Mangroves comprise different species of salt tolerant trees and shrubs…
…that inhabit coastal areas of tropical and subtropical regions of the world.
In total there are 80 known species of mangroves of which 60 species live exclusively on coasts between high and low tide lines.
Mangroves can even flourish along riverbanks far inland where freshwater meets ocean tides.
This image shows the conditions that need to be satisfied for mangroves to thrive:
Perhaps the most iconic of all mangroves is the red mangrove (Rhizophora mangle), which is instantly recognisable by its aerial stilt roots.
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.
By way of comparison tropical rainforests contain thousands of tree species.
Although mangroves are few in species they provide a habitat for a great diversity of organisms including fish….
….. and birds that live off those fish.
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….
Image 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.
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:
Each of these species has evolved unique adaptions- adaptions which enable them to inhabit distinct zones of intertidal areas.
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.
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.
After entering roots through lenticels above ground, oxygen is transferred to the below ground roots through aerating tissue called aerenchyma.
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).
Black mangroves (Avicennia germinans) have evolved a different type of aerial root- the pneumatophore roots.
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.
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 mangroves grow landward of black mangroves in wet, stagnant soils above the high water mark.
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.
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).
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.
Whereas red mangroves exclude salt other species, such as white mangroves (Avicennia marina), excrete salt.
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.
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.
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.
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.
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…..
…..while still attached to its parent.
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.
In this vertical position….
….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.
Whereas the seedlings of red mangroves are 15 cms long, the seedlings of black mangroves are 2 to 3 cms long….
…and the seedlings of white mangroves are even smaller at 0.5 cm long.
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.
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.
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.
In contrast waves more easily pass through forests in which there are few or no aerial roots.
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.
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.
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 sequester (capture) atmospheric carbon dioxide in their trunks, roots, branches, stems and leaves.
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.
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.
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.
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….
Different types of mangrove forest at mangrove.at
Conserving and restoring mangroves at mangroveactionproject.org