Immense tracts of coastal wetland exist in the east coast of Sumatra and around the Borneo coast. I present hereby the geomorphological history of the coastal wetlands which I elucidated in the Batanghari river basin of East Sumatra.
Due to the rising sea level in the post-glacial period, which reached its peak in c. a. 6000 years ago, by 1.9m higher than present one, Plio-pleistocene terraces were drowned by the Sunda Sea. Under humidified climate, tropical rain forest expanded to the wetlands, and peat deposition started to take place on the former terraces. In the coast, brackish water sediments deposited in the lagoons occluded by sand ridges and on tidal flats. After the final sea regression which occurred c.a. 1500 years ago, the exposed brackish sediments were covered by relatively thin peat of 2 to 3 meters depth.
Zonation of the coastal wetlands was proposed, depending on microtopography and stratigraphy. (1) Flood zone: River water flowing down from the steep slopes of the middle reach have repeated floodings and caused the river meanderings when it enters the low-lying and level lower reach. On the basement of Plio-pleistocene terraces, silt and clay have deposited on the natural levees, and eutrophic woody peat have developed in the river-affected backswamp. (2) Central zone: In the farther terrains of the backswamp, ombrogenous peat have developed with remarkable peat-dome microtopography. The basal peat rests on the Plio-pleistocene terraces, and is enriched in the fern spores and Gramineae pollens, while the upper peat is predominantly composed of swamp forest vegetation with varying degree of decomposition. (3) Tidal zone: Several sand ridges which are parallel to the present coast line have remarkably been developed, with inter-ridge level plains composed of mud clay. Inter-ridge level plains are classified to the recent lagoons and very recent tidal flats, depending on the different degrees of the ripening of the mud clay. This zonation itself represents the geomorphological processes.

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Regional characteristics of the landform changes in the Bengal lowland, including Ganges delta are closely related to the characteristics of the floods and storm surges of the region. Due to the remarkable increase of the discharge in a rainy season, most part of Bangladesh except Hill Tract is covered by floods. The flooding condition in the Bengal lowland shows regional differences
Landforms of the southern Ganges delta are characterized as the deltaic and tidal lowlands with low and flat surface. Ground height of the region is 2-3 m. The flooding inundation depth in the delta region facing the Bay of Bengal is not so deep and the coast line along the region except around the mouth of the Meghna is almost stable during these several decades. In the region of the active delta around the mouth of the Meghna (mouth of the Ganges), landforms are extremely unstable due to the strong river flow and large supply of sediments. The region is characterized by low and flat land with deltaic islands such as Hatia, Bora, Sandwip and so on. Height of the islands is lower than 3 m, and mostly 1-2 m above mean sea level. Sediments of the islands are very soft unconsolidated silt and clay. Then the landforms of the active delta are extremely temporal and have been easily changed in their forms owing to the flood flow and attack of cyclones. Especially, the islands around the mouth of the Meghna have been easily eroded by river flow and wave action.
The region along the southeastern coast of Bangladesh from Feni to Cox’s Bazar is a coastal plain. Landforms of the plain are characterized by rows of beach ridges and low tidal plains. Two or three obscure rows of beach ridges can be seen in the region between Feni and Chittagong. Broad tidal plain is seen in the region south of Chittagong. Rows of beach ridges and coastal sand dune are also seen in the region near Cox’s Bazar
Regional characteristics of the coastal changes of the Ganges delta are closely related to the flooding and landform conditions. Landforms of the central and western coastal regions of the Ganges delta are characterized as the tidal and deltaic lowlands with mangrove vegetation. Abundant growth of mangrove vegetation in the region west of the active delta is effective for the deposition of sediments. The mangrove forests in the region are also effective against the tidal surge erosion caused by cyclones.

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Geomorphological evolutions of mangrove habitats on the Pacific islands and prediction of the habitat dynamics were discussed using the examples of Kosrae Island in Micronesia, Bohole Island in the Philippines and Iriomote and Ishigaki Islands belonging to the Yaeyama Islands in Japan.
Most of the mangrove forests of Kosrae have been developed during the last 2000 years by accumulating mangrove peat with the gradual sea-level rise of 1 to 2 mm/yr. During the period of rapid sea-level rise of about 10 mm/yr between 4100 and 3700 yr B.P., the mangrove forests ceased peat accumulation and retreated landward. These evidences suggest that the critical rate of mangrove peat accretion with sea-level rise is between 2 mm/yr and 10 mm/yr. If the velocity of the anticipated sealevel rise exceeds the critical rate, all of the mangrove forests of Kosrae will retreat landward and reduce rapidly.
The mangrove habitats on Bohole Island have been formed by the same processes as on Kosrae Island except the expansion of clay layer overlying a mangrove peat layer during the last several hundred years. The clay deposit, which was inferred to have derived from the deforested slopes, supported Nypa fruticans thicket. Rapid sea-level rise over the critical rate of the mangrove peat accumulation will induce the retreat of mangrove forests from their seaward margin, however, the invasion of mangrove trees into the Nypa habitat will not occur immediately because the habitat conditions of the clay deposited area is no longer suitable for the growth of mangrove species except Nypa.
Mangrove habitats of Iriomote and Ishigaki Islands abruptly moved seaward by a relative sea-level fall that occurred about 1000 and 2000 yr B.P., apparently caused by seismic uplift. The thickness of the layers containing mangrove organic materials in present mangrove habitats were less than 1 m. The organic layers have deposited during the last several hundred years. During the last few decades, input of sediments caused by soil erosion has affected the seaward expansion of mangrove forests. However, the sediments supply will result the reduction of mangrove area because the seaward limit of mangrove expansion is restricted by the geomorphological framework.

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Atmospheric concenhation of methane (CH₄) has been increasing rapidly in recent years and the influences on the global environment, in particular on global warming, are concerned, Rice paddy fields are considered as an important source of atmospheric CH₄ because of the past increase in harvest area of the world. CH₄ is produced by the activity of methanogenic bacteria under strictly anaerobic conditions in paddy soils. The results from the field measurements showed that there are pronounced temporal and spatial variations in CH₄ emission rates being controlled by various factors, including soil properties, plant activity, agronomic practices, and climate. Candidates for the options that mitigate CH₄ emission from paddy fields are water management, soil amendments, organic matter management, different tillage, rotation, and cultivar selection. Adopting these mitigation option to world rice cultivation would reduce the global CH₄ emission rate significantly. However, there are several formidable obstacles to adopt the mitigation options into local rice farming. The options should be in accordance with sustaining or rather increasing rice production.

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During the time of the Vietnam War, mangrove forests were destroyed by “operation defoliation”. 125,000 ha of mangroves, about a half of the area of south Vietnam, was killed. Reforestation activities started soon after the end of the war, and it is estimated that approximately 100,000 ha has been recovered in 15 years.
It should be remembered that Vietnam is a nation whose mangrove forests have suffered the worst destuction in the world, and at the same time, where the greatest reforestation has been made without foreign support. However, mangrove forests in Vietnam are still damaged and degraded even at the present time.
Destruction of mangrove forests and their ecosystems caused shortages of charcoal and firewood, decrease in marine products harvests, and increase of coastal erosion. In addition, damage to seadykes and damage from sea air to the crop and paddy fields increased particularly in northern and central Vietnam where typhoons are frecuent.
The majority of the population in Vietnam lives in the coastal areas, where they, economically poor farmers and fishermen, suffered from the serious damage caused by mangrove destruction. The target of the project is restoration of mangrove forests to their condition before the Vietnam War started. About 200,000 ha should be planted by supporting the reforestation activities of Vietnamese Villagers. As the result of some three years preparation activities in research, reforestation support, etc. in Vietnam, we concluded that the restoration is possible. Labour for planting will require 2-4 million people. day. This is the work that 500 organizations, each of 400 workerswould be able to execute in 10-20 days. Women’s Unions and secondary schools, which are spread all over Vietnam, will be expected to cooperate in this project. Project term is scheduled for 10 years and the cost estimatd twelve hundred million yen.
Mangrove forests in the tropics have been decreased and degraded year by year. We expect that this project will be a “model” for mangrove restoration on a global level.

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