To offer essential data to evaluate how the forest affected the slope failure in Izu Oshima Island, we estimated both above- and below-ground biomass per unit area for each vegetation type. Aerial photographs and vegetation map were overlaid on GIS, and the vegetation types at the slope failure site were estimated. Seven quadrats were established adjacent to the slope failure sites, and the diameter at breast height was recorded for each tree stems more than or equal to 10cm in circumference. The former vegetation types before the slope failure determined by vegetation maps were as follows: Cerasus speciosa (hereafter Cerasus) forest, Eurya japonica - Ilex crenata var.hachijoensis (hereafter Eurya-Ilex) forest, and Castanopsis sieboldii (hereafter Castanopsis) forest. The estimated above-ground biomass was the highest in the Castanopsis forest (276t/ha), followed by the Cerasus (145–183t/ha) and Eurya-Ilex (97–197t/ha) forests. The T/R ratios (top weight / root weight) were 2.9–3.8 and 2.7–2.8 in the Cerasus and Eurya-Ilex forests, respectively. Although there was no difference in aboveground biomass between the Cerasus and Eurya-Ilex forest, the T/R ratio tended to be higher in the Cerasus forest. In this study, the woody biomass (raw weights) at the time of the failure was estimated. The results should be useful for evaluating how much load the biomass weigh on the ground of collapsed.
