Holocene vegetation history based on phytolith records in Asodani Valley, northern part of the Aso caldera, Japan

Abstract

Vegetation history during the Holocene has been reconstructed using phytolith records obtained from three tephra sections in the Asodani Valley basin, northern part of Aso caldera, southwestern Japan. The topography of the Asodani Valley basin is divided into four types: somma (caldera rim and pyroclastic-flow plateau), caldera wall, caldera floor and post-caldera central cone slope. The Holocene vegetation transitions in the basin vary between these topographic types. At the northern caldera rim, Sasa (cool-temperature dwarf bamboo) grassland existed prior to 8 ka, and thereafter Pleioblastus (warm-temperature dwarf bamboo) grassland dominated to the present. At the steep caldera wall, forest with understory of Bambusoideae plants (Sasa and Pleioblastus) existed during the Holocene, and mixed forests composed of evergreen trees, deciduous trees and conifers appeared after 6 ka. On the post-caldera central cone slope, grassland vegetation composed of Sasa existed prior to 10.2 ka, and Miscanthus (pampas grass) grassland with sparse trees established around 13 ka and flourished in the last 4 ka. The center of the Asodani Valley was occupied by the last intra-caldera lake prior to 8.9 ka, and thereafter changed to swampy and fluvial environments. Phragmites occurred continuously in and around the intra-caldera lake and the subsequent marshes and swamps in the last 11.4 ka. The difference of vegetation transition in the Holocene between topographic types can be explained as difference of human activities in and around the Aso caldera. Grassland dominated by Sasa, Pleioblastus and Miscanthus occurred on pyroclastic-flow plateau around the caldera and on the central cone slope, which were strongly affected by human activities including burning for a long time, and forest with understory Bambusoideae plants was preserved on the caldera wall, which is unapproachable because of the steep topography.