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On the basis of 90 releves, rock vegetation and succulent communities were analysed in SW Arabia on a transect between 14 degrees and 16 degrees n. l. to see 1) which communities and water strategy types occur along an altitudinal range from sea level to 2500 m and its climatic gradient? 2) whether the chasmophytic flora mirrors the shift from palaeotropical lowlands to afro-montane phytogeographical relations in mid altitudes’ ? 3) whether the floristic patterns in SW Arabia confirm the observations in other areas that rocks are evolutionary traps, have an outstanding rate of stenochorous species, and shelter palaeo- and neoendemics?

The saxicolous flora is highly diversified. Euphorbia, Kleinia, Caralluma and Aloe have evolved ecological and geographical vicariant species. Euphorbiaceae and Asclepiadaceae are substituted in high altitude by frost tolerant Crassulaceae. The different bioclimatic niches of diagnostic species result in a number of syntaxa, replacing each other in altitude. The Kleinio odorae-Carallumetea penicillatae cl. nov. are characterized by succulent Stapelieae, candelabrous Euphorbias, and by Kleinia and Cissus ssp. Classification results in three clusters (alliances) and a number of associations, some of them new. The Euphorbion cacti occurs in the lower submontane belt, the Euphorbion inarticulatae in the Tihama Foothills. Both alliances grow on lithosols, on run-off sites and in ruderalized habitats. They are embedded as pedoclimax in the climatic climax of open woodlands. The Euphorbion adenensis is the climatic climax in the arid Eastern High Plains. Most of the associations are endemic in SW Arabia. The higher syntaxa are of Eritreo-Arabian distribution and have equivalents at the Horn of Africa.

Rocks in the upper montane zone are colonized by the Centaurothamno maximi-Aeonion leucoblephari. This new alliance typifies the Crassulo albae-Aeonietea cl. nov., a class of Arabo-Afro-montane distribution. Terrace walls and rock fissures shelter small succulents (Dorstenia), poikilohydric ferns (Actiniopteris, Cheilanthes, Asplenium) and spike mosses (Selaginella), e.g. the Cheilantho coriaceae-Actiniopteridetum semiflabellatae (Selaginello yemenensis-Actiniopteridion, Cheilantho-Actiniopteridetalia). This new order has floristic relations with fern communities in the Mediterranean, in SW Australia and in the Andean Cordillera. They can be summarized in a Sinopterido-Aspleniea-class group.

The rock vegetation in SW Arabia confirms 1) the high degree of endemism in the chasmophytic flora, 2) the importance of cliffs a refugium for Palaeoendemics and the role of the rock habitats for allopatric speciation and genetic drift.

Close floristic relations and disjunctions exist between SW Arabia, NE Africa and Macaronesia, to a lesser extent with SW Africa. Common taxa for the class groups Kleinio-Euphorbiea and Crassulo-Aloea are the name-giving genera. These similarities have a phylogeographic background, and the phenomenon of synvicariance is obvious. Clear differences can be stated in the niche of dwarfish succulents: Caralluma and other Stapelieae evolved and radiated in Arabia, the Aeonium clade on the Canary Islands, and Aizoaceae in the Capensis. Succulent vegetation in the New World (class group Agavo-Opuntiea) has a totally different floristic basis.