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 only trees that are capable of thriving in salt water. 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. The word mangrove is derived from the Portugese word mangue which means “tree” and the English word grove which is used for trees and shrubs that are found in shallow, sandy or muddy areas (Karleskint, 1998[2]). They replace salt marshes in tropical and subtropical regions. They are salt-tolerant forested wetlands at the interface between the terrestrial landscape and the marine environment. The dominant vegetation are several species of mangrove: woody trees and shrubs with a thick, partially exposed network of roots that grow down from the branches into the water and sediment. They settle 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[3]). 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[2]).


Fig. 1. Mangal in Thailand.


Requirements for development

Requirements for the development of mangroves are:

  • Average temperature of the coldest month higher than 20°C; the seasonal temperature range should not exceed 5°C. They are not resistant to freezing.
  • A fine-grained substrate. Exceptions are the development of mangroves on corals, as for example in Papua New Guinea and Kenya.
  • Shores must be free of strong wave action and strong tidal currents.
  • Saline water; they are facultative halophytes.
  • Deposition of sediments by small-moderate waves and tides.

Due to these processes, a well-marked zonation is seen. Each of the zones is dominated by a different mangrove species and associated fauna and flora. Red mangroves (Rhizophora) are usually found closest to the edge of the water, where the greatest degree of tidal flooding occurs. More landwards are the black mangroves (Avicennia). These areas receive only shallow flooding during high tide. The upper limit of the mangroves is occupied with white mangroves and buttonwoods. The buttonwoods are not really a mangrove species, but are a transition species between the mangrove and the terrestrial vegetation.


Distribution

Estimates of the area covered by mangroves worldwide range between 84,000 and 136,000 km2 (Hamilton and Casey, 2016[4]; Worthington and Spalding, 2018[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 (Berness et al., 2001[6]). 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 a good indicator of the distribution limit. 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

Mangroves have several functions and adaptations for thriving in saline intertidal zones. They grow in an environment whose salinity ranges between freshwater and seawater. For this reason, they have to take up water against the osmotic pressure. To overcome the negative osmotic pressure, they generate a negative hydrostatic pressure (by transpiration processes). Roots or 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 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[10]).

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.

Because salt tolerance is costly, a greater relative root mass is needed to recover the demand for water. When it rains, drop roots that descend from the branches absorb the freshwater that runs down from the stem through a special superficial layer. In this way, no high-energy inverse osmosis is needed.


Salt crystals on a mangrove leaf [11]
Extended root mass. Photo credit Eric Coppejans [12].
Drop roots from branches [13]


Deposits of fine grained (muddy and sandy) sediments lack oxygen (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. One type of roots is the prop root or rhizophore. This root diverges from the tree and anchors into the bottom to stabilize the tree in the soft, muddy substrate. The rhizophore has lenticels on the upper surface, large pores with a corky layer enabling the uptake of oxygen. Seawater cannot get into the lenticels. 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 [12]
Drop roots or rhizophora [11]


  • 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 [11].
Buttress root. Photo credit Eric Coppejans [12].
Propagules. Photo credit Eric Coppejans [12].


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.

Mangroves aid soil formation by trapping debris. Prop roots and pneumatophores accumulate sediments in protected sites and form mangrove peats. Filamentous algae also help to stabilize the fine sediments trapped by mangroves. They usually form a green-to-red mass over the substrate. They are also a filtering system for the land run-off and remove the terrestrial organic matter. They are very important habitats for many species of small fish, invertebrates and various epiflora and epifauna as well as larger birds. This is called a nursery function. The mangrove is a major producer of detritus that will contribute to offshore productivity in some seasons (Coppejans [14]).

Biota

Epiphytic micro-algae grow on the aerial roots of the trees and on the sediments. The algae are green ( Chlorophyta), brown (Phaeophyceae), red (Rhodophyta) and blue-green (Cyanophyta). The dense biomass on the aerial roots causes the water to remain on the pneumatophores; the lenticels are no longer functional and oxygen cannot penetrate into the roots. For this reason, the bark regularly falls off the root. This process is called decertification. Vertical zonation along a single pneumatophore occurs, but there is also a zonation from the upper limit of the mangroves to the lower limit.

Several invertebrate species are found in the mangrove ecosystem. Macrobenthic species are commonly or occasionally present because they are migrating or because they are living in adjacent environments. Crabs are keystone species in the mangrove ecosystem. 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. An example of a mangrove crab is the fiddler crab Uca lacteal.

An important bivalve is the purple oyster Lopha frons. This species encrusts the pneumatophores and prop roots. When the tide is high, barnacles and mussels compete with the oyster for space on the roots. Periwinkles also occur on the roots and stems, as well as on the shells of sedentary organisms attached on them. Snails are important in the turnover of the organic material. Other species that occur in the mangroves are tunicates, sponges, ants, hermit crabs, shrimps, fishes,… They are a source of nutrition for higher level predators. Species that cannot tolerate changing saline conditions can survive in the forest. These species are sea stars, brittle stars and sea squirts. Predators are clapper rails, diamondback turtles, water moccasins, raccoons and killifishes. Bacteria and fungi initially break down the leaf litter (decomposition). In the tree canopy, vertebrate fauna and birds are common. Examples of birds are pelicans, wood ibises, herons, egrets and roseate spoonbills (Hogarth, 1999 [8]).


Fiddler crab Uca lacteal [15].
Egret Egretta alba [16].
Water moccasin Agkistrodon contortrix [17].
Diamondback turtle Malaclemys terrapin [18].


Ecosystem services

  • Coastal protection. Mangroves help to mitigate the threat of inundation and devastation by storm surges and cyclones, building a living seawall that can slow or halt erosion, rapidly diminish wave-energy, and temper flood flows driven by storm surges. 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[5]).
  • Carbon sequestration. Deposition of plant litter and woody debris, root accumulation and algal mat development contribute to carbon storage. Belowground biomass represents 85% of the total biomass in many mangrove forests (Kauffman et al., 2020[19]). Alongi (2020[20]) estimated the carbon burial by mangroves at about 180 gC/m/year, yielding a global carbon sequestration of about 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[21]), this figure is underestimated by a factor 2-3 because the burial rate in the tropics is much larger than average (i.e. of the order of 400 gC/m/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, see 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 and live animals (fish, shellfish). The detritus serves as a nutrient source and is the base of an extensive food web in which organisms of commercial importance take part. In addition, mangrove ecosystems serve as shelter, feeding, and breeding zones for crustaceans, mollusks, fish of commercial importance, and resident and migratory birds (Holguin et al., 2001[22]).
  • Denitrification. Anaerobic denitrification and nitrogen-fixation by certain bacteria and cyanobacteria associated with mangrove mud and with above-ground root systems can improve water quality from wastewater inputs (Romanacha et al., 2018[23]).


Threats

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

  • Variations in river and surface run-off, that deprive tropical coastal deltas of fresh water and sediment, entails reductions of species diversity and organic production. This results in alterations of both the terrestrial and aquatic food web and reduces habitats available to species of higher trophic levels.
  • 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 coastal safety and biodiversity.
  • Clearcutting mangrove forest and replacement with dikes creates ponds with anoxic water that increases the level of sulphide in the soil and increases the pH leading to major shrimp losses.
  • Another negative impact of humans on the mangrove habitat is the use of pesticides and fertilizers. The products that are used in the upstream agriculture end up in the water around the mangroves. This causes an increased nutrient concentration, especially nitrogen and phosphorus. These nutrients cause oxygen depletion in the water and promote the growth of algae. As a result, the ecosystems will be no longer in equilibrium. See also Possible consequences of eutrophication.
  • A further 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 with large concentrations of termites. 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[24]).
  • A similar effect as with the added fertilizers and pesticides is the use of mangroves for waste-water treatment. Nutrients are added to the water and the equilibrium in the food web is disturbed. Mangroves no longer can survive in this environment and die off. The organic matter, normally stored in the mangroves, will be transported to open water and increases the aquatic primary production. This results in a huge amount of phytoplankton and water turbidity. Corals and seagrasses are strongly affected by these processes and will be deteriorated.
  • Other coastal development activities influence on the quality of the water. Industry, tourism and port development involve land reclamation and dredging. This causes resuspension of the sediment and increases the water turbidity. Light penetration in the water will be limited causing damage to the mangroves.
  • 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.
  • Climate change. Mangroves raise the soil level and thus can keep pace with a moderate sea level rise. Elevation gain by physical and biological processes generally exceeds subsoil compaction by dewatering and organic matter decomposition. Net elevation rates exceeding 5 mm/year have been recorded (Krauss et al., 2014[25]). Sediment trapping plays an important role, but other processes such as plant litter and woody debris deposition, root accumulation and algal mat development also contribute. Mangroves do not survive when the rate of sea level exceeds the soil raising capacity. Another threat is the impact of storms that may become more severe and more frequent so that periods for recovery will become shorter. Inland migration of mangroves is possible where the landward margin of mangroves is unimpeded by artificial or natural barriers (Friess et al., 2019[26]).


Mangrove loss and preservation

Worldwide, a total area of almost 10,000 km2 (about 7%) of mangrove has been lost between 1996 and 2016, while an estimated 1,400 km2 of remaining mangrove forests are identified as degraded (Worthington and Spalding, 2018[5]). 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[26]). The main causes of mangrove loss are transformation of forests into economic land use such as aquaculture and agriculture, wood production, and (urban) infrastructure. There is little chance of spontaneous mangrove regeneration in the deforested lands (Coppejans[14]; UNEP, 2006[27]), due to increased wave agitation and soil erosion (Winterwerp et al. 2013[28]). Non-native plant invasions may also restrict the re-growth of mangroves (Romanacha et al. 2018[23]).

Several measures can be taken to preserve the mangrove ecosystem. An example is the FAO Code of Conduct for Responsible Fisheries [29]. The FAO Code regulates fishing techniques and eliminates destructive fishing gears. The Code also provides guidelines for setting up management plans, for establishing no-take areas and involving fishermen and other users. Mangrove ecosystems can also be protected by creating National Protected Areas, World Heritage Sites or Ramsar Sites, that provide a legal protection framework.


Mangrove restoration and rehabilitation

Causes of failure

Mangrove replanting projects have been undertaken in many places worldwide (Friess et al., 2019[26]). However, few replanting programs have proven successful. A first major issue is that many rehabilitation projects start planting before studying the original cause of mangrove loss to find out why there is no natural regeneration on site (Lewis, 2005[30]). 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[22]). A second major issue is that species chosen for replanting may not be appropriate for the currently prevailing 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[31]). 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[32]).

Principles of Ecological Mangrove Restoration (EMR)

EMR aims at restoring the favorable habitat conditions for mangroves, and generally no planting is done (Lewis, 2005[30]). In this way, EMR strives for natural zonation and optimized species site matching. This results in fast growth of the forest and high survival rates. If abiotic conditions are favorable, mangroves generally recruit spontaneously and grow naturally. This is often preferable to planting, as natural recolonization can be very fast if conditions are favorable, whereas up to 85% of planting efforts fail.

If regeneration through natural recruitment is not an option, species must be carefully selected for planting. Planting mixed species produces a richer mangrove community and higher plant success rates, provided it is accompanied by biotic and abiotic research, while often also requiring hydrological restoration (Primavera et al., 2012[33]).

Sites that have been altered by previous reclamation and interventions require prior physical restoration. Altered physical conditions include: 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. Bamboo is a suitable material for the construction of such dams. Detailed instructions for the construction of impermeable dams are given by Winterwerp et al. (2020[32]). The configuration of the dams must be designed such that the original sedimentation regime is also restored, as mangrove degradation may cause accreting/stable coastlines turning towards an erosive state (Besset et al., 2019[24]). Disturbance of the fine sediment balance is a also an important cause of the poor success of rehabilitation efforts on eroding coastlines (Winterwerp et al., 2020[32]). 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[30]).

Rehabilitation of mangroves and their habitat is rarely successful without the involvement of local stakeholders. Socio-economic aspects should be included in restoration projects so that local communities benefit from sustainable mangrove use. It is essential to introduce sustainable economic activities alongside mangrove restoration, such as sustainable aquaculture and integrated mangrove-aquaculture schemes, fisheries, eco-tourism and non-timber forest products (Primavera et al., 2012[33]).


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


Related articles


External links


References

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The main author of this article is Töpke, Katrien
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Citation: Töpke, Katrien (2021): Mangroves. Available from http://www.coastalwiki.org/wiki/Mangroves [accessed on 19-10-2021]