Bulletin 36 - November 1988: Adaptation of Plants to a Desert Environment
Adaptation of Plants to a Desert Environmentby R.A.Western
The climate of the UAE is classified as arid to semi-arid but tempered by the maritime influences of the Arabian Gulf and Gulf of Oman. Temperatures can rise to 49 C in July, but humidity remains relatively high throughout the year. Rainfall is very variable from year to year and from place to place; the annual total may fall in as few as three days in parts of the country. The physiography of the UAE is diverse, however, ranging from vast sabkha plains along the Gulf coast, to more fertile rangelands in the centre-north, to mountains parallel with the eastern seaboard.
No complete survey of the country's vegetation has yet been completed, but some 400 species have been identified and recorded by members of the ENHG and others. A guide to some of the plants of the Al Ain region was published in Arabic on behalf of the Emirates University in 1983. This article outlines some of the ways in which vegetation has adapted to the local environm2nt, though these adaptations of course are not unique to the UAE.Characteristics of Desert Vegetation
To a newcomer, plant life in the UAE seems scarce, not particularly attractive, and very seasonal. Compared with temperate lands there is a distinct lack of trees, of continuous grassland, and plant communities appear stereotyped and rigid, leading perhaps to the assumption that there exists here a primitive vegetation climax. This is not so. The plants which grow here, whether individually or in communities, are the result of a series of very successful adaptations to local conditions. Each habitat, bordering salt flats, among the high dunes of Liwa, in crevices on Jebel Hafit, in the steep wadis of Fujeirah, or overlooking the east coast, contains its own distinct plant association, but this remains only while prevailing conditions last. The vegetation cover of an arid zone fluctuates according to a number of variables, notably climatic change and the influence of man. Millennia ago huge river and wadi systems crossed the Arabian peninsula from west to east, and these relatively narrow channels were the conduits along which some of the present-day species of the UAE reached eastern Arabia. There is also evidence of migration of plant species in the past from the Horn of Africa and up through Yemen and Oman; and also from the high mountains of Southern Iran across the Strait of Hormuz. As far as man is concerned, his influence in the past can be seen in the denuded forests of Oman and eastern UAE, during periods (notably around 2500 BC and 1200 AD) when vast amounts of charcoal were required to promote the local copper ore extraction industry. On a more recent and smaller scale, many of the 'wild' desert species of plant recorded on Abu Dhabi Island in the late 70's and early 80's no longer exist; their habitats are now houses or parking lots.
Just as man's influence can be very destructive in the short term, so a temporary climatic change can also affect plant life. For example, it is probable that a steady increase in rainfall over just a few years would very likely result in the establishment of a greater number of perennials. This is dramatically illustrated by the vast spread of Zygophyllum hamiense along the Abu Dhabi to Al Hair road, attributable to the heavy spring rains of 1982 and 1983, reinforced by those of 1987.
The raking of surface soils and gravels for the construction industry has left large patches of eroded and depleted desert, an increase in windborne particles and the disappearance of some species of the natural vegetation, which relies on a minimum soil depth. In areas Where there has been severe flooding, a similar effect is observed. Even in the most violently-disturbed habitats, however, the resilience of some species to make a comeback, apparently against the odds, is a tribute to nature's pattern of survival.
There is not the same distinction of seasons in the UAE as in temperate climes. The transition from winter to summer can be rapid and plants adapt by responding to certain stimuli, of which a rise in temperature may be more vital than the presence or lack of rain. Given a sufficient soil depth, well-established species as well as seeds experience a catalytic impetus towards a period of growth. Of course soil quality -- depth, coarseness, acidity and salinity -- is also important. Throughout the UAE natural organic matter and nitrogen are content in soils are limited, and very few localities possess any significant depth of soil stratification. Extremely arid areas may have blown, unsifted soils as with the red aeolian quartzite dunes of the Liwa, and less marginal habitats will still have a thin depth, while calcium carbonate deposits occur at a shallow depth in most areas of the countries.
There are three basic types of vegetation that are able to cope with such conditions: ephemerals, succulent perennials and woody perennials.
These consist of herbaceous, non-woody species and their method of adaptation is literally to opt out for a certain period of the year. They constitute at least half of all plant species present in the region, and typically have a very short growing and reproductive season. Root systems are shallow and the plants themselves mainly small, but they grow rapidly, mature in a short period and produce copious amounts of seed. The seed numbers are necessary to permit a sufficient amount to survive and so perpetuate the species. Many are of course eaten by animals, birds and insects and some fall onto the proverbial stony ground. In the UAE there are both spring and winter ephemerals, but the vast majority and those making the biggest visual impression appears as the temperatures gradually rise from the end of February through to the end of April.b) Succulent Perennials
Succulence occurs when the outer leaf or stern cells enlarge to increase water storage volume. A waxy layer on the outside surface prevents moisture loss as well as lending extra physical support to the leaf or stern. The leaf stomata, or breathing pores, are usually closed in the heat of day. Such plants are very common along the coasts and bordering the sabkhas and inland depressions. They can usually tolerate relatively high salinity levels, and may remain evergreen and fleshy throughout the year. Many of them flower in late summer and autumn, but the petals are generally miniscule, yellow or white. These petals should not be confused with the papery fruit wings which follow on several species, such as Salsola and Suaeda.
c) Woody Perennials
These are the dominant plant type in terms of size, since trees and most of the larger shrubs are included in this category. Numerically, however; they constitute the smallest type. All species within this type are tough, able to cope with heat, wind and drought, as well as herbivores. These plants are slow growing, but when they appear dormant or even lifeless, growth usually has been transferred to the root systems in order to tap deep aquifers. Surface activity is thus much reduced during periods when the equilibrium is disturbed; the plant goes into a period of semi-'hibernation'. Seed production is less than for the two previous types but tend to be individually larger and very tough. Some seed coats need to be damaged, e.g. by rolling stones or even by the digestive juices of certain animals, before germination can occur.
A high proportion of plant species in the UAE differs markedly in appearance from plants in temperate countries. There are far fewer tall trees, some plants appear leafless, hairs and spines and prickles are more numerous, and flowers are often inconspicuous. While general aridity can never be overlooked as an important factor, the local habitat and especially soil conditions are vital influences in determining individual species adaptations, which are specifically designed to conserve and control the movement of water, oxygen, carbon dioxide and other chemicals through the cell systems.
Woody perennials such as trees usually have deep root systems. This is true of local species such as Acacia tortilis, Prosopis cinerea and the date palm Phoenix dactylifera. The roots of the younger plant are often longer than the shoot in order to seek out a water layer which the plant can then tap permanently. Hence such species are slow-growing above ground while they are becoming well-established below the surface. In shallower soils some species possess large lateral root systems which are tough and spongy. This applies to many perennial desert grasses, and the roots are often exposed by sand movements, sometimes covered with damp sand particles.
In many parts of the country the vegetation is typically scrub-like, with plants dotted all over but with patches of bare sand or soil between. Beneath this surface there exists an intricate lacework of lateral roots interspersed and not in competition with the long vertical roots of larger perennials.
The short shoot, long root pattern is commonly in the ratio of 1 : 6. The shoot is usually hard and tough and may bear spines or prickles, perhaps as a deterrent to herbivores. An example is the succulent Anabasis setifera. It is probable that some chemical compound provides a further deterrent. The seedlings of Hammada elegans can appear as a reddish carpet in their tens of thousands, yet these succulent patches do not seem to be attractive to grazers; rather, the majority die in the competition for available nutrients and moisture before the onset of summer.
Leaf modification is one of the most visually-obvious examples of adaptation. Many salt-tolerant species, or halophytes, found near the coasts and in inland depressions, bear leaves which are globular, or at least very fleshy in appearance. When squeezed these leaves produce copious sap at any time of the year. The expansion of the outer leaf cells means that water can be stored for long periods, an obvious benefit during long, dry summers. These outer surfaces usually appear shiny because of the presence of a waxy cuticle designed to prevent water loss. The amount of stored water varies but can be much more in terms of volume by night than by day. Hence many herbivore species such as camels and hares tend to be largely nocturnal in feeding habits.
Some desert plants have very reduced leaves. This is true of several bush-like species such as Leptadenla pyrotechnica and Calligonum comosum subsp. comosum, both common in the Liwa and of trees such as Periploca aphylla ('aphylla' = leafless) and Moringa peregrina, both common in the northern hills. In some species leaves are needle-like, such as Blepharis ciliaris and the older leaves of Zilla spinosa. A small leaf area means less surface exposure and therefore less risk of too much moisture loss. In plants which are virtually leafless, normal physiological processes such as photosynthesis are carried out by the stems. In times of prolonged drought, leaves of some species curl or roll up or die off slowly from the tip back to the stalk. In such plants only some leaves are affected, and younger leaves tend to survive, thus ensuring survival for the plant. This is a notable feature of Oleander species and Dodonaea viscosa.
These may be deeply grooved, as is the case with some halophytes such as Anabasis setifera. The stomata in these grooves are unable to open widely, thus reducing the risk of over-loss of moisture. Some stems are jointed at frequent intervals, particularly so on fleshy succulents. This lends extra support to the plant since such joints are usually quite rigid.
Stems and leaves may be covered with dense hairs, forming a micro-habitat which traps moisture and also acts as an insulator. Hairs are also unattractive to smaller herbivores. Numerous small spines, as on Cornulaca monacantha, may have a similar effect. Some species, when young, bear fleshy leaves which only become spiny later. An example is Zilla spinosa, which looks very thorny, but a closer inspection reveals fleshy, flattened leaves on the protected inner part around the main stem. The stems of true xerophytes are often very woody as a result of lignification, or the hardening of the outer cells which die to leave a rough bark, while the inner cells continue to function normally. Calligonum comosum subsp. comosum may appear to be completely dead except for a single tuft or two of needle-like leaves, but each branch is full of sap and very difficult to twist off. Such branches are a common sight wherever camels have passed by and fed on these bushes.
The presence of conspicuous or miniscule flowers is largely determined by the pollination method adopted. Desert plants which flower in the summer or autumn are more likely to be wind-pollinated because of a relative lack of suitable insect agents at that time of year. There is thus no need for showy inflorescences. Most of the Salsola and Suaeda species present in the UAE are good examples. Exceptions are Tribulus species in the sands and Capparis species in the mountains, both of which bear large attractively-coloured flowers. Needless to say, these two species are insect pollinated.Photosynthesis
Normal temperate region plants absorb carbon dioxide by day via the leaf stomata. Plants with a very strong resistance to arid conditions, however, do not conform to this pattern. In these plants there is a delay in the transfer of carbon dioxide, and hence the process of photosynthesis is delayed. Instead of opening by day, the stomata open by night when gas interchanges take place, and water is used much more efficiently. The absorbed carbon dioxide is temporarily stored overnight and then transfused through the plant during the day while the stomata are closed. This phenomenon occurs in several common UAE species including Portulaca oleracea, Aizoon canariehse and Citrullus colocynthis. This chemical adaptation is interesting for it is a step beyond the normal physiological modifications to leaves, stems and roots, but it also restricts plants to their arid environments. Hence the difficulty of transplanting desert species to more nutritious soils in temperate regions of the world. However, there is some evidence to suggest that plants have the ability to choose their photosynthetic cycle according to environmental conditions and pressures. If aridity increases, some species can apparently adapt correspondingly and use water much more sparingly -- hence they become less useful as fodder, containing less sap. Several species which are continually over-grazed (especially grasses) also tend to conform to this pattern.Conclusion
This short account does not attempt to give justice to the subject and a lot more work needs to be done with individual species in different parts of the country. Plants are supremely adaptable and have evolved mechanisms of survival over an extremely long period. Members of the ENHG are encouraged to select plants of the same species in different parts of the UAE arid to observe their progress over a period of time, comparing and noting growth patterns, leaf shape and size, flowering and fruiting periods and so on in various habitats, from plantations and home gardens to the open desert. Only by meticulously recording such progress can data be obtained from which it may be possible to draw further conclusions.
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