Brackish Water and Soil Components of Mangrove Forests on Iriomote Island, Japan

Abstract

Fringe and riverine forests of mangroves occur along the Shiira River of Iriomote Island, Japan. Sonneratia a/ba is distributed around the river mouth as a fringe forest. A mixed forest of Rhizophora stylosa and Bruguiera gymnorrhiza, a pure forest of Bruguiera, and a mixed forest of Bruguiera and nonmangrove plants are seen between 0 and 0.4, 0.4 and 1.5, and 1.5 and 2.0 km from the river mouth, respectively. During high tide and 2 h before on the days of flood tide, the salinity of brackish water at the river mouth was almost the same as that of seawater. However, the invasion of upstream fresh water in the 2 h after high tide reduced the salinity at the river mouth to about half that of seawater. The water at 1.6 km upstream maintained low salinity, from 4 to 0. 1Noo, throughout the survey period. Salinity and salt concentration changed more in the brackish water along the riverine forests than in the groundwater. During high tide, the ratio of divalent to monovalent cation in brackish water collected from above the soil of both a Rhizophora-Bruguiera mixed forest and a Bruguiera pure forest was approximately ten times that of seawater. However, this ratio in brackish water collected from fringe forests at the river mouth was similar to that of the seawater. Although generally high in the Bruguiera area, the di-to-monovalent cation ratio was lower in soil water than in surface water. The soils of Bruguiera-dominated forests contained less sand and more clay for a lower bulk density than fringe forest soils. Furthermore, the soils of Bruguiera-dominated forests had more humus, lower phosphate absorption, and a higher cation exchange capacity than those of the fringe forests. Finally, the soils of Bruguiera-dominated forests were always in a highly reduced state. These results suggest that Bruguiera plants can grow better in restricted brackish water and soil possessing certain physical and chemical properties specific to Iriomote Island. MANGROVE FORESTS DEVELOP AT TROPICAL and subtropical seashores that are inundated periodically. Mangroves also are found along the Nansei Islands, just north of the Tropic of Cancer, although the adjacent forests are typically temperate, evergreen, laurel forests (Miyawaki et al. 1983). Phytosociological studies of the mangroves on the Nansei Islands have been published (Kikuchi et al. 1978, Miyawaki et al. 1983, Odani 1964), but they do not examine the ecological and physiological factors that would account for the growth of mangroves in a temperate habitat. It has been proposed that the growth of mangroves in tropical and subtropical zones is dependent on salinity (Bunt et al. 1982; Chapman 1976; Cintr6n et al. 1978; Clarke & Hannon 1969; Conner 1969; Davies 1970; Galloway 1982; Lugo & Snedaker 1974; Macnae 1966, 1968; Macnae & Kalk 1962; Spenceley 1976; Thom 1967; Walter & Steiner 1936; Watson 1928), salt components (Chapman 1944, 1976; Chapman & Ronaldson 1958), and soil characteristics (Chapman 1976, Clarke & Hannon 1969, Holdridge 1940, Thom 1967, Troll & Dragendorff 1931). However, because the salinity and soil characteristics of mangrove forests in temperate zones have not yet been examined, the present study was undertaken to identify which ecological and physiological factors accounted for the growth of mangroves on Iriomote Island in the Nansei Islands. MATERIALS AND METHODS LOCATION AND SURVEY.-The Shiira River on Iriomote Island (24'19'N, 123?53'E) lies in southernmost Japan (Figs. 1 and 2). Iriomote Island has no dry season throughout the year and no frost even in the winter, since the average monthly temperature does not exceed 15?C (Fig. 3). Fringe and riverine mangrove forests occur north to the Tropic of Cancer. Sonneratia a/ba is distributed around the river mouth as a fringe forest (Cintr6n et al. 1978, Lugo & Snedaker 1974). A mixed forest of Rhizophora stylosa and Bruguiera gymnorrhiza, a pure forest I Received 30 August 1983, revision accepted 31 October 1984. BIOTROPICA 17(4): 277-286 1985 277 This content downloaded from 207.46.13.184 on Sun, 10 Apr 2016 04:17:35 UTC All use subject to http://about.jstor.org/terms