In most areas of the world, mangrove formations have simply been exploited with little or no attempt to manage the resource on a sustainable basis. The management of mangrove forests under selection and clear-felling silvicultural systems is practiced in a limited number of countries, mostly in Asia, although in recent years several countries have seriously taken up mangrove plantation establishment and the rehabilitation of degraded mangrove formations. This article examines some of these situations and their potential applicability on a wider basis.
M. Zakir Hussain is the World Conservation Union (IUCN) Asian Regional Coordinator-wetlands. He is based in Bangkok, Thailand.
Note: This article draws heavily on the recent FAO publication Mangrove forest management guidelines FAO Forestry Paper No. 117), to which the reader is referred for more detailed information.
The development of a qua culture represents a major threat to mangrove ecosystems
Mangroves are one of the most familiar forms of vegetation occurring in the intertidal zones along sheltered coasts and river banks in coastal areas in tropical and subtropical countries of the world. Mangrove forests are highly productive ecosystems and are a natural, renewable resource. They provide essential goods and services and play a very important role in the lives of coastal communities. Through adaptations such as viviparous germination, the separation of freshwater from salt water and the conservation of freshwater, the ability to strike roots soon after coming into contact with soil and the ability to exchange gases through specialized root systems, mangrove species have been able to deal with a very adverse environment where few other plants would have survived.
However, in many areas of the world, mangrove habitats are being destroyed as rivers are dammed, their waters diverted and the intertidal zone extensively developed for agriculture or aquaculture and generally dried up. Large tracts are being converted to rice fields, industrial and land development and other non-wood uses. In response to the lucrative shrimp export trade, a new breed of small- and large-scale farmers are carving out vast chunks of tidal flats for shrimp farming and pisciculture. Remaining mangrove resources are overexploited for fuelwood and charcoal-making. The depletion of mangroves is also a cause of serious environmental and economic concern for many developing countries, given the pivotal role of this vegetation in coastal protection.
The mangrove vegetation of the world can be divided into two broad groups. The old world mangroves occurring in the Indo-Pacific region extending from the east coast of Africa to Samoa in the South Pacific. The second group, the new world mangroves, occurs along the west coast of Africa between Mauritania and Angola, in the Americas on the east coast between Barbados and Brazil and on the west coast between Mexico and northern Peru. Indonesia has the largest total area of mangrove forest while the Sundarbans swamp region in Bangladesh and India is the largest single chunk of mangrove forest in the world.
Mangrove vegetation primarily comprises trees and shrubs, with a limited number of palms and lianas. The World Conservation Union's report on the global status of mangroves (IUCN, 1983) lists 61 species. Major mangrove species belong to less than 15 families but the most frequently occurring mangroves belong to the Rhizophoraceae, Sonneratiaceae and Avicenniaceae. The Sundarbans, where members of the families Sterculiaceae and Euphorbiaceae predominate, is an exception.
Mangroves grow in conditions where few other plant species can survive (Hutchings and Saenger, 1987). The occurrence of mangroves is contingent on a number of factors and important physiological adaptations which enable the flora to thrive in the exacting environment and which also have silvicultural implications.
The mangrove environment is primarily saline, and the vegetation grows and flourishes by using three different mechanisms which cope with excess salt. The roots of salt-excluding species of Ceriops, Excoecaria and Rhizophora can absorb only freshwater from the saline water through a process of ultrafiltration (Scholander, 1968). Species of Avicennia and Sonneratia can regulate the salt content of their tissues by glands in their leaves. Xylocarpus sp., Lumnitzera sp. and Sonneratia sp. deposit salt in older leaves, roots and bark (Josh), Jamale and Bhosal, 1975). Mangrove plants also display features similar to those in desert plants which tend to conserve water (Hutchings and Saenger, 1987).
Mangroves occur in areas where strong wave actions are absent. The most extensive growth of mangroves can be seen in estuaries of rivers and protected lagoons and coastal lakes. Mangroves occur in areas of high humidity and their luxuriant growth is often associated with a high rainfall. Minimum air temperature and seasonal variations in temperature are important in the growth of mangroves. As cited by Chapman (1975; 1977), the best mangrove growth and development occurs where the seasonal temperature variation does not exceed 10°C and where the air temperature in the coldest month is higher then 10°C.
Mangrove soils are characterized by high salt and water, low oxygen and high hydrogen sulphide contents and contain a high proportion of humus (Macnae, 1968). The best growth and development of mangroves takes place on alluvial and muddy soils which are generally formed by the deposition of water-borne soil particles. Mangrove soils are mostly anoxic except for the surface layer in which roots spread (Rag, 1987). As a result, mangroves generally have shallow root systems and therefore cannot withstand strong wind and grow better in a sheltered habitat.
Mangrove seeds and propagules are dispersed exclusively by water and their distribution is therefore greatly influenced by tides which carry them both upstream and downstream. The tidal range, together with the topography, of an area regulates the lateral extent of mangrove development. The greater the tidal range, the greater the vertical range available for mangrove communities (Hutchings and Saenger. 1987). Tides also bring about changes in the salinity concentration of water in mangrove areas.
Dense natural regeneration of Rhizophora spp. in Matang, Malaysia
Mangrove forests have an efficient mechanism for natural regeneration, particularly in areas within mangrove stands where site degradation has not taken place. Because of the daily or periodic inundation by tidal water as well as the deposition of water-borne soil particles, the forest bed is soft and always suitable for regeneration and normally does not require any site preparation.
One other notable phenomenon in a large number of important mangrove species (Rhizophora, Ceriops, Bruguiera, Kandelia and Nypa) is that their seeds develop into seedlings while they are still attached to the mother tree. This phenomenon is known as vivipary where the embryo ruptures the pericarp and grows beyond it. In a second group, which includes species of Aegiceras, Laguncularia, Pelliciera and Avicennia, the embryo, while developing within the fruit, does not enlarge sufficiently to rupture the pericarp. This has been termed as cryptovivipary by Hutchings and Saenger (1987). Species of Excoecaria, Sonneratia, Heritiera and Xylocarpus are not viviparous. However, all of the above-mentioned species produce buoyant seeds which are dispersed exclusively by water and are capable of reaching any corner of the forest inundated by tidal water.
Most seeds that fall off mother trees during low tide stick to soft mud and quickly strike roots. In the case of viviparous seeds, the adventitious roots already present in the hypocotyl emerge and anchor the seedlings (Chapman, 1976). However, in all cases the deposition of silt by subsequent high tides helps the seeds or seedlings to secure a better hold.
In instances where seeds or seedlings drop during high tides, they continue to float in the water until they come into contact with a soil substrate and strike roots. It is interesting to note that most mangrove seeds retain viability for a long period while still in the saline environment, but lose viability soon after they are removed from that environment.
Profuse natural regeneration seems to occur in areas under mangrove formations. In many instances, advance growth establishes itself and waits for an opening in the canopy to emerge. On the other hand, radical changes in site conditions through the clearing of existing vegetation or alterations in the water regime seem to affect the regeneration process adversely. Dry soil in open areas often does not favour natural mangrove regeneration and is invaded by the undesirable species of Achrostichum.
With the exception of Asia, in most countries with mangrove formations no systematic silviculture or management is applied to the resource. However, the mangroves in countries where the formations are not managed scientifically are harvested and used very extensively as fuelwood, charcoal, thatching material, construction and boat-building timber, poles, a source of tannin, etc. Fuelwood for domestic cooking and fish curing remains the most extensively used mangrove wood commodity.
All over the tropical world, mangroves are regularly harvested by coastal communities to meet their needs but mangrove use is not confined to subsistence, small-scale harvesting. For example, before the discovery of oil in Nigeria, about 20 000 m³ of mangrove wood was utilized annually in the coal mining industry as pit-props (Adegbehin and Nwaigbo, 1990). Even though the demand for coal has drastically declined, mangrove wood is still extensively used in Nigeria and the extraction of much higher volumes of wood is undertaken exclusively as selection harvesting under licence agreements with the competent authorities (Isebore end Awosika, 1993).
Rhizophora racemosa propagules ready to be planted in Sierra Leone
Traditionally, because of their strength and durability, small-sized mangrove poles of 2.5 to 14 cm diameter are extensively used for making the frames for mud-plastered walls and structures for supporting roofs (generally made of palm leaves) in Kenya and the United Republic of Tanzania. These poles are also in very high demand in Arab countries to which they are exported in large numbers. In 1982, Kenya exported almost four million poles to Arab countries (FAO, 1991). This large-scale operation has also continued to be carried out through the selection of trees that need market specifications, without due silvicultural consideration.
In spite of this ad hoc harvest, which can be termed selection harvest, it appears that mangrove forests have not suffered as much as expected. Even though such activities may have resulted in a reduction of trees of certain size classes and poorer-quality tree crops, there are very few instances where such felling, unless very drastic and resulting in the complete denudation of sites, has hindered the establishment of regeneration.
In Asia, on the other hand, mangrove forests - including those in Pakistan, India, Bangladesh, Myanmar, Thailand, Malaysia and Indonesia - have been under management for a long time. For example, the Sundarbans in India and Bangladesh has been under sustainable management for more than 100 years (Hussein and Ahmed, 1994). In this region's forests, silvicultural systems have been developed based either on selection or clear-felling. The modification of silvicultural systems to suit local conditions has been carried out in many cases.
The selection system is practiced in Pakistan, India, Bangladesh and Myanmar where trees above certain predetermined diameters are harvested from the annually stipulated coupes.
The Sundarbans was managed under a selection system in 1892-1893 when the first management plan for the forest was implemented (Curtis, 1933), and the modified selection system, which was fine-tuned and adapted during the first three decades of this century, has proved to be very appropriate for the sustainable management of the forest. The depletion that has occurred in the growing stock has resulted from faulty management decisions rather than silvicultural practices (Hussein end Ahmed, 1994).
Rhizophora racemosa two years alter planting
The mangroves of the Sundarbans are managed under a selection-cum-improvement silvicultural system. Separate annual coupes are laid out for timber, fuelwood and pulpwood extraction. All types of harvest are carried out in a 20-year cycle where a single harvest operation is carried out once in each 20-year period.
The specific process for Heritiera fomes, which is the principal timber species in the forest, involves the laying out of the annual coupe, the marking of sound trees above a predetermined exploitable diameter provided their removal will not create any permanent gap in the canopy - the harvesting of timber followed by the removal of dry lops and tops and, finally, improvement felling of all deformed trees and the thinning of dense stands. This conservative opening of the canopy ensures an adequate regeneration of this species, which thrives in partial shade in its early stages, while discouraging the regeneration of strong, light-demanding and economically less desirable species.
Excoecaria agallocha is exclusively used as pulpwood and matchwood. All trees above the exploitable diameter are harvested in a single operation. In the case of Sonneratia apetala, all trees above 30 cm in diameter are removed, provided such removal does not create any gap in the forest canopy. Within annual coupes where H. fomes, E. agallocha, Ceriops decandra and other mangrove species have been established as understorey, clear-felling of all Sonneratia apetala trees is carried out to assist in the establishment of a more valuable crop. S. apetala seedlings do not establish under a mature crop of the same species.
C. decandra is used both for poles and fuelwood. Poles in a coupe are removed following selection felling rules as described in the case of H. fomes. During the fuelwood harvest, at least one healthy shoot is left in each branch.
The mangroves of the Rakhine and Irrawaddy regions of Myanmar are similar in composition to those of the Sundarbans and they too have been managed under a selection system. However, because of the severe depletion of the forest, the opportunity for managing it for productive purposes has become limited.
Another modification of the selection system is practiced in Indonesia (Soemodihardjo and Soerianegara, 1989) where 50 and 10 m wide "no felling zones" are maintained along the coasts and river banks.
Forty Rhizophora, Bruguiera and Ceriops trees above 20 cm in diameter and distributed at a distance of about 17 m from one another are retained as seed trees in each hectare of the forest. The forests are worked on a 30-year rotation, with a single thinning at age 15, and all trees above 20 cm in diameter except the seed trees are removed in the final felling.
Different adaptations of the clear-felling system are in practice in Thailand, Malaysia and Indonesia and, in the past were also used in Viet Nam. This system has been found suitable particularly in the case of economically valuable species such as Rhizophora apiculata and R. conjugata, which are strong, light-demanding species and can withstand competition in open areas.
The Matang mangrove forest in the State of Perak in peninsular Malaysia was initially managed under a shelterwood system where two thinnings were followed by a regeneration felling to enhance seedling establishment. It was observed subsequently that adequate regeneration could be achieved without a regeneration felling and, in the absence of regeneration, felling trees could grow for a few ´'extra" years and gain in volume (Hasan, 1981). This resulted in the silvicultural system being changed to one which has been termed as clear-felling with the retention of standards. The practice involves the retention of seven trees per hectare at the time of final felling when all trees above the diameter of 7.5 cm are removed. A narrow 3 m belt of trees is retained adjacent to river banks or the coast to prevent erosion(FAO, forest is currently worked on a rotation 30 years, with a thinning at ages 1 Sand 20 when 50 and 25 percent of the trees, respectively, are removed. At the end, about 1680 trees per hectare are found in a fully: stocked stand (Hasan, 1981).
In order to avoid damage to regeneration during harvest operations, extraction is mostly carried out with axes and trees are cut into 1.6 m billets which are carried out of the forest manually so as to minimize damage to the remaining vegetation. Immediately after the final felling, Acrostichum ferns are uprooted from infested sites.
An Acrostichum fern-infested area
About a year after the final felling, the coupe area is inspected to assess the status of regeneration. If necessary, efforts are made to assist the process of natural regeneration by eliminating competing vegetation. If this fails, then the area with unsatisfactory regeneration is planted with Rhizophora apiculata and R. conjugata. Propagules are collected from the forest and planted within three days. A vacancy filling or beating-up is carried out where necessary in the following year.
A different version of the clear-felling system is currently practiced in predominantly R. apiculata forests in Thailand. The system known as clear-felling in alternate strips. The rotation has been fixed at 30 years and the system is practiced by dividing the area into 15 coupes, each of which is further divided into 40 m wide strips forming an angle of 45° to the tide. Alternate strips are cut once every 15 years, giving a rotation of 30 years (FAO, 1985). In places where satisfactory natural regeneration is not established, replanting is carried out subsequently. A similar system is also in practice in Venezuela. However, the width of felling strips is 50 m (Aksornkoae, 1993).
The clear-felling system with artificial regeneration of stands was practiced in Viet Nam before the Second World War when a French charcoal company clear-felled large areas of mangrove forest in Minh Hai province in southern Viet Nam and artificially regenerated them through direct sowing of Rhizophora apiculata propagules. A 1940 plantation established at Thanh Tung in Ngoc Hien district is still in existence and could be the oldest commercial-scale mangrove plantation.
The process of establishment of mangrove plantations includes: enrichment planting to supplement natural regeneration particularly in areas under a clear-felling management system such as that practiced in Thailand and Malaysia; reforestation of degraded mangrove stands, for example in Pakistan, Sri Lanka, Myanmar, Viet Nam, Indonesia and a host of other countries; and afforestation in newly accreted coastal land, as is done in Bangladesh where more than 120 000 ha of such plantation have already been established.
In most instances very limited, if any, site preparation is necessary, and propagules or pregerminated seed and seedlings can be easily collected from nature and planted to achieve good results.
Viviparous and cryptoviviparous seeds can be dibbled in seed beds which are located in coastal areas so that they can be inundated by tidal water. Sonneratia apetala seeds need pretreatment and, in Bangladesh where large-scale plantations of this species are established, mature fruits collected directly from the trees are dumped in 0.6 to 1.3 m deep pits and are covered with thick brushwood and watered with saline water regularly for four to seven days. The fruits are then taken out, bruised lightly by hand and washed in river water to obtain white-coloured pregerminated seeds which are immediately broadcasted in seed beds or sown in plastic bags. The viability of pregerminated seeds is completely lost in a few hours and it is therefore important to sow them immediately. None of the seeds of the other species used in plantation establishment require pretreatment (Choudhury, 1991). Seedlings raised in soft nursery beds can be easily pulled out without any damage to the roots.
Raising seedlings in plastic containers has become more popular. However, most mangrove species have an elaborate root system and root development is much faster than the shoot development; as a result, roots may develop into balls and, when the plant is transplanted, its growth may be hampered.
In most cases satisfactory results can be achieved by directly sowing pregerminated seeds or fruits in the plantations. However, monkeys and crabs may cause extensive damage to fruits and seeds. Because of this, it is advisable to sow seedlings in areas infested with monkeys and crabs.
Planting is done mainly in the monsoon season. Seedlings are generally planted 1.2 to 1.5 m apart in most Asian countries, except in the Philippines where mangrove plantations are raised with a spacing of 0.5 x 0.5 m (PCARRD, 1991). In Sierra Leone, Rhizophora racemosa is raised with 2 x 2 m spacing (FAO, 1989). The size of seedlings is important, as small seedlings often get buried under silt.
Depending on the abundance of weeds, one to three weedings are generally carried out every year during the first two to three years. In cases where colonizing grasses or Achrostichum ferns are present, it is necessary to eradicate these to ensure the success of plantations. The best results are obtained when these weeds are uprooted.
Beating-up operations are generally taken up in the second and third year of planting. In Bangladesh, a beating-up is carried out within two months of the original planting. Some vacancy filling is also done during the actual planting operation if some seedlings are seen to have been washed away by the tide.
Thinning is not carried out in the cases of Sonneratia sp. and Avicennia sp. because such a practice is not economical end product is either fuelwood or pulpwood. However, in the case of Rizophora sp. plantations in Malaysia and Indonesia, a thinning is carried out at age 15 and 10, respectively, while in Malaysia a second thinning is earned out at age 20. The products are used for piling in the construction industry.
Harvesting mangrove timber
Diverse goods and services are provided by mangrove forests and many communities are dependent on these formations for survival. In spite of this, only a small portion of global mangrove resources is currently under any active management. However, considerable information and knowledge on the sustainable management of this forest type is available. Using simple, easy-to-follow silvicultural regimes, forests such as the Sundarbans and Matang have been managed for several decades for sustainable production. Moreover, techniques for the rehabilitation of sites that have been cleared of mangrove vegetation and put to some other land use have been developed and implemented in a number of countries. It is important that due attention be given to the management of this important resource and that all viable areas of mangrove forests be brought under active management through clearly formulated silvicultural regimes. At the same time, further investigation is required to increase the understanding and knowledge of appropriate silvicultural practices suited to the wide variety of situations in which mangroves occur.
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