Abstract:Intensive field observations as well as monitoring of discharge, water quality, and soil erosion have been conducted in forest plantations in order to identify the effects of forest cover and management practices on runoff generation, sediment transport, and downstream environmental issues. Five experimental catchments, each with rather uniform lithology, were established in both managed and unmanaged plantations of Japanese cypress and cedar, as well as broadleaf forests. Field monitoring was conducted from sub-plots (e.g. splash cups) to small hillslope plots (0Ð5 ð 2 m) to large hillslope-scale plots to first order streams (0Ð1-4 ha) and finally to larger catchments (>4 ha) in a nested structure. Remote sensing techniques were employed to identify broad scale forest stand and soil surface conditions. As part of this integrated study, these field-based monitoring and remote sensing techniques provide information for modelling runoff generation and developing adaptive management schemes with respect to catchment-scale water resources.

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In order to evaluate the effects of settings of digital fisheye photography on estimates of relative illuminance (RI) within a forest under low light conditions, we assessed the effectiveness of some estimators, i.e., canopy openness (CO), sky factor (SF), and diffuse transmittance (DIF). The estimates were calculated from digital hemispherical photographs taken with two different exposure settings, i.e., the reference exposure setting and the autoexposure setting. We also presented a new estimator (%Exposure), which does not require image analysis, and compared it with other estimators calculated from a digital hemispherical photograph using image analysis. The CO, SF, and DIF calculated from digital hemispherical photographs taken with auto-exposure setting were not significantly correlated with RI. However, those calculated from digital hemispherical photographs taken with reference exposure settings were significantly correlated with RI. On the other hand, %Exposure showed a slightly higher correlation with RI compared with the other estimators.

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The objective of this paper is to investigate the role of a tree roots system in slope stability through an analytical method. Variations of the diameter and the area ratio of the roots depending on its position in the soil were determined using two actual Japanese cedar trees (Cryptomeria japonica D. Don). The shape of the whole roots of a tree was assumed to be a hemisphere centered at the middle of the tree stump, thus the diameter and the area ratio of the roots is distributed on the surface of the hemisphere with an arbitary radius. The role of the tree roots system was clarified by the Spencer method for two-dimensional analysis. The conventional studies of many researchers have adopted fixed and linear slip surfaces in stability analysis. In this study, the safety factor against slope failure was optimized to determine the critical slip surface of the non-circular shape. The analytical examina tion using model slopes of homogeneous soil material revealed that a tree located at the toe of a slope is far more effective in slope stability than at any other position of the slope, and the role of this system has the function of increasing the critical safety factor against slope failure, on the contrary the depth of the slip surface (i. e. the scale of slope failure) will increase.