Bulletin 32 - July 1987: Avicennia Marina -- The Grey Mangrove

Avicennia Marina- The Gray Mangrove

General Notes and Observations

Mangroves fall into at least twelve genera in eight different families.

In the past the mangroves of Abu Dhabi have erroneously been called the Black Mangrove, the correct name for which is Avicennia officianalis. Those present in Abu Dhabi are the Grey Mangrove, Avicennia marina.

Mangrove is a West Indian name given to a formation of trees inhabiting the coasts of tropical or sub-tropical seas. Coastal marshes of tropical regions are usually characterized by a mixture of mangrove communities and other salt-tolerant species or halophytes. Certain areas of the UAE intertidal zones conform to these characteristics.

The northern global limits of distribution are generally between the latitudes 29° - 30'°N. In the southern hemisphere distribution is about 38'S in Australia and 44'S in New Zealand.

The UAE coastal areas lie roughly between 24° and 20°N, though the country as a whole is between 23° and 26°N. The northernmost stands are found in tidal lagoons in Ras al Khaimah, about 26°N. It is suggested in one report that the mangroves of the UAE cover an area of a few thousand acres, from Kalba in the east, Ras al Khaimah in the north, to Abu Dhabi and other islands in the west. The bulk of the trees are in Abu Dhabi Emirate.

Geographically it is temperature that limits the distribution of various mangrove species. In areas where the average temperature is below 19°C, mangroves will not survive. Although the mangrove is a tropical plant, in areas where the waters are constantly warm certain species may move into bordering climatic regions beyond the tropical regions. One variation of Avicennia marina, var. resinfera is found on sites in New Zealand that have occasional freezing temperatures.

In the habitats where mangroves are found, mainly the intertidal zone, they are subjected periodically to quite large variations in water levels. As a result of these tidal movements, at high tides some of the smaller mangroves are fully immersed, some have only the lower foliage covered while the fully-grown have their canopies above water level. To give some indication of the varying depths to which the Abu Dhabi stands are subjected, on 18th January 1987 the variation between low and high tide was from 0.23 m to 1.88 m.

Generally the formation of new stands of mangroves, away from the area of origin, is dependent on the seedlings being carried by tidal action. Their ability to survive periods of immersion allows them to invade suitable coastal niches where they become established and commence the development of a new formation. Areas around Abu Dhabi Island where new stands are developing are quite numerous.

Once the plants are established the accompanying growth of dense patterns of pneumatophores and root systems restricts the tidal flow of water throughout the developing habitat, causing a build up of silt and fine clay minerals. Consequently, a new shallow, muddy coastal habitat is formed, thereby encouraging additional plant species to establish themselves. With the build up of mud the coastal area becomes stabilized, allowing other marine organisms to develop within the habitat.

From a distance, a stand of mangroves looks just like a small wood on the seashore, and closer inspection does not initially reveal its inner secrets. The habitat shelters a complete ecological system and provides the first link in the detritus food chain of marine organisms that give nourishment to the inhabitants of the intertidal zones.

All the organisms that are present play an important and contributory role in the life cycles of the habitat. The algae covering the mud flats between the mangrove stands helps to bind mud particles together. At low tide it becomes exposed, dries and cracks but when washed by the rising tide it is rejuvenated. Even the colour of the mud is dictated by diatoms, microscopic organisms that release oxygen.

The small crabs which inhabit the mud flats and creek banks play a dual role. They remove rotting organic matter, and with their burrowing to make shelters for themselves they aerate the substrate. The mangroves contribute too; their falling leaves fall among the pneumatophores, become trapped and decompose, while the flowers provide nectar for the honeybees that establish colonies in the trees.

The shallow waters provide a haven for small fish during their formative years until they are large enough to make it to the open sea. Once there, of course, these fish have commercial value as part of the food chain for the human population. This marine life in the habitat also provides abundant food for birds on migration as well as residents.

The floor of the substrate of the habitat is of a heavy silt or mud composition containing large amounts of organic material. The consumption by microorganisms of the small amount of oxygen in this soil makes it practically oxygen free. When disturbing the mud of the substrate sometimes a strong sulphur-like smell is produced. This is the gas hydrogen sulphide that is produced below the mud surface by bacteria living there without light or oxygen. Plants that have not developed a system to cope with these saline and anaerobic conditions would be unable to survive.

Mangroves, along with other plants of the swamps, have developed systems to withstand such conditions. The depth of the mud and silt is governed by the maturity of the stands; from being shallow during the early formative years it builds up gradually until, as in the older stands in Abu Dhabi, it becomes quite deep and makes human progress difficult. As the stands grow, their outer limits expand as they reclaim more land from the sea.

A very critical period in the life cycle of a plant is the time of dispersal and germination of the seed. Dispersal by wind, water etc. is a major factor determining the existence of plants in a specific location. The saline and anaerobic conditions in which the mangroves thrive are completely unsuited to plants that have not developed a survival mechanism adapted to such conditions. Even some of the other halophytic (salt tolerant) species will not establish themselves in an area until the mangroves have first colonized and created adequate conditions. Among the halophytes, various methods of seed dispersal are employed, most of which make use of running water. This dispersal, with the aid of currents and tides, and the attendant ability to survive under these conditions, has led in numerous cases to worldwide distribution of certain species.

To overcome the problem of a seed being able to germinate under saline and anaerobic conditions, which are the norm for the mangrove, a special mechanism is required. Otherwise, young seeds would fall into the mud, sink and suffocate.

The mangrove solves this problem by being viviparous, which means producing seeds that germinate while still attached to the parent plant. During germination, the seedling receives nutrients and water from the parent tree and it grows uninterrupted until, becoming too heavy to bear, it falls off into the mud beneath. Seedlings, or propagules, thanks to their ability to respire anaerobically, can stand long periods of inundation. Some of the seedlings are trapped in the web of pneumatophores beneath the parent tree while others are carried away by tides to near or distant new sites where, if conditions are correct, a new community is established. The seedling feeds off the propagule until such times as its roots develop sufficiently to anchor it in the mud and supply nutrients from the substrate. The remnants of the propagule eventually drop off.

Fig. 1 A seedling with an established root system. The seedling is now anchored and receiving nutrients. The young shoot can be seen emerging.

Fig. 2 A seedling with well-established root systems showing its growth beyond the propagule plus the first two leaves.

Fig. 3 Continuing seedling growth but now with four leaves. The propagule remains attached for quite some time but at this stage possibly all nutrients are being received via the root system.

Fig. 4 A very healthy seedling showing a well-established root system and plan diagram of the propagule.

Although the vast majority of new growth is from seedlings, some growth does originate from the pneumatophores. This of course prevents dispersion beyond the root system of the parent plant.

Fig. 5 Simplified diagram of the mature mangrove root system. 1. Main trunk. 2. Pneumatophores. 3. Nutritive roots. 4. Support roots. 5. Cable roots.

The system of pneumatophores, or aerial roots, allows the mangrove to survive in conditions of minimal aeration. The roots contain a highly developed system that enables the rapid transport of gases through a pneumatic tissue and contains air ducts, which connect the aerial roots with the main root system and the atmosphere. During high tide water is unable to enter these air chambers.

The mangrove has also developed a very complicated and as yet not fully understood method of surviving in saline conditions. Secretion of salt via glands on the undersides of the leaves is often obvious to the interested observer.

Below the surface, in the substrate, the mat of roots is woven from the pneumatophores, 'nutrient cable' and support roots. It must also be remembered that these root systems hinder the current flow, thereby also contributing to build up of silt.

Fig. 6 An illustration of the actual root system as found in the substrate. All roots noted in Fig 5 are visible. As mentioned, propagation can be via the pneumatophores; proof was difficult to find and this one was the result of 20 attempts. In this case the growth is distorted when compared to the seedling growth in Figs. 1 - 4. On the pneumatophore and plant stem barnacles are shown. These are related to crabs; instead of using their legs for walking, they use them for catching, and they are only extended when covered by water. Figs. 6-1 and 6-2 show cross sections of pneumatophores and roots. These are of a spongy, flexible material, given strength by tiny cylindrical wooden strips within.

Fig. 7 Simplified diagram of a salt crystal that has been secreted from a gland on the underside of the leaf.

Fig. 8 Formation of buds in late April.

Fig. 9 Flowers and buds in late May. Not long after this, in June, it becomes apparent that the majority of flowers had died on this particular plant.

The mangrove stands in the northern and eastern areas of Abu Dhabi Island are well established and expanding within the intertidal zones. Interspersed with other halophytes, such as Arthrocnemum macrostachyum, these areas present a very delicately balanced ecosystem, susceptible to disturbance and change. Man, unfortunately, is the agent of greatest change.

One example of the disturbance of this balance by man stands out. A few years ago a motor vehicle was driven through the mud flats destroying part of the algae mat and bushes, leaving deep tracks and finally became bogged down. Today, those same tracks are still clearly visible and the damaged areas have, as yet, shown no signs of recovery, and possibly they will not for years to come.

Mangroves are also subject to various forms of stress that can affect their development. During 1986, in the stands closest to the old Abu Dhabi sewage farm, it was observed that very few propagules formed after flowering, and those that did were of very poor quality. During the season from bud to flowering stage progress appeared normal. Insects and bees associated with the pollination of the flowers were active within the stands. The majority of flowers reached full bloom. A few were affected prior to this stage and died; others turned brown and died after full bloom. In some cases the flowers were covered with a white dust or powder, and others with cobwebs.

Whether the construction of the new road in close proximity to the stands, the discharge of pollutants by tankers or a purely natural phenomenon affected the apparent disease is not known. The stands to the north were not investigated but it is known that the growth of the stands on the island of Abu Al Abyad several miles to the west of Abu Dhabi town was normal and healthy. The problem was probably of a localized nature, and confined to the sewage farm area.

In the past mangroves have been utilized, often to their detriment, as fodder, fuel and occasionally commercially for timber and until recently were ignored scientifically. But now more time is being spent on research and experimentation into what one scientific paper referred to as a "botanically very monotonous" plant. Fortunately worldwide as well as in the UAE and Abu Dhabi in particular, experts are at work researching with mangrove and other halophyte cultivation techniques that must ultimately benefit mankind. The research is exciting in that indigenous species are being examined. Although to some the natural species is not as pleasing to the eye as some imported exotics, the vast savings on the economic side, in water and maintenance alone, will be of great consequence. Regretfully, in many parts of the world, economic aspects still dictate to ecological considerations.

Fig. 10 Enlargements and cross section of stems showing box-type construction.

Fig. 11 Cross-sections of flower, showing the four stamens.

These notes and observations are intended only as a simple thumbnail sketch, which might hopefully prove of interest to members. Literature from which to explain and fill in gaps in the observations is scarce in Abu Dhabi. If nothing else, perhaps this article will provoke interest in order to correct any erroneous information or, better still, to continue the research and amplify upon points made in future Bulletins.

One anomaly that needs explaining is why, in 1985, propagules on one tree were seen fully formed and ready to drop, when surrounding trees were only in flower. In October one tree was observed in full bloom while the others were bare.

The illustrations, with the exception of 9 and 10, are my own and cover a period of a few months in 1985 and 1986.

My thanks go to Ted Burton for his great patience in answering my whys and hows.

If you do decide to walk about in the mangroves, please follow wherever possible the gravel-bottomed waterways in order to minimize damage.

Burton, E., 'Wetlands Conservation Project on Abu Dhabi Island'; initial report presented to H.E. Sheikh Nahyan bin Mubarak al Nahyan, November 1986.

Kurschner, H, 'A Study of the Qurm Nature Reserve, Muscat Area, Oman', in Arab Gulf Journal of Scientific Research, Vol.4, No.1, June 1986.

Waisal, Y. 'Biology of Halophytes', Academic Press.