con caracteres din?micos y ed?ficos. Fitosociologicamente las incluimos en una nueva subasociaci?n: Sax?frago tridactylites-Hornungietum petraeae Izco 1974 subas, galietosum muralis nova, que refleja sus afinidades con las de territorios mesomediterr?neos m?s meridionales.