Effects of microtopography and erosion on seedling colonisation and survival in the volcano Usu, northern Japan, after the 1977-78 eruptions

The effects of microtopographical characteristics of the thick accumulation of ash and pumice, which were produced from 1977-78 eruptions, on seedling emergence and survival were monitored on the volcano Usu, northern Japan, 10 years after the eruptions. The characteristics included inicrotopography, texture of volcanic deposits on ground surface, volcanic-deposit movements, water content on ground surface and chemical nature of volcanic deposits (ignition loss, P, K and pH). Polygonum sachalinense showed a higher seed germination percentage on finer-particle ground surface under lab conditions, and Petasites japonicus var. giganteits germinated well on the ground surface with any particle sizes. However, in the field, the seedling densities of those two species and Anaphalis margaritacea var. angustior (three dominant species in this region) were higher on the sites with coarse particles and/or the rill inside the crater basin. The seedling germination timing and space overlapped among these species in the field. Those facts suggested that the seedling colonisation pattern was influenced more by the topographical characteristics of ground surface rather than the germination abilities. The amount of organic matter in the volcanic deposits was very low, even I I years after the eruptions, and was not related to microtopography, suggesting that the effects of organic matter on seedling emergence did not differ along microtopographical characteristics. Inside of rills, higher water content seemed to be more advantageous for seed germination of those species. Volcanic deposits were physically stable in summer, but were heavily eroded after winter due to snowmelt. Most seedlings did not overwinter in the inside of rills with pumice where the volcanic deposits eroded intensively, indicating that the patterns of land degradation by melting snow should be mentioned carefully even with narrow scale. Copyright (c) 2007 John Wiley & Sons, Ltd.