屋久島 : 花之江河湿原 : 九州山地湿原の植生(I)

抄録

In Hokkaido and northeastern regions of Japan Proper, we can find many moorlands, and numerous investigations on the moorland vegetation have hitherto been made. But to our regret, little has been known as to the peat bogs of south-west Japan, especially in Kyusyu proper and the Island of Yaku. Although there are fewer occurrences of moorland in south-west Japan, when compared to those in northern regions, we can find some good examples. Hananoego Bog is the largest one which lies at some 1600m altitude near the wood limit against the uppermost vegetation zone, the zone of Pseudosasa Owatarii. This moorland is surrounded by woodlands consisted of the scrub forest of Juniperus Sargentii, and evergreen shrubs like Rhododendron yakusimanum, Eurya yakusimensis under the canopy of Cryptomeria japonica. It is about 60m long and 40m wide, and its east and west periferies are bordered by two rivulets. The annual rainfall in Yaku Island is tremendous, attaining 4,305mm at Anbo near the seacoast and 8,299mm at Kosugidani 600m above the sea-level. Judged from the vegetational aspect and the experience during the field work, larger amount of precipitation is quite sure at Hananoego, although meteorological observations are lacking. Five lines of peat profiles were observed at 10m intervals, and as the result (Fig. 1) we came to the conclusion that this moorland must have developed by bog formations in several small basins, of which the lowest level with the deepest peat formation was that of the east side of the western rivulet attaining as deep as 1m. The pH-values ranged between 5.0 to 5.4,and the lowest value was measured at Sphagnum papillosum growth. Temperature observed on one summer day reached its maximum at 20cm depth. Several moorland communities were detected there. The Juniperus Sargentii scrub accompanied by Lycopodium and Bazzania is found in cushion-like fragments. They might be the remain of the previous woodlands burled by peat formation. The Sphagnum papillosum bog with co-dominating Juncus Koidzumii and Carex hakonensis presents the optimum phase of peat formation and is developed on the bog-plateau, especially along the rivulet. But in the flat facing to the middle stream of east rivulet, Sphagnum is almost replaced by Rhacomitrium lanuginosum. Rhacomitrium lanuginosum community was probably produced by the colluviation of granite debris, where peat formation is restricted and even surface erosion is prominent here and there. Consequently the moss growth is interrupted by depressions dominated by Eriocdulon decem. florum, and elevations occupied by the dominance of Zoisia japonica. The Campylopus japonicus community occupies the flat of the southern part. Here erosion is completely lacking. The component flora is poor in species, and the peat deposit is not deep. This is assumed to have been formed by the deposition of sand particles and lowering of water table. From the facts mentioned above we can reconstruct the genesis of this moorland. Owing to the high rainfall and flat topography the Sphagnum growth might have started on the slope and the depression covered by Juniperus and in the valley bottom Juncus Koidzumii might have predominantly developed. Thus the peat formation was commenced, but it was frequently disturbed by the deposition of sand. Phragmites and Molinia which are very common in Japanese moorland are substantially absent in this moor.