Mangroves 1: Mangrove as Forest

MANGROVES 1: Mangrove as Forest


Mangroves, Channel Island (Ian Morris)

Mangroves are forests that are associated with the sea. Located between the high tide levels and the low tide levels, they are capable of growing in soils which are at times covered by salt water. This in itself makes them a unique type of plant community, as plants are usually killed by excess salt levels in the soil. Within the plant world the mangrove forest represents a high degree of adaptation to extreme conditions.

Mangals, a term sometimes used to distinguish mangrove forests, may line coastal lagoons and estuaries, grow on the shores of sheltered islands, create a strip of vegetation that runs along the coastline or follow tidal rivers for some distance inland. They may be only metres wide or form dense forests that are almost impenetrable. Although mangals seem unwelcoming to humans they are fascinating places containing a rich variety of animal life..

Mangals can be composed of one species of tree or they can contain many species, depending on their location. In the temperate areas of Australia mangals are made up of the one species of mangrove, Avicennia marina. 

Zonation

In tropical areas the mangals have a number of species, grouped into zones which are distinct belts of vegetation growing parallel to the shore. In the zones there are associations of trees, so that one or two species may be the dominant ones out of a number of others, in the same area. Each zone is named after the dominant species in it, but there can be considerable overlap as when for example numbers of Ceriops plants mix in with the Bruguiera on the landward side.

 
Zonation of mangroves at Channel Island (Michael Michie)
 

The Sonneratia/Avicennia zone has large but sparsely distributed trees growing just above the mud flats which appear at low tides. Avicennia marina has the lightest green foliage of all the mangroves and grows to 15 m in height. Sonneratia alba grows to 8 m. Both have pencil-like pneumatophores (breathing roots). Associated species can be the shrub Aegialitis annulata and the small tree Aegiceras corniculatum

The Rhizophora zone is occupied by Rhizophora sp. alone. The zone is easy to recognise because of the dense growth and distinctive prop roots of the trees.

The Ceriops zone is distinguished from the others because of the small size of the Ceriops tagal species. Ceriops thickets are often found lining small tidal channels and in open flat areas. Avicennia marina can also be found in the Ceriops zone along with Xylocarpus mekongensis.

The landward Bruguiera zone has the most number of species of all zones. The dominant species are Bruguiera exaristata and B. parviflora. Both are moderately tall trees of about 5 m and have knobby knee-like pneumatophores. Xylocarpus mekongensis and Excoecaria ovalis grow in this zone and Lumnitzera racemosa usually occupies the outside landward fringe. There may be salt pans bare of vegetation (samphire flats) inland from the Bruguiera zone.

 

Sea Sonneratia zone     Rhizophera zone    Ceriops zone                  Bruigera zone            Land

Typical zonation in Darwin harbour (M. Michie after NTDE, 1994)

 

Tides

The zones described above do not occur in a random or haphazard way. They are directly influenced by the rhythm of the tides, and it is the rise and fall of the sea water upon the shore that helps to create the clearly defined zones of mangrove communities. Over the years the mangrove plants have responded to the influence of the tides and each species has evolved so that it fits into a particular place. Some trees may grow in different parts of the intertidal habitat; others are restricted to one particular level of the tidal range.

In Darwin Harbour there are two unequal tides each day, the first high tide occurring about forty minutes later than that of the previous day. The difference between the high tide and the low tide is the range of the tide. In Darwin, a range of six or seven metres is big, and a range of two or three metres is small. Tides with a big range occur twice a month and are called spring tides, whilst the tides of smaller range are called neap tides. The height of both spring tides and neap tides vary from month to month.

The measurements in a tide chart are given in metres, and they represent the distance above a datum point which has been chosen as the lowest level that the sea in Darwin Harbour can reach (lowest astronomical level). The datum point has the value of zero. Although this is more of a mathematical concept than anything, tidal measurements are visibly shown on a painted marker on Fort Hill wharf. A more sophisticated electronic gauge is installed in the harbourmaster's office and the levels of the tides are recorded on a rotating cylinder.

There are five main tide levels which influence life in the intertidal (between tides) zone. They are

MHWS            (mean high water springs): the average of spring tides' high waters over a year

MHWN           (mean high water neaps): the average of neap tides' high waters over a year

MSL                (mean sea level): the calculated average level of the sea

MLWN           (mean low water neaps): the average of neap tides' low waters over a year

MLWS            (mean low water springs): the average of spring tides' low waters over a year

There is also a Highest Astronomical Tide (HAT) which is the highest possible tide according to calculations. The word 'astronomical' gives a clue that bodies such as the Sun, Moon and planets enter into the calculations. In practice, strong winds can cause sea levels to be above the HAT and flying salt spray can also affect plants in the vicinity.

  2. Mangrove as plant 3. Mangrove as habitat 4. Mangrove as resource 5. Use by Northern Aboriginal Clans 6. Activities

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