Sugi (Cryptomeria japonica var. japonica) plantations are the predominant evergreen coniferous forest ecosystems in Japan. Sugi is a unique evergreen tree with no abscission layer in the leaf. However, the lifespan of sugi leaves is unknown. In this study, we described sugi leaves, investigated the age structures of living and newly deceased leaves, and estimated the lifespan and death process of the leaves. The lifespan of sugi leaves was 1–8 years, with an estimated mean of 4.3–5.3 years. Then, we modeled patterns of leaf replacement and cesium-137 (¹³⁷Cs) concentrations and estimated the potential ¹³⁷Cs supply to the forest floor through leaf shedding based on ¹³⁷Cs concentration data from leaves of various ages sampled after the Fukushima Daiichi Nuclear Power Station (FDNPS) accident. Around 90% of leaves that had sprouted before March 11, 2011 (ante -3.11 leaves) were predicted to die within 4 years of the FDNPS accident. Moreover, ~90% of the ¹³⁷Cs in ante -3.11 leaves was predicted to be removed within 3 years of the FDNPS accident. The predicted trends of stand leaf ¹³⁷Cs concentrations were verified with ¹³⁷Cs concentrations measured in four permanent sample plots in Fukushima Prefecture. This study revealed that ¹³⁷Cs translocated from the canopy to the forest floor at a faster rate than the ante -3.11 leaves that had been directly contaminated by the FDNPS accident. Including sugi leaf mortality and replacement in future models will allow for more accurate predictions of the fate and persistence of radiocesium in sugi forests affected by the FDNPS accident.

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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.

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We investigated the distribution of root systems of fallen trees around the slopes of Izu-Oshima Island where landslides occurred because of Typhoon No. 26 (Wipha) on October 16, 2013. We surveyed tree species, tree height, root depth, and root width of fallen trees. Distribution of root systems was different for different tree species. Species of the surveyed fallen trees were Eurya japonica, Ilex crenata var. hachijoensis, Prunus lannesiana var. speciosa, Camellia japonica, etc. Distribution of E. japonica and ,I. crenata var. hachijoensis was high on the slopes. These trees had wide roots in the shallow range under the slope surface. In contrast, P. lannesiana var. speciosa, and C. japonica had relatively wide roots in the deep range under the slope surface. These findings suggested that tree species that took roots in the shallow range under the slope surface could not prevent the occurrence of landslides.

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We investigated the distribution of root systems of wind-fallen trees on the not-landslide slopes near the slopes where landslides occurred by the Typhoon 26 (Wipha) in October 16, 2013, in Izu-oshima Island. Wind-fallen trees were scattered in the study area. We surveyed the species, tree height, root depth, and root width of wind-fallen trees, in order to clarify the difference of root systems between the wind-fallen trees and the not wind-fallen trees around them. Many of the wind-fallen trees were Ilex crenata var. hachijoensis. The wind-fallen tree produced even on the gentle slope and the wind-fallen tree’s roots were distributed in a shallow range than 50cm from the ground. Therefore, it was guessed that trees that roots are distribution in a shallow range were damaged by wind. Tendency of windthrow was often southwest-north direction. For this reason, it was considered that the trees might have fallen by the wind from the southeast before the maximum wind speed at the time of the passage of the typhoon is observed.

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2013 Typhoon Wipha has brought serious landslide and debris-flow hazard on the northwestern hillside of Mt. Mihara which caused 36 deaths in Motomachi District in Izu-Oshima Island. This debris flow was accompanied by copious amount of driftwood that accelerated the debris flow damage. This study has reconstructed recent change in tree height around the landslide area by using aerial photogrammetry and airborne LiDAR data to clarify background of the hazard. A digital surface model (DSM) was produced from three air photos taken in 1976 by Geological Survey Institute (GSI). The 2013 DSM and digital elevation model (DEM) were surveyed by the airborne LiDAR on the second day after the landslide. Vegetation height was calculated from the difference between 1976 DSM and 2013 DEM derived from the airborne LiDAR by Tokyo Metropolitan Government. Comparison between 1976 and 2013 DSM clarified that tree height around the landslide area has increased 50 – 100% over the past 37 years. This tree growth would be mostly induced by tree growth and decrease of charcoal production due to the fuel revolution since 1960’s.

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