Bioclimatology and Phytogeography of the Red Sea and Aden Gulf Basins: A Monograph (with a Particular Reference to the Highland Evergreen Sclerophylls and Lowland Halophytes)

Abstract

The Red Sea Basin occupies an area of ca. 810,000 km 2 , including essentially the Red Sea coastal plains (210,000 km 2 ) and the highlands bordering them on both sides (600,000 km 2 ). The bioclimate of the lowlands is of the Mediterranean desert type to the north and equatorial desert to the south. These lowlands are among and perhaps the hottest large area of the planet, with a mean annual temperature approximating 30°C and extremes above 55°C, perhaps reaching 60°C in the Dallol and Assale´ depressions. In the highlands the bioclimate is arid Mediterranean to the north, and semiarid to humid equatorial with a bimodal rainfall pattern to the south. The lowlands vegetation is characterized by desert contracted vegetation with many halophyte communities to the north and tropical Acacia-Commiphora savanna in addition to halophytic communities to the south. The highlands vegetation results from vestigial Podocarpus gracilior and Juniperus procera (J. excelsa subsp.polycarpos) forests. Podocarpus, however, is restricted to the moistest places in Ethiopian highlands. These forests are degraded into various stages of evergreen sclerophyll woodland, bushland, and shrubland, physiognomically reminiscent of, and botanically akin to Mediterranean shrublands (garrigue, maquis, etc.). These in turn may evolve towards montane acacia-grassland (Acacia abyssinica,A. negrii, A. origena, A. pachyceras, A. xiphocarpa, Faidherbia albida, Andropogon spp., Eleusine jaegeri, Hyparrhenia spp., Pennisetum schimperi, P. villosum), succession patterns. These patterns are almost similar in botanical composition and structure on both sides of the Red Sea as mentioned by many authors over a couple of centuries. This leads to the definition of a phytogeographic entity labeled either as the Eritreo-Arabian Domain or Somalo-Arabian Domain, which is a part of the Somalia-Masaï Province and center of endemism, and thus of the Sudano-Deccanian Kingdom. The transition between the Sudano-Zambesian region of Intertropical Africa and the Deccanian Regions of South Asia, albeit fairly discreet, is quite clear on several accounts. On the other hand, Mediterranean and related taxa are over 500 species in SW Arabia highlands. The Somalo-Arabian phytogeographic entity thus appears as resulting from the confluence of five floristic regions: Sudano-Zambesian, Deccanian, Saharo-Arabian, Mediterranean, and Irano-Turanian. The total flora is ca. 5,000 species; nearly 1,500 of which are endemic, one endemic family and a dozen endemic genera. Particular reference is made to the evergreen sclerophyll flora and vegetation characteristic of the highlands and to the halophytic communities characterizing the lowlands, including mangroves.