Mangrove Forests Supported by Peaty Habitats on Several Islands in the Western Pacific.

Abstract

Six types of mangrove forests were distinguished on Kosrae Island (5' 20' N.L.; 163"00' !.L.) and Pohnpei Island (6"55'N.L.; 1.58"14'E.L.) in Micronesia and Bohol Island (i0"00'N.L.; L24"30' E.L.) in the Philippines based on phytosociological investigations. They are the Rhizophora stylosa forest on an inorganic habitat along the seaward fringe of a mangrove area, the Nypa fruticans thicket and the Barringtonia racemosa forest supported by clay deposits at the landward fringe of a mangrove region, and the Rhizophora apiculata-Avicennia marina forest, the Sonneratia alba-Bruguiera gymnorrhiza forest and the Xyrocarpus granatum-B. gymnonhiza forest characteristically inhabiting peaty habitats. Accumulation of mangrove peat should be limited to a range conesponding to the upper half of the intertidal zone because mangrove forests themseves are restricted to this range. This range is estimated to be 75 cm or so in the areas investigated. A habitat with a peat layer equivalent to this range in thickness was usually found under the S. alba-B. gymnorrhiza forest. This type of forest is thought io have developed under the present, stable sea level condition through autogenic succession from the R. stylosa forest. On the other hand, a much thicker peat layer supporting the X. granatum-B. gymnorrhiza forest was found in the investigated area' Development of such a thick peat layer is assumed to have been proceeded by a rise in sea level, followed by filling up of the area with organic matter provided by the mangrove forest itself. Thus, this type of forest was considered to have been formed from the S. alba-B. gymnonhiza forest through an autogenic process of succession in spite of the extemal effect of a rise in sea level. Key Vlords: allogenic succession / autogenic succession / mangrove / Northwestern pacific / peat accumulation / rise in sea level Mangrove forests have persisted through the Quaternary despite substantial sea level fluctuations, although these must have caused repeated total disruption of mangrove shorelines (Woodroffe, 1992). According to Kawana et al. (L995), the sea level rose to the present level during the past 2,0(X) years on Kosrae Island, Micronesia, after a fall following the maximum rise about 4,000 years B.p. in the Hypsithermal Era of the Holocene. Some of the present mangrove forests may have survived this recent sea level rise in their own habitats where sedimentation can keep the pace with the rise in sea level. This sedimentation includes accumulation of organic matter provided by the mangrove forest. This process results in the formation of a peat layer under the mangrove forest as its own habitat.