Vegetation and fire history during the last 30,000 years based on phytolith and macroscopic charcoal records in the eastern and western areas of Aso Volcano, Japan

Vegetation and fire history during the last 30,000 years has been reconstructed using phytolith and macroscopic charcoal records obtained from two tephra sections around Aso caldera, southwestern Japan. Phytolith assemblages from both sites revealed that grassland vegetation dominated by Gramineae consistently occurred for more than 30,000 years. Prior to 13.5 ka, vegetation both east and west of Aso caldera was composed mainly of Sasa (cool-temperate dwarf bamboo) although the vegetation east of the caldera considerably declined by intense volcanic activity between 30 and 13.5 ka. In the Holocene (<13.5 ka), Miscanthus (pampas grass) grassland was established and continued consistently to the present in the eastern area whereas Sasa and Pleioblastus (warm-temperate dwarf bamboo) grassland has existed in the western area of the volcano. The difference of grassland vegetation transition in the Holocene between the eastern and western regions can be explained as difference of fire regime between both areas inferred from macroscopic charcoal records. Prior to 13.5 ka fire activity was infrequent east of Aso caldera whereas fire events moderately occurred west of the caldera. Fire frequency in the eastern region rose abruptly at 13.5 ka and fire activity was most frequent in the last 5 ka while Holocene fire frequency in the western area was similar to that of 32-30 ka. The consistent presence of Miscanthus grassland in combination with high fire activity east of Aso caldera after 13.5 ka suggests that the Miscanthus grassland was attributed to burning as a consequence of anthropogenic activities.