Community Changes in Mires in Volcanic Areas Caused by Frequent Volcanic Activity and a Consideration of Conservation Possibilities (pp. 219-230)
Authors: (Akira Haraguchi, Faculty of Environmental Engineering, The University of Kitakyushu, Japan)
Abstract: Mountain mires near volcanos are strongly affected by volcanic disturbances such as deposition of volcanic ash. In this study we investigated relationships between volcanic activity and vegetation changes with special reference to chemical environmental changes in peat soil caused by volcanic activity in the Tadewara mire in the Aso-Kujyu volcano area of south-western Japan. Soil and water chemistry in the mire had high spatial and temporal heterogeneity because of the strong chemical and physical disturbances resulting from volcanic activities. Vegetation in the mire also showed high spatial heterogeneity, and vegetation change was observed over the short-term. Dominant species in these mires were Phragmites australis, Moliniopsis japonica, Sphagnum palustre and S. fimbriatum, and these species established mixed communities, i.e., a minerogenous Phragmites community and an ombrogenous Sphagnum community constituted a mixed community.
Peat cores were collected for peat-forming plant species and chemical analysis was carried out in the Tadewara mire. A clear peak of sulfur content in peat cores was found at the depth of 105-115 cm from the surface. Contents of carbon, nitrogen, and hydrogen in the peat were low in comparison to the high content of sulfur. Chronological changes of dominant peat-forming species showed both progressive and retrogressive successions. Vegetation change in the Tadewara mire showed retrogressive succession from the Sphagnum-dominated community to M. japonica and P. australis communities correlated with the sulfur compound deposition in sediment. Furthermore, vegetation showed
progressive succession corresponding to a sulfur content decrease. Enrichment of sulfur in sediment cores appeared just over the water impermeable layer with a heavy clay texture on the volcanic ash horizon, and so vegetation change could be induced by the paludification caused by the deposition of that heavy clay layer. Deposition of the volcanic ash layer and the subsequent formation of a water impermeable layer were dated at ca. 970 ± 40 yBP, which corresponds to the eruption of Mt. Kurotake recorded at 980 ± 30 yBP. Retrogressive succession could be the result of hydrological change rather than direct chemical changes in soil.
Mires frequently disturbed by volcanic activity showed heterogeneous succession including both progressive and retrogressive successions among mire vegetation, and so the succession does not directly correspond to environmental change. Dominant species in the present mire vegetation established a mixed community of ombrogenous Sphagnum spp. and minerogenous Phragmites australis. Change in the abundance of Sphagnum spp. was not directly affected by the abundance of P. australis. Species replacement was evident especially in the marginal area of the mire which were directly affected by stream water. Vegetation and succession in these volcanic mires are quite different from that found in mires without volcanic disturbances. Conservation plans for mires without volcanic influence are not applicable for the conservation of volcanic mires, and as a result conservation of volcanic mires should be planned by considering these features specific to volcanic mires.