Difference between revisions of "Mangroves"

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This article describes the [[habitat]] of the Mangrove forests. It is one of the sub-categories within the section dealing with biodiversity of [[marine habitats and ecosystems]]. It gives an overview about the characteristics, distribution, biota, functioning and adaptation to general problems the organisms are facing with. A short discussion about the threats is also present.
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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.
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==Introduction==
 
==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). <ref name="vliz">http://www.vliz.be/vmdcdata/mangroves</ref>
 
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). <ref name="vliz">http://www.vliz.be/vmdcdata/mangroves</ref>
  
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 [[mud]]dy areas. <ref>Karleskint G. 1998. Introduction to marine biology. Harcourt Brace College Publishers. p.378</ref>  They replace [[Salt marsh]]es in tropical and subtropical regions.  
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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 [[mud]]dy areas (Karleskint G. 1998<ref name=K98>Karleskint G. 1998. Introduction to marine biology. Harcourt Brace College Publishers. p.378</ref>). They replace [[salt marsh]]es in tropical and subtropical regions.  
They are salt-tolerant forested [[wetlands]] at the sea-land interface which forms the link between the terrestrial landscapes and the marine environment. The dominant plants are several species of mangrove (for a species overview, check the Mangrove Species Database <ref name="vliz"/>).
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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 [[waves|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<ref>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</ref>). Mangals therefore fulfill an important coastal protection function.  
Mangroves are 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 [[waves|wave action]] and where [[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<ref>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</ref>. Mangals therefore fulfill an important coastal protection function. The deposits of fine grained (muddy and sandy) sediments lack oxygen. <ref>Hogarth P.J. 1999. The biology of mangroves. Oxford University Press. p.228</ref>
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Mangroves are frequently associated with saline [[Lagoon|lagoons]] and are regularly found on protected sides of islands, [[Island atolls|atolls]] and tropical [[estuaries]] (Karleskint, 1998<ref name=K98/>).
Mangroves are frequently associated with saline [[Lagoon|lagoons]] and are regularly found on protected sides of islands, [[Island atolls|atolls]] and tropical [[estuaries]]. <ref>Karleskint G. 1998. Introduction to marine biology. Harcourt Brace College Publishers. p.378</ref>
 
  
  
 
[[image:mangrove thailand.jpg|center|thumb|450px|caption|Fig. 1. Mangal in Thailand.]]
 
[[image:mangrove thailand.jpg|center|thumb|450px|caption|Fig. 1. Mangal in Thailand.]]
  
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==Requirements for development==
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Requirements for the development of mangroves are:
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* Average temperatures of the coldest month higher than 20°C; the seasonal temperature range should not exceed 5°C. They are not resistant to freezing.
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* A fine-grained substrate. Exceptions are the development of mangroves on [[Coral reefs|corals], as for example in Papua New Guinea and Kenya.
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* Shores must be free of strong [[waves|wave]] action and strong [[tidal current]]s.
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* Saline water; they are facultative halophytes.
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* Deposition of sediments by small-moderate waves and tides.
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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.
  
  
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The distribution, density and species composition are determined by the water and air temperatures during the winter, exposure to [[waves|wave action]] and [[tidal current]]s, 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 distribution, density and species composition are determined by the water and air temperatures during the winter, exposure to [[waves|wave action]] and [[tidal current]]s, 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 Malaysia and Indonesia. Over the world, 54-70 species (for a species overview, check the Mangrove Species Database <ref name="vliz"/>)(and hybrids) in 20-27 genera and 16-19 families are found. <ref>Bertness M.D. Gaines S.D. Hay M.E. 2001. Marine Community Ecology. Sinauer Associates, Inc. p. 550</ref>
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The most highly developed and most species-rich mangals are found in Malaysia and Indonesia. Over the world, 54-70 species and hybrids in 20-27 genera and 16-19 families are found (Berness et al., 2001<ref>Bertness M.D., Gaines, S.D. and Hay, M.E. 2001. Marine Community Ecology. Sinauer Associates, Inc. p. 550</ref>). A species overview is given in the Mangrove Species Database <ref name="vliz"/>.
  
  
They are almost exclusively tropical, but are also seen in the subtropics. Mangroves are intolerant of frost, but can tolerate air temperatures as low as 5°C. They are most closely correlated with the seawater temperature. The 20°C isotherm in the winter is a good indicator for the limit of distribution. The [[Species diversity|number of species]] tends to decrease with the distance from the equator. In the southern hemisphere, ranges extend further south on the eastern margins of land masses than on the western. This is because of the pattern of warm and cold [[Ocean circulation|ocean currents]]. But local anomalies of [[currents|current]] and temperature or the local evolution can create local changes. <ref>Pinet P.R. 1998.Invitation to Oceanography. Jones and Barlett Publishers. p. 508</ref> <ref>Hogarth P.J. 1999. The biology of mangroves. Oxford University Press. p.228</ref>
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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 [[Species diversity | 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 circulation | ocean currents]] (Pinet, 1998<ref>Pinet P.R. 1998.Invitation to Oceanography. Jones and Barlett Publishers. p. 508</ref>; Hogarth, 1999 <ref name=H99/>).
  
  
 
[[image:MangroveWorldMap.jpg|center|thumb|700px|caption|Fig. 2. World distribution of mangroves.]]
 
[[image:MangroveWorldMap.jpg|center|thumb|700px|caption|Fig. 2. World distribution of mangroves.]]
  
==Requirements for development==
 
  
Mangroves have several requirements to develop.
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==Functioning and adaptations==
  
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Mangroves have several functions and adaptations for thriving in saline [[intertidal]] zones.
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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. The lower surface of the leave is highly covered with hairs to raise the secreted droplets of salty water away from the leaf surface. This prevents osmotic withdrawal of water from the tissue. They also restrict the opening of their stomata (only on the lower surface of the leaf), have a tick cuticle with a waxy layer, and orientate their leaves to avoid the burning sun. This also reduces evaporation. 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.
  
* They need average '''temperatures''' of the coldest month higher than 20°C. The seasonal temperature range should not exceed 5°C. They can tolerate temperatures of 5°C, but the development will be affected. They are not resistant to freezing.
 
* In general they need a '''fine-grained substrate'''. But there could be some exceptions. This is the case in Papua New Guinea and Kenya, where the mangroves grow on [[Coral reefs|corals]].
 
* The shores must be '''free of strong [[waves|wave]] action and [[tidal current]]'''.
 
* They need salt water. They are '''facultative halophytes'''.
 
* They need a '''large tidal range'''. This causes limited [[erosion]] and deposition of sediments.
 
  
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{| style= border="0" align="center"
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|-
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| valign="top" align=centre|
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[[File:Salt crystals.jpg|thumb|120px|Salt crystals on a mangrove leaf <ref name="wikipedia">http://en.wikipedia.org/wiki/Mangrove</ref>]]
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| valign="top"|
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[[File:Extended root mass.JPG|thumb|250px|Extended root mass. Photo credit Eric Coppejans <ref name="eric"> Eric Coppejans - http://www.vliz.be/imis/imis.php?module=person&persid=134</ref>.]]
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| valign="top"|
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[[File: Drop roots.jpg|thumb|250px|Drop roots from branches <ref>http://sofia.usgs.gov/virtual_tour/ecosystems/index.html</ref>]]
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|}
  
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 the 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.
 
  
==Functioning and adaptations==
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Deposits of fine grained (muddy and sandy) sediments lack oxygen (Hogarth, 1999<ref name=H99>Hogarth P.J. 1999. The biology of mangroves. Oxford University Press. p.228</ref>).
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Mangroves therefore have to cope with anoxic conditions. The tissue of the plants requires oxygen for [[respiration]] which cannot diffuse into soils that are waterlogged. Even if the water is saturated with oxygen, its concentration is too low. This is why mangroves develop various forms of aerial roots.
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* 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, [[mud]]dy 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 '''[https://en.wikipedia.org/wiki/Aerenchyma  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'''.
  
The mangroves have several functions and adaptations to a life in an [[intertidal]] ecosystem.
 
They need to conquer some problems to be resistant to the environment.
 
The first problem is that mangrove trees are '''freshwater''' riverine trees. They grow in an environment whose [[salinity]] ranges between that of 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). They developed a mechanism to '''exclude salt''' by the roots or leaves. In this way, they are tolerant for saline conditions. Even with exclusion of most of the salts, concentration of chloride and sodium ions in the tissue is higher than other plants. This high concentration can inhibit many enzymes. To protect the enzymes, the salt is stored in '''vacuoles'''. 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 exclude salt. This can be seen as salt crystals. The lower surface of the leave is highly covered with hairs to raise the secreted droplets of salty water away from the leaf surface. This prevents osmotic withdrawal of water from the tissue. They also restrict the opening of their stomata (only on the lower surface of the leaf), have a tick cuticle with a waxy layer, and orientate their leaves to avoid the burning sun. This also reduces evaporation. Because salt tolerance is costly, a greater relative root mass is needed to recover the demand for water. When it rains, drop roots, descending 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.
 
  
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{| {| style= border="0" align="center"
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|-
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| valign="top"|
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[[File: Knee Roots detail.JPG|left|thumb|300px|caption|Knee roots. Photo credit Eric Coppejans <ref name="eric"/>]]
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| valign="top"|
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[[File: 799px-Mangrove.jpg|none|thumb|250px|caption|Drop roots or rhizophora <ref name="wikipedia"/>]]
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|}
  
<gallery>
 
Image:Salt crystals.jpg|Salt crystals on a mangrove leaf <ref name="wikipedia">http://en.wikipedia.org/wiki/Mangrove</ref>
 
Image:Extended root mass.JPG|Extended root mass. Photo credit Eric Coppejans <ref name="eric"> Eric Coppejans - http://www.vliz.be/imis/imis.php?module=person&persid=134</ref>.
 
Image:Drop roots.jpg|Drop roots from branches <ref>http://sofia.usgs.gov/virtual_tour/ecosystems/index.html</ref>
 
</gallery>
 
  
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* 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'''.
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* The '''buttress root''' is a root that covers the whole space between the upper part of the root and the bottom.
  
A second problem is the [[anoxic water|anoxic environment]]. The underground in which they root is saturated with water. The tissue of the plants requires oxygen for [[respiration]]. Gas diffusion between gas particles can only supply this need in soils that are not waterlogged. Even when the water is saturated with oxygen, its concentration is too low and the diffusion in water is very slow. This is solved by various forms of aerial roots.
 
  
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{| style= border="0" align="center"
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|-
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| valign="top" align=centre|
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[[File: Pneumatophores.jpg|left|thumb|300px|caption|Pneumatophores on cable roots <ref name="wikipedia"/>.]]
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| valign="top"|
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[[File: Buttress roots.JPG|none|thumb|200px|caption|Buttress root. Photo credit Eric Coppejans <ref name="eric"/>.]]
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| valign="top"|
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[[File: Propagules.JPG|center|thumb|150px|caption|Propagules. Photo credit Eric Coppejans <ref name="eric"/>.]]
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|}
  
  
* The most 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, [[mud]]dy substrate. The rhizophore has '''lenticels''' on the upper surface. Lenticels are large pores with a corky layer and enable the exchange of air. Seawater cannot get in the lenticels. The tissue of the prop roots consists of '''aerenchyma''' and is connected with the lenticels. Through this aerenchyma, air can be provided to the submerged parts of the tree. The root can periodically break the soil surface and submerges again. This forms a '''knee root'''.  
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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'''.  
  
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Mangroves aid soil formation by trapping debris. Prop roots and pneumatophores accumulate sediments in protected sites and form mangrove peats. The 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 [[habitat]]s 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 [[Biological productivity|productivity]] in some seasons. <ref name=E>Eric Coppejans – Course Biodiversity of aquatic food webs: from algae to marine mammals UGent</ref>
  
[[image:Knee Roots detail.JPG|left|thumb|300px|caption|Knee roots. Photo credit Eric Coppejans <ref name="eric"/>]]
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==Biota==
[[image:799px-Mangrove.jpg|none|thumb|300px|caption|Drop roots or rhizophora <ref name="wikipedia"/>]]
 
  
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'''Micro-algae''' are important in mangrove ecosystems. They are [https://en.wikipedia.org/wiki/Epiphyte epiphytic] and 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 can not 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.
  
* 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. They also have lenticels and aerenchyma. This can create a huge network of vertical roots. The horizontal root is called the '''cable root'''.
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A number of invertebrates 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''.
* At last, the '''buttress root''' is a root that covers the whole space between the upper part of the root and the bottom.  
 
  
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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.
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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).
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In the tree '''canopy''', vertebrate fauna and birds are common. Examples of birds are pelicans, wood ibises, herons, egrets and roseate spoonbills (Hogarth, 1999 <ref name=H99/>).
  
[[image:Pneumatophores.jpg|left|thumb|250px|caption|Pneumatophores on cable roots <ref name="wikipedia"/>]]
 
[[image:Buttress roots.JPG|none|thumb|250px|caption|Buttress root. Photo credit Eric Coppejans <ref name="eric"/>]]
 
  
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{| style= border="0" align="center"
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|-
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| valign="top" align=centre|
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[[File: Uca lacteal 1.jpg|right|thumb|200px|caption|Fiddler crab ''Uca lacteal'' <ref>http://www.fiddlercrab.info/u_lactea.html</ref>.]]
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| valign="top" align=centre|
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[[File:Egretta alba.jpg|thumb|200px|Egret ''Egretta alba'' <ref>http://en.wikipedia.org/wiki/Snowy_Egret</ref>.]]
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| valign="top"|
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[[File:Agkistrodon contortrix.png|thumb|200px|Water moccasin ''Agkistrodon  contortrix'' <ref>http://en.wikipedia.org/wiki/Agkistrodon_contortrix</ref>.]]
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| valign="top"|
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[[File:Diamondback turtle.jpg|thumb|200px|Diamondback turtle ''Malaclemys terrapin'' <ref>http://en.wikipedia.org/wiki/Diamondback_turtle</ref>.]]
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|}
  
The pollination of the trees is 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'''.
 
  
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==Threats==
  
[[image:Propagules.JPG|center|thumb|250px|caption|Propagules. Photo credit Eric Coppejans <ref name="eric"/>]]
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Mangroves are threatened in their existence by several causes, generally related to human activities.
 
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* 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.
 
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* Soil reclamation for agriculture and aquaculture reduces regional biodiversity due to loss of mangrove [[habitat]]s. Many mangrove forests worldwide have been cleared to make way for shrimp aquaculture, with a strong negative impact on coastal safety and biodiversity.
Mangroves aid soil formation by trapping debris. Prop roots and pneumatophores accumulate sediments in protected sites and form mangrove peats. The 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 [[habitat]]s 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 [[Biological productivity|productivity]] in some seasons. <ref name=E>Eric Coppejans – Course Biodiversity of aquatic food webs: from algae to marine mammals UGent</ref>
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* 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.  
 
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* 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]].
==Biota==
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* Another 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<ref name=B19>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</ref>).
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* 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. [[Coral reefs|Corals]] and [[seagrass]]es are strongly affected by these processes and will be deteriorated.
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* 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.
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* [[Oil spills|Spills]] of oil, toxic chemicals and dumping of waste into the water causes localized impacts on the mangroves. The introduction of [[Non-native species invasions|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.
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* Another threat is '''[[climate change]]'''. The sea level rises and storm impacts may become more severe. Storms cause damage to the mangroves and due to the higher frequency, periods for recovery will become shorter.
  
'''Micro-algae''' are important in mangrove ecosystems. They are epiphytic and 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 can not 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.
 
  
A number of invertebrates are found in the mangrove ecosystem. Macrobenthic species can be commonly found or they could only occasionally been found because they are migrating within it or because they are living in adjacent environments. '''Crabs''' are commonly found in the mangroves. They are '''keystone species'''. 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 are crucial in the processing of the leaf litter.  They can reach high densities. Burrow activities also take place. The micro-topography of the bottom will be modified and the soil will be aerated. 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''.
+
==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<ref>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</ref>). 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<ref name=F19>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</ref>). 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<ref name=E/>; UNEP, 2006<ref>UNEP. 2006. In the frontline: shoreline protection and other ecosystem services from mangroves and coral reefs. p.36</ref>).
  
[[image:Uca lacteal 1.jpg|right|thumb|200px|caption|Fiddler crab ''Uca lacteal'' <ref>http://www.fiddlercrab.info/u_lactea.html</ref>]]
+
Several measures can be taken to preserve the mangrove [[ecosystem]].  
 +
An example is the FAO Code of Conduct for Responsible Fisheries <ref>[http://www.fao.org/resilience/resources/resources-detail/en/c/273397/ FAO Code of Conduct for Responsible Fisheries]</ref>. 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 [[Marine Protected Areas (MPAs)|National Protected Areas]], World Heritage Sites or [[Ramsar Convention for Wetlands|Ramsar Sites]], that provide a legal protection framework.
  
  
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 extremely important in the turn over of the organic material.
+
==Mangrove restoration and rehabilitation==
'''Other species''' that occur in the mangroves are tunicates, sponges, ants, hermit crabs, shrimps, fishes,… They may be a source of nutrition for higher level predators. Species that cannot tolerate the 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. <ref>Hogarth P.J. 1999. The biology of mangroves. Oxford University Press. p.228</ref> <ref>Karleskint G. 1998. Introduction to marine biology. Harcourt Brace College Publishers. p.378</ref>
 
  
 +
===Causes of failure===
 +
Mangrove replanting projects have been undertaken in many places worldwide (Friess et al., 2019<ref name=F19/>).
 +
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<ref name=L5>Lewis III, R.R. 2005. Ecological engineering for successful management and restoration of mangrove forests. Ecol. Eng. 24: 403–418</ref>). 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<ref>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</ref>). 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<ref name=W19>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</ref>).
 +
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<ref name=W20>Winterwerp, J.C., Albers, T., Anthony, E.J., Friess, D.A., Mancheño, 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</ref>).
  
<gallery>
+
===Principles of Ecological Mangrove Restoration (EMR)===
Image:Egretta alba.jpg|Egret ''Egretta alba'' <ref>http://en.wikipedia.org/wiki/Snowy_Egret</ref>
+
EMR aims at restoring the favorable habitat conditions for mangroves, and generally no planting is done (Lewis, 2005<ref name=L5/>). 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.
Image:Agkistrodon contortrix.png|Water moccasin ''Agkistrodon  contortrix'' <ref>http://en.wikipedia.org/wiki/Agkistrodon_contortrix</ref>
 
Image:Diamondback turtle.jpg|Diamondback turtle ''Malaclemys terrapin'' <ref>http://en.wikipedia.org/wiki/Diamondback_turtle</ref>
 
</gallery>
 
  
==Threats==
+
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<ref name=P12>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.</ref>).
  
The mangroves are threatened in their existence by several causes. The main source of these threats are induced by '''humans'''.
+
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<ref name=W20/>). 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<ref name=B19/>). 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<ref name=W20/>). 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<ref name=L5/>).
* Variations in river and surface run-off, that inhibit the tropical coastal deltas of fresh water and silt, cause losses of mangrove species diversity and organic production. This results in alternations in both the terrestrial and aquatic [[food web]]. This has an effect on the types of refugees available to consumers.
 
* People will always be engaged in making projects. Soil reclamation for '''agriculture''' and aquaculture reduce the regional levels of biodiversity due to loss of mangrove [[habitat]]s. Many mangrove forests worldwide have been cleared to make way for shrimp '''aquaculture''', with a strong negative impact on coastal safety and biodiversity. 
 
* People are '''clearcutting''' the mangrove trees ('''deforestation''', '''habitat loss''') and are building dikes. This forms ponds with [[anoxic]] water. These anoxic conditions increase the level of sulphide in the soil and increase 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 [[nutrient]]s cause oxygen depletion in the water and promote the growth of [[algae]]. As a result, the ecosystems will be no longer in equilibrium.
 
* Another problem is the clearcutting of the mangrove for their '''hard wood'''. This wood is an important export product for building constructions in areas with large concentrations of termites. This is because the wood is resistant against these termites. The wood can also be used as charcoal and fuelwood. The substrate will be no longer stable when the trees are cut away. The result of this unstable condition is '''[[erosion]]'''.  
 
* A similar effect as with the added fertilizers and pesticides is the use of mangroves in '''waste-water treatment'''. [[Nutrient]]s are added into 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 causes water turbidity. [[Coral reefs|Corals]] and [[seagrass]]es are influenced negatively by these processes and will be deteriorated.
 
* Other '''coastal development activities''' have an influence on the quality of the water. Industry, [[tourism]] and port development involve land reclamation and [[dredging]]. This causes resuspension of the [[sediment]] and makes the water turbid. Because of this, light can not penetrate enough in the water and causes in this way damage to mangroves.
 
* [[Oil spills|Spills]] of '''oil, toxic chemicals and dumping of waste''' into the water causes localized impacts on the mangroves. Also the introduction of '''alien species''' by [[ballast water]] of on the hulls of vessels will have negative effects on the mangrove [[habitats]]. They will compete with indigenous species for space and food.
 
* Another threat is '''[[climate change]]'''. Storms will become more frequent and more intense and the sea level rises. The storms cause damage to the mangroves and due to the more frequently appearance, the recovery periods will become shorter.
 
  
There is little chance of mangrove regeneration in the remaining arid lands. <ref name=E></ref> <ref>UNEP. 2006. In the fronline: shoreline protection and other ecosystem services from mangroves and coral reefs. p.36</ref>
+
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<ref name=P12/>).
  
  
In the past, several measurements have been taken to preserve the mangrove [[ecosystem]].
 
An example of such a measurement is the FAO Code of Conduct for Responsible Fisheries. The FAO will regulate the fishing techniques and eliminate the destructive fishing gears. They also create management plans, establish no-take areas and involve the fishermen and other users. Mangrove ecosystems can also be protected by setting up National Protected Areas, World Heritage Sites and Ramsar Sites. These special sites have a legal framework. There were created guidelines for the people that use the [[ecosystem]]. But there are frequently problems with the implementation of these guidelines. The UNEP Regional Seas Programme is an organization that helps countries to work together to protect the ecosystems. A good management plan is essential to the protection of every ecosystem. In the future, we have to develop much more mechanisms to protect the ecosystem. Rates of mangrove deforestation have decreased globally during the past decades (from about the year 2000). The degradation of remaining mangrove areas is a major concern but receives less attention than deforestation<ref>Friess, D.A., Rogers, K., Lovelock, C.E., Krauss, K.W., Hamilton, S.E., Lee, S.Y., Lucas, R., Primavera, J., Rajkaran, A. an Shi, S. 2019. The State of the World's Mangrove Forests: Past, Present, and Future. Annual Review of Environment and Resources 44: 89-115</ref>.
 
  
 
==Further reading==
 
==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
* http://www.ozcoasts.gov.au/indicators/mangrove_areas.jsp
 
 
 
* http://www.vliz.be/vmdcdata/mangroves/index.php
 
 
 
* http://www.glomis.com
 
  
  
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*[[Potential Impacts of Sea Level Rise on Mangroves]]
 
*[[Potential Impacts of Sea Level Rise on Mangroves]]
  
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==External links==
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* http://www.ozcoasts.gov.au/indicators/mangrove_areas.jsp
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* http://www.vliz.be/vmdcdata/mangroves/index.php
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* http://www.glomis.com
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* https://en.wikipedia.org/wiki/Mangrove
  
  
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[[Category:Coastal and marine ecosystems]]   
 
[[Category:Coastal and marine ecosystems]]   

Revision as of 16:23, 18 February 2021

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 G. 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 temperatures 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 [[Coral reefs|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

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 Malaysia and Indonesia. Over the world, 54-70 species and hybrids in 20-27 genera and 16-19 families are found (Berness et al., 2001[4]). 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[5]; Hogarth, 1999 [6]).


Fig. 2. World distribution of mangroves.


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. The lower surface of the leave is highly covered with hairs to raise the secreted droplets of salty water away from the leaf surface. This prevents osmotic withdrawal of water from the tissue. They also restrict the opening of their stomata (only on the lower surface of the leaf), have a tick cuticle with a waxy layer, and orientate their leaves to avoid the burning sun. This also reduces evaporation. 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 [7]
Extended root mass. Photo credit Eric Coppejans [8].
Drop roots from branches [9]


Deposits of fine grained (muddy and sandy) sediments lack oxygen (Hogarth, 1999[6]). Mangroves therefore have to cope with anoxic conditions. The tissue of the plants requires oxygen for respiration which cannot diffuse into soils that are waterlogged. Even if the water is saturated with oxygen, its concentration 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 [8]
Drop roots or rhizophora [7]


  • 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 is a root that covers the whole space between the upper part of the root and the bottom.


Pneumatophores on cable roots [7].
Buttress root. Photo credit Eric Coppejans [8].
Propagules. Photo credit Eric Coppejans [8].


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. The 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. [10]

Biota

Micro-algae are important in mangrove ecosystems. They are epiphytic and 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 can not 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.

A number of invertebrates 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 [6]).


Fiddler crab Uca lacteal [11].
Egret Egretta alba [12].
Water moccasin Agkistrodon contortrix [13].
Diamondback turtle Malaclemys terrapin [14].


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.
  • Another 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[15]).
  • 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.
  • Another threat is climate change. The sea level rises and storm impacts may become more severe. Storms cause damage to the mangroves and due to the higher frequency, periods for recovery will become shorter.


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[16]). 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[17]). 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[10]; UNEP, 2006[18]).

Several measures can be taken to preserve the mangrove ecosystem. An example is the FAO Code of Conduct for Responsible Fisheries [19]. 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[17]). 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[20]). 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[21]). 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[22]). 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[23]).

Principles of Ecological Mangrove Restoration (EMR)

EMR aims at restoring the favorable habitat conditions for mangroves, and generally no planting is done (Lewis, 2005[20]). 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[24]).

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[23]). 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[15]). 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[23]). 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[20]).

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[24]).


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 12-04-2021]