Rapid Sea Level Rise and Mangrove Habitat

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Mangrove forests and their habitats were studied on Pohnpei and Kosrae Islands in Micronesia and Cebu and Bohol Islands in the Philippines, to clarify the accumulation process of mangrove peat in relation to sea-level changes. There are two types of peaty habitat in Micronesia : one with thin peat layr about 30-50 cm thick and the other with thick peat layr as thick as 2 m. The former supports a mangrove forest dominated by Sonneratia alba and the latter another mangrove forest characterized by Xylocarpus granatum. The accumulation of mangrove peat is limited to a range corresponding to a part of the intertidal zone because the ground level of mangrove habitats is itself restricted to this range. Consequently, the sea level must rise higher and higher or the level of the land must fall lower and lower for the development of such a thick peat layr as 2 m. In fact, according to the evidences of landforms and C-14 dates, sea level was at the level about 2 m below present one about 1800-2000 yr B.P.Then, it rose to the present level.The rate of peat accumulation calculated from information obtained from our research was about 2 mm per year. This value does not necessarily indicate the limit of habitat maintenance capability of mangrove forests because it is the average for hundreds or thousands of years and some bigger values within a short period might be included. At least, a mangrove forest is capable of maintaining its own habitat against a sea level rise as rapid as 2 mm per year.