90,000-year phytolith records from caldera rim to western foot of Aso Volcano, Japan: Implications for vegetation history since catastrophic eruption

Abstract

Vegetation history after the Aso-4 ignimbrite eruption (89 ka) has been reconstructed using phytolith records obtained from three tephra sections from caldera rim to the western foot of Aso Volcano, southwestern Japan. The ca. 90,000-year vegetation transitions vary between these sites and elevations. At the northern caldera rim (800–900 m asl), sparse vegetation composed of Sasa sect. Crassinodi (cool-temperate dwarf bamboo) and Zoysia was considered to be established within 1000 years after the catastrophic Aso-4 eruption. The Sasa grassland dominated by Sasa sect. Crassinodi existed continuously at the northeastern caldera rim between 89 and 13.5 ka, whereas Pleioblastus sect. Nezasa (warm-temperate dwarf bamboo), Andropogoneae (including Miscanthus Japanese pampas grass) and Zoysia existed in the Sasa grassland between 66 and 30 ka. Although Sasa grassland existed after 13.5 ka, Pleioblastus dwarf bamboo became a constituent of the grassland around 8 ka and continued flourishing until the present. On the western slope (ca. 150 m asl), sparse vegetation existed prior to 55 ka after a large outburst flood generated by a breach of caldera rim of intra-caldera lake (ca. 89–72 ka). The vegetation after 55 ka was dominated by grassland accompanied with moderate amounts of trees. Zoysia prevailed from about 55 to 40 ka, although Sasa was predominant during the period of 40–5 ka. The Sasa grassland containing Andropogoneae pampas grass declined around 5 ka, and Pleioblastus dwarf bamboo became a constituent of the grassland. The Pleioblastus grassland including Miscanthus Japanese pampas grass, Paniceae and trees (Lauraceae etc.) continued to the present. At the western foot of the volcano (∼100 m asl), Zoysia grassland became established after the large outburst flood. The grassland gradually declined from ca. 60 ka and almost disappeared around 40 ka. Instead, Sasa dwarf bamboo grassland, which accompanied Pleioblastus dwarf bamboo, increased after 60 ka. The Sasa grassland accompanying sparse trees continued, but decreased after 13.1 ka. Miscanthus grassland occurred after 29 ka, and flourished especially in the last 8 ka. The consistent presence of Miscanthus grassland for 29,000 years may be related to frequent fire episodes due to human activities including burning at the low elevation of the western foot of Aso Volcano.