Bulletin 21 - November 1983: Vegetation of the Arabian Gulf Coast of the UAE
Vegetation of the Arabian Gulf Coast of the UAE
by R.A. WesternTopography and Soils
Despite its overall low relief the Arabian Gulf coastline from the Qatar border to north of Ras Al Khaimah presents a variety of plant communities which, though bewildering in its associations and seasonal differences, is extremely interesting as a study of the country's flora. For most of the year it is the perennials which dominate, but in Spring some regions exhibit a vivid display of annuals. The average elevation of the coastal highway is under 10 metres, being lowest in the south-west and highest just south of Ras al Khaimah town.
Large tracts of l:he coast between Silaa, the border post with Qatar, and Abu Dhabi town, consist of saline flats (subkha), just a metre or two above sea level. After prolonged rains these areas may be inundated and waterlogged for several weeks, as happened in the Spring months of both 1982 and 1983. The true subkha, with its evaporitic crust of gypsum, anhydrite and calcite, supports no plant life except for occasional subsurface algae, but wherever there exists an outcrop or accumulation of aeolian or even mixed calcareous sand, halophytic species usually colonise. Inland of the subkha lies a region of semi-settled low dunes, rising gradually to the more mobile sand seas south and west of the Liwa crescent.
Further northeast, at Mafraq, the coastal road begins to rise, and from there undulates from 20 - 40 metres above sea level to Jebel Ali, where the elevation again drops. Subkha still exists in many areas towards the coast, along with dune associations formed as a result of surface intermingling of calcareous and aeolian sands. The road itself runs along a low limestone plateau, severely degraded and flattened, with an extensive surface horizon of sand.
Plantations have been introduced between Zibara and Sameeh, but with little success so far. The coastline consists of barrier beaches comprising ooliths and shell debris, or merges into the subkha with occasional outcrops often forming raised flat-topped platforms just above high water level. Higher outcrops are numerous but small; many of them are virtually islands at low tide.
As the road approaches Dubai subkha depressions again become noticeable, though separated from the coastline by low dune barriers, particularly between Jebel Ali and Jumeirah. From Ajman northwards there is a fairly clear distinction between the seaward, marshy side of the road, and the landward side with its undulating but stable dunes. In places the road reaches the coast itself. Fossil dunes can be seen at Abraq, while the vast number of shell middens testify to a former shoreline up to five kilometres further inland than at present. Dunes become dominant again near Jazirat Al Hamra, but they are for the most part fixed by vegetation. A ten-kilometre strip of high beach dune just south of Ras Al Khaimah town effectively separates the road from the coast.
Beyond this the road is gradually squeezed between the sea and the northerly extension of the Hajar Mountains, the two finally meeting at Shaam. The road runs close to the coast apart from a stretch north of Rams, where it skirts an occasionally inundated marshy area. On the landward side the soil is pebbly with surface alluvial deposits, supporting date palm and acacia associations.
The soils of the coast are generally poorly graded, thin, and saline. This is most noticeable in the west where the subkha is in places 25 -30 kilometres wide, and the flat landscape is broken only by a series of low, flat-topped buttes which often carry a thick layer of shattered chert fragments and pockets of aeolian sand. The whole of the UAE is classified as semi-arid or arid, and organic matter in soils is low and mineral content high. Clay content is minimal.
Bordering the subkha salt tends to collect at a shallow depth and forms a clear horizon, effectively barring perennial plant growth. Further north, there is some improvement in soil quality with a higher concentration of non-calcareous sand and limestone deposits, while the proximity of the mountains in Ras Al Khaimah has resulted in the accumulation of thin alluvial deposits.Climate
The Arabian Gulf climate is characterised by low rainfall and high temperatures. Since records began (1966 in Tarif; 1972 in Abu Dhabi) the coast has witnessed an average of 10 days with rainfall per year, the number decreasing further west with Bu Hasa and Tarif each recording approximately 5 days. The annual total, however, can be very variable. The Old Abu Dhabi airport recorded 195.2 mm of rainfall in 1982, whereas the combined total for the years 1977 -1981 inclusive was 151.9 mm at the same station. 1982 was also a record year for monthly extremes with February recording 95.5 mm compared with the previous February maximum of 55.1 mm in 1976. The figures for March 1982 and March 1972 were respectively 86.5 mm and 55.3 mm. The total rainfall at the same station for the whole of 1981 was just 36.4 mm, of which 26.3 mm was deposited in one night in early May. Over a 36 hour period between 13th and 14th February 1982 the Old airport recorded 91.5 mm. 1983 also began with rainfall records well above the long-term average, with 92.7 mm up to the end of April. The variability and unpredictability of rainfall in adjacent areas is apparent from the records at the Old and New Abu Dhabi airports only some 12 kilometres apart. In January 1983 10.1 mm was recorded at the New, but only 5.0 mm at the Old. Further north rainfall tends to be higher, with Sharjah recording a long-term annual average of 95.8 mm. Fujeirah, on the east coast, regularly records 150 mm plus. Most precipitation comes in the first four months of the year, with occasional light falls in autumn. Significantly rainfall is of the cold-front type and in itself acts indirectly upon the up-surge of annuals which require an increase in both ground and air temperatures in late Spring. For this reason February and March may be very wet, but the main activity of annuals may be delayed until late April.
Absolute maximum temperatures rise as high as 49° C in June/July and can drop as low as 4° C in January, though such extremes are rare. The highest temperatures tend to be in the Western Region. Temperatures in Abu Dhabi and Dubai reached 47° C in July 1983, the highest at these locations since 1974. The average maximum in Abu Dhabi for June is 40.4° C, and the average minimum for January, is 12.8° C. Corresponding maximum and minimum daily means for the year are 32.2° C and 21.0° C.
Mean annual relative humidity is over 60% the figure for winter months is generally over 70%. Diurnal means show great variability, often up to 50%.
Foggy days are recorded in all months, but again the variability is high. In March 1983 here were 5 such days but in April only 1 (visibility less that 1000 metres). However it would seem that local fogs are more frequent inland particularly where sand dunes rise from the edge of the subkha. There is no doubt that this dewfall is an important source of moisture for desert vegetation. Smooth outer surfaces, low incidence of broad leaves and upright stance all help to concentrate the flow of droplets from the branches down to the central stem and further to the roots.
Winds are generally north to northwesterly during the day, reverting to southeasterly by night. A southerly or southwesterly wind may introduce fine aeolian sand into the coastal areas.Plant Associations
The assumption that the desert is devoid of vegetation is totally incorrect. Driving along the UAE's highways it is inevitable that much of the local ecology is missed and there is a tendency to see only drabness in the landscape. Yet a brief stop anywhere (except on the true subkha plains) will show how misleading this attitude can be. At any time of the year a count of separate plant species will show what variety exists, even if the individual species are not recognised. Even in July and August up to 20 species can be counted within a few minutes on the Abu Dhabi - Dubai road, and at least three of them are likely to be in flower. The number of species is easily doubled when the Spring annuals appear. The regions bordering the subkha too possess a variety of perennials depending on the local environment and microhabitat. There is plenty of scope for recording and the Group is still turning up odd and new records.
The following notes outline the chief associations as one progresses from Silaa (Qatar border) to Sili (the most northerly hamlet in Ras Al Khaimah). Inevitably there are a lot of Latin names - apart from the local vernacular this is the only way of giving internationally understood identifications. Most of the species mentioned can be viewed in their dried state in the Group's Workroom, along with notes, photographs and distribution maps.a) Silaa -- Jebel Dhanna
Despitethe low elevation of much of this region the road from the Qatar border to Jebel Dhanna is surprisingly rich in its flora. The area is dominated by succulent halophytes plants that can tolerate high levels of salinity and aridity, and these give a visual conformity to whole stretches of plain on either side of the road.
The border region includes the green/fleshy shrub Zygophyllum hamiense*, the dirty grey and spiny Cornulaca monacantha (which tends to collect any paper or other rubbish flying around in the wind), the sometimes black Suaeda vermiculata, the ubiquitous sedge Cyperus conglomeratus, the fleshy Seidlitzia rosmarinus with its smooth white branches, the low spreading shrub Pagonia ovalifolia and the very fleshy Anabasis setifera sometimes quite red. Z. hamiense is the largest and commonest of this group, especially in high summer when it is one of the few flowering species in this area. As the level rises slightly around Al Hamra, with its distinctive brick-red soil, this plant is totally dominant with rare patches of the related Z. simplex in summer, and very rare examples of another fleshy halophyte, Halopeplis perfoliata, which can be any combination of bright red, green or orange, depending on local soil characteristics. Most of these perennial halophytes flower in the autumn. Further inland, where the broken limestone ridges are less saline, stunted specimens of Hamada elegans and the small desert grass Astenatherum forsskalii are dominant. In summer this whole region between the Subkhat Mutti and the coast is extensively roamed over by large herds of camels (a herd of 120 plus was counted in June 1983), but the succulent Z. hamiense is generally avoided because of its purgative effects.
The coastline as far as Jebel Dhanna supports a narrow band of vegetation wherever tidal flooding into the subkha is not a danger. Z. hamiense and C. monacantha are the dominant perennials, within a metre or two of high tide line on deep calcareous soil. Scattered among these are individual clumps of H. elegans, rare stands of Cornulaca aucheri (up to 15 cms. only, much smaller than the erect, spreading C. monacantha), and odd shrubs of Heliotropium kotschyi which are very occasionally locally dominant in association with individual S. vermiculata.
Lotus garcinii and Neurada procumbens have been recorded on more settled sandy/gravelly surfaces, along with the annual Z. simplex which here often has a purplish tinge to the leaves. The parasite Cistanche phelypaea ('Desert candle'; 'Desert hyacinth') is common right along the water line, apparently hosting on Z. hamiense and possibly the large fleshy Bienertia cycloptera in late winter months, providing a vivid splash of yellow. Individual Crotalaria aegyptiaca .specimens are not uncommon, but never dominant.
The subkha areas contain a number of small buttes, flat-topped and up to 60 metres in height, which are relic formations of a depleted land surface.
These buttes are ecologically very interesting since their isolation on the subkha plains has led to very specific microhabitats and plant associations. The severely eroded side gullies and lower .pediments contain pockets of soil and growing conditions are quite different from those on the exposed coasts and plains. Grasses are much more common, and besides many of the chenopods mentioned above there are more annuals in evidence, including Savigna parviflora, Eremobium aegyptiacum, Arnebia hispidissima (also a biennial or even perennial version) and several composites including Launea mucronata. On the lower pediment slopes the distinctive three-valved fruits of the lily Dipcadi erythreum are a very common site in summer. The seeds and bulbs are probably a major food source for the ubiquitous jerbil Gerbillus cheesmani.
Jebel Dhanna itself is unique along this coastline in its geological formation and blackened 'volcanic' appearance. The hill itself is bare with narrow gullies and the only perennial that can survive is Capparis spinosa, just as on similar islands offshore. This flowers in summer months and even at Jebel Dhanna attracts numbers of bees and wasps.b) Jebel Dhanna -- Abu Dhabi
Between Jebel Dhanna and Tarif there is a little more variation in scenery with a mixture of depressions (some permanently filled with water and thickly encrusted with salt deposits), low bluffs and small dunes. Suaeda aegyptiaca and Salsola tetrandra now become common. The latter sometimes appears a mass of bare dead twigs in summer, but it comes to life and flowers in the autumn. Monsonia nivea and Moltkiopsis ciliata are other late-flowering species while Panicum turgidum and A. forsskalii remain the dominant grasses.
Northeast of Tarif the landscape is particularly barren as the subkha widens. Inland bluffs continue to carry the same plant associations, but between the road and the sea only a few Salsolas survive. Closer to Abu Dhabi there are the first of the major mangrove (Avicennia marina) stands, particularly around Abu Al Abyad and Zubbaya. It is interesting to note that since this stretch of road was converted to a dual carriageway in 1982, the central reservation with its imported sand and rubble has attracted a number of species, including Salsola baryosma, A. setifera, Tamarix, A. hispidissima, H. elegans and Heliotropium brachycarpa. Nearer Tarif the central reservation has been planted with Salvadora persica at five metre intervals. Inland from the subkha is the UAE's only major stands of Hamada persicum.
Approaching Mafraq the bluffs rise slightly on the landward side and the subkha narrows. The bluffs, which stretch back to a gradually rising plateau all the way to the Eastern Region, are full of flowering Limonium axillare ('Sea lavender') in early Spring but unfortunately access is limited. F. ovalifolia is extremely common.c) Abu Dhabi -- Dubai
The major cities of the coast have been excluded from this account as they constitute essentially disturbed localities where the natural vegetation has either adapted or been destroyed. Some indigenous species have not survived, others are under threat. Anagallis arvensis, Sporobolus arabicus, and Halopyrum mucronatum fall into this latter category on Abu Dhabi island. Other plants, such as Chenopodium album, Convolvulus arvensis, and Alhagi maurorum can be classified as weeds; they have colonised large, though different, areas. The desert east of the new airport accommodates a community comprising the dominate C. monacantha, with C. conglomeratus on deeper sand. H. elegans is poor and stunted; it seems that this species prefers less saline conditions further inland where it forms huge phytogenic mounds, as along the Sueyhan road. F. ovaliforia, Z. hamiense, H. kotschyi and Dipterigium glaucum remain well represented but a number of legumes now appear in quantity for the first time. These include the silvery Indigofera argentea and Astragalus haurensis, both with blood-red though tiny flowers. C. phelypaea and another parasite, Cynomorium coccineum, are also frequent in early Spring. Rather surprisingly, Calligonum commosum is not at all uncommon, though it is far from its traditional habitat among the dunes of the Western Region around the Liwa. This bush is very heavily grazed, however, and not easily identifiable without flower or fruit. Pennisetum divisum and Cenchrus ciliaris take over from P. turgidum as the dominant grass, though Eragrostis cilianensis, Eremopogon foveolatus, Eleusine compressa and Stipagrostis plumosa have all been recorded.
The vegetation along the Abu Dhabi - Dubai road appears very dense but on examination there is at least as much bare sand showing as plant cover, except during the season of Spring annuals.
Rainfall here is a little better than in the Western Region, but many of the larger perennials have shallow lateral as well as vertical root systems, and this pattern inhibits close growth. It is a question of survival of the fittest which enables soil and water resources to be most effectively used.
There is a greater variety of perennials closer to the Dubai border, as the shallow dunes give way to more stable gravel and sand areas. Sphaerocoma aucheri, which is unrecorded south of Abu Dhabi town, is now common, along with Helianthemum lippii, Salsola rubescens and Atriplex leucoclada. Less frequent is L. axillare and Pergularia tomentosa with its strange green fruits. In low-lying areas the halophytes are, as usual, dominant, especially A. setifera. C. monancantha is far less frequent as Dubai gets closer, though C. conglomeratus remains very common on dunes.
The road really becomes a pleasant drive when temperatures rise in the weeks following heavy Spring rains, as in 1982 and 1983.
A large number of new perennials become well established, though they have to compete at first with the masses of brightly-coloured but short-lived annuals. The most numerous ephemerals include S. parviflora, A. hispidissima, M. nivea, E. aegyptiacum, Silene villosa and Launea capitat~. In good seasons millions of seeds are released to await their turn, sometimes for years.
Between Zibara and Samih Clinic extensive plantations have been tried on either side of the road but without much success. However they provide protected environments for some species, and C. prostratus and L. capitata in particular flourish. The aim seems to be to create a green belt, fenced to discourage camels from wandering onto the road.
Closer to Dubai town, where the ground is frequently marshy, there is little besides the usual halophytes, and the dominant grass is S. arabicus.
For the first time there are trees, individual Prosopis spicigera up to 6 metres being noticeable northeast of Jebel Ali. Along the Jebel Ali - Hatta road there are a few small wooded areas of this species.
This area is also noted for the summer-flowering legume Rhynchosia schimperi with its yellow petals and dark stubby pods. This plant has spread down to the Abu Dhabi border, following especially the line of the new by-pass east of Jebel Ali.
The coastline between Abu Dhabi and Dubai bears its own plant associations. Actual species represented depends on whether the soil is sand or mud and their proximity to t.he coast. S. aucheri, A. marina, S. vermiculata and H. kotschyi are well documented, but other less common species include Frankenia pulverulenta (often minute plants with miniscule purple flowers on outcrops surrounded by water at high tide), Herniaria hemistemon and Farsetia linearis on limestone rocks away from the coast, Rumex dentatus and Gisekia pharnaceoides. The muddy lagoons are surrounded by extensive tracts of Arthrocnemum macrostachyum which gives protection to a number of bird and crustacean species. The graceful warbler nests among these bushes. A related but smaller shrub, Halocnemum strobilaceum, is less common but can also tolerate shallow tidal flooding on a regular basis. In deeper but well-protected lagoons on the landward side of islets the mangrove is again very frequent, especially near Gharab. North of Ras Ghanada the coastline has a clearly-defined barrier beach, behind which the grass Halopyrum mucronatum, with its long stolons (up to 15 metres is common) is dominant for up to 100 metres inland in a broad belt. It is so dense that nothing else survives, but C. conglomeratus, S. baryosma, C. monacantha, S. vermiculata and A. setifera are all commonly associated immediately inland of the belt. In early Spring the vivid yellows of C. phelypaea are to be seen the whole length of the coast, on sand and mud, preferably hosting on A. macrostachyum.d) Ajman -- Ras al Khaimah
The seaward side of the road north of Ajman consists of mud and salt subkha, with occasional patches of dark red and purple H. perfoliata. In contrast the landward side is mostly low dunes stabilised by a variety of grasses, including P. turgidum, P. divisum, S. arabicus, Coelachyrum piercii and Dactyloctenium aegyptium. These dunes delineate a former beach line and many of them are topped with old shell middens. A few Acacia and Prosopis trees are scattered around opposite Hamriyah, and these shelter micro-environments which include the weed Chenopodium murale, the prickly shrub Lycium shawii (very commonly associated with acacias elsewhere in the UAE) and Malva parviflora, the common local mallow with its attractive white flowers in the stem and branch axils. This region, inland of the road between Ajman and Umm Al Qawain, is noted locally for truffles ('fougah') which are associated with H. lippii. Immediately after a Spring shower whole families can be seen walking over the scrubby desert with sticks, looking for the tell-tale cracks in the wet sand that might reveal the presence of the delicacy, small though they are (2 cms. in diameter is a good specimen). These are then washed, steeped in very salty water overnight, and then sliced and fried. Perhaps because the dunes have levelled out here, C. conglomeratus is a dwarf variety, rarely more than 15 cms. tall, compared with up to 60 cms. in mobile dunes elsewhere. In late Spring the ephemerals make this a very pleasant drive, with the yellow carpet effect of A. hispidissima, particularly on the flat plain around Al Raafah. Other familiar annuals include the thin, straggly plantain Plantago boissieri, the creamy-flowered S. villosa, Ononis serrata, Senecio glaucus, Hippocrepis bicontorta with its very distinctively-twisted legumes on long trailing stalks, Lotus halophilus and the red and green-veined Emex spinosus with its soft but prickly branches. Around Abraq, where there are many sand cliffs, the fleshy pink and red perianths of the wild docks Rumex vesicarius and R. pictus stand out in February and March. The leaves are said to be edible, though tart. Salsolas and Zygophyllaceae are still represented, also Convolvulus deserti, plus the usual halophytes in depressions. Occasional poor specimens of the tree-like shrub Calotropis procera appear, though they are stunted compared to the taller, healthier trees further inland towards Falaj Al Moallah.
Further north, towards Jazirat Al Hamra, the road runs very close to the sea, and this region contains a large number of L. shawii shrubs which themselves support the creeper Ephedra foliata. The large shrub Grewia erythrea is also common, recognisable by its four-lobed orange fruits. Heliotropes .are frequent plus a variety of grasses and low acacia scrub. Among other annuals, Trigonella hamosa is common.
Inland from Jazirat Al Hamra, where the dunes are higher, C. aegyptiaca and C. comosum are noticeable among the larger species but the dominant plant is H. kotschyi. In the vicinity of the two old watch towers Moltkiopsis, Convolvulus and Cyperus are the main species.
The high sea dunes just south of Ras Al Khaimah town form an interesting ecological niche with a number of large Acacia and Prosopis trees, a mass of E. foliata on the sea-facing slopes and large areas of Lycium shrubs. Erodium neuradifolium and M. nivea are two abundant members of the Geranium family in Spring months, usually tucked away among the grassy tussocks (mostly C. ciliaris), while on the landward slopes Leptadenia pyrotechnica can be seen growing to three metres. Beneath the trees, especially Prosopis, there is the usual Spring carpet of E. spinosa, C. murale and M. parviflora in a neat circle approximating the circumference of the foliage above. Ras Al Khaimah itself has been extensively developed. The old road past the hotel to Al Nakheel is worth mention for its Tamarix and mangroves in the creek, but these are unlikely to survive the gradual backfilling of the lagoon.
Al Nakheel, as its name implies, was once a centre of date cultivation, but the major groves are now further north.e) Ras al Khaimah -- Shaam
The alluvial gravels and pockets of date palm cultivation north of Al Nakheel make this a relatively fertile stretch of the coastline. Annual rainfall totals are higher and there is the benefit of runoff from the nearby hills. On the narrow alluvial plain acacias (A. tortilis and A. arabica) with their flat tops lend an East African dimension to the landscape. These trees are well-spaced along pebbly runnels and around the remains of abandoned walled field systems. In Spring a thin carpet of annual grasses appears after rain, followed by little patches of crucifers and other ephemerals: Sisymbrium erysimoides with its serrated leaves, pale yellow flowers and thin, elongated fruits; Erucaria crassifolia with its clustered purple-white flowers and conical pods; Brassica tournefortii, up to a metre tall if supported, with pale yellow flowers. All these however, fade into insignificance against the dominance of Asphodelus fistulosus in February and March. This asphodel covers literally acres of the stony ground between the road and the hills, growing as vast spreading colonies with individuals up to 40 cms., their white flowers streaked red or orange. Among the perennial crucifers Physorrynchus chamaerapistrum, strong, woody plants with pale white or lilac flowers, and Farsetia heliophlli. with pale creamy pink petals are not infrequent. This latter, first recorded in February 1983 is a new record for Arabia. Occasionally the prostrate gourd Citrillus colocynth is with its yellow flowers and green or yellow apple-sized fruit is seen, but never as large or in the same quantities as found in the sandy wadis of the central plains. In the runnels themselves Tephrosia persica is frequent, a reminder that this region fringes the mountains.
The coastal lagoons north of Rams are rimmed with A. marina, regularly flooded to a full metre, while those mudflats which are only occasionally inundated contain A. leucoclada and also Limonium carnosum, the only recording we have of this particular sea lavender for the UAE.
Where the road-side falls away to the flats, Mesembryanthemum nodiflorum and Suaeda aegyptiaca have formed extensive colonies. At Shaam the hills and the sea meet, and mountain vegetation is dominant apart from the coastal acacias. Tephrosia apollinea, Cassia italica and Euphorbia larica dot the rugged slopes, while the visciously-spiny Astragalus fasciculifolius gives colourful splashes of purple in early Spring.
The total length of this coastline from Silaa to Shaam is about 550 kilometres. There is no doubt that the plant associations in many locations are being altered by man's interference, while the vagaries of climate and grazing further affect the composition of the plant communities. Yet it is a mistake to view the present pattern of distribution in terms of tough survival in a hostile environment. Most of the species mentioned in this account have adapted supremely well to their surroundings; plants find their way into some forbidding niches but adjust, reproduce and increase to the limits of the available resources. There exists a situation of equilibrium, not a climax, and it is hoped that man's effect will not be too detrimental to what is to begin with a rather fragile environment.
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