The Archipelago of Socotra is well-known for its assemblage of endemic and unusual species. Plant species found here have evolved morphological and physiological adaptations to cope with the dry climate and fierce monsoonal winds. The succulents display several morphological adaptations: plant bodies are globular or columnar, with reduced surface areas that decrease transpiration. Glaceous wax surfaces and microanatomical epidermal emergences reflect radiation. Umbrella-shaped shrubs form dense thickets, with all plants reaching the same height, a structure that protects them from strong winds. Adenium socotranum has a special cell sap cycling within the caudex which prevents overheating: it is one of the most famous bottle-trees species that store water within their trunks to help them survive the more arid enviroments of Socotra.
Because of such high diversity and endemism in plants, the island has been declared a WWW Global 200 Ecoregion in addition to designations of UNESCO World Heritage Site and UNESCO Man and Biosphere Reserve.
The native plant species of Socotra reflect the island’s geological history and demonstrate links to the Horn of Africa, as well as to the more distant Macaronesian islands. Socotra separated from the Arabian mainland in the Tertiary by the same series of dislocations that produced the Gulf of Aden. Before that, its position on the supercontinent Gondwana meant that Socotra was nearest to Madagascar, India and the east coast of the African mainland. Many of the endemic plants found on Socotra were previously widespread: flora has strong links with adjacent parts of Somalia and Arabia but some species and genera have interesting disjunctive distributions:
- Dendrosicyos socotrana was recorded in Djibouti and is now regarded as extinct on the African mainland.
- the genus Dracaena contains xeromorphic species distributed in the Macaronesian islands, Madagascar, and along the African coast from southern Africa into Arabia; Dracaena cinnabari, is a tertiary relict with related species in southern Arabia, north-east Africa and the Canary Islands;
- species of Kalanchoe and Helichrysum show strong links with southern African species
- Thamnosma socotrana and related species in southern Arabia, south-west Africa and south-west North America
More research is needed to reveal the biogeographical circumstances that gave rise to these systematically unique and locally isolated species. Socotran’s flora also includes plants which can be considered taxonomic relicts, that is with no close relatives:
- Dirachma socotrana, one of only two species in the Dirachmaceae, a family related to the Malvaceae but with an interesting mixture of characters including 8 merous flowers, stamens opposite the petals and fruits with a dehiscence similar to that found in Geranium;
- Dendrosicyos socotranus the only arborescent member of the Cucurbitaceae
- Wellstedia a small shrub of boraginaceous affinities but which is sometimes placed in a family of its own.
Approximately 900 plants have been recorded from Socotra, of which at least 300 are found nowhere else (endemic species) making the archipelato the world’s tenth richest island. Botanical field surveys led by the Centre for Middle Eastern Plants indicate that 307 out of the 825 (37%) plant species on Socotra are endemic, i.e., they are found nowhere else on Earth. There are ten endemic genera: Ankalanthus, Ballochia and Trichocalyx (Acanthaceae), Duvaliandra and Socotranthus (Asclepiadaceae), Haya (Caryophyllaceae),Lachnocapsa (Cruciferae), Dendrosicyos (Cucurbitaceae), Placoda (Rubiaceae), Nirarathamnos (Umbelliferae) and one near-endemic family (Dirachmaceae, recently a second species has been found in Somalia). The families richest in endemics are Compositae (26), Acanthaceae (24), Euphorbiaceae (21), Labiatae (20) andAsclepiadaceae (11).
Some of the plants on Socotra represent the last surviving members of their genus. The entire flora of the Socotra Archipelago has been assessed for the IUCN Red List , with 3 critically endangered and 27endangered plant species currently recognised. The endemic and monotypic Dirachma socotrana is considered Vulnerable by IUCN while Croton pachyclados survives only in one location. Dendrosicyos is the only representative of the cucumber family to grow in tree form. Euphorbia abdelkuriensis grows only on Abd al Kuri. This endangered plant is an unusual Euphorbia, known for its spineless columnar stems, all linked by a single rootstock. In total, IUCN names 52 endemic Socotran plants in the Red List of Threatened Species.
One of the most famous botanical curiosity of Socotra is the Dragon’s blood tree (Dracaena cinnabari). The trees are slow-growing and long-lived (possibly a few hundred years) and in maturity their crowns have an upside-down umbrella shape. The dense Dracaena woodland found at Firmihin is globally unique – no other arborescent (tree forming) Dracaena species are found in woodland as they are in Socotra. The red sap (cinnabar-like resin) of these trees was thought to be the dragon’s blood of the ancients; it was also compared to the “blood of Abel” since in Arabic language it is called Dum al Axwein, “blood of the two brothers, Cain and Abel”. The Sucotri name Arriyahib has no connection to the Arabic. The dragon’s blood was once prized in the outside world as a pigment, a glue, and a dye in the manufacture of English pound notes – and is still used on the island to dye wool, decorate pots, paint lips, and cure stomach pains.
The Adenium obesum (or desert rose), a weirdly bloated, grotesquely humanoid-looking succulent with a smooth skin, thick white sap, and small pink flowers, is found in the most indecent postures. And the overgrowncucumber tree, which reaches almost 12 feet high, looks like an immense, oddly inverted parsnip; it is festooned with poisonous cucumbers that look like plums and smell like bad fish.
Some socotran plants, such as frankincense and myrrh, were treasured in times past and are still valued locally for medicinal and domestic purposes. Others, including the Socotran begonia and the so-called Persian violet – have long been popular houseplants in the Western world.
Other endemic plants includ:
- the giant succulent tree Dorstenia gigas;
- the cucumber tree Dendrosicyos socotranus which somewhat resembles a small baobab, is one of the largest trees in Socotra (6m in height).
- the rare Punica protopunica, a species of flowering plant in the Lythraceae family, related to the pomegranate (P. granatum) but has smaller and less palatable fruits;
- species of plant in the genus Aloe, most represented by Aloe perryi;
- Boswellia socotrana, species of plant in the Burseraceae family, trees that produce frankincense or olibanum.
Remarkable and beautiful smaller plants inculde many succulents such as Caralluma socotrana, 5 endemic species of Echidnopsis and 2 endemic genera with single species, Duvaliandra dioscorides and Socotrella dolichocnema and 3 endemic species of Aloe.
Pronounced local variations in climate have resulted in a broad mosaic of plant communities on te island. Topographically Socotra can be divided into three main zones: the coastal plains, a limestone plateau and the Hagghir Mountains. The coastal plains tend to be fairly arid and vegetation is sparse. The foothills of the mountains display a shrubby landscape with incense trees and bizarre bottle-trunked trees. The species for which the island is renowned in evergreen woodland over the centre and east of the island and is the dominant tree in some areas. The limestone plateau and the Hagghier Mountains are the richest areas for endemic plant species, but endemics are found throughout the island in every type of vegetation.
The costal planis and low inland hills consist of open deciduous shrubland dominated by the endemic Croton socotranus and scattered trees of Euphorbia arbuscula, Dendrosicyos socotranus and Ziziphus spina-christi. Grasses and herbs develop after sufficient rainfall. The most widespread vegetation type is a distinctive species-rich open shrubland found on the coastal foothills and the limestone escarpments. Two endemics,Croton socotranus and Jatropha unicostata, are the main shrubs present and are the most abundant plants on Socotra. Euphorbia arbuscula, Dracaena socotranus, the “desert rose” Adenium obesum spp. sokotranumand emergent trees, such as Boswellia spp.*, Sterculia africana var. socotrana and Commiphora spp., are also present.
*out of a total of 24 species, 7 are endemic to Socotra, with some species growing only on rocks and cliff faces, other from the ground. The tallest species are Boswellia elongata, many of which are found at Homil andBoswellia ameero, whic is common at the higher altitutdes of the walking route from wadi Ayheft to Meqadrihon pass.
On the limestone plateau and upward to the middle slopes of the Hagghier Mountains there are areas of semi-deciduous thicket dominated by Rhus thyrsiflora, Buxus hildebrandtii, Carphalea obovata and Croton spp. The higher montane slopes support a mosaic of dense thickets, dominated by Rhus thyrsiflora, Cephalocroton socotranus, and Allophylus rhoidiphyllus with the emergent dragon’s blood tree (Dracaena cinnabari), lowHypericum shrubland and in many areas anthropogenic pastures. Open rocks are covered by lichens and low cushion plants, including an endemic monotypic genus of Umbelliferae (Nirarathamnos asarifolius) and several endemic species of Helichrysum.
Threats and conservation
Agriculture is not a traditional activity in Socotra: apart from dates, some people have small gardens with banana, tomatoes, tamarind and few other fruits.
Several species on Socotra are of horticultural interest for instance Begonia socotrana, the hybrid parent of winter-flowering begonias, and Exacum affine – the Persian violet, but not for local people.
The people living on Socotra have a thorough knowledge of the flora, and many of the plants have traditional uses, such as providing livestock fodder, fuel, building materials, foods, gums or resins. Plant extracts are still used in medicines, cosmetic and hygiene preparations, and in the manufacture of cordage, as a source of insecticide, and in tanning and dyeing.
The flora of Socotra faces some real threats: many trees are suffering from a lack of regeneration, possibly caused by climate changes and overgranzing, leading to loss of vegetation cover and serious soil erosion in some areas. Large scale infrastructer projects, such as road building, are damaging habitats as well as the aesthetic beauty of the island. Although there are some potenitally invasive species present, they are either being actively controlled by Socotra Governance Biodiversity Project or are confined to the heavily-grazed, degraded areas around the main town of Hadibo.
Populations of endmic and endangered plants are veing established at Adeeb’s nursery, just outside Hadibo, both as an educational resource and an ex-situ conservation collection. It contains 120 species of the island’s plants. The plants are distributed freely for those who want to plant them at home or in the island’s sanctuaries.
Reports from visitors
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