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  » Where do Mangroves occur?
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  Coping with salt
 
  Specialized Root System
 
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Mangrove Ecology...........


       
Where do Mangroves occur

The richest mangrove communities occur in tropical and sub-tropical areas, i.e., between the 30°N and 30°S latitudes where the water temperature is greater than 24ºC in the warmest month, where the annual rainfall exceeds 1250mm and mountain ranges greater than 700m high are found close to the coast. Mangroves are found practically in almost all the continents, excepting Europe, the Arctic and Antarctic. Luxuriant patches of mangroves are found on all the other continents but the best mangroves are found in Asia, especially in India and Bangladesh - the Sunderbans are the largest mangrove forest in the world both in size as well as biodiversity

The total area of mangroves in India is about 6,740 sq. km, which is about 7% of the world's total area of mangroves. Of the total mangroves 80% are present along the east coast, mostly forming the Sunderbans, Bhitarkanika and the Andaman & Nicobar mangroves. The Gangetic Sunderbans is about 4,000 sq. km whereas Andaman & Nicobar is about 700 sq km. Besides, large rivers like Mahanadi, Krishna, Cauveri, Godavari also harbour major mangroves in their estuarine regions.

The remaining 20% mangroves are scattered on the west coast from Kutch to Kerala. The reason for such a restricted mangrove cover is the peculiar coastal structure and the nature of estuaries formed by the relatively small and non-perennial rivers except Narmada and Tapi.

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How do they establish?

Under the right conditions like the formation of a mud-flat, growth of mangroves is initiated. Stabilization of mud-flats is a preliminary process in the establishment of mangroves. Pioneer plant species initiate this process. The roots of these plants help in binding the soil and also help the establishment of micro-organisms which further help in stabilizing the area. Stabilization starts from the land side and gradually shifts towards the sea. The pioneer plants are species like Porterasia coarctata and some members of the Cyprus family. These are slowly replaced by other mangrove plants and then these mangroves gradually spread towards the sea.

Once mangroves grow, the submerged banks are fully stabilized. Then the plants slowly reach a stage which is called the climax vegetation. A climax vegetation of mangroves is represented by the complete circle of life where there are different species of plants, animals (both terrestrial and aquatic) and micro-organisms forming an ecosystem called the tropical salt marsh or the mangrove ecosystem. In case the sediments are not stabilized, submerged banks are washed out. Thousands of deltas are formed and washed out every year before they can be stabilized. In the Gangetic delta this situation is quite common.

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Zonation in Mangroves

Mangal along a tropical bay characteristically shows zonation. In India this zonation may be very distinctive (east coast of India) or merging (west coast of India). A very broad and general distinction would be:-

1. Proximal Zone (Front mangroves)
This zone is towards water front, subject to regular tidal effect where intensity of soil accumulation and inundation is a continuous process. The mangrove species in this zone are specially adapted with stilt roots, prop roots for stability and anchorage. Main species with these features are Rhizophora apiculata and Rhizophora mucronata. On rocky and coral reef substrata, Avicennia Spp, Sonneratia Caseolaris are also found. Both Avicennia and Sonneratia produce pneumatophores.

2. Middle Zones (Mid mangroves)
Above the Rhizophora/ Avicennia line luxuriant group of Bruguiera gymnorrhiza, B. Cylindrica, Lumnitzera racemosa, L. littoralis, Ceriops tagal and Aegiceras corniculatum occur. Ceriops and Bruguiera develop a strong hold fast in the form of knee roots or bent roots as a special adoption for supporting the erect bole.

3. Distal Zone (Back mangroves)
Towards island area mangroves like Excoecaris agallocha, Heritiera littoralis and Xylocarnus spp occur. Both Heritiera and Xylocarpus produce buttresses. Generally the salinity is on lower side in this zone occurring towards hill sides where run off of fresh water is for a prolonged period. The duration of tidal submersion is low in this zone compared to front mangroves.

However, the zonation in mangroves is not so simple and varies from place to place. Every species has its own level of salinity tolerance. Estuaries on east coast show distinct zonation. The high salinity range on the east coast estuaries may be the principal reason for distinct zonation there. The range and force of tidal action also play a determinant role in creation and maintenance of zones as distribution of seeds or propagules is influenced by tidal action. Also, tides do influence the salinity in an estuary.

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Mangrove Adaptations

Mangrove plants live in hostile environmental conditions such as high salinity, hypoxic (oxygen deficient) waterlogged soil strata, tidal pressures, strong winds and sea waves. To cope up with such a hostile environment mangroves exhibit highly evolved morphological and physiological adaptations to extreme conditions.

Do mangroves need salt?

The answer is no. Mangroves are facultative halophytes, i.e., the presence of salt in the environment is not necessary for the growth of mangroves and they can grow very well in freshwater. One particular advantage to growing in a salty environment is the lack of competition! Only a limited number of plants have invested evolutionary energy into adapting to intertidal conditions. In the optimum conditions of a tropical rainforest, diversity is great and competition fierce.

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How do Mangroves cope with salt


Coping with salt

• The first line of defense for many mangroves is to prevent much of the salt from entering by filtering it out at root level. Some species can exclude more than 90 percent of salt in seawaters (Rhizophora, Ceriops, Bruguiera species are all ‘salt-excluders’.)

• Another method is the retention of water in the leaves giving rise to leaf succulence in many species, viz., Sonneratia apetala, S. alba, Lumnitzera recemosa, Salvadora persica etc. These species show remarkably high concentration of salts stored in their tissue. To avoid the toxic effects of salts, these plants absorb a large quantity of water for dilution of salt.

• The leaves of many mangroves have special salt glands, which are among the most active salt-secreting systems known. It is quite possible to see and/or taste the salt on the leaf surfaces of species, which choose this method. (Examples of ‘salt-secretors’ include Avicennia, Sonneratia and Acanthus).

• Fourth method of coping with salt is to concentrate it in bark or in older leaves which carry it with them when they drop. (Lumnitzera, Avicennia, Ceriops and Sonneratia species all use this) .

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Specialized Root System in Mangroves

Specialized Root System

The major plant species forming the mangrove ecosystem have aerial roots, commonly prop roots or even stilt roots (Example: Rhizhophora spp). Stilt roots serve, of course, to anchor the plants, but also are important in aeration, because the mangrove mud tends to be anaerobic.

Rhizophora spp (Red mangroves) have prop roots descending from the trunk and branches, providing a stable support system. Other mangrove species, including the white mangroves (A. marina) obtain stability with an extensive system of shallow, underground ”cable roots” that radiate out from the central trunk for a considerable distance in all directions: pneumatophores extend from these cable roots.

Breathing Roots (Pneumatophores) : Special vertical roots, called pneumatophores, form from lateral roots in the mud, often projecting above soil (to a height of 20-30 cms, e.g. Avicennia, Sonneratia ) permitting some oxygen to reach the oxygen-starved submerged roots. Roots also can exhibit development of air cavities in root tissues, designs that aid oxygenation of the tissues. The density, size and number of pneumatophores vary per tree. They are green and contain chlorophyll.


 

Stilt roots are the main organs for breathing especially during the high tide. They are very common in many species of Rhizophora and Avicennia (Avicennia marina and Avicennia offficinalis). The stilt roots of Rhizophora mucronata extend more than a meter above the soil surface and contain many small pores (lenticels) which at low tide allow oxygen to diffuse into the plant and down to the underground roots by means of open passages called aerenchyma. The lenticels are highly hydrophobic and prevent water penetration into the aerenchyma system during the high tide. In Brugeira and Ceriops they become hollow and malfunctional after some stage.

Aeration occurs also through lenticels in the bark of mangrove species, e.g., species of Rhizophora.

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Reproductive Strategies of Mangroves

Reproductive Strategies:

Virtually all mangroves share two common reproductive strategies: dispersal by means of water and vivipary.

Members of the Rhizophoraceae family (Rhizophora, Bruguiera and Ceriops species) have an intriguing viviparous method for successfully reproducing themselves. Vivipary means that the embryo develops continuously while attached to the parent tree and during dispersal. They may grow in place, attached to the parent tree, for one to three years, reaching lengths of up to one meter, before breaking off from the parent and falling into the water.

These seedlings (propagule) then travel in an intriguing way. In buoyant sea water they lie horizontally and move quickly. On reaching fresher (brackish) water however, they turn vertically, roots down and lead buds up, making it easier for them to lodge in the mud at a suitable, less salty. Once lodged in the mud they quickly produce additional roots and begin to grow.

Some other species (Avicennia and Aegiceras) also produce live seedlings but these are still contained within the seed coat when it drops from the plant. The seed of Avicennia floats until this coat drops.

 

 

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