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Abstract By virtue of their nature, mangrove trees grow in tropical and subtropical areas. With the availability of these shrubs naturally in several coastal areas in UAE, and neighboring countries, it is believed that if they are properly utilized, they can reduce desertification impacts. They also contribute in improving coastal areas as natural habitat for birds, fisheries and other forms of life. In addition to providing beautiful view of the shores. The availability of a long coastline, and a large number of islands, lagoons and natural water impoundment in UAE, and the ease with which these shrubs can be cultivated, tempts to consider planting them in large numbers, in every possible location, so as to utilize their potential to reform the environment. In view of their physiological adaptation to local climatic conditions, and to their contribution in preserving the bio diversity, of both marine and terrestrial lives, facts indicating the importance of incorporating mangroves in plans to reduce desertification effects, and increasing the green area in UAE are outlined. The consequences of such undertakings on fisheries and marine life, and their use in areas remote from the seashore, are particularly emphasised. Cultivation and salinity tolerance of the local mangrove species Avicennia marina has been reviewed also. Introduction Mangroves are a group halophytic species of trees that grows mainly in tropical and subtropical areas. They usually grow between high spring tides and mean seawater level. There are several definitions for this group of plants, but we would like to refer to a definition stated by Choudhry (1997) …"Mangroves are the characteristic littoral plants, grouped under unique ecosystems, inhabiting mostly the sheltered coastlines, within the intertidal zone of tropical and subtropical regions". Mangroves are generally found along the coastlines of tropical and subtropical regions, usually between 32úN and 38úS latitude (Anwahi et al, 1998). Choudhry (1997), however, indicated an exception to these areas, where mangroves could be found as far as 25úS, in New Zealand, and as far as 25úN, in Japan. As such they cover a wide area of distribution, worldwide. The distribution, zonation, and diversity of mangroves received thorough investigation by several authors ( Macnae 1968, Walsh 1974, Chapman 1976, Semeniuk 1983, and Ruwa 1993). Fresh water in the form of rain, frost, dew, or fog is crucial to mangrove, since these amounts of fresh water, however small, contribute to washing their leaves from the salt accumulating on them due to their respiration and salt exclusion. Benefits and Uses Mangrove trees and shrubs constitute a very useful ecosystem, in addition to being of substantial economic value in many ways. Its most important benefits are
Of the benefits that characterize mangrove swamps is their high primary productivity to their immediate environment. The falling leaves of mangrove trees constitute the greater element in this respect. Several estimates of mangrove areas productivity were reported for different areas, specially the estimate of 930 gm/m2/year (Golley et. al. (1962). They are equipped with natural mechanisms to absorb various degrees of saline waters in order to extract water from varying degrees of saline water, and for their roots to live in oxygen deficient soils. As a group, they are known to live in several coastal areas, and river basins. Usually they grow in wetland and marshes of saline and brackish water impoundments. They require calm areas and enclosures to grow, but they can not tolerate exposed areas of rough wave action (Embaby, 1993). Those plants are naturally equipped with special mechanisms enabling them to uptake salt water in different concentrations, extracting water molecules from it, and excreting the salt from their leaves. Mangroves in the United Arab Emirates In the United Arab Emirates, which is country characterized by arid climate, mangrove shrubs and trees of the species Avicennia marina (gray mangrove) occur naturally along several coastal areas and islands in the country. Mangrove colonies, showing signs of healthy growth are present along the near shore islands and lagoons of: Abu Dhabi, Umm Al Qaiwain, Khor Kalba, Ras Al Khaima. The estimated mangrove areas in these localities are: 2500 ha, 200 ha, 150 ha, respectively. With narrow colonies in Ajman ( 20 ha), Ras Al Khaima (20 ha) (Rabanal and Beushell, 1978). In Dubai, efforts to restore the natural mangrove swamps were successfully carried out. For example, Dubai Municipality reported planting over 40000 seedlings in Ras Al Khor area during 1993, with majority of seedlings brought from Abu Al Abiad island in Abu Dhabi. Follow up of these cultivation indicated that the shrubs are growing very well, and they have flowered and produced porpagules. Currently, Ras Al Khor area is considered a flourishing protected mangrove swamp, that hosts a variety of marine and bird ( flamingos) types. A nursery for mangrove seedling production have been established by Dubai Municipality, and on Abu Al Abiad Island (in Abu Dhabi), which provide a good number of seedlings for cultivation each year. Embaby (1993) reviewed the geographical distribution of mangrove in UAE, using satellite images and topographic maps. He found that certain environmental aspects characterizes the areas where mangrove colonies are present. He further classified these aspects into four categories: climatic conditions, geomorphological features of the area, water characteristics, and man made modifications. He concluded that the environmental and geomorphological conditions prevailing on UAE coasts are favorable for the growth of A.marina, which can apparently tolerate water of high salinity, and dry weather conditions satisfactorily. These trees and shrubs have been traditionally used by the natives as a source of: fire wood, housing material, charcoal, and fodder for their animals. Far important than these obvious forms of utilization are the ecological gains that these mangrove colonies have sustained the environment in the country. The substantial fishery resource the country enjoys, which amounts to an annual production of about 114000 tons in 1998, could partially be due to the positive effect these colonies provided in the form of a very productive and protected nurseries for juveniles of many fish and crustacean species. Not only fisheries which have might benefited from these ecosystems, it is believed that they have contributed to a great extent in protecting the coastal areas from erosion by wave action. In addition they are providing a natural habitat for nesting birds. The Ministry of Environment and Water , UAE, realized at an early stage, the importance of mangroves as an important component of the arid environment of the country, and noted their natural occurrence in several parts of the coastal area in the country. On basis of that, a mangrove afforestation program was launched as early as 1985, where thousands of propagules and seedlings are cultivated at different localities in the country every year. While in Abu Dhabi, afforestation efforts started much earlier. A prominent feature of this program is the continuous monitoring of a colony of A.marina cultivated around the outlets of the fish culture grow out ponds, part of the facilities of the Marine Resources Research Center( MRRC) of the Ministry of Environment and Water , in Umm AL Qaiwain. The size of this colony is approximately one hectare, and it could be taken as evidence to the success of such endeavors in the country. Furthermore, and as an indication to the role mangrove swamps can play as a nursery ground for various fish and crustaceans species, Tamaei (unpublished data) reported the occurrence of several important fish species larvae in mangrove cultivated area off MRRC, such as mullets, snappers, groupers, sea breams, rabbitfish, baracuda, and majarra. These observations might have a profound dimension on the Ministry of Environment and Water endeavors to conserve and develop fishery resources in the country. A sowing and cultivation technique for A.marina have been developed, the technique involves culturing propagules, and transplanting seedlings. Summary of this technique is provided below (Tamaei, 1999): The season for cultivating A.marina should be carried out during the period August to October. For seed sowing
For transplanting seedlings Seedlings should first be cultivated in agricultural pots of 10 cm diameter, and 20 cm height, filled with normal beach sand. The pots should be kept in shade with running seawater for 30 days to avoid dehydration. Water should not exceed half the size of the pot. For planting seedlings do not remove the plant out of the pot. Make many holes in the pot, and then implant the entire pot with the seedling. Several experiments on the emergence and growth of A. marina in UAE were conducted (Anwahi, 1994). He found that propagule emergence of the gray mangrove stopped at salinities of 60 ppt, or higher. While at salinity of 40 ppt. or lower, seed emergence was optimum, indicating a very well adaptation of A. marina to the local salinities prevailing in most areas of the country. He also found that seedling emergence rate decrease with increasing distance from seawater line, the best distance to plant seedlings is 0- 2 m from sea water line. Likewise, the best shoot height observed for seedlings implanted at different distances from sea water line, was 0 -2 m. In an experiment to investigate the shoot growth rate of A. marina on a slopping beach, Anwahi (1994) found that 0 -2 m distance from the seawater line gave the best growth. Further experiments on several aspects of the local mangrove species were also carried out, specifically an experiment on the effect of crude oil pollution on the propagule emergence and growth of A. marina in Umm Al Qaiwain (Anwahi et al. 1998a). Results of the previous research indicated that no propagule emergence was observed when the propagules were kept in crude oil for more than 12 hours or when the oil pollution in the soil exceeded 6.63% v/v which is equivalent to 25% of oil saturation of sowing soil. Another very important aspect of mangrove cultivation in UAE was investigated by Anwahi et. al.(1998b). They investigated the feasibility of cultivating A. marina below sea level. In their work, Anwahi et.al indicated the possibility of cultivating the mangrove successfully in a dug out pond 500 m away from the shore, and below ground level, with groundwater made available to these plants naturally through percolation. On February 2000, these trees were more than four years old and with a height of 3.7 m. They were found to host some bird's nests. The significance of the above experiment is the contribution of its findings in promoting the widespread cultivation and afforestation of A. marina in areas below sea level, and away from the seashore provided water percolates to the intended area. Conclusions The natural occurrence of the mangrove species A. marina in the United Arab Emirates is an indicator on the adaptation of this species to the local climatological, hydrographic, and topographic characteristics prevailing in different areas of the country. These colonies provide very productive ecosystems which sustains a productive swamp rich with organic matter due to the leaves falling in the water. This, consequently, aids substantially in increasing the primary productivity of these swamps and improves the area for serving as a nursery and/or a spawning ground for various fish species. Mangrove forests in UAE are believed to have contributed greatly to preserving coastal areas and coasts of islands and impoundment against erosion caused by the wave and wind action. As such they serve a crucial element in counteracting desrtification processes. A visit to Abu Dhabi island, to Umm Al Qaiwain lagoon, and Ras Al Khor area in Dubai, reveals the recreational value of these trees in this arid area, apart from their mere ecological benefits. Therefore, their utilization for all of the above purposes should be promoted up to the level that every suitable area for A. marina growth is cultivated by the species. This, and the findings of Anwahi et.al.(1998b) calls for investigating the cultivation of the mangrove in areas away from the sea shore, specifically in salt marshes (locally called "Sabkha"). These areas are characterized by a soil with high salt content, and with an underground source of seawater infiltrating by percolation. These areas, and because they are very hard to reform using ordinary plant species, are likely to be developed and restored as areas of mangrove forests, taking into account the requirements to succeed in doing so. It is believed that these "Sabkhas", and similar areas, represent naturally potential areas suitable for mangrove cultivation. This undertaking will help reduce desertification effects of land in the UAE, and it will also help in restoring a thriving and productive ecosystem that is hard to reform otherwise. In areas with weak or no underground influx of seawater, a surface canal can be dug out to these areas to ensure a continuous flow of seawater to reduce salinity. At the time of preparing this paper, a group of researchers at MRRC were conducting experiments on culturing A. marina in a given number of "sabkhas" in the country. Preliminary results are indicative of good success. Transplanting of seedlings and direct propagule sowing showed 60% and 73% survival respectively after one year of sowing, and a plant height of 35.5 cm at an inland area with surface water of 26.4 to 36.7 ppt salinity, and 5 km away from the sea (Tamaei, personal communications). References:
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