In 1923, P.A. Buxton wrote ‘Animal Life in Deserts’, a pioneering work in which he defined a desert as an area where the climate is hostile to plant and animal life, in which normal agriculture is impossible, and where nearly all forms of life are modified to endure their peculiar environment.
In Abu Dhabi we have a variety of deserts.To the south we border on the Rub al Khali, the great desert of the Arabian Peninsula in which some areas experience drought lasting many years.Within the borders of the UAE we have sand desert with dunes extending throughout much of the interior. On the northeastern border we have stony, scrub desert criss-crossed with wadi beds, showing the lines of surface drainage. There is also the coastal strip of saline oolitic sand and subkha.
Professor John Cloudsley-Thomson in his book ‘Life in Deserts’ describes another method of delimiting arid regions. In tropical regions desert conditions are said to prevail where the average precipitation is under 5”(127mm) a year. From information provided by the meteorological office at the airport in Abu Dhabi, it is interesting to note that in the last eight years the highest rainfall recorded for any one year was 3 5/8” (90.2mm) that fell in 1976. Abu Dhabi is therefore will within the definition of a desert quoted by Cloudsley-Thompson. (Semi-desert conditions are said to exist in tropical regions where less than 15” (381mm) of rain fall in a year)
Desert environments are typified by intense heat, drought, high salinity in places and heavy grazing. Plants have evolved various mechanisms to cope with these. Let us take water shortage as the main problem of a desert environment. In this context Cloudsley-Thompson considers three categories. He discusses mechanisms that enable plants to tolerate a reduction of water in their tissues without permanent damage; methods by which plants are able to survive drought by modifying themselves in some way; and ways in which plants avoid drought by passing the driest season in their hardiest forms.
a) Not many plants can endure a reduction of water in their tissues without suffering permanent damage. Those that do are often found in moist habitats. These are the fungi, bacteria, mosses and liverworts. They become air dry in times of drought but can continue growth without damage when the dry period is over. The creosote bush, larrea divaricata, of North America and Mexican deserts, has the same property. Some onifers are also able to adopt this technique in times of drought.
Most plants however fall into the other two categories.
b) The ephemerals avoid the worst season by germinating quickly after the first shower of rain and completing their life cycles within a few weeks. The seeds are then left to endure the summer until rain starts off the next cycle.
In this connection we should mention the interesting methods some plants have devised of dispersing their seeds. Some plants, on the assumption that the conditions they successfully grew up in are probably the best for their offspring, retain their ripened seeds on the parent plant through the dry season until moisture in the atmosphere triggers their dispersal and the seeds fall close by and germinate rapidly. Other plants favor a wide dispersal. There are burrs that hook onto animal hair and thistledown parachutes that carry seeds on the wind. The wind is also used in other ways. Citrullus colocynthis, the Desert Melon, has a gourd fruit that becomes detached in the dry season and is blown about the desert by the wind until caught by a rock or in a hollow where it slowly disintegrates. The seeds are deposited in a sheltered spot in which rain will collect when the time comes and the seeds will germinate in favorable conditions. Anastatica hierochuntica, Rose of Jericho or Kif Miriam, is another interesting plant. After the flowering season the plant drops its green leaves and the naked woody branches dry and curl up like a fist enclosing the seeds. In time, moisture in the atmosphere then uncurls the fingers and the seeds disperse ready to germinate.
c) The last category deals with those plants that have evolved mechanisms for survival during the dry season.
Why is water vital to plant life? A plant’s protoplasm, or living material, cannot survive without water; in fact most tissue will not survive a reduction of water content or less than 10 percent of normal. Nutrients in the soil are dissolved in water, enter the plant through the roots and move from cell to cell within the plant by means of water. Chemical reactions take place in a watery medium. Water is a raw material in photosynthesis, and it keeps the plant in a healthy and turgid state. It is in a wilted state that heat, for example, can do a plant permanent damage. Plants can absorb heat and remain at a fairly constant temperature by giving off water through the breathing holes, stomata, in their leaves. This is call transpiration and is a normal and vital part of a plant’s functioning. However, plants generally lose about 90 percent of their absorbed water through transpiration. In fact, some plants can lose an amount of water equivalent to five times their own dry weight in one hour. The consequences in a desert environment are obvious.
Some plants have evolved methods of tackling this problem. In some the root systems have been increased to improve the intake of water. Some trees let down taproots to the water table below. (In some species this can be a depth of 150). Other plants have extensive but shallow roots to take advantage of each shower of rain, e.g., Cacti grasses and euphorbias. This is one reason for the spacing of plants in arid regions.
Apart from increasing intake, it is also possible to reduce the water lost. Some plants can control the stomatal openings in their leaves. Others reduce leaf size and by-pass the problem. Calligonum comosum and Leptadenia pyrotechnica are two plants that employ this method. Plants with “aphylla” in their name suggest the same mechanism is in use as in Periploca aphylla. (“Aphyllous” means naturally leafless.). Succulents such as one finds in the mesembrianthemum family, euphorbias and cacti all store water in their stems.
Improved translocution of water from cell to cell to cell thereby ensuring that supplies quickly reach the place where they are most needed is another means of avoiding permanent damage and keeping the plant turgid.
Finally plants in deserts are under great pressure from grazing animals. With less foliage to choose from, animals are more prepared to eat whatever is available. Some plants cut down the number of species that feed on them by making themselves as unpalatable as possible. They put out prickles and spines, exude bad smells and have unpleasant tastes. Some are poisonous, irritation or frequently, purgative. But they rarely manage to put off all predators. Other plants try to avoid notice by remaining very small, so that it is common to see desert plants in flower and fruit while still bearing their cotyledons, their embryo leaves.
The struggle for life in a desert environment produces some fascination adaptations. It is often possible to guess at the family of a plant but to be at a loss over the genus and species. The identification of the plants we have found in the U.A E. comes largely from Dr. Christopher Grey-Wilson at the Herbarium at Kew and the books listed below. I have also benefited from the advice of several of our own Natural History Group. This list is by no means complete nor are the identifications a hundred percent certain as most were made from slides rater than specimens. However, I hope this will give the amateur botanist something to work on.
|Genus and Species||Family||Common and (Local) Name|
|Anastatica hierochuntica||Cruciferae||Mustard (Kif Miriam)|
|Boerhavia elegans||Nyctaginaceae||Four o’clock|
|Calligonum comosum||Polygonaceae||Dock (Arta)|
|Calotropis procera||Asclepiadaceae||Milkweed (Ushurr)|
|Cistanche sp||Orobanchaceae||Yellow Broomrape (Thanoon)|
|Dipterygium glaucum||Capparidaceae||Caper (Ailqi)|
|Leptadenia pyrotechnica||Asclepiadaceae||Milkweed (Markh)|
|Monsonia nivea||Geraniaceae||Cranes bill|
|Rumex sp (cf.vesicarius)||Polygonaceae||Dock|
|Silene sp. (cf. Villosa)||Caryophyllaceae||Pink|
|Tribulus terrestris||Zygophyllaceae||Caltrop (Zahr)|
|Zygophyllum coccineum||Zygophyllaceae||Caltrop (Harm)|
This article and the talk given on 8th January were based largely on Life in Desert by J.L. Clouds-Thompson and M.J. Chadwick. Details of this and the other publications used are given below.
Cloudsley-Thompson, J.L. and Chadwick, M.J. 1946 Life in Deserts London.
Cloudsley-Thompson, J.L. 1977, The Desert London.
Dickson, V. 1955, The Wild Flowers of Kuwait London.
Fitter, R, Fitter, A, and Blamey M. 1974 The Wild Flowers of Britain and Northern Europe London.
Lipscombe Vincett, Betty A. 1977(?) Wild Flowers of Central Saudi Arabia Milan.
Mandaville Jr, J.P. The Journal of Oman Studies Special Report England.
Mandaville Jr, J.p Wild Flowers of Northern Oman London.
Migahid, A., and Hammouda, M.A. 1974 Flora of Saudi Arabia Riyadh.
Polunin, O. 1969 Flowers of Europe; A Field Guide OUP.