Truly Amazing Mangroves

Mangroves comprise different species of salt tolerant trees and shrubs…

mangrove forest shenzhen china

…that inhabit coastal areas of tropical and subtropical regions of the world.

world_map_mangrove_distribution

In total there are 80 known species of mangroves of which 60 species live exclusively on coasts between high and low tide lines.

different species of mangrove

manguemarpernambuco.blogspot.co.uk

Mangroves can even flourish along riverbanks far inland where freshwater meets ocean tides.

mangroves flourish along river banks so long as sea water meets fresh water CC BY-NC-ND 2.0

This image shows the conditions that need to be satisfied for mangroves to thrive:

summary of conditions that mangroves need to survive in Northland New ZealandImage Credit  nrc.govt.nz/for-schools/

Perhaps the most iconic of all mangroves is the red mangrove (Rhizophora mangle), which is instantly recognisable by its aerial stilt roots.

Red Mangrove Rhizophora mangle in Queensland Australiablueventures.org

Four different species of salt tolerant mangrove trees can be found in the coastal channels and winding rivers around the southern tip of Florida, USA.

Mangrove_plants_swamp_in_Florida

By way of comparison tropical rainforests contain thousands of tree species.

Rio Pinquen meandering in lowland tropical rainforest, Manu National Park, Madre de Dios, Peru.© Andre Baertschi | wildtropix

Although mangroves are few in species they provide a habitat for a great diversity of organisms including fish….

red_mangrove and biodiversityImage credit: naturefoundationsxm.org

….. and birds that live off those fish.

Birds that live im mangrove forests of Florida spider monkey artwork at ecologic.org

  • Why are there so few species of mangrove?

Living an intertidal existence is very challenging.  High tides bring about cooling and flooding by salt water, a lack of oxygen (anoxia) and increased salinity….

 Red Mangrove at high tide showing roots below water lineImage Credit: Bahamas National Trust

…whilst low tide brings about an increase in temperature and desiccation. At low tide concentrations of salt in the soil increase as water in the soil evaporates.

red mangrove roots exposed at low tide at Cape Tribulation, Daintree National Park AustraliaImage Credit: holeinthedonut.com

Growing in coastal regions where land meets sea, mangroves are constantly battered by ocean storms and hurricanes. Only a very few select species that comprise the mangrove tree community can tolerate such broad ranges of salinity, temperature and frequent tidal inundation.

Those select few species from Florida, USA that possess the physiological adaptations which enable them to overcome all the problems associated with living in intertidal regions are shown below:

Florida_mangrove_zonation

Each of these species has evolved unique adaptions- adaptions which enable them to inhabit distinct zones of intertidal areas.

  • So what are these physiological adaptions?

Aerial stilt roots, such as the aerial stilt roots of this red mangrove, support and spread the weight of the trunk, branches and leaves. Aerial stilt roots enable the tree to stay upright in muddy, tidal and windy conditions.

Red Mangrove Tree and its stilt roots in Singaporemangroveactionsquad.files.wordpress.com

Aerial roots also play an important part in providing oxygen, essential for respiration, to underground roots. Underground roots need oxygen to survive waterlogged soils.

Aerial roots absorb oxygen through thousands of small breathing pores called lenticels. It is only at low tide that the aerial roots actively absorb oxygen through lenticels; at high tide the lenticels close preventing the roots absorbing water and drowning.

Lenticels

After entering roots through lenticels above ground, oxygen is transferred to the below ground roots through aerating tissue called aerenchyma.

Aerenchyma of bullrush Schoenoplectus tabernaemontani CC BY-NC-ND 3.0

Aerenchyma tissue comprises honeycomb shaped air spaces running down the length of the roots; aerenchyma tissue facilitates the passage of oxygen to the roots below ground where oxygen levels are very low (hypoxic).

Pneumatophore root of mangrove tree showing lenticels and aerenchymamesa.edu.au

Black mangroves (Avicennia germinans) have evolved a different type of aerial root- the pneumatophore roots.

Avicennia germinans black mangrove showing pneumatophores Avicennia germinans in Guanacaste, Costa RicaCC BY-SA 3.0

Pneumatophores are vertical, unbranched roots sticking out of the ground; they increase the surface area exposed for the uptake of oxygen at low tide. More than 10,000 pneumatophores may be found on a single tree.

Pneumatophores and lenticles of black mangrove tree shown in diagramatic form

White mangroves (Laguncularia racemosa) sometimes develop peg roots depending on habitat conditions. Peg roots are similar to the pneumatophores of black mangroves, except they are shorter and thicker in appearance.

This image shows a white mangrove (with no peg roots) on the Isla Corazón in Ecuador.

White mangrove tree shown on higher ground on Isla Corazon Ecuadorislacorazon.com

White mangroves grow landward of black mangroves in wet, stagnant soils above the high water mark.

White mangrove grows on dry salty landmangrovesforfiji.com

Buttonwoods (Conocarpus erectus) grow landward of black mangroves. Like white mangroves, buttonwoods have hidden roots and can also thrive in dry, saline soil. The buttonwoods in the image below inhabit ‘No Name Key’ in the lower Florida Keys, USA. They are sometimes inundated by sea water following violent storms. An occasional inundation by sea water brings about hypersaline soil conditions in which only mangroves can survive.

buttonwood flat showing buttonwoods (Conocarpus erectus)dep.state.fl.us

  • So how do species of mangrove cope with levels of salinity that would kill other vascular plants and trees? Different species use different techniques.

Red mangroves exclude salt from entering the xylem vessels in their roots. It is the xylem vessels which transport water and soluble minerals to the leaves and replace water lost by transpiration (evaporation).

xylem vessel are one way flow only and transport water lost through transpiration of leaves

  • How do red mangroves prevent salt solutes from entering the xylem?

The pressure of water in the xylem is less than the pressure of the sea water outside the root. It is transpiration at the leaf surface that creates this negative pressure. Water flows from high pressure to low pressure regions so it flows into the xylem from outside the root.

A series of semi-permeable membranes create a filtration system which allows for the passage of water, but which prevents salt, from entering the xylem. The whole process is called reverse osmosis.

Reverse Osmosis in tree root of red mangrove shown in diagramatic form

Whereas red mangroves exclude salt other species, such as white mangroves (Avicennia marina), excrete salt.

White Mangrove (Avicennia marina), Urunga Coast, Australia

They accomplish this by excreting salty water from glands on their leaves. The water then evaporates and salt is left behind on the leaf surface which is then washed off the leaf when it rains.

Saltcrystals on_avicennia_marina grey mangrove leaves in St Kilda South Australia

A third strategy used by several mangrove species, including black mangroves, is to concentrate salt in older leaves. When these older leaves drop off the tree salt stored in the leaves is removed for ever. Leaf drop often increases during summer which also helps the mangrove reduce the impact of losing water through evaporation.

dead mangrove leaves in sea Port Douglas AustraliaImage Credit natureinfocus.wordpress.com

  • How do mangroves conserve fresh water?

Just like desert plants, mangroves store fresh water in thick succulent leaves. A waxy coating (or cuticle) on the leaves of many mangrove species seals in water and minimizes evaporation.

waxy cuticle on mangrove leaf reduces water losshsieteachers.com

Like other vascular plants and trees, tiny pores (stomata)  on the leaves close up when the conditions are not right for photosynthesis. Small hairs on the leaves of many mangrove species deflect wind and sunlight, further reducing water loss. Mangroves can also turn their leaves away from the sun to keep from drying out.

Mangrove Reproduction

The mangroves’ ecological niche between land and sea has led to unique methods of reproduction. Some mangrove species germinate within their fruit while still attached to the parent plant, a condition known as vivipary.

The seed of a viviparous red mangrove tree germinates and grows out of the fruit…..

Development of viviparous seedling of Red Mangrove

…..while still attached to its parent.

Rhizophora mangle seedlings on treewildlifeofhawaii.com

After falling into the sea off the parent plant the elongated seedlings initially float horizontally rather vertically. Once the seedlings (or propagules) are ready to root, after a minimum of 40 days have lapsed, their density changes so they float vertically rather than horizontally.

Red Mangrove propagules floating in sea Great Barrier Reef AustraliaFlikrlickr.com/photos/briangratwicke/5421446571

In this vertical position….

propagules sea pencils or sea pens of red mangrove bobbing up and down in the seaImage credit: thegoatthatwrote.net

….the seedlings, often called ‘sea pencils’, are more likely to lodge in the mud and take root. If they do not root, seedlings can alter their density and drift   horizontally once again in search of more favorable conditions. Seedlings can survive desiccation and remain dormant for over a year before laying down roots.

Red mangrove propagules rooting in soil

Whereas the seedlings of red mangroves are 15 cms long, the seedlings of black mangroves are 2 to 3 cms long….

Viviparous black mangrove seeds (Avicennia germinans) which have germinated beachbeans.blogspot.co.uk

…and the seedlings of white mangroves are even smaller at 0.5 cm long.

red white and black mangrove propagules compared in size

The seedlings of black and white mangroves takes root above the low tide mark and as a consequence are not as elongated as those of red mangroves.

Diagram showing propagules of red black white mangroves and buttonwood propagules establishing themselves in coastal mud

The seedlings of black mangroves need to be immersed in water for 14 days and those of white mangroves for 8 days before they can take root.

While red, black and white mangroves are all viviparous, the seeds of buttonwoods are non viviparous. Dry fruits (drupes)  of buttonwoods typically contain clusters of 35 seeds. The dried fruits burst open when ripe and the seeds are dispersed by gravity or water.

buttonwood dry fruit drupe containing seeds backyardnature.net/yucatan/buttwood.htm

  • So how do mangroves benefit communities?

Mangroves reduce wave damage

Waves enter mangrove forests during rising tides. As they pass through the tangled above-ground roots and branches of the mangroves, waves rapidly lose energy and height.Tightly packed arching prop roots and low branches present a solid obstacle to waves. The presence of mangroves reduces the extent of wave damage in low lying areas behind mangrove forests.

illustration of how mangroves reduce wave damage

In contrast waves more easily pass through forests in which there are few or no aerial roots.

open structure with few or no aerial roots water from sea passes more easily

Mangroves reduce storm damage

Tropical storms, including cyclones and hurricanes, are widespread in many tropical and subtropical areas.  High winds and low atmospheric pressure create raised sea levels (‘storm surges’). Torrential rainfall and strong winds often cause widespread flooding and extensive damage.

diagram showing how mangroves reduce storm damage

Where mangroves are extensive they are able to reduce the depth of water from a storm surge as the storm surge flows inland. This reduction in the depth of water can greatly reduce the extent of flooding in low lying areas behind mangrove forests.

This satellite image shows The Sundarbans mangrove forest of Bangladesh containing 100,000 hectares of mangrove forest habitat. This habitat provides an excellent buffer against severe storms and typhoons heading up the Bay of Bengal.

satellite image of the Sundarbans on the India Bangladesh border showing mangroves

Mangroves reduce erosion

Mangrove trees create soils that are rich in organic matter- including living roots but also dead leaves and fallen branches. The dense network of roots helps protect the soil against erosive forces and traps soil particles. Because mangrove soils are waterlogged and anaerobic (low oxygen content) much of this organic matter accumulates and forms a layer of peat that increases in thickness over time.

This accumulation of sediments allow mangroves to keep pace with rising sea levels. Some mangroves sit on top of deep layers of mangrove peat, that may be 6 meters deep or more, built up over thousands of years. These mangrove soils typically grow vertically at rates of up to 10 mm per year.

mangroves build up land by sedimentation, litterfall and trapping and binding

Mangroves capture carbon

Mangroves sequester (capture) atmospheric carbon dioxide in their trunks, roots, branches, stems and leaves.

Mangrove sequesters atmospheric CO2 in leaves, stems, branches, trunks and rootsoceanhealthindex.org

As leaves, branches and stems fall to the ground they are buried by accumulating sediments. Anaerobic soils with low oxygen content decompose very slowly. The captured carbon is stored away in dead tree parts in the peat where it can remain for millennia.

Storage of carbon in mangrovesoceanhealthindex.org

Disturbance of peat and sediments by natural events (hurricanes and typhoons) or human activity (excavation, dredging logging ) exposes buried carbon to oxygen. The stored carbon is rapidly released back into the atmosphere as carbon dioxide.

carbon remains in storage in mangroves until released by human or storm activities oceanhealthindex.org

Mangroves and water purification

Mangrove forests can improve water quality. In polluted areas agrochemical and heavy metal contaminants stick to sedimentary particles within the mangrove root system trapping the pollutants. These pollutants removed from offshore waters, thereby protecting coastal ecozystems.

Mangroves and the algae, bacteria and filter-feeding animals that live within the mangroves, play an important role in removing excess nutrients from the water. Sources of nutrient pollution include surface run off of pesticides from farm fields, discharges from septic tanks and animal feedlots.

polluted water around red mangroves in BaliLawrence Hislop at www.grida.no

Mangroves under threat!

The destruction of the world’s mangrove forests is happening faster than destruction of the world’s land forests. They are fast disappearing! From this….

intertidal mangrove forest

..to this!!

Mangrove forest destroyed as a resulty of shrimp farminggoodplanet.info

Further Reading

Different types of mangrove forest at mangrove.at

Conserving and restoring mangroves at mangroveactionproject.org

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