Effects of volcanic activity on mire development: case studies from Hokkaido, northern Japan

Field stratigraphy and macrofossil composition of four mires in Hokkaido, northern Japan, were investigated to test whether tephra deposition induces long-term vegetation shifts. Macroscopic tephras of known origin ranging from 0.5 to > 25 cm in thickness were found in two or more of the study sites, allowing correlation of the stratigraphic development of the mires. Major peat types and position of macroscopic mineral layers were determined in the field, and selected cores were analysed by quantitative macrofossil analysis. Some macrofossil groups and the content of unidentifiable organic matter showed changes in the vicinity of tephra layers, but they did not indicate a clear response pattern of the vegetation or the decomposition dynamics of plant material. Mosses were expected to be most strongly affected by tephra deposition, but even in cases where tephra fell on Sphagnum-dominated vegetation, no fundamental shifts to new plant communities were found. This suggests that disturbance by widespread tephras less than 10 cm thick did not alter site conditions sufficiently to induce shifts in the peat-forming plant communities, or short-term changes did not become manifest in the macrofossil record. A correlation between tephra deposition and vegetation change from nutrient-and mineral-poor bog communities to richer fen communities that had been postulated by some authors was not found at our study sites.