Abstract
Arrangement of mangrove forest is discussed with relation to its geomorphological conditions in the present report, as a part of a study on vegetation-landform relationships of an alluvial plain in the lower reaches of the titular river. This plain of about 6km in length and 0.1 to 2km in width can be divided into three parts; a flood plain, a delta and an estuarine section (Fig. 1). It is estimated in terms of the distribution of deposits that during the period of the Holocene transgression an estuary extended about 4km from the present rivermouth just over the deltaic and the estuarine stretches on which the mangroves discussed here are found. The deltaic and the estuarine sections have been differentiated in the course of deposition after the maximum of the transgression, and the difference is reflected on the characters of the mangrove forests of the areas.
The depositional surface of estuarine deposits during the transgression appears to have reached to the elevation of 2m above the present sea level. This surface remains on the fringing area of the delta as very flat mounds and scattered on the delta, too. Pandanus tectorius forest and bare grounds occupy such flat mounds (Figs. 2, 3 and 5).
The greater part of the delta is a flat basin which is filled with clay, silt and fine sand from the rivers (Fig. 2). A wide mixed forest of Bruguiera gymnorhiza and Rhizophora stylosa extends on the basin. Along the main stream, branches and tidal channels, the mixed forest is dominated mainly by the former species and its pure stands are often found. Stands dominated by the latter, on the contrary, occupy the areas away from the channels (Figs. 3, 4, 5 and 6). On channel bars, some little stands of Kandelia candel and Rh. stylosa are found in pioneer state. However, a successional change from these stands to the mixed forest could not be recognized.
The lowermost part of the Nakama about 2km in range might be called an upper estuary transitional to a typical one. In this estuarine area rather small mangrove forests develop on thin deposits of medium to coarse sand which cover storm benches cut into limestone. In such mangrove stands a typical zonation is found; Avicennia marina and Sonneratia alba constitute a seaward fringe, Rh. stylosa makes a next zone and Br. gymnorhiza the landwardmost one.
