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This chapter gives an overview of mangroves with focus on the Rhizophora mucronata species. It starts with an introduction then continues with the botanical description of Rhizophora mucronata. It also highlights different aspects such as mangroves distribution, factors affecting mangroves, mangroves adaptation, mineral nutrition and roles. It also elaborates on the threats affecting mangroves. Finally it ends with a brief on mangrove management and restoration.
Mangroves are plant species that are adapted to grow in tropical coastal zones (Kathiresan and Bingham 2001). Mangroves forest has an area of about 15 million hectares worldwide. Although they amount to only 1% of the total area of tropical forest, they are highly productive. Around the world there are 9 orders, 20 famillies, 27 genera and approximately 70 species (Alongi, 2009). In Mauritius, two species of mangroves are found namely the Bruguiera gymnorrhiza and Rhizophora mucronata. The latter being the most common one. Mangroves are rich in biodiversity and support complex communities. Mangroves provide important ecological and socioeconomic role. They also contribute to the livelihoods of many people by providing goods like timber, firewood and thatching materials. Mangroves are also known as an important greenbelt and carbon sink which has the potential to protect coastal areas from natural disasters such as tsunamis, erosion and cyclone (Spalding, 2010). Despite their importance, mangroves areas have declined alarmingly around the world.
Mangroves are predominantly tropical and show variation in area. They are found in 123 countries and territories. They are found in higher abundance in the wetter coastline of south and Central America and west and central Africa. They are also found in the northeast of India and northern Australia. Mangroves are more abundant in the indo west pacific region (Alongi, 2009). On a finer scale, they can be found on oceanic island in the western pacific and the central Indian Ocean.
Figure 1: map showing global distribution of mangroves. Red line shows distribution of all mangroves and blue line show Rhizophora mucronata in particular. Pink and green line show Rhizophora apiculata and Rhizophora stylosa
Source: Duke, 2006
Mangroves distribution in Mauritius
Rhizophora mucronata is found on the Northeast, East, and Southeast coast of Mauritius. They also occur in the South-Southwest coasts of the island. Small patches of Bruguiera gymnorrhiza occur at Pointe la Fayette, Trou D'eau Douce, Ferney and Mahebourg, on the East Coast. (Annon, 1991a, cited in Appadoo, 2003)
Mangroves at Pointe D'esny wetland
Pointe D'esny wetland is located in the south east of Mauritius and is among the largest remaining wetlands of the island. The site has been proclaimed as Ramsar site on 2 February 2012 under the treaty of the Convention on Wetlands. This organisation provides support and cooperation for the sustainable use of the wetland. The wetland has an area of 21.5 hectares. Both Rhizophora mucronata and Bruguiera gymnorrhiza are can be found there. The wetland has a rich biodiversity where a variety of fauna and flora exist. Amongst, there are many crustacean, native butterflies. In 2006, the Mauritius Wildlife Foundation rediscovered the Zornia vaughaniana which was thought to be extinct on the island (source: Information Sheet on Ramsar Wetlands). The wetlands help in protecting the lagoon of Mahebourg by filtering water and absorbing excess nutrients. The site is also used for fishing activities and bait collection.
Taxonomy of Rhizophora mucronata
Genus: Rhizophora L.
Species: Rhizophora mucronata
Botanical Description of Rhizophora mucronata
Rhizophora mucronata trees may range from medium to tall that may reach up to 30-40m. The diameter can around 15-35 cm.
They are stilt mangrove with above ground props. They can be both single stemmed of multi stemmed. The latter are more common in arid region.
Flowers and Leaves
Leaves are opposite. They may be light or dark green in colour. They are glossy and have mucronate tips which are about 1-7cm. Petioles ranges from 2.5-5 cm. Flowers are wind pollinating. Flowering and fruiting occur throughout the year (Aksornkoae, 1993)
They have aerial stilt roots.
Fruits and seeds
The number of seeds per fruit is 1 and the number of fruit per branch range between10-25 (Thampanya, 2006). Seeds are viviparous.
In Rhizophora mucronata the progation unit is a seedling. This seedling is buoyant and has the ability to photosynthesize. It has a length of 30-70 cm and a width of 2 - 4 cm. (Tomlinson 1986). After loosing buoyancy, propagule anchor horizontally in the sediment.
Adaptation of mangroves
Growing at the transition between land and sea, mangroves have developed special and unique morphological and physiological adaptation that enable them to survive in harsh conditions (Kathiresan and Bingham, 2001). These adaptations vary among species and even between individuals of the same species.
Different mangroves species have different types of roots (stilt type, knee type and pneumatophores). Rhizophora mucronata has stilt or prop roots that enable them to anchor properly in the sediment. These roots do not penetrate deep into the soil but instead they extend sideways in order to provide support. It also helps to withstand strong winds and to obtain oxygen directly from air. Komkris (1993) found that tannin which are present in large amount in the periderm and cortex of Rhizophora mucronata gives strength to the roots. Lenticels are present on the stilt roots and hypocotyls of Rhizophora mucronata. They enhance uptake of oxygen.
All mangroves depend on the movement of tides to disperse their progagules. Rhizophora mucronata exhibit vivipary during which the seed germinate while it is still on the on the tree. Viviparous embryo allows rapid establishment and better chance of regeneration of seedlings. Propagules that are released are buoyant that float on the water. In Rhizophora mucronata, propagules are released in large numbers. They can survive for a long time until conditions are favourable for establishmen. Field (1995) found that the propagules of Rhizophora mucronata have a longetivity of more than 1 year.
Mangroves have both halophytic and zerophytic characteristic that give then the ability to thrive in water of high salinity. They have specialised glands that excrete excess salt or they can store the salt in senescent leaves. These leaves are waxy like and are positioned in such a way so as to reduce the loss of water (Kathiresan & Bingham, 2001).
Mangrove ecology and dynamics
Mangroves forests are never homogeneous and the species it contains are not uniformly spread (Spalding, 2010). Environmental factors and their gradients across time and space determine the population dynamics of mangroves. The physical environment of mangroves is often subjected to regular inundation and changing salinities. The composition of mangroves forest is dependant on temperature, salinity, type of soil, inundation (Lugo and Snedaker, 1974). Sedimentation has profound effect on the dynamics of mangroves forest. They create new environment which mangroves can colonize. However high sedimentation caused by heavy rainfall can cause mortality by blocking aerial roots. Under normal condition sedimentation rate can be up to 1.5cm per year permitting the root system to adapt and thus allowing aeration to continue.
Different approaches have been used to study mangrove forest dynamics. One approach is the conventional measurement of tree abundance and structure over time (Clough, 1992).
Types of mangroves forest
Fringe mangroves They are found along the coast, in bays and lagoons. They can also be found in estuarine. They are periodically inundated by tidal change.
Basin mangroves They are found away from water edge normally where there is shallow depression.
Riverine mangroves They are found along the banks of rivers and stream.
Overwash mangroves They occur in patches. They are always entirely covered by water. (Lugo and Snedaker, 1974)
Mangrove forest can be described as having zone. They also show pattern of succession. These zones are arranged along tidal gradients which are usually dominated by one or two species (Feller, 2010). Environmental factors like salinity, nutrient availability and flood influence patterning of the vegetation (Ball, 1996). Others factors that causes zonation is competition among species and predation.
At Pointe D'esny, the mangroves forest has 4 zones which are dominated by Rhizophora mucronata and Bruguiera gymnorrhiza. Studies have shown that many species may co-occur because of similar physiological requirements (Ball, 1996).
Factors affecting mangroves structures
Various factors such as solar radiation, nutrient, temperature, water and oxygen availability affect growth and occurrence of mangroves (Ball, 1988, 1996 cited in Alongi 2009). Other factors like changing salinity, soil or sediment type, temperature, nutrient availability, predation and competition all affects mangroves.
Extensive mangroves have been found to occur in areas where the water temperature exceeds 240 C. In water where the temperature is below 240 C, mangroves are absent (Blasco et al 1996).
Mangroves thrive better in region where there is high rainfall. It influence salinity and also contributes to silt formation.
Salinity is one of the most important factors that help mangroves establishment and development (Ball, 2002). Mangroves are facultative halophyte which means that they can thrive in areas of high salinity but does not necessarily require it for their normal growth.
The effect of shade and light intensity differs among mangroves. Rhizophora seedling grows and sprouts better under shady canopy.
Mangroves can often be subjected to oxygen stress. Different species of mangroves have the ability to cope with low oxygen levels. McKee ( 1993) found that under anoxia condition, the biomass of rhizophora can increase by 9-24%. Due to loss of CO2 during respiration, pressure may drop in the aerial roots causing rapid influx of air when high tide recedes. Anoxia can also be caused by high methane levels.
Tides and currents
Tide is a very important feature in mangrove forest. The rise and fall of tides facilitates movement of water and sediment (Alongi, 2009). Tidal circulation is affected by the ebb and flood tides. Ebb tides has stronger current velocity than flood tides (Alongi, 2009). This velocity depends on the geometry of the forest. Presence of roots, decaying vegetation and burrows also affect movement of water by exerting a drag force (Mazda et al 1997).
Mineral nutrition in mangroves
Like all other trees, mangroves have a requirement for nutrients for their proper growth and development. (Aerts and Chapin 2000 cited in Alongi, 2009). Mineral nutrient can be divided into 2 groups namely macronutrient and micronutrient.
Micronutrients that are required are: iron, manganese, copper, zinc, nickel, molybdenum, boron, chloride, sodium, silicon, cobalt, selenium, and aluminum. The macronutrients are nitrogen, phosphorus, sulfur, magnesium, calcium, and potassium. (Alongi,2009).
Phosphorus is needed for the formation of nucleic acid and is an important part of phospholipids Plant obtain phosphorus in the form of phosphate. Phosphorus deficiency can lead to stunted growth, delayed maturity and necrotic spots in leaves.
Nitrogen is a major component of amino acids. Mangroves obtain nitrogen from the decomposition of organic matter. It is taken up in the form of nitrate or ammonium. Its deficiency can cause chlorosis.
Potassium helps in the regulation of osmotic potential. It has also been found that potassium activate various important enzymes involved in respiration and photosynthesis. Deficiency of potassium can lead to chlorosis and weak stem.
Calcium is needed for formation of calcium pectate which is important constituent of cell wall. They are also important secondary messenger. A deficiency of calcium can caused stunted growth.
Magnesium is an important structural component of chlorophyll. It is also required for metabolism of carbohydrates. It also has a role in nucleic acid synthesis. Its deficiency can cause chlorosis.
Sodium is needed only in small amount. Deficiency of sodium can cause chlorosis and later necrosis.
IMPORTANCE OF MANGROVES
Mangroves are important sites used as nursery and breeding ground for birds, reptiles, crustacean, mammals and fish. (Alongi,2009). They provide temporary or permanent habitat to a wide range of fauna. Many juvenile fish live in mangroves environment to protect them themselves from predators and once they reach maturity they move to open sea.
They protect shores against erosion, wave action by acting as physical buffers. It has been shown that mangrove can absorb up to 70- 80% of wave energy. They also trap sediment thus protecting the lagoon from sedimentation and improve water quality. They also protect the coast from solar UVâ€B radiation. In addition they also protect from green house effect, floods, wave action, and coastal erosion (Kathiresan, 2000). Recent research has shown that mangroves have the higher levels of primary biomass productivity than any other tropical or temperate forest.They also help in carbon sequestration
Socioeconomic value roles
In addition to their importance in sustaining biodiversity, mangroves also provide direct and indirect benefits to man (Feller et al, 2010). They provide highly valued resources such as forestry product and fisheries product. (Kathiresan and Bingham, 2001).Since long, mangroves have been used by man both traditionally and commercially. It has been estimated that mangroves provide more than 70 direct human activites (Dixon, 1989; Lucy, 2006 cited in Kathiresan) In Thailand and Malaysia, mangroves forest are managed to provide commercial timber (Blasco et al 1996). Timber is used making furniture, houses In some Bangladesh and India mangroves are used for honey production which is an important local industry (Miththapala, 2008). Mangroves provide woods that are used for cooking and building purposes. Leaves of some species are used for weaving and thatching. Mangroves may also have medicinal properties. Leaves of Bruguiera species can be used to treat high blood pressure and barks of rhizopora species have antidiarrhoeal, astringent and antemetic properties. Other mangroves species are used for treating leprosy, rheumatic disorders (Kathiresan, 2000). As cultural services, it promotes eco tourism since more and more people understanding the uniqueness and importance of this ecosystem. Through the various sustainable management program, it is helping to conserve traditional fishing in many areas.
Threats to mangroves
Despite continuous demonstration of mangroves ecological and socioeconomic values, they have been declining alarmingly. More than 50% of the world's mangroves have been destroyed (Feller, 2009). These damages have been cause mainly due aquaculture, coastal development, altered hydrology and overexploitation. Rhizophora species have been heavily exploited due to its tannins which are used in leather industry (Miththapala, 2008). Introduction of invasive alien species can also affect mangroves population. This have been the case in some part of China where the introduction of common cordgrass has cause the death of many mangroves (Xia et al, 2006) Other causes are due to altered hydrology, pollution sea-level rise, global warming and nutrient overenrichment. (Alongi,2009). In Senegal, Brazil and Venezuela, large areas of mangroves have been devastated by shoreline erosion (Blasco et al, 1996) Pest and diseases can also affect mangroves. Disease can slow down growth or death. Alongi (2009) also predicted that with the increase in population, mangroves will be more threatened in the future. Further degradation of mangroves habitats will result in loss of ecological functionality. This will ultimately affect millions of people living in coastal areas.
Rehabilitation and sustainable management of mangroves
Worldwide many attempts are been made to restore and rehabilitate mangroves. However many of such project has result in failure due to improper planting technique and site selection (Elison, 2000).
Mangroves communities are now been recognized not only for their economic value but also as living systems of scientific interest which can useful indicators of coastal change to which they are particularly vulnerable (Blasco et al 1996).
In Mauritius mangroves propagation programme began in 1995 by the Ministries of Fisheries. Its aim was to protect and preserve the affected areas and also to sensitise the public about the importance of mangroves.