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Prepared By: Abstract An experimental cultivation of local gray mangrove trees, Avicennia marina (Forsk.) Vierh, on a natural sandy beach was practiced utilizing discharged seawater from aquaculture tanks and ponds, an artificial forest of gray mangrove trees was established along the drainage without any maintenance from the beginning. Some seedlings sprouted from seeds reached more than 5 m in height after 10 years, and produced abundant seeds at 5 years of age. Although about 95 % of crypto-viviparous seeds of gray mangrove in sand germinated at the middle of inter-tidal zone of the drainage, 89 % of young trees were killed when sea algae and plastic rubbish got deposited on them during the initial 2 years. No mortality happened when trees reached more than 1 m in height 3 years later. Abundant fish fry and other aquatic animals were observed in the drainage when both banks of the drainage were fringed by gray mangrove trees, and many birds visited for prowling about for foods. The result of this experiment suggests strong encouraging and productive procedure for the development and greening of large arid lands along the UAE coast.Key words: Avicennia marina, gray mangrove, afforestation, aquaculture, Sabkha Introduction There is enormous difficulty to plant trees on coastal arid land (Sabkha)due to high salty marsh, and it is impossible to improve the area as an agriculture land in a large scale by reclamation work. According to the fact that some gray mangrove forests grow widely at inter-tidal zone along coastlines in UAE (Rabanal & Beushel 1978; Embabi 1993), which indicate that only mangrove trees have possibility to grow on Sabkha (Satchell 1978, Lieth & Lieth 1993). Only single species forests, Avicennia marina (Forsk.) Vierh, gray mangrove is covering 2,930 hectare along inter-tidal zones of some lagoons and islands (Rabanal & Beushel 1978). The gray mangrove trees get wide tolerance to various concentration of seawater, thus the tree has thousands of micro-desalination systems in its leaf-tissues which are removing the salt from seawater (Macnae 1968; Ghowail et al 1993). But mangrove trees can not grow under the present condition in Sabkha due to lack of seawater and nutrient resources. According to our ecological observation in various lagoons in UAE, abundant fish resources were found in areas where well grown gray mangrove forests were present. It could be stated that dredged canal combined with fish farms can provide both seawater and nutrient (fish feces) supply at the same time for culturing gray mangrove trees in Sabkha. The discharged seawater from fish/shrimp farms contains abundant feces and dissolved feed which are usually considered as pollutants for the marine environment. Especially, shrimp/fish farming is called the most subversive activity against mangrove ecosystem in tropical reigns in spite of its strong effect of large economical impact (Matthes & Kapetsky 1988; Cintron-Molero & Schaeffer-Novelli 1992; Field 1995). But we considered that this enriched seawater can be utilized for mangrove cultivation in Sabkha, and UAE has a great potential of fish/shrimp farm industry on its wide and flat coastal areas. Mangrove cultivation might be possible to be carried out utilizing such pollutant seawater as a fertilizing source to establish low coast greening at the same time creating economical activity on Sabkha areas in UAE. Materials and Methods Gray mangrove trees in UAE
Seeds for cultivation Gray mangrove seeds (Fig. 3) for cultivation were collected during fruiting season from late-August to mid-October in gray mangrove forests in Umm Al Qaiwain lagoon, and seeds were healthier and gave quicker germination in September than in October. In October, many seeds got pin holes on their surface indicating the entry of larvae of leafhopper, and these larvae preferred to eat inside of cotyledons including plumules (Fig. 4).Both mature (yellowish color) and immature (light green color) seeds more than 3 g in weight covered with pericarp on trees were good for planting. All collected seeds were spread on the ground for avoiding to fungi infection by their inner moisture, and seeds could be stocked 15 days under the roof with normal temperature (Fig. 5).While naked seeds without pericarps can be stocked only a few days. Seeds even with pericarps could not be stored in a refrigerator due to getting chilblains. Pre-treatment of seeds for cultivation Seeds covered with pericarps are difficult to stay in sand due to their floating mechanism. If these seeds would be planted in deep sand more than 10 cm, seeds might get fungi infection from their decayed pericarps in sand. Therefore pericarps of seeds should be removed to avoid their washing away by waves and tidal currents before planting in sand. Although it is very troublesome to peel pericarps from seeds by hands, large number of seeds' pericarp can be peeled easily by the following method. a. Keep necessary number of seeds in running/standing seawater/fresh water 6-24 hours according to their freshness (Fig. 6). Remove floating pericarps a few hours later. b. Remove pericarps from seeds next morning, it is easy to peel pericarps by fingers. c. Wash all seeds by fresh seawater/water. Floating seeds infested by larvae should be excluded. Keep seeds under the roof until planting without seawater/fresh water. d. Planting should be done same day or next day. The best timing of planting is just after removing pericarps. Although a few days can be stored in seawater, these seeds in seawater should be planted before the opening of cotyledons. Once the cotyledon and hypocotyl developed in the seawater, the seeds get out of joint during handling of transportation and planting.
Nursery for gray mangrove
Location of cultivation Experimental afforestation of gray mangrove was executed along a shallow long drainage of the UAE Marine Resources Research Center in Umm Al Qaiwain. The drainage was dug on the empty sandy beach behind fish/shrimp ponds, and the water of drainage was coming from these ponds and facilities of fry production and aquarium. The drainage and its surrounding areas were inundated by seawater from the sea at high tide period. The drainage is less than 1.2 m deep, two wide openings around months of two ponds, and 5-20 m width with 615 m long from the interior to its outlet. The facilities of these fish/shrimp rearing tanks and ponds flow out 350 m3 seawater per hour continuously to the drainage. Chemical and antibiotic treatments for fish/shrimp have never been used in the center when mangrove seedlings grow along the drainage. Gray mangrove cultivation The gray mangrove planting areas along the drainage were between low and high tide line, and nest holes of pea-sized crab, Scopimera scabricauda, on the bank slopes are good indicators for planting place. The drainage is protected from strong waves, current and cattle, and there were no aquatic animals to eat these planting seeds. The salinity of the drainage water at in 1990 was between 39.5 ‰ and 41‰ (Fig. 9), there was not much difference between summer and winter season. Air temperature fluctuated from 18.7? to 34.7?, and the highest air temperature was in August (Fig. 10). pH of the drainage water (pH 8.0-8.2) was slightly higher than that of Umm Al Qaiwain beach water (pH 7.5-8.1). Direct seed planting along the drainage was tried during fruiting season of seeds in September 1990, 1,000 seeds without pericarps were buried in sand at 3 cm deep and 20-25 cm interval (Fig. 11).For the comparison, 1,000 seeds buried in sand at 5-10 cm deep and 20-25 cm interval were planted along sandy coast of the Gulf in front of the center. No maintenance activities were done after planting seeds.
Result Growth of mangrove The germination of seeds along the drainage was observed after 20-25 days, and they grew smoothly until December (Fig. 12). Sea algae (Cladophora sp. and Sargassum sp.) and drifting rubbish got deposited on young trees and killed them during months from January to April (Fig. 13). The photosynthesis activities of young trees were shut down by these covering obstacles. Beach sands blew by strong north winds hit young trees caused heavy bruises on the leaves and buds which later died. All seeds planted along sandy coast of the Gulf were washed away by waves. Mortality less than 5 % of seeds were observed in germination around the middle region of low and high tidal lines, while all seeds planted at low and high tidal lines died before or after germination. There were no animals to eat young trees. The height of these mangroves trees, the seeds of which were planted in September increased steadily to around 40 cm in average in a year (Fig. 14 & 15).A high mortality always occurred in the winter season due to covering with sea algae, fouling of rubbish and sand storms, but young trees could survive 40 % after winter season. Two-year old mangrove trees survived 21 %, and no mortality occurred after the following years, bushy trees became strong enough to obstacle of sea algae and sand storms. Three-year old trees started flowering and some fruiting.Five-years old mangrove trees started to produce large number of seeds from late August to early October, and mangrove trees become strong and grow steadily as the years pass. The formation of small forests more than 5 m in height after 10 years was established without doing any maintenance or nourishment during the period of the experiment (Fig. 16 & 17).
After the afforestation The soil color of mangrove growing areas changed from whitish sand to brown soft sand due to decayed mangrove foliage when area was covered by mangrove trees after 10 years. Abundant pea-sized crabs were observed under the forest. Many young fish including mullet, black seabream, mangrove snapper, sweetlips, grouper, milkfish, barracuda, etc. started to appear in the drainage canal when mangrove grow up around the drainage after 10 years (Table 1), and many birds were attracted to this mangrove forest and the drainage since it formed an ideal feeding place. Green sea algae, Cladophora sp. and Chaetomorpha sp. were always vegetating around roots of mangroves. Many halophytes, Suaeda vermiculata, Salsola barysoma, Zygophylum hamience and Cyperus conglomeratu appeared behind gray mangrove trees, and sand movement by strong north wind have never happened on the beach after growing mangroves trees and those halophytes. Although the mangrove afforestation areas are declared protected areas, some farmers sometimes invaded and cut mangrove braches for cattle feeding without permission. Fishermen were allowed to enter the drainage for collecting green sea algae. Discussion Although mangrove afforestation activities are being conducted in many tropical countries (Qureshi 1990; Field 1995), a few trials were practiced in desert coasts (Kogo & Tsuruda 1995). The most obstructive factors for gray mangrove planting at sabkha is extra ordinary high salt in soil due to high evaporation through the year (Satchell 1978). Kogo & Tsuruda (1995) indicated high possibility for gray mangrove cultivation of five salt tolerant species including A. marina at Khafji in Saudi Arabia. Gray mangrove tree has strong resistance to high saline seawater comparing to other species (Macnae 1968), and only this mangrove is grown naturally along coasts of the Gulf. Therefore gray mangrove is the most suitable species for afforestation on Sabkha in UAE. Mangrove need nutrient for its growing (Karim & Karim 1992; Saintilan 1998)) which were provided from rivers, recycling of mangrove forests and living animals in tropical regions (Macnae 1968; Odum & Heald 1975;Mirton 1990; Cintron-Molero & Schaeffer-Novelli 1992). There are no resources of nutrient for mangrove growing in Sabkha. Result of well grown gray mangrove along the drainage shows that dissolved feces flooded out from fish and shrimp rearing tanks and ponds might be utilized by mangroves as their nutrient. Although they should need adequate amount of nutrient from the seawater for growing, we still could not analysis relation between amount of rearing fish and growth of mangrove. While sewage water and agriculture fertilizer are not recommended for mangrove cultivation since their effect are unknown, and cost of mangrove afforestation might be increased when fertilizer spread on the water. The main causes of mortality of young mangroves are from green sea algae, rubbish, and sand storm. The growing period from seedling to two-year old mangroves needs a little care for removing sea algae and rubbish trapped on it. Three- year old trees become bushy and are strong enough to live with adverse natural conditions. The sand storm is the most dangerous phenomenon for young trees; blowing sands hit leaves and buds, inducing high mortality of trees. The areas liable to have frequent sand storms need a fence to protect young trees from this calamity until the second winter. When mangrove forest was established along the drainage after 10 years, the foliage of mangrove provided a good source of food for many aquatic animals, and the drainage became a nursery for fish. The result of mangrove cultivation along the drainage shows that the high possibility of greening Sabkha by mangrove afforestation combined with aquaculture in UAE. When many large mangrove forests along all coastlines of large canals will be created in Sabkha, many inhabitants such as birds, fish and various animals might circulate which will induce moderate climate and natural life ecosystem on Sabkha. These forests will provide large quantity of forage to dairy farmers, and flowers to beekeepers. While a large amount of fish production could be expected from these canals. References
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