Underground Vertical Distributions of Macrofauna and Root in a Mangrove Forest of Southern Thailand

Abstract

In a mangrove forest of southern Thailand, underground vertical distributions of macrofauna and root biomass were investigated to a depth of 1 m. While smaller roots were distributed more or less evenly, larger roots were restricted to shallower layers. Animals were more abundant in shallower layers, but some of them penetrated into the deepest. Relation between the distribution of animals and that of larger roots is discussed. Cross-shore and for along-shore distributions of the benthic macrofauna in mangrove swamp have been studied quantitatively in various parts of the world (e.g. Warner, 1969; Day, 1974; Sasekumar, 1974; Frith et al., 1976; Shokita et al., 1983; Shihe & Fuxue, 1985). But the quantitative data on the underground vertical distribution of it have not so far been presented. One of the reason is probably that matted roots of mangrove trees make it difficult to dig the substrate deeper. In the temperate salt marsh habitat, the root mat density of the plants has been known to play an important role in controlling the distribution of fiddler crabs (Ringold, 1979; Bertness & Miller, 1984). Likewise, the underground root system of mangrove forest is assumed to influence more or less the distribution of the burrowing animals there. In this paper, we report the vertical distribution of benthic macrofauna in relation to the root biomass in a mangrove forest of southern Thailand. Study site and methods The study site was located in a mangrove forest at Hatsaikhao village near Ranong (9°58'N, Publ. Seto Mar. Biol. Lab., 32 (4/6), 329-333, 1987. (Article 12) 330 K. WADA, A. KOMIYAMA & K. OGINO 98°38'E), southern Thailand. In December 1982, a plot of 15m X 20m was established about 50 m landward from the seaward fringe of the forest. The plot area was not flooded at normal high tides and the substratum was sandy mud, with scattered puddles in places. The temperature and salinity of the water in a puddle measured at 10:00 on 1 Dec. were 27.0°C and 27.28%0, respectively. The dominant tree species in the plot was Rhizophora apiculata Blume with only a few smaller trees of Bruguiera gymnorrhiza (L.) Lam. and B. cylindrica (L.) Blume. The maximum diameter and height of the trees were 44.5 em and 37.2 m, respectively. The basal area totalled 31.3 m2/ha and the stand density was 733 treesjha. Shrub measurements were 31500/ha in the density, 7.5 m2jha in basal area, 2.4 em in mean diameter and 3.1 min mean height. A trench (0.2 m X 15.5 m) was laid out in the plot between two big trees of R. apiculata (diameter: 44.5 and 43.6 em). Fresh weight of prop roots/pneumatophores above the ground of the trench totalled 63.8 kg. The water table of the trench was 28-30 em deep and the temperature and salinity of the underground water were 25.0°C and 22.98%a, respectively (10: 10 on 9 Dec.). The trench was subdivided into 31 compartments, each 0.5 m long and each compartment was further subdivided vertically into 10 layers of soil blocks to a depth of 1.0 m. The 310 soil blocks, each 0.2 m (width) X 0.5 m (length) X 0.1 m (depth), were cut by a hand saw from the smoothed wall of the trench. Each soil block was sieved by a net with I mm opening to collect living roots and animals. Living roots were sorted into eight diameter classes ( 50 mm) and weighed fresh. Animals were fixed in 10% sea-water formalin and the number of individuals of each species was recorded for each soil block.