Bulletin 19 - March 1983: Grasses of Abu Dhabi Island


Grasses of Abu Dhabi Island

by R.A. Western

The rapid urbanisation of the Island in recent years had led to a large increase in grass (Gramineae) species represented. Twenty, possibly ten, years ago the number of species could probably have been counted on two hands; the absence of any records means that we have to rely on assumptions based on the present state of the nearby coastline and islands. The original sparse grass cover would most likely have included Sporobolus spicatus, S. arabicus, Cynodon dactylon, panicum turgidum, pennisetum divisum, Aeluropus lagopoides and perhaps Phragmites australis. Plant density in general was thin, and some of these species would only have survived in the shelter of huts or walls, where dampness percolated into the saline sand. The grasses would have been hardy, and the perennials, at least, stunted from overgrazing. The nearest open desert areas was, and is, Mafraq, a long journey across the subkha for a few days' foraging by flocks of sheep and goats.

In the 1980s, however, the picture is quite different. In addition to the cultivated grasses in parks and roadside verges, a number of indigenous species have colonised the Island and, in spite of frequent disturbance, survive and indeed flourish. It is true to say that grasses are among the most successful of plant families, being extremely adaptable to, and tolerant of, a rapidly changing environment. They evolved very early in the evolutionary recqrd, and most of the major groups are represented on all continents. The development of grasses since the end of the Cretaceous (where the fossil pollen record begins) has continued in genetic isolation on each continent. However, none of the species on Abu Dhabi Island are unique to this locality; all are recorded elsewhere in Arabia. The supreme adaptability of grasses to soil types, climatic variations, competition with other plants and resistance to persistent overgrazing makes it inevitable that they would spread throughout the town once the soil was disturbed and water and shelter were introduced on a large scale. The structure of grasses is such that root systems can in different ways utilise all available surface water; "many have developed rhizomatous systems. All grasses have resilient stems which grow only from the ba"se of the plant, plus reproductive systems that result in very high seed production. Many grasses have also developed a much higher rate of carbon dioxide intake by means of a system which operates at its most efficient only in hot climates.

The following lists are from locations on Abu Dhabi Island, drawn from ENHG records since 1978. No overall conclusions are drawn since the records are random in time and space. While some localities may disappear in their present form, however, it is reasonable to assume that all species cited will continue to exist in Abu Dhabi town in the foreseeable future, while others will no doubt be introduced.

A- Annual
B- Perennial
C- Common
R- Rare


1. Afforested Areas, Ungrassed and Irregularly Weeded/Irrigated

The main tree species here are Acacia, Ziziphus, Casuarina, Tamarix, Terminalia and Eucalyptus. Common perennial shrubs include Conyza pyrrhopappa, Cornulaca monacantha, Suaeda aegyptiaca and Heliotropium lasiocarpum.

Gramineae

Aeluropus lagopoides (P,C) -forms large mats, individual plants diverse in appearance, some being very postrate, others with long stolons and semi-ascending.
Cenchrus ciliaris (P,R) - only in remote corners. Much commoner in the open desert.
Dectyloctenium aegyptium (A,C) - ubiquitious throughout the island where surface water is freely available.
Echinocloa colonum (A) - not very common; thrives in very damp but not over saline sites, such as around young trees watered by drip irrigation.
Eleusine compressa (P)
Eragrostis barrelieri (A) - none of the Eragrostis species is common; found in isolated clumps in well-irrigated localities.
E. cilianensis (A)
E. papposa (A)
E. pilosa (A)
Eremopogon persica (A,R)
Phragmites austral is (P,R) - only recorded as deliberately planted in odd corners, especially as a windbreak around huts.
Setaria verticillata (A,R)
Sporobolus spicatus (P,C) -forms extensive clumps on compact soils; can tolerate high salinity.


I I. Old Sewage Farm

Most of these species have disappeared since the demise of the farm and saltwater flooding as a result of dredging and backfilling activities, but they were all represented up until early 1982 and all have been recorded elsewhere on the Island. Associated vegetation includes Typha domingensis, Lippia nodiflora, Acacia tortillis, Conyza pyrrhopappa, Frankenia pulverulenta, Scirpus maritimus, Fimbristylis sieberama and Suaeda vemiculata.

Gramineae

Aeluropus lagopoides (P,C) -both in the occasionally inundated areas close to the main outfall.
A. Lagopoides var. massaueniss (P,C)
Chloris virgata (A) - in isolated clumps.
Cynodon dactylon (P,C) - large colony in vicinity of C. virgata.
Dactyloctenium scindicum (A)

Echinocloa colonum (A) - beside outfall amongst Typha domingensis only. Polypogon monspeliensis (A) - conspicuous in March/April in scattered clumps. Rare elsewhere on the Island.
Sporobolus spicatus (P,C) - the commonest grass and the first to colonise mud flats reclaimed by dredging activities. The dominant grass species near the intertidal lagoon, in association with Arthrocnemum macrostachyum.

III. Ec-camel Yard Adjoining Al Manhal Palace

Abandoned at the end of 1981 this tiny area of some 1500 square metres or so was a haven for a number of annuals in 1982 because of the scattered heaps of ageing dung and straw and a leaking pipeline. Species included Melilotus indicus, Lotus cf. halophilus, Vicia monantha, Erodium neuradifolium, Malva parviflora, Trigonella hamosa and Anagallis arvensis. The site is now being built upon.

Gramineae

Bromus madritensis (A,R) - on dung heaps only.
Cenchrus pennisetiformis (A,R)
Chloris virgata (A,R)
Cynodon dactylon (P,C) - on fringes, able to tolerate poorer soil and more open conditions.
Eragrostis cilianensis (A) - mostly among M. parviflora.
Lophochloa phleoides (A,R) - on dung heaps only.
Phalaris minor (A,R). - on dung heaps only.
Phragmites australis (P) - planted against Palace wall.
Sporobolus spicatus (P,C) - in association with C. dactylon.

These are the Group's only recordings so far for the Lophochloa and Phalaris species.




IV. Various Minor Locations

a) Arabian Gulf Street
Aeluropus lagopoides (P,C)
Chloris virgata (A, C)
Dactyloctenium aegyptium (A,C)
Digitaria sanguinalis (A)
Echinocloa colonum (A)
Eragrostis barrelieri (A)

All were in prime condition in January 1982. The roadsides and central reservations have recently been regraded after severe flooding, and regeneration so far (Jan/Feb. 1983) is restricted to E. colonum.

b) Muqta Bridge

Phragmites australis (P)

A large area, approx. 80 by 50 metres is dominated by a colony of this species, flowering annually in December. Individual stolons are up to 12 metres long. The location is barely above high tide level, on mud.


c) Defence Area

Sporobolus arabicus (P,R)

A few clumps survive among Zygophyllum spp. in an area shortly to be developed.

d) Corniche, Old Radio Station

Halopyrum mucronatum (P,C)

Until early 1982 a large but isolated colony existed on calcareous sand just to the southeast of the mast, in association with Suaeda vermiculata, Dipterigium glaucum, and Halopeplis perfoliata. The area was levelled and backfilled in mid 1982 and all spp. apart from this grass were apparently destroyed. Many shoots reappeared towards the end of 1982 and there is no doubt that only severe disturbance will prevent the colony from re-establishing itself.



e) Corniche, East end

panicum turgidum (P,R)

This sp. survived along undisturbed parts of the Corniche until its redevelopment in 1982. P. turgidum is extremely rare on the Island and no specimens have yet been recorded in 1983.

The ENHG is indebted to the Royal Botanic Gardens at Edinburgh and Kew for their invaluable help in providing positive identifications.

 


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