Belowground carbon storage was examined for mangrove forests on Pohnpei Island, Micronesia. Stored carbon in a coral reef‐type mangrove habitat consisting of a 2 m thick mangrove peat layer, which is a type of mangrove habitat in tropical Pacific islands, was estimated at 1300 t C ha−1. The carbon burial rate during the phase of gradual sea‐level rise, which was calculated at 93 g m−2 year−1 between 1800 and 1380 years BP using the medians of the radiocarbon ages, was significantly higher than that between 1380 years BP and present in a stable sea‐level phase.

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The phytosociological data of the mangrove forests in the Malay Peninsula were collected from 2L6 releves, and so they were phytosociologically divided into 6 community types as follows: (L) the Sonneratia alba\4vicennia alba commtnity, (2) the Rhizophora apiculata community, (3) the Rhizophora apiculata\Bruguiera spp. community, (4) the Ceriops tagalVylocarpus spp. community, (5) the Lumnitzera littorea cnmmrtnity and (6) the Excoecaria agallocha community. These 6 community types distributed from the marginal lower part to inner of the upper part and were well correlated to the level oftidal zone and the sediment types. Furthermore 1.3 transects were examined, our investigations were able to clarify the relationships between the floristic composition, the mangrove zonation, sediments type, major land use and location in the tidal range of the mangrove ecosystem in estuary and delta types. Key Words: mangrove / phytosociological study I zonation / habitats / tidal condition / micro-topography / sediments / major land use / Malay Peninsula The authors carried out investigations on mangrove vegetation in the southwestern archipelago of Japan (Kikuchi et al., 1978, 80), western Pacific and Philippines (Mochida et al., !994), Indonesia (Mochida, 1992), Malaysia and Thailand. There is a close relationship between the floristic composition of mangrove, the forest structuro, and their habitat (Kikuchi et al.,1995). Phytosociological studies on mangrove vegetation in Southeast Asia and on islands in the Pacific Ocean have been reported by Suzuki & Mochida (1982), Miyawaki et al,(1983), Suzuki & Nakamura (1985), Sasaki (1992 a, b, c, d and e), Nakamura et al.(L993), Mochida a al. (1994) respectively. In their study, Miyawaki et al.(1983) classified the mangrove forests in Thailand into eight associations, four alliances, three orders and two classes. We had an opportunity to investigate the southwestern part of Thailand along Andaman sea coast, Satun and Khlong Thom, and the eastern part of Malaysia

Stand structure dynamics of rnangrove forest induced by sedimentation was studied on a small river-mouth bar in Sematan, Sarawak, Malaysia as a case study conceming dynamics between habitat and vegetation development in deltaic setting. The active invasion of Mizophora spp. into Sonneratia alba stand was observed. Furthermore, decline of S. alba at inner part of the forest was observed. Grain size analysis suggested that the complicated root systems of Rhizophora spp, had accelerated the sedimentation of finer material in the mangrove forest. As this sedimentation had progressed, pneumatophores of S, alba were buried and S, alba trees were in decline. Judging from the above-ground biomass and tree density, this vegetation change had taken place before well development of S, alba pure stand. Root productivity of Mizophora spp. resulted in increments of organic carbon content in sediments corresponding with this vegetation change,

Surface sediments were collected to determine the distribution of sediments inside a mangrove area. Mean sediment size, skewness, sorting and percentages of silt and clay were parameters measured and analyzed from 137 surface sampling points. The sampling points were located along 7 transects from the front to the back and were divided into 4 zones: mud flat, front, middle and back mangroves according to distance from the front water interface zone and the height of each point. The mean sediment size, skewness and percentage of silt and clay of surface sediments between four zones were found to be significantly different statistically (P < 0.05). Mud flat has the finest surface sediments and greatest amount of silt and clay. Contrary to normal trend, the sediments at the back mangrove is coarser than the front. This may be attributed to bioturbation by crabs causing the coarser sediments from the bottom to be brought up to the surface.