Mt. Usu, in southernHokkaido, Japan, erupted in March 2000, and in July 2000 a rainfall-induced mudflow occurred on the northern slopeof Mt. Konpira, near to the area that was active in the recent eruption. To investigate the effect of this mudflowsediment on gas exchange by tree root systems and tree mortality rates, gas diffusion rates were measured at twostudy sites at the foot of this slope in July 2001. The tree species, their diameter at breast height, and sedimentthickness were also scrutinized. The relative gas diffusion coefficients in soil air (D/D₀) of sediment samples wereanalyzed in the laboratory. The mortality rate depended on the surface sedimentary structure after the recenteruption. At the first site, where the mortality rate was almost 100%, the rainfall-induced mudflow sediment wasabout 20 cm thick on a pyroclastic fall deposit about 10 cm thick. At the second site, where the mortality rate was36%, the sediment layer consisted solely of a pyroclastic fall deposit about 10 cm thick. While the D/Do values in thepyroclastic fall deposit covered with the mudflow sediment were less than 0.005 during September 2001, the values atthe site not covered by the mudflow exceeded 0.02. This suggests that the mudflow sediment cover retarded gasexchange in the rhizosphere, asphyxiating root systems ; consequently, the trees died after the rain-induced mudflowoccurred.