Vegetation of Offshore Islands in the Gulf

Vegetation of Offshore Islands in the Gulf

The Gulf waters of the UAE contain a number of islands in two major categories, inshore and offshore. The inshore islands, including Abu al Abyad, Al Bahreni, Futaisi, Abu Dhabi, Sadiyat and Ghurab, are essentially part and parcel of a drowned coastline extending from Qatar in the west to the Abu Dhabi - Dubai border in the east. Their vegetation is intimately linked with that of the mainland itself and at times of exceptionally low tide some of them are hardly true islands at all. This coastline consists mostly of pure carbonate sedimentation interspersed with large areas of subkha which have developed within the last eight thousand years. The offshore islands, by comparison, were formed by a process of salt diapirism and are known as salt domes though little or no salt is visible. These islands, including Dalma, Sir Bani Yas, Arzanah, Qarnain, Das, Zirku and Sir Bu Nuair, frequently attain a greater height than the mainland coast. The highest point of the Sila - Mafraq road is in the region of 30 metres above mean sea level, compared with 161 metres on Zirku. During the Pleistocene glaciations the sea more than once receded and the Gulf was a valley (on the Iranian side) through which the Euphrates/Tigris ran to an estuary at Hormuz. During the Holocene, i.e. since the end of the last major glaciation around 18 - 16,000 BC, the sea gradually rose worldwide to reach its approximate present level around 4000BC. That the sea was on occasion higher is evidenced not only by the inland subkhas but also by the existence of raised terraces on the offshore islands. In this article the vegetation of three of the islands is discussed; Sir Bu Nuair, Qarnain and Das. The first two are now relatively undisturbed, whereas Das is extensively industrialised and altered.

All three islands consist of a core of pre-Cambrian Hormuz outcrops which have pierced the surface at the highest point of the salt dome and which now constitute heavily-eroded roughly conical hills, plus a small surrounding 'plain' of Pleistocene carbonates, including some coral reefs, with a very thin veneer of mixed carbonate and aeolian sands. The Hormuz series includes crystalline gypsum, red and brown haematites and black speckled dolomite. In addition Sir Bu Nuair displays evidence of some igneous rocks. The hills are steep and deeply incised largely as a result of water runoff erosion. There is no present source of fresh water on any of these islands. The hills are completely devoid of perennial vegetation, and even a favourable rainy season only attracts a thin layer of grasses and limited annuals. The coastlines, particularly where there are low ledges (1 - 3 metres) abutting into the sea, plus the shallow runoff wadi areas, are the regions 'of densest vegetation, particularly perennials.

Although rainfall is scant by temperate standards, high summer temperatures are somewhat offset by the reducing effects of the maritime environment which creates very humid conditions. Of the three islands Das is the only one where regular weather records have been kept but there is insufficient data on rainfall. Records do show an average of 0.51 ins. per year between December and May, with rainfall present on about 10 days during this period. However, individual years can have a major effect on averages - 4 ins. fell in one day in March 1982. The maximum summer temperature recorded on Das is 115' F in the shade, and a minimum of 45' F. Humidity reaches 100% on several days during the summer. The other islands have a very similar regime, though rainstorms are localised enough for one island to be free of rain on the same day that another is breaking a record. The dominant wind is the northwesterly 'shamal'.

Sir Bu Nuair

Situated approximately 75 kms. offshore midway between Abu Dhabi and Dubai, this is the largest of the three islands, located at 250 13' Nand 540 14' E. It is roughly pear-shaped, about 4 by 4.25 kms., with a low protruding spit to the southeast. The highest point, just west of centre, is 81 metres, though there are four other points in the central hilly zone over 75 metres high. These hills are typically steep with large amounts of scree. Between this hilly, severely eroded region, and the sea the terrain slopes in a fairly gentle gradient, a little steeper to the south compared with the north and east. Shallow wadi systems in this region have filled with coarse gravels and sands, but contain isolated rocks and smaller stones of haematite and dolomite. The crust immediately surrounding the hill zone is severely eroded and contains numerous fissures, niches and holes.

There is no airstrip on the island, but local fishermen use it as a temporary base; a few plywood and tin shacks dot the southeast tip along with the wire fishing cages so typical of this part of the Gulf. In the past mining for red ochre (haematite) took place and quarries are evident in the hilly zone. The island is used as a roost by several thousand cormorants which occupy a sizeable area of open rocky ground below the hills to the west. There is a population of tiny stumpy-tailed mice which might be a sub-species, and rumour has it that feral cats exist.

The island was visited only once, on 29th December 1982, when it was exceptionally 'lush' as a result of early winter rains. A transect was made from a point on the southwest coast towards the centre, and later the whole coastline was briefly surveyed. The low cliff on the south side is dominated by Suaeda vermiculata, forming a thick barrier up to one metre high and impenetrable in places. This species forms a belt overhanging the ledge and stretching inland irregularly for some ten metres. The same plant dominates the seaward side of some of the undisturbed inshore islands, in association with Halopyrum mucronatum, which was not recorded on any of these offshore islands. Where the limestone ledge on Sir Bu Nuair gives way to gravels and rougher terrain, the vegetation is less thick and annuals were conspicuous including Malva parviflora (in flower and fruit) in abundance, up to 20 cms. tall in relatively sheltered spots; large patches of Argyrolobium roseum (flower and fruit) on softer sand; distinct but small clumps of Lotus schimperi and Lotononis platycarpa (both generally in late flower); and the whole area interspersed with tiny patches of Zygophyllum simplex (in full flower), occasionally linking to form larger mats. Moving further inland the ground becomes rougher still with a series of clefts and jagged-sided mini wadis aligned approximately from the centre of the island to the sea. The major plant associations here were the Salsolas (S. baryosma, S. schweinfurthii and S. tetrandra) along with minor patches of A. roseum and Z. simplex (this latter only in sand in the wadi beds). The broken mini plateaux between these fissures was dominated in this zone by Capparis spinosa (in full flower and fruit), with individual shrubs up to I metre across and 50 cms. high, and even more so by Reseda aucheri up to 90 cms., especially in more open areas. Numerous Zygophyllum hamlense, densely covered wlth white flowers, were also greatly in evidence with their twisted gnarled stems. These plants were fairly fragile and outer branches would easily break off when brushed against. In this same zone, but less prolific, were a number of individual specimens of Convolvulus cf. prostratus, in full flower, with stems up to 65 cms., semi-ascending. Dotted among these were several small patches of Polycarpea repens in flower.

Towards the north coast of the island are the remains of an old stone cistern, full of tall grasses and shaded by the only tree on Sir Bu Nuair, a 4 metre tall Zizyphus spina-christi, presumably planted to provide shade. This plant was very healthy, and it would seem that this immediate locality remains a small catchment area for water runoff.

A brief survey of the coastline showed a very bare western and northern side, with occasional clumps of S. vermiculata and rare patches of annuals. The projecting sandspit to the southeast is also generally bare, with just a few Salsola species dotted here and there.

Qarnain lies much further west, some 140 kms. northwest of Abu Dhabi town, centred on 240 56'N and 520 51' E. It is smaller than Sir Bu Nuair, and is roughly rectangular in shape on a NW/SE axis, measuring approximately 1 by 2.5 kms. The rocky core is at the north end with two adjoining high points of 58 and 53 metres respectively. Elsewhere the island is of low relief and wadi systems are recent, being extremely shallow and fairly broad. From the foot of the core towards the south the island consists largely of carbonate sands and semi-subkha with an outer low shelf of limestone around the whole coastline.

The island was briefly surveyed on 28th December 1982, but specimens had been forwarded to the ENHG in April 1982 by Messrs. Harbison and Clarke who were employed by DECCA on the Island. A small navigational aid station is situated on the northeast side facing Das, and a short unsurfaced airstrip runs approximately NS down the west side. In spite of these intrusions, Qarnain remains fairly undisturbed (there is a weekly flight from Abu Dhabi with a two hour stopover), and during the visit an osprey was sitting tight on two eggs in a raised nest at the south end of the runway.

The southern half of the island is dominated by Suaeda vermiculata, Salsola baryosma and Salsola tetrandra, lending an overall flat, dull-grey tone to the landscape from a distance. Just as on Sir Bu Nuair, S. vermiculata was closest to the water's edge, successfully colonis.ing niches in the 1 imestone shelf. The two Salsolas were dense across the island within this perimeter.

The major vegetation variations occur in the sandy areas of shallow wadi systems around the hilly core. On deeper sand Aizoon canariense was conspicuous (in flower) along with Malva parviflora which was being ravaged by the caterpillars of (suspected) Painted Lady butterflies. On more compact but gravelly horizons, Zygophyllum simplex, Lotus schimperi and Argyrolobium roseum were the dominant annuals in mixed associations; these three species had been collected in April 1982, also in flower.

In sheltered runnels and cracks at the foot of the scree slopes a less common species was the delicate Spergula fallax, just coming into flower. Young plants had been collected the previous April.

This third island is 25 kms. almost due north of Qarnain, centred on 25 09' Nand 52 52' E; it is thus some 136 km west of Sir Bu Nuair. In the early 1950s Das was in much the same condition as the other islands - windswept, arid and supporting meagre vegetation except for an added scattering of annuals in favourable years. However, over the past 25 years Das has been extensively built over and the original landscape is largely unrecogniseable apart from one or two minor patches of coastline. The total area remains much the same, 1.5 by 0.75 kms. on a north-south axis, with the hilly core to the north where the coast is rocky, and small sandy coves on the other three sides, especially the south and east. The core has been quarried and lowered for building purposes, and the present maximum height for a natural feature is 31 metres.

Around the accommodation and office areas efforts have been made to create small gardens and to line some of the roads with both annuals and perennials which can adapt to the saline environment. Most of these localities are protected from the full force of the 'shamal', and include Aloe sp., Mesembryanthemum nodiflorum and Flaveria trinervia, as well as the odd Nerium sp. Zizyphus spina-christi and Phoenix dactylifera. These are all imports and are not a fair reflection of the island's original indigenous biomass.

Vegetation

There is virtually no area that has not been affected, apart from a tiny walled graveyard on the southwest side where, in times past, seamen were buried. It is no longer used, but significantly the ground within this cemetery is at least a metre higher than the surrounding built up area. Inside the walls a few straggling Anabasis setifera, Salsola baryosma and Suaeda aegyptiaca survive.

It is clear that some species, regarded as natural elsewhere in the UAE, have been introduced to Das, even if accidentally. Fertilisers, foodstuffs and materials brought from Abu Dhabi are probably responsible to a greater or lesser degree, though transport by migrating birds cannot be categorically ruled out in some cases.

The nature of the vegetation cover varies both with the season and the amount of clearance activity. Thus the open area around the north end of the runway is at times a haven for Salsola baryosma and Anabasis seti;era, but these plants are occasionally totally destroyed and the ground soaked with crude oil as a safety measure; where there is no vegetation there is not much likelihood of birds collecting.

As a broad generalisation there are two categories of 'natural' vegetation; those species on the shoreline and those elsewhere on the island where there is a supply of water. The major coastal species are Anabasis setifera, followed closely by Salsola baryosma and Suaeda aegyptiaca. These are all prevalent along the south and southeast sides and survive because they are outside the sphere of development activities. Among the 'cultivated' areas and where pipes are continually dripping, there are several species of grass, all recorded in similar environments in Abu Dhabi town. The commonest grass is Sporobolus spicatus, but areas of Echinochloa colonum, Cynodon dactylon, pactyloctenium aegyptium and Chloris virgata are not difficult to find. At one spot on the west side, on deep sand above the high tide line there is a colony of Euphorbia hirta, flowering in early summer, and also one or two minor patches of struggling portulaca oleracea. In especially damp areas Chenopodium murale is a common annual.

The vegetation of Das has been checked on numerous visits between 1978 and 1983 and it is expected that the species enumerated above will survive in greater or lesser proportions whatever the scale of industrialization on the island. Any expansion of the garden areas will offer scope for most of these species to expand. From time to time other species may gain temporary footholds (Sonchus oleraceus was recorded on a single occasion) but there is unlikely to be any greater change until the island is eventually abandoned.

The arbitrary choice of these three islands is due to the fact that the author has not yet visited any others offshore. Several species are common to all three while some are more restricted, particularly where Das is concerned, because of its different environment. Sir Bu Nuair and Qarnain were each visited only once, and it is quite possible that some species were overlooked but this brief article introduces the vegetation in the absence of any other known published records. The ENHG is very grateful to the staff of the Royal Botanic Garden at Edinburgh for all their patient help in giving positive determinations to the collections.