Location and General Description
The most remarkable feature of this ecoregion is the luxuriant vegetation and dense woodland supported by the coastal fogs, in direct contrast to the adjacent desert interior. The heavy fogs on Oman’s Jiddat al Harasis stony plateau can limit visibility to less than 10m; on occasion, only the sound of footsteps made by nearby oryx (Oryx leucoryx) and gazelle (Gazella gazella) through the gloom and complete silence.
This ecoregion covers much of the western and eastern coasts of the Arabian Peninsula. In Oman, it starts close to the Bar al Hikman, opposite Masirah island, and extends southwards. It reaches up to 120 km inland over the stony plateau of the Jiddat al Harasis, home to the re-introduced Oryx leucoryx and other wildlife of the White Oryx Sanctuary. Continuing southwards it covers the escarpment mountains of Dhofar, consisting of three ranges that together form a chain 290 km in length: Jebel Qamar in the west; Jebel Qara in the centre; and Jebel Samhan in the east. Thereafter, across the border into Yemen, the region runs along more steep escarpments, followed by coastal plain as far as Aden and beyond. It then turns north to follow the Tihamah coastal plain to Jeddah in Saudi Arabia.
The east coasts of Oman and Yemen receive dense fogs influenced by the southwest monsoon, whereas the Tihamah coastal plain receives little or no fog but can record high levels of humidity.
Various climatic and topographic factors are responsible for fog formation, which in turn supports a varied fauna and flora in this ecoregion. On Oman’s Jiddat al Harasis, fogs are particularly prevalent between March and October, when the prevailing wind comes from due south off the Indian Ocean. Inland fogs form as the sea breeze causes a rapid drop in air temperature while relative humidity increases rapidly. As the breeze drops below 8 knots, the moist cool air condenses and forms a fog bank at ground level (Stanley Price et al. 1988). The fogs make an important moisture contribution in the Jiddat al Harasis area, where the average annual rainfall is less than 50mm. The high density of trees, particularly Acacia tortilis, but also A. ehrenbergiana and Prosopis cineraria, ends abruptly, probably reflecting the maximum inland extent of fog moisture (Stanley Price 1989).
The escarpment mountains of Dhofar, south of the Jiddat al Harasis, are mainly of lower Tertiary and Cretaceous limestone overlying a basement complex of Pre-Cambrian rocks. For three months of the year, from mid-June to mid-September, the mountains are under the influence of the southwest monsoon. During this period, an upwelling of cold water off the Dhofar coast rapidly cools the moist winds to dew-point. This causes dense fog to build up against the seaward facing escarpments, condensing on vegetation or forming drizzle or rain. The fogs disappear once they pass over the summit ridges. At other times of the year, cyclonic storms bring more rain. The mean annual rainfall is 100 mm on the coast at Salalah and between 200-500 mm on the mountains (Sale 1980). The quantities of fog precipitation are among the highest for any part of the world (Miller 1994).
A zonal vegetation pattern is evident from the coastal plains to the desert in the north (Miller & Morris 1988). Dense deciduous woodland dominated by the regional endemic tree Anogeissus dhofarica, together with Acacia senegal and Commiphora spp., occurs on seaward-facing slopes. A semi-deciduous thicket replaces this formation at higher altitudes, comprised mainly of Olea europaea, Dodonaea viscosa, Carissa edulis and Rhus somalensis. Summit plateaus are covered by grassland and scattered trees of Ficus vasta, F.sycomorus and patches of evergreen thicket. As the influence of fog decreases further north, grassland is replaced by low succulent shrubland dominated by Euphorbia balsamifera. Beyond this, the vegetation becomes sparser and finally gives way to open desert (Miller 1994).
The escarpment continues southwest of Dhofar, across the Oman-Yemen border and along the Yemeni coastline for 30 km, extending inland for approximately 5 km (Evans 1994). There is little or no coastal plain as the escarpment descends directly down to the sea. It continues to receive the southwest monsoon mists and rain, which are responsible for the dense woodland vegetation growing on the hillsides. Further to the southwest, the escarpments give rise to coastal hills, sandy wadis (water courses) and gravel plains. Near Aden, there is an extensive Acacia savannah plain which may be the most extensive and intact woodland savannah in Yemen (Evans 1994).
The Tihamah coastal plain of Yemen and Saudi Arabia is up to 50 km wide. It is hot year round, with a mean annual temperature of 30.20C. Mean annual rainfall is 26.7mm, with most rainfall occurring in winter and spring (Hegazy et al. 1998). The area can record high levels of humidity but no coastal fogs. The plain comprises two main habitat types. First, along the coast are sandy beaches, sabkah and intertidal mudflats and areas of mangrove (Avicennia marina). Offshore there are more than 25 islands. The second habitat occurs inland and consists of sandy wadis and stony plains with halophytic and xerophytic plant associations and open woodland of Commiphora and Acacia. Other common trees are doum palms (Hyphaene thebaica), and tamarisks (Tamarix spp).
The Arabian Oryx Sanctuary, covering c. 25,000 km2, is a world heritage site made famous for the successful re-introduction of Oryx leucoryx (Stanley Price 1989). It supports a diverse wildlife community and contains the largest population in the Arabian Peninsula of Arabian gazelle (Gazella gazella ssp. cora). Also present are Arabian sand gazelle (Gazella subgutturosa ssp. marica), the Arabian wolf (Canis lupus ssp. arabs), caracal (Caracal caracal), honey badger (Mellivora capensis), Ruppell’s sand fox (Vulpes rueppelli) and the red fox (Vulpes vulpes). The Sanctuary’s Huqf escarpment contains one of the main populations in Oman of Nubian ibex (Capra ibex ssp. nubiana).
The Jebel Samhan in Oman’s Dhofar region, which is subject to little human disturbance, is thought to contain Arabia’s largest population of Arabian leopard (Panthera pardus ssp. nimr), possibly its last stronghold in the wild (Spalton 1999). It is listed on the IUCN Red List of threatened animals as critically endangered (Baillie & Groombridge 1996; IUCN 2001). The area also contains a unique assemblage of larger mammals, including Blanford’s fox (Vulpes cana), genet (Genetta felina grantii), hedgehog (Paraechinus aethiopicus), Capra ibex ssp. nubiana, Cape hare (Lepus capensis), Mellivora capensis ssp. pumilio, striped hyaena (Hyaena hyaena ssp. sultana) and wildcat (Felis sylvestris ssp. gordoni) (Spalton 1999).
The fog-affected escarpments of Dhofar have the highest number of endemic species and some of the most species-rich habitats in Oman. About 900 vascular plants have been recorded from the fog oasis of Oman and Yemen, of which c.60 are endemic, including two endemic genera, Cibirhiza and Dhofaria (Miller & Morris 1988). Many of the plants have important medicinal or economic properties, the most famous of which is the frankincense tree (Boswellia sacra), which in the past brought great wealth to the area.
The volcanic massif of Jebel Areys, 150 km east of Aden on the southern coast of Yemen, is frequently shrouded in fog. Its upper seaward-facing slopes are covered with a succulent shrubland dominated by Euphorbia balsamifera. The 1700 km2 area contains 15 endemics, including the extraordinary leafless succulent Kleinia deflersii (Boulos et al. 1994).
In Oman, the Jebel Samhan of Dhofar was declared a National Nature Reserve by Royal Decree in June 1997, mainly due to its leopard population. The Arabian Oryx Sanctuary provides protection for Oryx leucoryx and other wildlife. However, poaching reduced the re-introduced oryx from over 400 in 1996 to 136 by January 1999 (Spalton et al. 1999). Several nature sanctuaries also exist on the Salalah coast. The Ministry of Regional Municipalities and Environment, through its Directorate of Nature Protectorates, designates protected areas and administers them together with the Office of the Adviser for Conservation of the Environment, Diwan of Royal Court.
In Yemen, the National Environment Council is responsible for nature conservation. Eighteen Important Bird Areas (IBAs), including islands just offshore, were identified along Yemen’s coastline (Evans 1994) but at the time of the survey none of the sites were legally protected for nature conservation purposes. There is little current information on the status of protected areas in Yemen or the fog-affected woodland on the country’s south coast. Along Saudi Arabia’s Tihamah plain and on its offshore islands, 11 IBAs have been identified. One of these is the two islands of Umm al-Qamari, officially protected as a Special Nature Reserve by Saudi Arabia’s National Commission for Wildlife Conservation and Development (NCWCD) (Evans 1994). An additional stretch of coastal plain is protected as part of the Asir National Park (Al Sayed 1984).
Types and Severity of Threats
The main threat to vegetation is from overgrazing by goats, camels, and cattle owned by tribal pastoralists. In Dhofar, socio-economic changes have brought improved veterinary services, cheap fodder and increased water availability though the sinking of boreholes, all of which have led to larger herds and greater grazing pressure. The cutting of wood for fodder, timber, and firewood is also a serious problem. Sale (1980) highlights the severe impacts of off-road driving on soils and vegetation on the Dhofar coastal plain and mountains. Other pressures in the Dhofar region relate to the increasing human population and associated increase in roads, housing and other development.
Justification of Ecoregion Delineation
This ecoregion encompasses the coastal tropical desert areas of southern Arabia characterized by thick summer fogs. The boundaries were defined using Zohary’s (1973) geobotanical map of the Middle East. They correspond to Zohary’s classified regions of Haloxylotea salicornici and Suaedetea deserti, with stands of Acacietea tortilis sub-sudanica scattered in lowland depressions.
Prepared by: Robert Llewellyn-Smith
Reviewed by: In process