Socotran Coastal Vegetation
The mangrove Avicennia marina occurs as local narrow belts, while on the salt-flats the salt-grass Heleochlora dura predominates together with other halophytes such the endemic Atriplex stocksii forma sokatrana (Chenopodiaceae) and species of Limonium. Other endemics or near endemics tolerant of these saline conditions include Fagonia socotrana (Zygophyllaceae) and Tamarix sokotrana (Tamaricaceae).
Socotran Sand Belt Vegetation
Many of the dunes on the coastal belt of Naukad are devoid of vegetation, but others support a sparse plant cover of perennials such as Calotrophis procera and the endemic grass Panicum rigidum (Poaceae). These are able to tap into moist gravel below the sand. There are also numerous annuals and ephemeral species such as the endemic Fagonia paulayana (Zygophyllaceae), while adjacent sand-free gravel includes the endemic Lindenbergia sokotrana (Scrophulariaceae), but there are no species confined to this zone.
Socotran Woody Herb Formation
On the coarse gravel and stony plains the first major vegetation zone inland from the sea is a woody herb community in which species Indigofera are conspicuous, together with other woody herbs such as Convulvolus fastigiatus and the endemic Campylanthus spinosus (Scrophulariaceae). Other endemics in this zone include Euphorbia arbuscula, E. septemsulcata, E. spiralis (Euphorbiaceae), Aloe perryi (Liliaceae) and the endemic grass Dactyloctenium hackelii (Poaceae).
Socotran Dwarf Shrubland
Landward of the woody herb zone, a dwarf shrub community has developed typically characterized by the endemic Croton socotranus (Euphorbiaceae). This species is in fact the most common shrub on the island often forming pure stands. Among associated shrublets are various endemics such as Asparagus africanus var. microcarpus (Liliaceae), Lycium sokotranum (Solanaceae), Placopoda virgata (Rubiaceae), Ballochia amoena, Justicia rigida and Trichocalyx orbiculatus (Acanthaceae).
Socotran Commiphora Transitional Community
At altitudes of between 100-250 m, a transitional community characterized by xerophilous species such as Commiphora parvifolia and the endemic Commiphora socotrana (Burseraceae) occur. Other associated endemics include Cissus subaphylla (Vitaceae), Barleria tetracantha and Blepharis spiculifolia (Acanthaceae).
Socotran Dendrosicyos Formations
On the lower slopes of the limestone plateau, endemic plants including Dendrosicyos socotrana (Cucurbitatceae), Adenium sokotranum (Apocynaceae), Cissus subaphylla (Vitaceae) and Euphorbia arbuscula (Euphorbiaceae) combine to form a landscape unique to Socotra. Dendrosicyos socotrana, commonly known as the cucumber tree, is the only known member of the cucumber family (Cucurbitaceae) that has evolved into a tree. Associated endemic shrubs include Maerua socotrana (Capparidaceae), Withania riebeckii (Solanaceae) and Cynanchum linifolum (Asclepiadaeae), while low growing species include endemic herbs such as Corchorus erodioides (Tiliaceae), Oldenlandia pulvinata (Asclepiadaceae), Trichodesma microcalyx (Boraginaceae), Lactuca rhynchcarpa, Pulicaria stephanocarpa and P. diversifolia (Asteraceae), and the endemic fern Adiantum balfourii (Adiantaceae).
Socotran Dorstenia-Kleinia-Ficus Formations
At heights above 500 m the characteristic limestone endemics include Dorstenia gigas and Ficus socotrana (Moraceae) and Kleinia scottii (Asteraceae). Other common endemics include Tatragonia pentandra (Aizoaceae), Euphorbia oblanceolata (Euphorbiaceae) and Hibiscus scottii (Malvaceae). The rock ledges support endemics such as Pseudomussaenda capsulifera (Rubiaceae), Haya obovata, Polycarpaea divaricata (Caryophyllaceae), while pockets of dark rich soil include the endemic Begonia socotrana (Begoniaceae), Exacum affine (Gentianaceae) and Pseuedanum caudatum (Apiaceae).
Socotran Succulent Shrubland
Where the soil is particularly poor succulents dominate the limestone flora. Endemics among these include Aloe perryi (Liliaceae), Kalanchoe farinacea and K. robusta (Crassulaceae). Also characteristic of these areas are various woody herbs such as the endemic Diceratella incana (Brassicaceae) and Teucrium sokotranum (Lamiaceae).
Socotran Limestone Valley Thickets
In sheltered valleys, especially where the soil is well developed, dense thickets occur. In the area around Reiged at an altitude of about 900 m the commonest shrubs and trees in these thickest include various endemics such as Acacia pennivenia (Fabaceae), Acridocarpus socotranus (Malpighiaceae), Croton sulcifructus, Dicliptera effusa (Euphorbiaceae), Rhus thyrsiflora (Anacardiaceae), Ruellia insignis (Acanthaceae), Psiadia schweinfurthii, and Vernonia cockburniana (Asteraceae). These in turn support various endemic lianas and creepers such as Cissus paniculata (Vitaceae), Discorea lanata (Discoriaceae) and Tragia balfouriana (Euphorbiaceae).
Socotran Dracaena-Boswellia Formations
The strange mushroom shapes of the endemic dragon tree, Dracaena cinnabari (Liliaceae) makes this one of the most distinctive communities in the world. The tree is also the source of the mysterious dragon’s blood, a type of resin that oozes from between the branches. The substance, originally thought to be made from dragons, turns red on cooking, and is in much demand throughout Arabia for its magical and curative properties. Other common endemics include Boswellia ameero, B. elongata, B. socotrana (Burseraceae), and Mitolepis intricata (Asclepiadaceae). The well-known aromatic resin frankincense, which is burned as incence, is derived from Boswellia socotrana. This community is characteristic of the limestone slopes of the Hamadera Hills.
Socotran Dirichletia-Punica-Rhus Mixed Thicket
The vegetation, found on the southern hills around Homhil, is largely composed of scrub thicket with the endemic Dirichletia venulosa (Rubiaceae), Punica protopunica (Punicaceae)and Rhus thyrsiflora (Anacardiaceae) forming the main species. The endemic Lannea aspleniifolia (Anacardiaceae) is also common in some of the inland zones while on the southern coastal plain endemic shrubs such as Dyerophytum pendulum (Plumbaginaceae) and Secamone socotrana (Asclepiadaceae) become some of the main species. It may be of some interest to note that Punica protopunica is the only known wild relative of pomegranate (Punica granatum).
Socotran Limestone Plateau Vegetation
In contrast to the slopes, the limestone plateau is remarkable arid mainly supporting just a few stunted bushes of the endemic Jatropha unicostata (Euphorbiaceae) together with occasional specimens of Adenium sokotranum, Croton socotranus and Ficus socotranus. Adenium sokotranum is commonly known as the sack-of-potatoes tree and has been described as the slug of the tree world - some individuals even suggest human form. The annual vegetation is largely composed of grasses such as Arthraxon lancifolius, Aristida funiculata and Pennisetum setaceum. However, the paucity of the vegetation is thought to be as much to do with the high winds experienced in this exposed area as it is to the aridity.
Socotran Granite Thicket
On the granite massif of the Hagghier Range the coastal vegetation of the northern slopes gives way to a mixed thicket of trees such as Aberia abyssinica, Sterculia rivae and Tamarindes indica and various shrubs such as the endemic Cordia obovata (Boraginaceae). There is also undergrowth of herbs such as the endemic Leucas virgata and Teucrium balfourii (Lamiaceae). At higher altitudes where there is increased rainfall the vegetation becomes more dense and may include small endemic trees like Clerodendrum galeatum (Verbenaceae), Cephalocroton socotranus (Euphorbiaceae), Indigofera sokotranus (Fabaceae), Boswellia ameero (Burseraceae), and small endemic bushes like Hedyotis stellarioides (Rubiaceae), Hypoestes pubescens (Acanthaceae) and Allophylus rhusiphyllus (Sapindaceae). The tangled undergrowth here includes the endemic Cocculus balfourii (Menispermaceae). Specimens of Dracaena cinnabari are less common here, but may be found associated with the endemic Commiphora planifrons (Burseraceae) and Haemanthus grandifolius (Amaryllidaceae) both of which have a similar flat-topped appearance. Commiphora planifrons is of value for the aromatic gum known as myrrh. On the steep stony slopes the thicket may give way to low scrub in which a variety of endemics occur including Aerva revoluta (Amaranthaceae), Gnidia socotrana (Thymedleaceae), Hypericum tortuosum (Hypericaceae), Euryops socotranus, Pluchea obovata, P. aromatica and Pulicaria vieraeoides (Asteraceae). On the high peaks of Hagghier, above 1200 m, the vegetation is characterised by Dichrocephala chrysanthemifolia and the endemic Nirarathamnos asarifolius (Apiaceae) and various species of Helichrysum.
Further information required.
References
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