This alliance includes vegetation of outcrops of a variety of different rock types, with the exception of strongly calcareous rocks and serpentine. Of the species confined to rock crevices, there are specialists of acidic substrata (e.g. Asplenium septentrionale. A. trichomanes subsp. trichomanes and Woodsia ilvensis) as well as species tolerant of a broad range of substrate pH (e.g. Cystopteris fragilis and Polypodium vulgare). The suite of other species mainly depends on the vegetation types occurring in the surroundings of the outcrops and the degree of shading, which can vary from fully exposed to deeply shaded under a forest canopy.