西表島仲間川下流の沖積平野にみられる植物群落の配列とこれにかかわる地形

II. サガリバナ林・アダン林

抄録

The alluvial plain along the lower Nakama River on Iriomote Island can be divided from the shore landward into the following four sections: 1) a section consisting of the estuary, the coast of which is occupied by mangrove forests with Avicennia marina and Sonneratia alba constituting the seaward fringe; 2) a section of the delta covered with mangrove forests, dominated by Bruguiera gymnorhiza and/or Rhizophora stylosa, but also with some Kandelia candel but not including Avicennia marina or Sonneratia alba; 3) a section consisting of low, flat mounds composed of estuarine deposits formed during the Holocene transgression and covered with Pandanus tectorius thickets; 4) a section of a floodplain on which Barringtonia racemosa forests are prominently distributed.
This paper, as the second part of a study on vegetation-landform relationships on this alluvial plain, deals mainly with detailed descriptions of the relationships in the third and the fourth sections mentioned above. In those sections, natural levees have developed along the Nakama River and its branches, and mixed forests of Machilus thunbergii and Mallotus paniculatus are found there. They are distinguishable from Barringtonia racemosa forests which compose a swamp forest in the back swamps and the Pandanus tectorius thickets on the flat mounds of the estuarine deposits. Colluvial slopes are occupied by other Barringtonia racemosa forests including Cyclobalanopsis miyagii.
Close correspondence between the areal extent of landform units and plant communities is found both in small-scale and in micro-scale. The landforms of the area were outlined in the course of deposition during the Holocene transgression and have been differentiated by the processes that followed. The arrangement of the plant communities on this alluvial plain can be interpreted in terms of both present and historical land-forming process types.