Mangroves are plants that grow between the sea and the land. These unusual
plants survive in an ever changing environment, affected by the tides and the
fresh water run-off from the land. All mangroves tolerate, to a greater or lesser
extent, salt water surrounding their roots. These plants are mainly found on
sheltered coastlines, deltas, estuaries and lagoons in tropical countries around
the world. Mangrove wetlands are remarkably diverse and are home to a
valuable range of animal, bird, fish and plant life
People who live on the tropical coasts and river estuaries of the world have had
a long association with mangroves. Sustainable use of the mangroves evolved
with time and they became an important resource for the local people. Yet in
recent times vast areas of mangrove land have been destroyed or seriously
degraded by the overriding needs of agriculture and urban development as
coastal populations have dramatically expanded.
Many scientists, planners, governments and international agencies have become
aware over the past twenty years that the thoughtless widespread destruction of
mangrove wetlands was removing a valuable biological and economic resource.
This prompted a surge in research and a plea for better management.
Today, there is more appreciation of the importance of mangrove, but the
perception still lingers that the mangrove wetland are little more than wastelands.
Mangrove trees and shrubs are a common sight on mudflats and banks of
tropical and subtropical coastlines in many parts of the world. They stand with
their roots in salt water. As such they are a special form of vegetation existing at
the boundary of two environments and receiving food from the land and the sea.
The species of trees and shrubs known as mangroves belong to a variety of plant
families. The common characteristic they all possess is tolerance to salt and
brackish water. Some sixty species of mangrove plants are recognized from
various regions of the world, with the highest concentration of species in SouthEast Asia.
A mangrove forest often possesses a strange and convoluted beauty and it
flourishes in conditions of heat, salinity and oxygen-starved mud and would
overwhelm other terrestrial plants. To cope with this hostile environment
mangroves have undergone selective changes. As time has passed they have
adapted and emerged as the most successful coloniser of tropical coastal
wetlands.
Aerial roots are the most noticeable adaptation. These come with various forms
of architecture such as hooped stilts, buttresses or single unbranched structures
rising elegantly from the mud to the sky that form breathing roots known as
pneumatophores. Other adaptations include glands on the leaves for excreting
salt, a tendency in some species towards succulence and roots that have an
ability to exclude salt. The seeds are often buoyant easily dispersed by tides and
shaped so that they anchor in the mud.
As a result of these adaptations a strange fringe system has developed on the
tropical shores and estuaries. It is important for timber and fish production, as
habitat for many forms of wildlife and has a role in stabilising river banks and
coastlines. The biology of the swamp forest is complex. Although in recent times
such systems have attracted a lot of scientific attention, the dynamics and
interrelationship of living organisms within the structure are still poorly
understood.
In regions with continuous high temperature and prolific rainfall, mangroves have
prospered. Slowly, huge productive forests have evolved and the resource has
been widely used by coastal people of the tropics for thousands of years. A
balance existed between the complex biological system that the tidal forests
represent and the swamp dwellers who learnt to exploit the system without
destroying it.
Historically, the human pressure on the mangrove swamps was limited as,
except for some subsistence populations, such places were seen as
inhospitable, unhealthy and dangerous. They were not easy places to penetrate
except by small boat and few communities of people actually lived within the
mangrove forests.
In recent years the pressure of increasing population, food production and
industrial and urban development have led to a large proportion of the world's
mangrove resource being threatened by destruction. Rapidly rising affluentce in
many developing countries has meant that areas previously covered with
mangrove forests have been converted to a luxury high rise apartments on the
sea front, marinas and tourist facilities.
As population have risen, the shortage of productive land in underdeveloped
countries has seen mangrove swamps converted for agricultural purposes.
During the Cultural Revolution of the 1960's large areas of mangroves were
destroyed along the coast and replaced by paddy fields for growing rice. The
reclaimed land proved unsuitable and much of it lies derelict. Large areas of
mangroves have been converted to fish and shrimp ponds for commercial
production, mangroves are also been chipped for paper production. In some
countries mangroves have been destroyed for mining, port construction and
replaced agriculture.
This intense exploitation of the mangrove lands has been relatively recent and
reflects the pressure of development extending into relatively difficult regions as
the need for more land and natural resources become imperative. Twenty years
ago mangrove forests were generally considered as wastelands with little
intrinsic value and their destruction was encouraged by governments and
planners. The usual approach was to dredge and drain the mangrove swamps
and then fill them, converting the natural habitat to dry land on which no
mangrove can survive. The construction of dams and other engineering projects
which divert fresh water from higher ground was another adverse influence on
mangrove communities.
Such attitudes and activities still exist but in the early 1970s scientific interest in
mangroves began to shift from a long established scholarly investigation of these
curious salt adapted collections of plants and animals to the more immediate
problem of their disappearance at an alarming rate without due consideration
being given to their importance as a naturally occurring, but unusual, biological
systems. At the same time mangroves began to attract the interest of ecologists
and conservationists.
History and evolution of mangroves
As is usual when one enters the realms of science one must first come to term
with the terminology. Scientists often tend to extend the mystique of their subject
by divising an elaborate set of terms. The treatment of mangroves has not been
immune to this approach.
The Shorter Oxford Dictionary describe the word "mangrove" as obscurely
connected with the Portuguese word "mangue" and the Spanish word "mangle"
and the English word "grove" and it dates its origin as 1613. Marta Vannucci in
her interesting book "The Mangrove and Us" points out that the word is neither
Portuguese nor Spanish and, after an exhaustive search, she concludes that the
word "mangue" derives from the national language of Senegal. She comments
that it was probably adopted by the Portuguese, and later modified by the
Spanish, as a result of their exploration of the coast of West Africa.
The term "mangrove" has been applied historically to plants which live in muddy,
wet soil in tropical or subtropical tidal waters. In the nineteen sixties the term
"mangal" was used for a community of mangrove plants and the term "mangrove"
for the plant species making up the forest. The terminology has tended to fall into
disuse recently and term such as "mangrove forest", "tidal forest" and "coastal
woodland" have begun to appear from groups of evergreen plants possessing
marked similarities in their physiological characteristics and structural
adaptations to habitats influenced by the tides. The scientific literature is divided
broadly into studies of the biology of individual species of plants or animals in the
mangroves and the study of communities that may involve just plants or the
relationship between plants and animals. The present intention is to sketch the
most important features of mangroves and mangrove communities in such a way
that they can be understood by the interested traveller.
Mangroves can be trees, shrubs, palms or ground ferns growing in the zone
between high and low tide. Every kind of plant has a Latin name or latinised
scientific name and mangroves are no exception.
The Swedish naturalist, Carl von Linne (Linnaeus) in 1735 devised a system for
classifying plants and animals in systematic way. Linnaeus assigned each
different kind of organism a latinised double name consisting of a genus name
followed by an exclusive species name. This system is known as binomial
nomenclature.
The classification of plants divides them into various categories, based on
increasing degree of similarity. The largest categories are division, which are
subdivided into classes, order, families, and genera. Each genus may contain
only a single species or many closely related species. Biological species are
physically and genetically similar to the extent they may interbtreed to produce
viable offspring.
An example of the taxonomic classification of a particular mangrove is as folloes :
Division
Class
Order
Family
Genus
Species
Scientific name
Common name
:
:
:
:
:
:
:
Spermatophyta
Dicotyledonae
Rhizophorales
:
Rhizophoraceae
Rhizophora
stylosa
Rhizophora stylosa Griff.
Spider mangrove
There areapproximately 70 species of true mangroves of which some 65
contribute significantly to the structure of mangrove forests. Approximately 15
species occure in South-East Asia, approximately 15 species occure in Africa,
and approximately 10 species occure in the America.
There are ninteen plant families with mangrove representatives and only two
families which are exclusively mangrove. There are no order or higher ranks that
are exclusively mangroves. Mangroves are not a single genetic group but
represent genetic adaptation of a large variety of plant families to a particular
environment. In case of plant family, Rhizophoraceae, often considered to be a
true mangrove family, only four of its 16 genera inhabit a mangrove habitat.
Extensive mangrove stands require a layer of earth or sand, usually deposited by
rivers and flood tides and shores free of strong wave and tidal action. The also
require salt and brackish water. Mangroves are often characterised by aerial
roots, seedling that germinate on the tree and bouyant seeds that can be
dispersed by water. Mangroves are often found in regions such as estuaries,
embayments and broad muddy tidal flats where the local terrain has led to the
build up of soil. They prefer sheltered places where tidal and wind are not too
destructive. The conditions in which mangroves grow also influence their
characteristics for survival, their size and the pattern in which they congregate.
On a global scale mangrove distribution is influenced by the presence of warm
and cold oceanic currents.
Mangrove trees and shrubs
Mangrove trees and shrubs come in many different shapes and forms.
Mangrove shores and forest
Mangrove forests are best developed on tropical shorelines where there are
large areas available between high and low tide points. Large mangrove
formation are typically found in sheltered muddy shorelines that are often
associated with the formation of deltas at the mouth of a river system. Mangroves
can also be found growing on sandy and rocky shores, coral reefs and oceanic
islands. There are instances where islands can be completely covered by
mangroves. It is impossible to describe a typical mangrove forest, as the
variation in height and girth, even for the same species, is immense, depending
on the many factors that control growth
All plants require various mineral elements to survive and these are absorbed by
the roots from the soil. Plants require nitrogen, phosphorus, potassium, calcium,
magnesium, sulphur and iron. Sodium chlorides required only in trace quantities
and this poses certain problems for mangroves due to high abundance of these
two elements in the sea water surrounding their roots. Other trace elements
required by the plants for successful growth are boron, manganese, zinc, copper
and molybdenum.
Mangrove soils are quite different from those that most other terrestrial plants
grow on. They are poorly drained, lacking in oxygen and are often fine grained
and rich in organic matter. In appearance the soils are often clayey mud or sand.
Mangroves grow on waterlogged soils that are often lacking in oxygen. These are
known as anaerobic soils, literally, soil without air. The lack of oxygen in the soil
is due to the slow rate of diffusion of oxygen in water and the biological activity of
micr-organisms in the soil which consumes oxygen. The amount of oxygen in the
soil varies according to how often and for how long tides cover the mud, how well
drained the areas are, and whether there are chemicals in the soil that absorb
oxygen. Oxygen in the soil could be expected to increase in proportion to the
amount of time that the soil is exposed to the air and the soil is covered by water.
Extreme lack of oxygen in the soil can lead to the formation of gas, hydrogen
sulphide, which has rotten egg smell often associated with mangrove swamps.
The various functinal types of mangrove forest can be briefly described as :
Overwash mangrove forests : These are small islands covered with mangroves
that are frequently washed by the tides. The dominant species is Rhizophora
mangle or the red mangrove.
Fringing mangrove forests : These strips of mangrove found along waterways
and covered by daily tides. The dominant species is Rhizophora mangle.
Riverine mangrove forests : These are luxurient stands of mangrove along tidal
rivers and creek with a good input of fresh water. Often compopsed of
Rhizophora, Avicennia.
Basin mangrove forests : These are stunted mangroves located in places such
as the interior of swamps. Often dominated by Avicennia.
Hammock mangrove forests : Similar to basin mangrove forsts but are found in
more elevated sites.
Scrub mangrove forest : A dwarfed stand of mangroves found on flat coastal
fringes.
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