Overstory #230 - Uses and functions of mangroves
Mangroves are coastal forests found in sheltered estuaries and along river banks and lagoons in the tropics and subtropics. The term ‘mangrove’ describes both the ecosystem and the plant families that have developed specialized adaptations to live in this tidal environment (Tomlinson, 1986). In a dense mangrove forest, lights and shadows reflect on the water and fish and crabs hide among the submerged roots and trunks. Moving forward may sometimes be possible only by climbing on giant roots or using small boats.
Mangroves have traditionally been widely used and exploited in the past in the majority of countries in which they exist. Knowledge of their current and past extent, condition and uses is essential for forest managers and policy- and decision-makers. The planning of sustainable forest management at the local and national levels depends largely on this information, and the lack of data on the status and distribution of mangroves makes it difficult to prepare successful plans for their conservation. Regular monitoring is thus necessary and may contribute to their conservation, but also to sustainable use of mangroves as a source of wood, food, income and recreational areas for present and future generations.
Although the literature on mangrove forests is extensive and numerous case studies describe their extent and losses over time, global, comprehensive information on the status and trends in the extent of mangroves has been lacking. The first attempt to estimate total mangrove area worldwide was undertaken as part of the FAO and United Nations Environment Programme (UNEP) Tropical Forest Resources Assessment in 1980. In that study, the world mangrove total was estimated at 15.6 million hectares, while more recent estimates range from 12 to 20 million hectares.
Countries with small areas of mangroves were excluded from many of the earlier studies, probably because of lack of information. The area of mangroves in these countries and areas is, however, relatively small and therefore did not significantly affect the world total. Tropical Timber Organization (ITTO). The main aim of this study was to facilitate access to comprehensive and comparable information on the current and past extent of mangroves in the 124 countries and areas where mangroves are known to exist, highlighting information gaps and providing updated information that may serve as a tool for mangrove managers and policy- and decision-makers worldwide.
Mangrove ecology and management have been described by many authors over time and the literature on this subject is substantial (for example, Hamilton and Snedaker, 1984; Aksornkoae, 1993; FAO, 1994; UNU, 2004). While it is not the aim of this report to provide a complete review of mangrove forest ecology, a brief overview of mangrove characteristics, distribution and services is provided.
The term ‘mangrove’ has been discussed by experts and scientists for years (Tomlinson, 1986). It is commonly used to identify trees and shrubs that have developed morphological adaptations to this tidal environment (e.g. aerial roots, salt excretion glands and vivipary of seeds), as well as the ecosystem itself.
Mangroves are salt-tolerant evergreen forests found along sheltered coastlines, shallow-water lagoons, estuaries, rivers or deltas in 124 tropical and subtropical countries and areas, mainly growing on soft substrates. Plants found growing on rocky shores, rooting in silt-filled depressions, may exceptionally be found. The mangrove ecosystem represents an interphase between terrestrial and marine communities, which receive a daily input of water from the ocean (tides) and freshwater, sediments, nutrients and silt deposits from upland rivers. Mangroves may grow as trees or shrubs according to the climate, salinity of the water, topography and edaphic features of the area in which they exist. Human pressure and disturbances also play a critical role in their development; plants living in stressed or polluted environments are often constrained in their development. A zonation in monospecific bands parallel to the shore is frequently visible, generally dictated by local topography, soil composition, tidal ranges and salinity.
Mangrove forests may be found as isolated patches of dwarf stunted trees – in very high salinity and/or disturbed conditions – or as lush forests with a canopy reaching 30–40 metres in height under suitable environmental conditions. In undisturbed and pristine estuaries, mangroves may extend for several kilometres inland. Examples of these extended forests are the Sundarbans, which lies in the delta of three rivers (the Ganges, the Meghna and the Brahmaputra – Bangladesh/India); the Mekong Delta (Viet Nam); the Gambia River delta (the Gambia); the Fly River (Papua New Guinea); and the Florida Everglades (United States of America). The different mangrove ecosystems contain a range of biodiversity.
Only a few plant families (e.g. Rhizophoraceae, Avicenniaceae and Combretaceae) have developed physiological and structural adaptations to the brackish water habitat in which mangroves live. The exact number of species is still under discussion and ranges from 50 to 70 according to different classifications (e.g. Tomlinson, 1986; Saenger, Hegerl and Davie, 1983; Lugo and Snedaker, 1975; Aksornkoae et al., 1992), with the highest species diversity found in Asia, followed by eastern Africa. When moving to the geographical limits of mangroves in subtropical countries and in arid zones, mangroves often appear only as small trees, but may still play an essential role for local communities.
At first sight, the most easily recognizable adaptation developed by mangroves to the tidal environment is the aerial rooting system, which is completely or partly exposed to the atmosphere at least part of the day, but covered by water during high tide. Its main functions are the exchange of gases, anchorage of the tree in the muddy soil and absorption of nutrients. However, only the most specialized species (i.e. the major components of the mangrove forest community – ‘strict or true mangroves’ according to Tomlinson, 1986) have developed this root system, and aerial roots may have different structures according to the species. For example, stilt roots grow from the trunk and lower branches of Rhizophora spp. and, to a limited extent, in the sapling stage of Bruguiera spp. and Ceriops spp. (they become shallow buttresses in old trees), while ‘pneumatophores’ – pencil-like extensions of the subterranean rooting system – rise from the ground and extend a long distance from the parental tree in the genera Avicennia, Sonneratia and Laguncularia. In the genera Bruguiera, Ceriops and Xylocarpus, the pneumatophores may form a series of arched or knee shapes during their horizontal growth (the so-called ‘knee roots’). According to Tomlinson (1986), minor components of the mangrove community may or may not have this aerial rooting system, while associated species develop it only in a very few species (e.g. in the genera Oncosperma, Phoenix and Raphia).
Processing excessive salt in the water absorbed is one of the biggest challenges in the salty environment in which mangroves live. These plants have developed several methods, according to the species, to desalinate ocean water. They may exclude the uptake of salt at the root level, or remove excess salt at the leaf level, by using salt excretion glands (species in the genera Avicennia, Aegiceras and Aegialitis), by cuticular transpiration at the leaf level, or by accumulating the salt in leaf tissues and then shedding the leaves.
To increase successful plant propagation, the most specialized mangrove families have developed some very efficient reproduction systems. In the Rhizophoraceae family, the fruit, and therefore the seed, is not released. It germinates on the parental tree, and the seedling itself is used as the propagule (vivipary) (Juncosa, 1982). In this viviparous species, the embryo has no dormancy and is detached only when mature and ready to be established. Other species, for example in the genera Aegiceras, Avicennia, Nypa and Pelliciera, have developed cryptovivipary (Carey, 1934), in which the embryo emerges from the seed, but not from the fruit until after it abscises.
Functions and uses of mangroves
Mangrove forest ecosystems fulfil a number of important functions and provide a wide range of services at the local and national levels (text box). Fishermen, farmers and other rural populations depend on them as a source of wood (e.g. timber, poles, posts, fuelwood, charcoal) and non-wood forest products (food, thatch – especially from nipa palm – fodder, alcohol, sugar, medicine and honey). Mangroves were also often used for the production of tannin suitable for leather work and for the curing and dyeing of fishing nets. However, this production has declined in recent years, mainly because of the introduction of nylon fishing nets and the use of chrome as the predominant agent for curing leather (FAO, 1994).
Mangroves support the conservation of biological diversity by providing habitats, spawning grounds, nurseries and nutrients for a number of animals. These include several endangered species and range from reptiles (e.g. crocodiles, iguanas and snakes) and amphibians to mammals (tigers – including the famous Panthera tigris tigris, the Royal Bengal tiger – deer, otters, manatees and dolphins) and birds (herons, egrets, pelicans and eagles, to cite just a few). A wide range of commercial and non-commercial fish and shellfish also depends on these coastal forests. The role of mangroves in the marine food chain is crucial. According to Kapetsky (1985), the average yield of fish and shellfish in mangrove areas is about 90 kg per hectare, with maximum yield of up to 225 kg per hectare (FAO, 1994). When mangrove forests are destroyed, declines in local fish catches often result. Assessments of the links between mangrove forests and the fishery sector suggested that for every hectare of forest cleared, nearby coastal fisheries lose some 480 kg of fish per year (MacKinnon and MacKinnon, 1986).
Mangrove ecosystems are also used for aquaculture, both as open-water estuarine mariculture (e.g. oysters and mussels) and as pond culture (mainly for shrimps). Because of its high economic return, shrimp farming has been promoted to boost the national economy and alleviate poverty in several countries. This activity is often an answer to the financial constraints on many farmers and local communities and represents a source of employment. However, if unsustainably planned and managed, it can lead to uncontrolled deforestation and to pollution of coastal waters, damaged or totally destroyed coastal ecosystems and the loss of the services and benefits provided by mangroves. A series of international principles for responsible shrimp farming have been prepared (FAO/Network of Aquaculture Centres in Asia-Pacific/UNEP/ World Bank/Worldwide Fund for Nature, 2006; FAO, 1995), with the main aim of offering guidance on reducing the sector’s environmental impact while boosting its contribution to poverty alleviation. The principles were welcomed by many countries (FAO, 2006)) and will hopefully provide support to the development of more ecofriendly shrimp production.
The increasing popularity of ecotourism activities also represents a potentially valuable and sustainable source of income for many local populations, especially where the forests are easy accessible.
Mangroves also help protect coral reefs, sea-grass beds and shipping lanes by entrapping upland runoff sediments. This is a key function in preventing and reducing coastal erosion and provides nearby communities with protection against the effects of wind, waves and water currents. In the aftermath of the 2004 Indian Ocean tsunami, the protective role of mangroves and other coastal forests and trees received considerable attention, both in the press and in academic circles. After more than two years, there are still contrasting views on this issue: eyewitnesses reported that coastal forests had saved villages from the destruction and lives, while some analyses asserted that elevation and distance from the coast were more significant determinants of protection than the forest cover itself.
Even though additional studies are needed to define specific details and limits of this protective function, the numerous studies and workshops undertaken on this topic over the past couple of years have brought to light a number of interesting factors. Experts and scientists agree that thick and dense coastal forest belts, if well designed and managed, have the potential to act as bioshields for the protection of people and other assets against some tsunamis and other coastal hazards (i.e. coastal erosion, cyclones, wind and salt spray). However, generalizations – and the creation of a false sense of protection provided by these bioshields – should be avoided, because mangroves and other coastal forests are not able to provide effective protection against all levels of hazards and may not be effective as shields against tsunamis as severe as the one that occurred in 2004. A full description of the factors to be taken into account with regard to enhancing the protective functions of mangroves and other coastal forests goes beyond the scope of this report. Interested readers are referred to FAO (2007) for further information.
MANGROVE USES – WOOD AND NON-WOOD FOREST PRODUCTS (Source: modified from FAO, 1994.)
Beams and poles
Thatch or matting
Fence posts, chipboard
Wood for smoking fish
Tannin for nets/lines
Synthetic fibres (rayon)
Dye for cloth
Tannin for leather preservation
Other natural products
Food, drugs and beverages
Other forest products
Wood for smoking sheet rubber
Paper – various
Despite the many services and benefits provided by mangroves, these coastal forests have often been undervalued and viewed as wastelands and unhealthy environments. The high population pressures frequently present in coastal zones have in some places led to the conversion of mangrove areas for urban development. In order to increase food security, boost national economies and improve living standards, many governments encouraged the development of shrimp and fish farming, agriculture, and salt and rice production in mangrove areas. Mangroves have also been fragmented and degraded through overexploitation for wood forest products and pollution. Indirectly, habitats have been lost because of dam construction on rivers, which often diverts water and modifies the input of sediments, nutrients and freshwater. Even though dense mangrove forests can be important in coastal protection, natural disasters should also be listed among the possible causes of degradation: several tropical countries are frequently hit by cyclones, typhoons and strong winds, and the trees in the front lines may be damaged and/or uprooted during these catastrophes.
Over the last few years, however, awareness of the importance and value of mangrove ecosystems has been growing, leading to the preparation and implementation of new legislation and to better protection and management of mangrove resources. In some countries, restoration or re-expansion of mangrove areas through natural regeneration or active planting has also been observed. In addition, many governments are increasingly recognizing the importance of mangroves to fisheries, forestry, coastal protection and wildlife. Despite these positive signs, much still needs to be done to effectively conserve these vital ecosystems.
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Carey, G. 1934. Further investigations on the embryology of viviparous seeds. Journal of the Proceedings of the Linnean Society N.S.W., 59: 392–410 (cited in Tomlinson, 1986).
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This article was excerpted with the kind permission of the publisher from:
FAO, 2007. The Worlds mangroves, 1980-2005, 2007. FAO Forestry Paper 153. FAO, Rome. http://www.fao.org/docrep/010/a1427e/a1427e00.htm
About the Author
The Food and Agriculture Organization of the United Nations leads international efforts to defeat hunger. Serving both developed and developing countries, FAO acts as a neutral forum where all nations meet as equals to negotiate agreements and debate policy. FAO is also a source of knowledge and information. We help developing countries and countries in transition modernize and improve agriculture, forestry and fisheries practices and ensure good nutrition for all. Since our founding in 1945, we have focused special attention on developing rural areas, home to 70 percent of the world's poor and hungry people.
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