Most of the world's deserts lie in the sub-tropical belt, and range from totally arid to semi-arid areas. They are the home of some 384 million people, half the world's cattle and one-third of the world's sheep. It is estimated that 50-70,000 square kms of land is added to the present total of over 48 million square kms annually; i.e., the earth's deserts are expanding.
Abu Dhabi is virtually all desert. The temperature range is from below zero to 50°C, with surface sand temperatures reaching 70°C on occasion. Rainfall is minimal. In the winter of 1978-79 20 millimeters was recorded at Medina Zayed Meteorological Station just north of the Liwa. Winds can regularly reach 25-30 knots with gusts of up to 40 knots or more. Soil is very sandy and salty, with little organic matter. Subsoil is very saline. In addition there is the pressure of man's presence and grazing animals.
The flat areas ('subkha') along the coast are completely devoid of vegetation. Mangrove vegetation consisting of Avicennia marina mostly is present on inundated muddy flats along the coast and coastal islands. Mangrove growth has been considerably destroyed and degraded on account of past over-exploitation.
In the comparatively elevated transition belt from subkha to sand dunes, the most commonly found species are Zygophyllum spp. ('harm'), Haloxylon salicornicum ('rims') and H. persicum ('ghadha').
In the interior sandy dune areas the sparse clump vegetation consists of trees such as Prosopis spicigera ('ghaf'), Acacia tortolis ('samar'), Acacia raddiana ('salam'), Phoenix dactylifera ('nakhal' the date palm), Zizyphus spinachristi ('sidr') and bushes and grasses such as Calligonum comosum ('arta'), Leptadenia pyrotechnica ('markh'), Tribulus terrestris ('zahra'), Cyperus longus ('khadram'), Pennisetum divisum ('haad'), Panicum turgidum ('thamam'), and Aristida spp. Whereas H. persicum is a plant indicator of highly saline subsoil water, C. Comosum indicates 'sweet' underground water.
The natural vegetation has been considerably damaged and in some places completely destroyed on account of overuse and lack of proper management.
In the wake of commercial exploitation of oil, resources became available for the taming of the inhospitable and advancing desert in Abu Dhabi. Sustained and steadily rising funds have been provided for afforestation and development of agriculture since the late sixties. In 1978 some 77 million dirhams, about 65% of the total allocation for the agricultural sector, was spent on afforestation.
A desert afforestation technique has been worked out in the course of time under the special environmental conditions prevailing locally. Because of the harsh climatic and edaphic factors strong plants are essential, so adequate nurseries have been established, as at Bujair between Bida Zaid and the Liwa, where there is a good supply of 'sweet' water (around 900 -1000 ppm). Four local species are concentrated upon. These are A. tortolis ('samar'), P. spicigera ('ghat'), and A. arabica ('qarat') which are all hardy but slow-growing and Z. spinachristi ('sidr') which is less hardy, requiring a water supply not exceeding 500 ppm in the early stages. Other local plants used less extensively are C.comosum ('artha') and L. pyrotechnica ('markh').
Water is pumped either from open shallow wells or from deep tube wells, and all irrigation is carried out with drip or trickle systems. The drip lines are 7 meters apart and the plant-to-plant distance is also 7 meters. Thus about 200 plants per hectare are raised. In the first year each plant is given about five gallons daily, subsequently increased to 10 gallons. .The date palm (P. dactylifera) requires two -- three times the water glven to the four arid zone species mentioned above.
The salinity of the sub-soil water varies considerably. Very near the coast the salinity can be as high as 50,000 ppm, at Habshan it is around 11,000 ppm, at Medina Zayed, it is 6,000 ppm and in the Liwa it is frequently less than 1000 ppm. With high salinity only high salt content tolerant species like Prosopis juliflori ('ghuaif') and Tamarix aphylla ('tamarix') can be successfully raised.
High winds and shifting sands also do a considerable amount of damage to young plants. For this reason the young plants are protected by providing them with suitable tree-guards. The saplings are later provided with wooden stakes until their sterns are firm enough to withstand the bending and tearing effect of severe wind storms. Wind breaks are also raised along the outer edges of plantations. The outer three rows of this shelter belt are grown with species like Eucalyptus spp, Casurina spp, z. spinachristi, P. spicigera and A. arabica. Plants and rows are three and one-half meters apart.
At the time of planting each pit is provided with about five kilos of compost. Subsequent. fertilization, when the plants are well established, is with urea in the irrigation water. Suitable pesticides have also to be used against any insect and fungal pests and diseases.
Tending operations in the form of scraping salt incrustations from arounct the base of the plants, hoeing and pruning are done as and when required. Interplanting with fodder species such as Atriplex spp, C. comosum and L. pyrotechnica is also done after the tree species have become well-established. The policy of interplanting was taken up in 1977, and the concentration on local species including grasses has meant a considerable natural recovery within fenced plantations. In some of the well-vegetated larger plantations, such as al Babha between Habshan and Bida Zaid, gazelle have been successfully introduced. As a consequence of these man-made plantations there has been a visible increase in the natural wild life populations. The wild hare is especially on the increase.
The plantations are being raised in the vicinity of new villages and towns where the nomadic beduin population is being settled. Besides a house, each beduin family is given a date garden of about two hectares. These gardens have a shelter belt of trees on their periphery and two rows of date palms. The middle portion of the garden is left open to be cultivated by the settled families for raising fodder, grain and vegetables.
It must be stressed that despite the successes so far, much of the work is still at an experimental stage and Abu Dhabi would benefit by an international symposium on desert afforestation. Points which could be raised and discussed include optimum water requirements for individual species, wind-break species, suitable ecotypes for both indigenous and exotic (imported) species and management of natural vegetation. There is also a strong need in Abu Dhabi itself for further ground water surveys and collection of agro-meteorological data particularly in the micro-climatic conditions obtaining in various parts of the Emirates.
(Much of this information is contained in an article by Mohamed Khan submitted to the Eighth World Forestry Congress held at Jakarta, Indonesia, October 16-28, 1978.)List of preserved plant specimens indigenous to Abu Dhabi