Japan has more than 80 active volcanoes in the small land. Most Japanese ecosystems have been affected somehow by volcanic activities for short and long terms. Soils originating from the volcanic ejecta are broadly distributed in Japan. Wetlands in Japan often contain volcanic-ash layers, in particular, in Hokkaido. Therefore, we need to clarify the effects of eruptions on vegetation dynamics and succession with various time scales. Here I firstly summarize the overall revegetation patterns after volcanic eruptions in Japan and secondly show the revegetation patterns on Mount Usu and Mount Koma. Mount Usu erupted on the summit in 1977-78 (U77) and on the foot in 2000 (U00). Mount Koma erupted in 1996, 1998 and 2000 on the summit (K96). The major volcanic ejecta was pumice and ash in all the eruptions. The climax is considered to be broad-leaved forests in all the sites. I set up permanent plots close to the craters in all of these three sites soon after the eruptions and monitored annually the cover on each species in the plots to the present. The data were summarized by temporal changes in growth form, derivation (native or exotic), etc.
The early stages of succession were characterized by the dominance of large perennial plants on tephra in many Japanese volcanoes, including U77, U00 and K96. Mosses were not predominant in the early stages. These suggested that the ground surface stability was the key of revegetation. Annual natives were poor because of the floras.
On the three surveyed sites, the trends shown above were observed well. However, the vegetation recovery was faster on U00, intermediate on U77 and slow on K96. Although species richness increased soon after the eruptions, the recovery paces differed between the sites. The species richness was increased by natives on U77 while it was by exotics on U00. Exotics were not observed on the summit area of Mount Koma, although a biologically-invasive tree, Larix kaempferi, was predominant on the mountainside. These results indicated that the succession patterns were determined greatly by plant sources. The forestation was determined not only by the environments but also by the interspecific interactions. Seedbank species occurring form the original topsoil slowed the forest development on Mount Usu and L. kaempferi altered the successional direction on Mount Koma. The long-term monitoring detected the patterns and mechanisms of succession.