Pollen analysis of the terrace deposits intervening Aira-Tn volcanic ash (AT) exposed along the Serikawa riverbank in Ohori-machi, Hikone, Shiga Prefecture, Japan, was carried out to investigate the vegetation history, the season of ash fall and the effect of ash fall on the vegetation in this area and its surrounding areas. Throughout the period of the transitioning from the interglacial MIS 3 to the coldest stage in the MIS 2, the mountains and hills around the Serikawa lowlands were covered with mixed forests of deciduous broad-leaved trees (mainly Quercus subgenus Lepidobalanus; in the following text, Lepidobalanus is used) and conifers (Pinaceae), while the lowlands were covered with swamps and marshes where Poaceae and Cyperacea thrived.

The presence of a mild period just prior to the AT, during which the climate was becoming colder and drier towards MIS 2, was indicated by an increase in Lepidobalanus and a decrease in Pinus and Tsuga in mixed coniferous and broadleaf forests. It is suggested that the fall of AT may have occurred during the summer, when the flowering season of Mallotus japonicus and Syringa coincided. The impact of AT fall on the mountain and hillside vegetation was greater for Pinus, Tsuga, and Lepidobalanus, and smaller for Betula and Ulmus-Zelkova. The forest vegetation such as Lepidobalanus almost recovered around the time of the Kitoragawa volcanic ash fall (KT) about 500 years after the fall of AT, but the recovery of Pinus and Tsuga was delayed with invasion of Betula and Ulmus-Zelkova. On the other hand, in the lowland Cyperaceae marshes, Alnus swamp forests expanded and grasslands shrank.