Rocks and outcrops colonized by pioneer communities, especially of Crassulaceae. The substrates are mostly siliceous, occurring in the alpine or montane levels of higher mountains of the nemoral zone. The communities are dominated by succulent Sempervivum arachnoideum ssp. arachnoideum, Sempervivum arachnoideum ssp. tomentosum, Sempervivum montanum ssp. montanum, Sempervivum montanum ssp. stiriacum, Sempervivum wulfenii, Jovibarba arenaria, Sedum montanum, Sedum anglicum ssp. pyrenaicum, Sedum sexangulare, Sedum album, Sedum annuum, Saxifraga aspera, accompanied by Silene rupestris, Scleranthus polycarpos, Veronica fruticans, Thymus praecox ssp. polytrichus, Viola tricolor ssp. saxatilis, by small crucifers, lichens and mosses.