Mangroves

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This article describes the habitat of the mangrove forests. It is one of the subcategories within the section dealing with the biodiversity of marine habitats and ecosystems. It provides an overview of the characteristics, distribution, biota, functioning and adaptation to habitat conditions. An introduction is given to management aspects, discussing threats, conservation and rehabilitation of mangrove forests.


Introduction

Mangroves are the characteristic woody plants of tropical and subtropical intertidal environments that can grow in saline water. They are generally found in relatively low or moderate salinity environments but many species can grow also in freshwater. They form unique intertidal forests at the edge of land and sea, see Fig. 1. They are represented on all continents with tropical and subtropical coasts, i.e. North and South America, Africa and Middle-East, Asia and Oceania (incl. Australia). [1]

Mangrove forests or mangals are a type of intertidal wetland ecosystems. In tropical and subtropical coastal zones, they prevail rather than salt marshes. They are salt-tolerant forested wetlands at the interface between the terrestrial landscape and the marine environment. Many species develop conspicuous above-ground root systems, including prop roots, pneumatophores, knee roots and buttress roots. They settle preferentially where there is little wave action and where muddy sediments accumulate. While growing, mangal forests further reduce waves and increase sedimentation. Wave energy reduction can be greater than 50% on average and increases with increasing offshore wave heights (Horstman et al., 2014[2]). Mangals therefore fulfill an important coastal protection function. Mangroves are frequently associated with saline lagoons and are regularly found on protected sides of islands, atolls and tropical estuaries (Karleskint, 1998[3]).


Fig. 1. Mangal in Thailand.


Requirements for development

Favorable conditions for the development of mangroves are:

  • Average temperature of the coldest month higher than 20°C; a seasonal temperature range that does not exceed 5°C. Mangroves are not resistant to freezing;
  • A fine-grained substrate, although mangrove species also establish on sandy or carbonate substrates;
  • Suitable species-dependent intertidal elevation and inundation duration;
  • Adequate tidal or freshwater hydrological connectivity and drainage;
  • Wave and current stresses do not cause persistent erosion or uprooting of seedlings;
  • Saline water (mainly because it provides a competitive advantage); mangroves are facultative halophytes.
  • A sediment balance that maintains a suitable intertidal elevation.


Red mangrove (Rhizophora)
Black mangrove (Avicennia)


These different requirements result in complex zonation patterns. Red mangroves (e.g. Rhizophora apiculata, Rhizophora stylosa, Rhizophora mucronata) and the grey mangrove Avicennia marina are often located close to the water's edge, where frequent tidal flooding occurs. Areas that receive shallow flooding during high tide are often home to mangrove species such as Bruguiera gymnorhiza. The upper tidal limit (spring tide) of the mangroves is often occupied by mangrove species such as Xylocarpus granatum, white mangroves and buttonwood. The buttonwood is not really a mangrove species, but a transitional species between mangrove and terrestrial vegetation. However, these zonation patterns are not universal; species position can vary between sites.


Distribution

The area covered by mangroves worldwide is estimated at almost 150,000 km2[4][5]. The distribution, density and species composition are determined by the water and air temperatures during the winter, exposure to wave action and tidal currents, the range of the tide, the type of sediment and the chemistry of the seawater. The global distribution of mangroves is shown in Fig. 2. The most highly developed and most species-rich mangals are found in Indonesia, Australia and Malaysia. Over the world, 54-70 species and hybrids in 20-27 genera and 16-19 families are found; the ranges reflect different taxonomic categorizations (Berness et al., 2001[6]). The genus Rhizophora with the characteristic drop roots is the most common mangrove species worldwide. A species overview is given in the Mangrove Species Database [1].


Mangroves are almost exclusively tropical, but also occur in the subtropics. They do not tolerate frost, but can cope with air temperatures down to 5°C. Their occurrence is most closely related to seawater temperature. The isotherm of 20°C in winter is an often cited indicator of the distribution limit. Avicennia appears to be more tolerant of low temperatures than Rhizophora or Laguncularia. The number of species tends to decrease with distance from the equator. In the Southern Hemisphere, mangals generally occur further south on the eastern edges of landmasses than on the western side. This is due to the pattern of hot and cold ocean currents (Pinet, 1998[7]; Hogarth, 1999 [8]).


Fig. 2. World distribution of mangroves.


As a result of global warming, the area with a suitable settling climate for mangroves is currently expanding in the polar direction. Colonization of salt marshes has been observed along several continents in recent decades, especially by the most cold-tolerant mangrove genus Avicennia (Saintilan et al., 2014[9]).

Functioning and adaptations

Although mangrove species are unrelated taxonomically, they exhibit similar morphological, physiological and reproductive traits. Mangroves have several functions and adaptations for thriving in saline intertidal zones. To take up water against the salinity-induced osmotic pressure, they generate a negative hydrostatic pressure (by transpiration processes) in the xylem[10]. Roots and leaves exude salt, which make them tolerant to saline conditions. Even after most of the salts have been removed, concentration of chloride and sodium ions in the tissue is higher than in other plants. Salt is stored in vacuoles and other tissues to protect enzymes that might otherwise be inhibited. The high cation concentrations are balanced by high non-ionic solutes in the cytoplasm. Several mangrove species deposit sodium and chloride in the bark of stems and roots. Other species deposit salt in senescent leaves, which later fall off the tree. Salt glands on the leaves also exude salt that forms crystals. In some species, fine hairs covering the lower leaf surface raise the secreted droplets of salt water away from the leaf surface in order to prevent osmotic withdrawal of water from the leaf tissues (Parida and Jha, 2010[11]).

Mangroves also need adaptations to conserve water. Leaves have a thick waxy cuticle (skin on the leaf) or dense hairs to reduce transpiration, or orientate their leaves to avoid the burning sun. Most evaporation loss occurs through stomata - pores in the leaves - so these are often sunken below the leaf surface where they are protected from drying winds. Leaves are also commonly succulent, storing water in fleshy internal tissue.


Salt crystals on a mangrove leaf [12]
Extended root mass. Photo credit Eric Coppejans [13].
Drop roots from branches [14]


Waterlogged fine-grained (muddy and sandy) mangrove soils are commonly oxygen-poor below a thin oxidized surface layer (Hogarth, 1999[8]). Mangroves therefore have to cope with anoxic conditions. The tissue of the plants requires oxygen for respiration which cannot diffuse sufficiently into soils that are waterlogged. Even if the surface water is saturated with oxygen, its concentration in the groundwater is too low. This is why mangroves develop various forms of aerial roots.

  • Most of the roots branch off from the stem underground. Another type of roots is the prop root (also called stilt root) that diverges from the tree and anchors into the bottom to stabilize the tree in the soft, muddy substrate. The prop root has lenticels on the upper surface, large pores with a corky layer enabling the uptake of oxygen. Lenticels can close or become functionally restricted during inundation. The tissue of the prop roots consists of aerenchyma and is connected with the lenticels. Through this aerenchyma, oxygen can be provided to the submerged parts of the tree. The roots that break at alternating places through the soil surface and submerge again form a knee root.


Knee roots. Photo credit Eric Coppejans [13]
Prop roots growing from the lower trunk [12]


  • Another type of roots is a shallow, horizontal root that radiates outwards. The vertical root is called a pneumatophore and can be as high as several decimeters. These roots also have lenticels and aerenchyma. They can create a huge network of vertical roots. The horizontal root is called the cable root.
  • The buttress root (also called plank root) is a root that covers the whole space between the upper part of the root and the bottom.


Pneumatophores on cable roots [12].
Buttress root (plank root). Photo credit Eric Coppejans [13].
Propagules. Photo credit Eric Coppejans [13].


Observations of the mangrove species distribution along the Mekong delta coast showed that pneumatophore species (e.g., Avicennia alba) were preferentially found in accreting sites where soil was often inundated and oxygen-poor mud had accumulated. Stilt root species (e.g., Rhizophora apiculata) occurred mostly in erosion sites where stabilizing support is important. Knee root species (e.g., Bruguiera parviflora) dominated in higher sites with more stable substrate[15]. Avicennia marina, Rhizophora apiculata, Bruguiera gymnorrhiza and Xylocarpus granatum are species with high carbon sequestration capacity. A. marina and R. apiculata strived on sand-silt soils in the low-medium intertidal zone, while B. gymnorrhiza and X. granatum were often found on silt-clay soils in the high-intertidal zone, especially in estuaries[16].

Mangroves aid soil formation by trapping debris. Plank roots and pneumatophores accumulate sediments in protected sites and form mangrove peats; sediment trapping is a function of the volume of aboveground roots (Du et al., 2021[17]). Root production also contributes to soil elevation, even more than leaf, twig and branch litter, due to slow decomposition (Krauss et al., 2014[18]).

Pollination of the trees is done by the wind or by organisms. All mangroves disperse their offspring by water. They produce unusually large propagating structures or propagules. The embryo initiates germination on the seed, still attached on the tree and further develops into a propagule. This phenomenon is known as vivipary.

Mangrove biota and food-web functioning

Mangrove forests support rich communities of aquatic, benthic and terrestrial organisms. The submerged roots, tidal creeks, muddy forest floor and canopy provide a wide variety of habitats[19]. Characteristic inhabitants include crabs, shrimps, molluscs, worms, insects, fish, birds and reptiles. The species composition varies with climate, salinity, tidal inundation, sediment type and the connection with rivers and adjacent coastal habitats. Fallen leaves, wood and roots enter the detrital food web, where they are fragmented by crabs and other detritivores and decomposed by fungi and bacteria. [20].

Crabs are keystone species in many mangrove ecosystems. This means that the presence of this animal in the community makes it possible for many other species to live there. The crabs go through their larval stages in the water beneath the mangroves. When they are mature, they crawl up on the mangroves and feed on the leaves. They can reach high densities and are crucial in the processing of leaf litter. Their burrowing activity modifies the micro-topography of the bottom and aerates the soil. This decreases the sulphide levels in the soil and positively influences the productivity of the trees[21]. These effects are not always beneficial. Crabs can consume large numbers of propagules and seedlings, and dense burrow networks may locally weaken creek banks or newly deposited sediment, causing mangrove uprooting, tree collapse and increased wave impact[22]. An example of a mangrove crab is the fiddler crab Uca lactea.

Mangrove oysters, such as the American species Crassostrea rhizophorae and the West African species Crassostrea tulipa, are edible cupped oysters that commonly attach to the aerial roots of mangrove trees in tropical estuaries. Barnacles, mussels and other sessile organisms compete with oysters for space on roots that are submerged at high tide. Periwinkles and other gastropods occur on roots, stems and oyster shells and contribute to the processing of algae and organic matter.

The submerged roots also support tunicates, sponges and other epibionts, while shrimps, hermit crabs and fishes move among the roots. Together, these organisms form an important food source for larger fishes, birds, reptiles and mammals. The composition of this fauna varies greatly among regions and with salinity, inundation and connectivity with coastal waters. Predators in American mangrove and adjoining wetland systems include rails, diamondback terrapins, water snakes, raccoons and killifishes; other regions support different predator communities.

Mangrove creeks and flooded forests provide feeding grounds and shelter for many fish and crustaceans. The shallow water and dense root systems offer refuge for juveniles, while algae, detritus, plankton and small invertebrates provide abundant food. Many species move between mangroves, tidal flats, estuaries, seagrass beds and coral reefs during tidal cycles or successive stages of their life history. Mangroves can therefore support coastal and estuarine fisheries far beyond the forest itself[20].

The canopy and upper forest are inhabited by insects, spiders, birds, bats, reptiles and other terrestrial animals. Birds use mangroves for feeding, nesting and roosting, while insects, birds and bats contribute to pollination. Examples of birds are pelicans, wood ibises, herons, egrets and roseate spoonbills (Hogarth, 1999 [8]). Together, these aquatic and terrestrial communities make mangroves important links between land, estuaries and the coastal sea.


Fiddler crab Uca lacteal [23].
Egret Egretta alba [24].
Water moccasin Agkistrodon contortrix [25].
Diamondback turtle Malaclemys terrapin [26].


Coastal protection

Coastal protection is an important ecosystem service provided by mangroves. It has been estimated that mangroves protect millions of people from flooding every year, notably in densely populated areas of southeast Asia and reduce the costs of flood damages worldwide (Worthington and Spalding, 2018[27]). The global flood protection benefits of mangroves have been estimated at more than $US 65 billion per year[28].

Mangrove forests efficiently dissipate the energy of incident waves. Wave height reduction of 0.2 – 1.2 m has been observed per 100 m forest width. It depends in particular on vegetation density (approximately linear scaling[29]), vegetation type[2] and several other factors. Even a narrow fringe of Rhizophora mangroves substantially reduces wave impulse forces on shoreward dikes or seawalls[30]. An accurate estimate of the rate of wave attenuation requires local investigation. Mangroves help to mitigate the threat of inundation and devastation by storm surges and cyclones, building a 'living coastal buffer' that can reduce erosion, and temper flood flows driven by storm surges. Storm surge attenuation depends on the width and density of the mangrove belt, with a stronger effect of short trees compared to tall trees; an order of magnitude estimate of the observed decrease of storm surge height in the South Florida mangrove zone for Hurricane Wilma (2005) was 0.2 m per km forest width (Chen et al., 2021[31]). See also Wave damping by vegetation. The reduction of storm surge level and wave height does not increase linearly with forest width.

The coastal protection function is further enhanced by sediment trapping in mangrove forests. Observed annual accretion rates from external sources are of the order of one up to a few cm[32]. Moreover, thick litter layers can build up over time due to accumulation of decaying organic matter, such as leaf and root litter and the formation of agal mats. However, in spite of substantial mineral and organic accretion, the observed net surface elevation change is much smaller - of the order of one or a few mm annually, generally positive and sometimes negative - due to mineralization, subsidence and compaction processes[33].

Even though mangroves reduce flood risk (provided width and elevation are sufficient), the risk is not eliminated and mangroves are therefore not a substitute for engineered protection in all settings.

Other ecosystem services

  • Carbon sequestration. Deposition of plant litter and woody debris, root accumulation and algal mat development contribute to carbon storage. Belowground carbon represents 85% of the total ecosystem carbon stock in many mangrove forests (Kauffman et al., 2020[34]). Alongi (2020[35]) estimated the carbon burial by mangroves at about 180 gC/m2/year, yielding a global carbon sequestration of 10-15 million tonnes C /year. This is of the order of 1% of the global carbon sequestration by forests. However, according to Wang et al. (2021[36]), this figure is possibly underestimated by a factor 2-3 because the burial rate in the tropics is much faster than average (i.e. of the order of 400 gC/m2/year). Carbon sequestration by mangroves may even be substantially larger than this burial estimate when taking into account the outwelling of dissolved inorganic carbon (DIC) to the deep sea, as discussed in the article Blue carbon sequestration.
  • Food production. Mangrove communities are recognized as highly productive ecosystems that provide large quantities of organic matter to adjacent coastal waters in the form of detritus, benthic microalgae, phytoplankton, epiphytes and animals (fish, shellfish)[20]. This organic matter serves as a nutrient source and is the base of an extensive food web in which organisms of commercial importance take part.
  • Nursery function. Mangrove creeks, flooded forest floors and submerged root systems provide shelter and feeding habitat for juvenile fish, crustaceans and molluscs, including many species of commercial importance. The shallow water and structural complexity of the roots reduce access by large predators, while algae, detritus, plankton and small invertebrates provide abundant food[37]. Filamentous algae and microbial mats can stabilize fine surface sediments and provide additional habitat and food for small fish and invertebrates. Mangroves also provide feeding, nesting and roosting sites for resident and migratory birds.
  • Nutrient uptake transformation. Anaerobic mineralization by bacteria and archaea in mangrove soils, together with nutrient uptake and transformation by microorganisms associated with the sediment and above-ground roots, can contribute to the removal or retention of organic matter and nutrients from wastewater inputs (Romanacha et al., 2018[38]). Within mangrove soils, microbial processes—including denitrification, ammonification dissimilatory nitrate reduction to ammonium, (DNRA), and anaerobic ammonium oxidation (anammox)—form a complex microbial N (nitrogen) transformation cycle (see Nutrient conversion in the marine environment). Nutrient enrichment (mainly from agricultural and urban effluents) stimulates growth of above-ground biomass and the ability of mangroves to act as nutrient sinks, due to high rates of nutrient uptake and denitrification. However, negative impacts have also been reported, see next paragraph.

Threats

Mangroves are threatened in their existence by several causes, generally related to human activities.

  • Soil reclamation for agriculture and aquaculture reduces regional biodiversity due to loss of mangrove habitats. Many mangrove forests worldwide have been cleared to make way for shrimp aquaculture, with a strong negative impact on biodiversity and coastal safety (increased erosion).
  • Another major issue is the clearcutting of mangals for their hard wood. This wood is resistant against termites and therefore an important export product for building constructions in areas where termites are abundant. The wood can also be used for charcoal and fuelwood. The substrate will be no longer stable when the trees are cut away and erosion will result (Besset et al., 2019[39]).
  • Reduction in river and surface run-off deprives tropical coastal deltas of fresh water and sediment. This results in coastal erosion, with loss of mangroves, reduced species diversity and less organic production. Both the terrestrial and aquatic food webs are altered and habitats for species of higher trophic levels are lost.
  • Replacement of mangrove forest with dikes can create ponds with anoxic water that increases the level of sulphide in the soil and lowers the pH leading to major shrimp losses.
  • Effluents from households, industries, agriculture and aquaculture containing nutrients and pesticides disturb the mangrove food web. Nutrient loading may increase productivity at moderate levels but can alter root allocation, soil stability, microbial communities and greenhouse-gas emissions. Several studies have found that nutrient enrichment, specifically N, leads to a reduction in overall soil microbial biodiversity and organic matter (OM) composition. This may decrease the potential for long-term carbon storage of root-derived OM while increasing carbon loss via soil respiration (Mack et al., 2024[40]). See also Possible consequences of eutrophication.
  • Spills of oil, toxic chemicals and dumping of waste into the water causes localized impacts on the mangroves. The introduction of alien species by ballast water or from the hulls of vessels will also have negative effects on the mangrove habitats. These species will compete with indigenous species for space and food.

Impact of climate change

Because mangroves raise the soil level, it is believed that mangrove forests can keep pace with a moderate sea level rise up till about 6 mm/year (Alongi, 2022[41]). Elevation gain by physical and biological processes generally exceeds subsoil compaction by dewatering and organic matter decomposition. Net elevation rates of about 5 mm/year have been recorded (Krauss et al., 2014[18]), but elevation rates are variable between regions. Survival of mangroves therefore depends on the rate of sea level rise. Another threat is the impact of storms that may become more severe and more frequent so that periods available for recovery will become shorter. More frequent and heavier rainfall in tropical and subtropical regions can lead to more productive and luxuriant mangrove forests, but longer periods of extreme drought can cause significant damage to mangrove forests in arid areas. Inland migration of mangroves in response to sea-level rise may occur where the landward margin of mangroves is unimpeded by artificial or natural barriers (Friess et al., 2019[42]; Bozi et al., 2021[43]). In some cases, warm temperatures and sea level rise may drive latitudinal expansion of mangroves into existing salt marsh habitat in subtropical and tropical areas. This may result in higher soil carbon concentrations and blue carbon accumulation in the newly colonized mangroves (Kelleway et al., 2016[44]). It is further likely that with small temperature increases, mangroves will produce more carbon via higher primary production, but rates of leaf photosynthesis decline as temperatures increase from 33 to 35°C[45].

Mangrove loss

Worldwide, a total area of over 5,000 km2 (about 3.4%) of mangrove has been lost between 1996 and 2020[4], while an estimated 1,400 km2 of remaining mangrove forests are identified as degraded (Worthington and Spalding, 2018[27]). Mangrove losses continue on every continent, although rates of loss have declined considerably from 1 – 3% in the late 20th century to 0.3 – 0.6% in the early 21st century (Friess et al., 2019[42]). The main causes of mangrove loss are transformation of forests into economic land use such as aquaculture and agriculture, wood production, and (urban) infrastructure. Spontaneous mangrove regeneration in the deforested lands rarely occurs (UNEP, 2006[46]) unless measures are taken against increased wave agitation and soil erosion (Winterwerp et al. 2013[47]). Non-native plant invasions may also restrict the re-growth of mangroves (Romanacha et al. 2018[38]).

Mangrove restoration and rehabilitation

Causes of failure

Mangrove replanting projects have been undertaken in many places worldwide (Friess et al., 2019[42]). However, many replanting programs have failed.

A major issue is that many rehabilitation projects start planting before investigating the original causes of mangrove loss, in order to find out why there is no natural regeneration on site (Lewis, 2005[48]). Often essential conditions are not met because previous reclamations and interventions may have rendered the site less suitable for mangrove regeneration. For example, compacted mudflats often have permanently saturated soil with poor drainage, leading to anoxic and potentially acidic soil (Holguin et al., 2001[37]). Besides soil degradation, other possible causes include altered hydrology (regarding e.g., flushing, salinity, sea level, hydroperiod), altered hydrodynamics (e.g., wave and current characteristics, dispersal of propagules), disturbed sedimentation regime (insufficient or excessive sediment supply) and pollution (e.g., fertilizers, pesticides) - see also #Threats. Any mangrove restoration plan should therefore first establish the causes why a previously existing mangrove forest was lost. The best option for mangrove restoration is generally the remediation of these causes, if possible. This may restore conditions in which mangrove forests recover naturally, without extensive replanting (Fig. 3).


Fig. 3. Hydrological rehabilitation of abandoned aquaculture ponds at Tiwoho, North Sulawesi, Indonesia. Left panel: In 2004, local residents breached pond dikes and partly filled artificial drainage channels to restore tidal inundation and drainage. The stakes visible belong to earlier unsuccessful planting campaigns. From some limited amount of planting, of over 14 mangrove species, natural mangrove recruitment started after approximately one year. Right panel: Substantial forest development had occurred after six years. Photo credit Ben Brown, Rignolda Djamaluddin[49]. CC BY 4.0. licence.


If the causes of mangrove forest loss cannot be completely eliminated, natural recovery will not occur and replanting may be necessary. Another important issue is that species chosen for replanting may not be appropriate for the current site conditions. Site conditions include: salinity, soil type, soil anoxia, sulphate levels, nutrient levels, pH, wave energy, temperature, light levels, inundation regimes, tides and wind distribution of propagules and seeds (Wodehouse, 2019[50]). Species must be selected that are adapted to the prevailing site conditions.

Other reported reasons for failure include: poor planting method, lack of aftercare (e.g. weeding) and monitoring, fresh water availability, lack of drainage and sediment availability and high wave energy (i.e. inappropriate site choice). Unfavorable biological conditions range from limited seed availability, insufficient seed transport capacity, to adverse biotic activity such as barnacle infestation, predation by crabs and bioturbation by worms, burying small seedlings. A further impediment is the large quantity of household waste, most notably plastic. Plastic getting stuck to seedlings increase the chance of uprooting, and covering pneumatophores causes deformation of the roots, while the tree attempts to outgrow the suffocating material (Winterwerp et al., 2020[51]).

Ecological Mangrove Restoration (EMR)

EMR aims at restoring the favorable habitat conditions for natural mangrove establishment, minimizing the need for replanting (Lewis, 2005[48]). In this way, EMR strives for natural zonation and optimized species site matching. If abiotic conditions are favorable, mangroves generally recruit spontaneously and grow naturally. This is often preferred over planting, because natural recolonization can proceed very rapidly if conditions are favorable, whereas the reported failure rate of replanting programs rises to around 80%. After restoration, it may take only 5 years to obtain a forest's almost full coastal protection capacity. Additionally, such capacity appears to be maintained throughout the forest's lifetime with minor variations, provided the forest is not affected by any conditions altering its health, such as extreme wave events or anthropogenic action that may modify its habitat (Maza et al., 2021[52]).

If regeneration through natural recruitment is not an option, species must be carefully selected for planting based on biotic and abiotic research. Planting mixed species produces a richer mangrove community and plant success rates are higher (Primavera et al., 2012[53]).

Sites that have been altered by previous reclamation and interventions require prior restoration including physical conditions such as wave climate, currents, flushing, sedimentation, and drainage. Wave conditions can be restored by the installation of permeable dams, consisting of horizontally placed brushwood, which damp the waves, and vertical poles to hold the brushwood. In mild wave climates, bamboo is a suitable material for the construction of such dams. The configuration of the dams must be designed such that the original sedimentation regime is restored, as mangrove degradation is often associated with accreting/stable coastlines turning towards an erosive state (Besset et al., 2019[39]). Detailed instructions for the construction of permeable dams are given by Winterwerp et al. (2020[51]). Disturbance of the fine sediment balance is a another important cause of the poor success of rehabilitation efforts on eroding coastlines. Newly formed mud flats should be protected from fishing and other bottom-disturbing activities, as frequent stirring up the fresh deposits will prevent mangrove recruitment. Project outcomes can further be improved by restoring appropriate hydrological connectivity with good tidal flushing and drainage (Lewis, 2005[48]).

An alternative method may exist to create conditions that stimulate mangrove regeneration in areas where coarse sediment (e.g. shell sand) is available. If the seabed in the vicinity contains such sediments, it can be winnowed for the stabilization or supply of possibly existing chenier remnants or otherwise for the construction of artificial cheniers some distance seaward of the shoreline. The protection provided by cheniers appears generally very beneficial for the development of mangrove forests, especially on microtidal coasts[54], see the article Chenier.

Programming mangrove restoration

For being successful at the long term, the social, institutional and financial components of mangrove restoration projects are crucial. These components are addressed in detail in the brochure Best practice guidelines for mangrove restoration[55]. Local and national stakeholders and financing organizations must be included in the project from the start and during all later project phases. Compliance with laws related to land tenure, customary rights and management responsibilities must be ensured. Thorough cost estimates are required for the financial planning of restoration projects and possibilities for financing through blue carbon sequestration should be investigated. A dedicated monitoring program should allow for iterative planning and adaptive management of the restoration project.

Community participation in mangrove management

In many countries, involvement and active participation of local communities are a major condition for sustainable management of mangrove forests[56]. Local indigenous communities have long been the guardians of the maritime and coastal environments in many parts of the world. These communities have utilized the resources in these regions responsibly based on ecological knowledge and cultural traditions developed as a result of historical and ongoing contact with the resources on which they rely[57].

Mangroves are a natural resource that can provide coastal communities with essential livelihoods. Government policies aimed at conserving mangrove forests by denying access to this resource are often doomed to failure. Given the right regulatory context, local communities are often best placed to develop and implement management practices appropriate to the ecosystem resource and social situation[56]. Uncontrolled exploitation of mangroves can lead to the loss of the natural resource due to the so-called 'tragedy of the commons'. Controlled exploitation, cutting down less healthy trees ('thinning') or the oldest trees ('selective harvesting') can even benefit the mangrove forest if young seedlings are planted in the cleared areas. Such controlled exploitation, based on enforced rules, can commit local communities to participate in sustainable mangrove management. Studies have reported enhanced stand biomass, improved growth conditions, increased carbon sequestration and improved forest productivity[58]. Stable or accreting coastlines are an important precondition[59].

Because rehabilitation of mangroves and their habitat is rarely successful without the involvement of local stakeholders, socio-economic aspects should be an important component of restoration projects. The socio-economics of sustainable mangrove use should provide benefits to local communities. Examples of economic activities alongside mangrove restoration include sustainable aquaculture and integrated mangrove-aquaculture schemes, fisheries, eco-tourism and non-timber forest products (Primavera et al., 2012[53]). Restoration projects must therefore be designed for community participation, co-creation, and engagement.


Cost-benefit of mangrove restoration

Estimates of costs and benefits of mangrove restoration widely diverge[60]. High-rated benefits are fisheries, timber, waste water treatment and coastal protection. An average estimate is 20,000 US$ per ha and per year, within a range of 500-100,000 US$. This large range can (partly) be explained by differences in valuation methods and site-specific factors such as vegetation characteristics (e.g. species, age, tree density, height, root), sediments (e.g. depth, nutrients) and various environmental variables (e.g. location, salinity, debris). Restoration costs reported in the literature fall in an even wider range of US$ 500-400,000 per ha, which can be (partly) explained by the type of engineering interventions, planting vs. natural regeneration and labour costs. Comparing the figures for restoration (per ha) and for benefits (per ha and per year), in almost every case well-designed and efficient restoration projects yield benefits that largely exceed the costs[60].


Further reading

  • Spalding, M. 2011. World Atlas of Mangroves: Mark Spalding, Mami Kainuma and Lorna Collins (eds.) London, Washington D.C.: Earthscan 2010. ISBN 978-1-84407-657-4
  • Beeston, M., Cameron, C., Hagger, V., Howard, J., Lovelock, C., Sippo, J., Tonneijk, F., van Bijsterveldt, C. and van Eijk, P. (Editors) 2023. Best practice guidelines for mangrove restoration. Global Mangrove Alliance, www.mangrovealliance.org


Related articles

Marine habitats and ecosystems
Blue carbon sequestration
Characteristics of sedimentary shores
Wave damping by vegetation.
Chenier
Coastal mud belt


External links


References

  1. ↑ 1.0 1.1 http://www.vliz.be/vmdcdata/mangroves
  2. ↑ 2.0 2.1 Horstman, E.M., Dohmen-Janssen, C.M., Narra, P.M.F., van den Berg, N.J.F., Siemerink, M. and Hulscher, S.J.M.H. 2014. Wave attenuation in mangroves: A quantitative approach to field observations. Coastal Engineering 94: 47–62
  3. ↑ Karleskint G. 1998. Introduction to marine biology. Harcourt Brace College Publishers.
  4. ↑ 4.0 4.1 Bunting, P., Rosenqvist, A., Hilarides, L., Lucas, R.M., Thomas, T., Tadono, T., Worthington, T.A., Spalding, M., Murray, N.J. and Rebelo, L-M. 2022. Global Mangrove Extent Change 1996 – 2020: Global Mangrove Watch Version 3.0. Remote Sensing. https://zenodo.org/records/6894273
  5. ↑ Jia, M., Wang, Z., Mao, D., Ren, C., Song, K., Zhao, C., Wang, C., Xiao, X. and Wang, Y. 2023. Mapping global distribution of mangrove forests at 10-m resolution. Science Bulletin 68: 1306–1316
  6. ↑ Bertness M.D., Gaines, S.D. and Hay, M.E. 2001. Marine Community Ecology. Sinauer Associates, Inc. p. 550
  7. ↑ Pinet P.R. 1998. Invitation to Oceanography. Jones and Barlett Publishers. p. 508
  8. ↑ 8.0 8.1 8.2 Hogarth P.J. 1999. The biology of mangroves. Oxford University Press. p.228
  9. ↑ Saintilan, N., Wilson, N., Rogers, K., Rajkaran, A. and Krauss, K.W. 2014. Mangrove expansion and saltmarsh decline at mangrove poleward limits. Glob. Change Biol. 20: 147–57
  10. ↑ Srikanth, S., Lum, S.K.Y and Chen, Z. 2016. Mangrove root: adaptations and ecological importance. Review. Trees 30: 451–465
  11. ↑ Parida, A. K. and Jha, B. 2010. Salt tolerance mechanisms in mangroves: A review. Trees 24: 199–217
  12. ↑ 12.0 12.1 12.2 http://en.wikipedia.org/wiki/Mangrove
  13. ↑ 13.0 13.1 13.2 13.3 Eric Coppejans - http://www.vliz.be/imis/imis.php?module=person&persid=134
  14. ↑ http://sofia.usgs.gov/virtual_tour/ecosystems/index.html
  15. ↑ Nguyen, L.T.M., Hoang, H.T., Choi, E. and Park, P.S. 2023. Distribution of mangroves with different aerial root morphologies at accretion and erosion sites in Ca Mau Province, Vietnam. Estuarine, Coastal and Shelf Science 287, 108324
  16. ↑ Syahid, L.N., Sakti, A.D., Ward, R., Rosleine, D., Windupranata, W. and Wikantika, K. 2023. Optimizing the spatial distribution of Southeast Asia mangrove restoration based on zonation, species and carbon projection schemes. Estuarine, Coastal and Shelf Science 293, 108477
  17. ↑ Du, Q., Qin,Z., Ming, S. and Zhang, C. 2021 Differences in the vertical accretion of sediment among mangrove species with different aerial root types. Estuarine, Coastal and Shelf Science 256, 107375
  18. ↑ 18.0 18.1 Krauss, K.W., McKee, K.L., Lovelock, C.E., Cahoon, D.R., Saintilan, N., Reef, R. and Chen, L. 2014. How mangrove forests adjust to rising sea level. New Phytologist 202: 19–34
  19. ↑ Nagelkerken, I., Blaber, S.J.M., Bouillon, S., Green, P., Haywood, M., Kirton, L.G., Meynecke, J.-O., Pawlik, J., Penrose, H.M., Sasekumar, A. and Somerfield, P.J. 2008. The habitat function of mangroves for terrestrial and marine fauna: A review. Aquatic Botany 89: 155–185
  20. ↑ 20.0 20.1 20.2 Hutchison, J., Spalding, M. and zu Ermgassen, P. 2014. The role of mangroves in fisheries enhancement. The Nature Conservancy and Wetlands International Report
  21. ↑ Xiao, K., Pan, F., Santos, I.R., Zheng, Y., Zheng, C., Chen, N., Lu, Z., Wang, F., Li, Z and Li, H. 2022. Crab bioturbation drives coupled iron-phosphate-sulfide cycling in mangrove and salt marsh soils. Geoderma 424, 115990
  22. ↑ Phong, N.T. 2024. Erosion of mangrove dominated muddy coasts in Vietnam and Indonesia: Processes and accelerating factors. Regional Studies in Marine Science 79, 103821
  23. ↑ http://www.fiddlercrab.info/u_lactea.html
  24. ↑ http://en.wikipedia.org/wiki/Snowy_Egret
  25. ↑ http://en.wikipedia.org/wiki/Agkistrodon_contortrix
  26. ↑ http://en.wikipedia.org/wiki/Diamondback_turtle
  27. ↑ 27.0 27.1 Worthington, T. and Spalding, M. 2018. Mangrove Restoration Potential: A global map highlighting a critical opportunity. Report, 26 October 2018. doi:10.17863/CAM.39153
  28. ↑ Menendez, P., Losada, I.J., Torres-Ortega, S., Narayan, S. and Beck, M.W. 2020. The global flood protection benefits of mangroves. Sci. Rep. 10, 4404
  29. ↑ Kelty, K., Tomiczek, T., Cox, D.T., Lomonaco, P. and Mitchell, W. 2022. Prototype-Scale Physical Model of Wave Attenuation Through a Mangrove Forest of Moderate Cross-Shore Thickness: LiDAR-Based Characterization and Reynolds Scaling for Engineering With Nature. Front. Mar. Sci. 8: 780946
  30. ↑ Tomiczek, T., Mitchell, W.T., Lomonaco, P., Cox, D.T. and Kelty, K. 2024. Prototype-scale laboratory observations of wave force reduction by an idealized mangrove forest of moderate cross-shore width. Ocean Engineering 310, 118740
  31. ↑ Chen, Q., Li, Y., Kelly, D.M., Zhang, K., Zachry, B. and Rhome, J. 2021. Improved modeling of the role of mangroves in storm surge attenuation. Estuarine, Coastal and Shelf Science 260, 107515
  32. ↑ Chaudhuri, P., Chaudhuri, S. and Ghosh, R. 2019. The Role of Mangroves in Coastal and Estuarine Sedimentary Accretion in Southeast Asia. Sedimentary Processes - Examples from Asia, Turkey and Nigeria. IntechOpen DOI: http://dx.doi.org/10.5772/intechopen.85591
  33. ↑ Krauss, K.W., Cahoon, D.R., Allen, J.A., Ewel, K.C., Lynch, J.C. and Cormier, N. 2010. Surface elevation change and susceptibility of different mangrove zones to sea-level rise on Pacific high islands of Micronesia Ecosystems 13: 129–143
  34. ↑ Kauffman, J.B., Adame, M.F., Arifanti, V.B., Schile-Beers, L.M., Bernardino, A.F.,Bhomia, R.K., Donato, D.C., Feller, I.C., Ferreira, T.O., Jesus Garcia, M.d.C., MacKenzie, R.A., Megonigal, J.P., Murdiyarso, D., Simpson, L. and Hernandez Trejo, H. 2020. Total ecosystem carbon stocks of mangroves across broad global environmental and physical gradients. Ecol. Monogr. 90, e01405
  35. ↑ Alongi, D.M. 2020. Global significance of mangrove blue carbon in climate change mitigation. Science 2020, 2, 57
  36. ↑ Wang, F., Sanders, C.J., Santos, I.R., Tang, J., Schurech, M., Kirwan, M.L., Kopp, R.E., Zhu, K., Li, X., Yuan, J., Liu, W. and Li, Z. 2021. Global blue carbon accumulation in tidal wetlands increases with climate change. National Science Review 8: nwaa296
  37. ↑ 37.0 37.1 Holguin, G., Vazquez, P. and Bashan, Y. 2001. The role of sediment microorganisms in the productivity, conservation, and rehabilitation of mangrove ecosystems: an overview. Biol. Fertil. Soils 33: 265–278 Cite error: Invalid <ref> tag; name "H01" defined multiple times with different content
  38. ↑ 38.0 38.1 Romanacha, S.S., DeAngelis, D.L., Koh, H.L., Li, Y., Teh, S.Y., Barizan, R.S.R. and Zhai, L. 2018. Conservation and restoration of mangroves: Global status, perspectives, and prognosis. Ocean and Coastal Management 154: 72–82
  39. ↑ 39.0 39.1 Besset, M., Gratiot, N., Anthony, E.J., Bouchette, F., Goichot, M. and Marchesiello, P. 2019. Mangroves and shoreline erosion in the Mekong River delta. Estuarine. Coastal Shelf Sci. 226, 106263
  40. ↑ Mack, M.R., Adam Langley, J., Feller, I.C. and Chapman, S.K. 2024. The ecological consequences of nutrient enrichment in mangroves. Estuarine, Coastal and Shelf Science 300, 108690
  41. ↑ Alongi, D.M. 2022. Impacts of Climate Change on Blue Carbon Stocks and Fluxes in Mangrove Forests. Forests 13, 149
  42. ↑ 42.0 42.1 42.2 Friess, D.A., Rogers, K., Lovelock, C.E., Krauss, K.W., Hamilton, S.E., Lee, S.Y., Lucas, R., Primavera, J., Rajkaran, A. and Shi, S. 2019. The state of the world’s mangrove forests: past, present and future. Annu. Rev. Environ. Resour. 44: 89–115
  43. ↑ Bozi, B.S., Figueiredo, B.L., Rodrigues, E., Cohen, M.C.L., Pessenda, L.C.R., Alves, E.E.N., de Souza, A.V., Bendassolli, J.A., Macario, K., Azevedo, P. and Culligan, N. 2021. Impacts of sea-level changes on mangroves from southeastern Brazil during the Holocene and Anthropocene using a multi-proxy approach. Geomorphology 390, 107860
  44. ↑ Kelleway, J.J., Saintilan, N., Macreadie, P.I., Skilbeck, C.G., Zawadzki, A. and Ralph, P.J. 2016. Seventy years of continuous encroachment substantially increase ‘blue carbon’ capacity as mangroves replace salt marshes. Glob. Change Biol. 22: 1097–1109
  45. ↑ Ball, M.C. and Sobrado, M.A. 2002. Ecophysiology of mangroves: challenges in linking physiological processes with patterns in forest structure. In: Scholes JD, BarkerMG, editors. PressMC. Blackwell Oxford: Physiological plant ecology 2002: 331–46
  46. ↑ UNEP. 2006. In the frontline: shoreline protection and other ecosystem services from mangroves and coral reefs. p.36
  47. ↑ Winterwerp, J., Erftemeijer, P., Suryadiputra, N., Van Eijk, P. and Zhang, L. 2013. Defining eco-morphodynamic requirements for rehabilitating eroding mangrove-mud coasts. Wetlands 33: 515–526
  48. ↑ 48.0 48.1 48.2 Lewis III, R.R. 2005. Ecological engineering for successful management and restoration of mangrove forests. Ecol. Eng. 24: 403–418
  49. ↑ Brown, B. and Djamaluddin, R. 2017. A site history and field guide for Ecological Mangrove Rehabilitation in Tiwoho Village, Bunaken National Marine Park, North Sulawesi, Indonesia. Bogor, Indonesia: CIFOR
  50. ↑ Wodehouse, D.C. and Rayment, M.B. 2019. Mangrove area and propagule number planting targets produce sub-optimal rehabilitation and afforestation outcomes. Estuar. Coast. Shelf Sci. 222: 91–102
  51. ↑ 51.0 51.1 Winterwerp, J.C., Albers, T., Anthony, E.J., Friess, D.A., Mancheno, A.G., Moseley, K., Muhari, A., Naipal, S., Noordermeer, J., Oost, A., Saengsupavanich, C., Tas, S.A.J., Tonneijck, F.H., Wilms, T., Van Bijsterveldt, C., Van Eijk, P., Van Lavieren, E. and Van Wesenbeeck, B.K. 2020. Managing erosion of mangrove-mud coasts with permeable dams – lessons learned. Ecological Engineering 158, 106078
  52. ↑ Maza, M., Lara, J.L. and Losada, I.J. 2021. Predicting the evolution of coastal protection service with mangrove forest age. Coastal Engineering 168, 103922
  53. ↑ 53.0 53.1 Primavera, J.H., Savaris, J.P., Bajoyo, B.E., Coching, J.D., Curnick, D.J., Golbeque, R.L., Guzman, A.T., Henderin, J.Q., Joven, R.V., Loma, R.A. and Koldewey, H.J. 2012. Manual on Community-based Mangrove Rehabilitation, Mangrove Manual Series No.1 London. 240 pp.
  54. ↑ van Bijsterveldt, C.E.J., van der Wal, D., Mancheno, A.G., Fivash, G.S., Helmi, M. and Bouma, T.J. 2023. Can cheniers protect mangroves along eroding coastlines? – The effect of contrasting foreshore types on mangrove stability. Ecological Engineering 187, 106863
  55. ↑ Beeston, M., Cameron, C., Hagger, V., Howard, J., Lovelock, C., Sippo, J., Tonneijk, F., van Bijsterveldt, C. and van Eijk, P. (Editors) 2023. Best practice guidelines for mangrove restoration. Global Mangrove Alliance, www.mangrovealliance.org
  56. ↑ 56.0 56.1 Rotich, B., Mwangi, E. and Lawry, S. 2016. Where land meets the sea: a global review of the governance and tenure dimensions of coastal mangrove forests. Bogor, Indonesia: CIFOR; Washington, DC: USAID Tenure and Global Climate Change Program
  57. ↑ Vierros, M. 2017. Communities and blue carbon: the role of traditional management systems in providing benefits for carbon storage, biodiversity conservation and livelihoods. Clim. Chang. 140: 89–100
  58. ↑ Phong, N.T. and Nuong, C.T. 2023. Thinning, selective harvesting and mangrove protection forests: Lessons learned and recommendations from the Vietnamese Mekong Delta. Estuarine, Coastal and Shelf Science 288, 108345
  59. ↑ Luom, T.T., Phong, N.T., Smithers, S. and Tai, V.T. 2021. Protected mangrove forests and aquaculture development for livelihoods. Ocean and Coastal Management 205, 105553
  60. ↑ 60.0 60.1 Su, J., Friess, D.A. and Gasparatos, A. 2021. A meta-analysis of the ecological and economic outcomes of mangrove restoration. Nature Communications 12: 5050


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