Effects of tephra deposition on mire vegetation: a field experiment in Hokkaido, Japan

1 The influence of tephra (aerially transported volcanic ejecta) on mire vegetation was investigated in a field experiment at Sarobetsu Mire, northern Hokkaido, Japan, which simulated relatively thin, widespread tephras. It was carried out in the centre of a raised part of the mire in a Carex middendorffii-Sphagnum papillosum community. 2 We tested the effects of varying tephra layer thickness, grain size and season of the simulated tephra impact. 3 Vegetation surveys and analyses of the mire pore water were carried out before and 1 and 2 years after tephra application. Redox potential, oxygen saturation and sulphide concentration were measured in the surface layer of selected plots after 10 months. 4 Pore water chemistry and oxygen saturation changed significantly in some treatments. Some plant species disappeared from certain treatments after tephra application, but the majority survived. Colonization by non-mire species played a significant role in only one treatment. Mosses were more strongly affected by the disturbance than vascular plants. 5 Tephra had stronger effects on the vegetation when layers were thicker, were more fine-grained and when applied at the beginning rather than at the end of the growing season. 6 Moderate tephra deposition is unlikely to cause long-term changes in mire development. Subsequent succession depends on properties of the tephra, on the vegetation type (e.g. life-forms) and on the season, but the original vegetation will probably recover even where the moss layer is severely damaged, as Sphagnum spp. can re-establish by growing through tephra at least up to 6 cm thick.