Mangroves Are Plants That Grow Between the Sea and the Land

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 South-East 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:Spermatophyta

Class:Dicotyledonae

Order:Rhizophorales

Family:Rhizophoraceae

Genus :Rhizophora

Species:stylosa

Scientific name:Rhizophora stylosa Griff.

Common name: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.

.