Halophytic vegetation along the Arabian coast azonal or linked to climatic zones?
After an introduction into the climatic differentiation of the Arabian Peninsula, the coastal plant communities - often regarded as azonal - are studied in order to find out, how the gradient from the extratropical to the subtropical climate and the transition from the holarctic to the palaeotropic kingdom is expressed by the halophytic vegetation. The available data from coastal-inland transects are handled like sigma-relevés. The associations and the vegetation complexes are derived from the floristic data set in an indirect way. Many taxa of the Arabian coastal vegetation, for example within some genera of the Chenopodiaceae-family and also in Zygophyllum, show a pronounced regional pattern. Saharo-arabian and tropical geoelements predominate. The extra-tropical elements (mediterranean and irano-turanian) are restricted to parts north of the Tropic of Cancer. The climazonal linkage of keystone species results in similar synchorological patterns of many plant communities. The separation is as clear as in the glycophytic vegetation types and the limits are congruent. Thermic and hygric parameters are effective. Only the two hygrohalophytic associations Avicennietum marinae (mangrove) and Halopeplidetum perfoliatae (periodically inundated interdune salt marsh) are of nubo-circumarabian distribution. They characterize the Arabian coastal vegetation mosaic in general. A clear climagenetic differentiation is visible in the coastal dune and salt-marsh vegetation. Three types of catenal vegetation mosaics (geosigmeta) can be distinguished: 1. The Zygophyllum qatarense-geosigmetum along the eastern coasts with a northern subtype at the Persian Gulf and a southern one at the Gulf of Oman: The first one is differentiated by Halocnemion strobilacei communities. This area gets winter rain and is not frost-free. The Sphaerocometum aucheri, a dune community with omano-macranian distribution, characterizes the southern subtype. Its synareal is frost-free and has a tendency to summer rain. 2. The Odyssea mucronata-geosigmetum: The Odysseo-Suaedetum fruticosae and the Aeluropo-Suaedetum are typical units within the coastal vegetation mosaic in the southwestern corner of the Arabian Peninsula. The northern fringes of the Indian monsoons cause a summer maximum of precipitation there. Like in the glycophytic vegetation, close phytogeographical relations to the Horn of Africa can also be stated for the halophytic flora. 3. The Zygophyllum album-geosigmetum is of nubian distribution (along both coasts of the northern Red Sea and the Gulf of Aqaba): Its character communities belong to the alliances Zygophyllion albi, Halocnemion strobilacei and Nitrarion retusae. Finally, some general synchorological features of aktobiomes (= coastal habitats) are discussed. Strong parallels (vicarious taxa und syntaxa, identical formations with corresponding species) and a similar latitudinal sequence can be stated for Arabia and Lower California. The azonal character becomes less and less evident, when passing from the hydro- to the geolittoral.