Northern Tihama

Unlike the southern Red Sea Islands, northern Red Sea Islands (Al Wajh Bank) are devoid of any dense vegetation. Majority of these islands are sandy while others are rocky with small hillocks of 5m high or less. Vegetation is almost nil in most of the islands except in a few where salt bushes such as Arthrocnemum macrostachyum, Halocnemum strobilaceum, Zygophyllum spp. etc dominate in patches. 

The plains of Northern Tihama are composed of stones and sand with differing soil depths. Vegetation between Jeddah and Yanbu is more or less similar to the that of the plains of southern Tihama. The most dominant among them is the widely spaced Acacia tortilis-Maerua crassifolia association. However, M. crassifolia is in a highly degraded state, mostly seen as heavily grazed and cushion-formed. Other Capparaceae species dominant in the sand dunes and stony plains of southern Tihama, such as Dipterygium glaucum, Cadaba glandulosa are less frequent north of Jeddah. Similarly Commiphora spp., dominant in the foothills of southern Tihama, is almost nil in the northern Tihama. Most of the areas during summer are dry and barren. Dominant species of the plains and foot hills are the scattered A. tortilis intermixed with A. ehrenbergiana.  Abutilon pannosum, Lycium shawii,  Ochradenus baccatus, etc. dominate in shallow silt depressions and Leptadenia pyrtotechnica and Rhazya stricta dominate in sandy hummocks. Common grasses in these areas are Panicum turgidum, Pennisetum divisum and occasionally Lasiurus scindicus and Aristida spp. Coastal areas are mainly dominated by salt bushes composed of chenopods and Zygophyllum spp. Limonium axillare, Arthrocnemum macrostachyum, Halopeplis perfoliata and Zygophyllum coccineum are the most dominant along the northern sabkhas. In general, L. axillare grow in low lying areas or in localized depressions while Z. coccineum dominate in areas covered by coral rocks or coral sands. The areas north of Jeddah the coastal zone is covered by white coral sand supporting dense stands of Suaeda monoica and Anabasis setifera. On the eastern side where the soil is less saline, the vegetation is dominated by Acacia ehrenbergiana, Anastatica heirochuntica, Senna italica and Hyparrhenia hirta.

Sabkhas of the coastal areas of Thuwal are densely vegetated with salt bushes. It supports Zygophyllum coccineum and Aeluropus lagopoides along with Desmostachya bipinnata, Indigofera oblongifolia and Astragalus spinosa in some locations. Sands overlain the crusty salt substratum support a number of annuals such as Sonchus oleraceus, Launaea mucronata, etc.

Trees are generally few in the entire Tihama area. Among the large trees, Acacia raddiana, A. johnwoodii, Capparis decidua, Maerua crassifolia and Hyphaene thebaica,  are prominent. Further north, the coastal zones near Rabigh and south of Yanbu are devoid of any appreciable level of vegetation due to the development of industries and corniches or occasionally with secondary vegetation dominated by Aeluropus lagopoides, Cressa cretica, Halopeplis perfoliata and Zygophyllum coccineum. At places where they form into dense stands sand accumulates at the base of these shrublets and provide habitats for other mesophytes such as  Fagonia indica,  Glinus lotoides, Lotus corniculatus, Picris cyanocarpa, etc. Ruderals and the common invasive species, Prosopis juliflora also dominate around new settlements and along road sides.

Coastal lines of Yanbu are with several creeks, most of which provide shelters for the growth of Avicennia marina. Sea in this area is very shallow. Cymodocea rotundata, the wide leaved sea grass occurs in such waters. 

The coastal zone is bounded on the east by salty flats or small dunes of coral sand. In some places north of Yanbu industrial city, coastal areas are often enclosed by coral rock and drained sand gullies. Saline coasts are dominated by Arthrocnemum macrostachyum, Tamarix aucheriana, Suaeda vermiculata, Sporobolus spicatus, while the sandy soil towards the interiors are dominated by Acacia ehrenbergiana and A. tortilis associated with Taverniaria spartea. During spring, most of the herbaceous flora of the plains and foothills near Wajh and Umlaj are mostly  seen along the drainage channels, alluvial plains and other silt filled depressions. Ravines are dominated by Gymnocarpos decander, Ochradenus baccatus, Zilla spinosa and Aerva javanica. Another striking species in the area is the Retama raetam, usually found in plains close to foothills.

Coastal areas of Duba is dominated by Aeluropus lagopoides, Juncus rigidus and Nitraria retusa. In some places it is seen in close associations with Haloxylon salicornicum, Seidletzia rosmarinus, Suaeda monoica or Zygophyllum coccineum. Open plains, not far away from the coastal zone, the branched palm tree, Hyphaene thebaica is seen intermixed with Acacia ehrenbergiana.  

The extreme northern side near Haql, Tihama is narrow with a rocky terrain. In some places the escarpment is seen very close to the sea shore. Haloxylon salicornicum dominate in these areas along with Acacia tortilis, Fagonia mollis, Capparis decidua and Stipagrostis plumosa. Small water channels and runnels originate along the slopes and reach the sea. During rainy season, silt accumulates in small ditches along the slopes and provide habitats for a diverse herbaceous flora comprised of Aizoon canariense, Arnebia hispidissima, Bassia muricata, Blepharis ciliaris, Caylusea hexagyna, Cucumis prophetarum, Diplotaxis harra, Fagonia bruguieri, Launaea capitata, Trigonella hamosa, Zilla spinosa and Zygophyllum simplex (Tetraena simplex).