Freezing and thawing processes play an important role for the gravitational transport of surface materials on steep mountain slopes in Japan. The effects of deforestation on frost heave activity were observed through the 2012/2013 winter season in Ikawa University Forest, a southern mountainous area in central Japan (1180–1310 m above sea level). During periods without snow cover, needle ice development prevailed at a clear‐cut site, and the downslope sediment movement of upper soil was 10 to 15 cm through the winter season. At a non‐cut site, rise and fall in the ground surface level prevailed on a weekly scale, with no evident downslope movements at the surface; ice lens formation in the soil layer is assumed. Abrupt changes in the radiation budget, such as the strengthening of nighttime radiative cooling and increases in daytime direct insolation, induced frequent development/deformation of needle ice at the clear‐cut site. In snow‐free periods, the day‐to‐day variability in needle ice growth length and in nighttime averaged net radiation showed significant correlations; cloudy weather with warmer and moist air intrusion associated with synoptic disturbances prevented the occurrence of needle ice. Namely, day‐to‐day weather changes directly affected the mass movement of the upper soil after deforestation. Shallow snow cover occurred discontinuously through the winter and is likely an important factor in keeping the soil moisture sufficiently high in the upper soil layer for initiating needle ice during snow‐free periods. We also discuss contributions of coastal extratropical cyclone activities providing both snow cover and cloudy weather in the southern mountain areas of central Japan to the intra‐seasonal variability in frost heave and its indirect effect on soil creep and landslides on the deforested steep slopes. Copyright © 2015 John Wiley & Sons, Ltd.

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Sediment transport activities in periglacial environments are controlled by microclimate conditions (i.e., air and ground temperatures, throughfall), which are highly affected by vegetation cover. Thus, there is the possibility that forest harvesting, the most dramatic change to vegetation cover in mountain areas, may severely impact sediment transport activities in periglacial areas (i.e., soil creep, dry ravel). In this study, we investigated changes in sediment transport activities following forest harvesting in steep artificial forests located in a humid periglacial area of the southern Japanese Alps. In the southern Japanese Alps, rainfall is abundant in summer and autumn, and winter air temperatures frequently rise above and fall below 0 • . Our monitoring by time lapse cameras revealed that gravitational transport processes (e.g., frost creep and dry ravel) dominate during the freeze-thaw season, while rainfall-induced processes (surface erosion and soil creep) occur during heavy rainfall seasons. Canopy removal by forest harvesting increased the winter diurnal ground surface temperature range from 2.7 to 15.9 • C. Forest harvesting also increased the diurnal range of net radiation and ground temperature, and decreased the duration of snow cover. Such changes in the microclimate conditions altered the type of winter soil creep from frost creep to diurnal needle-ice creep. Winter creep velocity of ground surface sediment in the harvested site (> 2 mm day −1 on the days with frost heave) was significantly higher than that in the non-harvested site (generally < 1 mm day −1 ). Meanwhile, sediment flux on the hillslopes, as observed by sediment traps, decreased in the harvested site. Branches of harvested trees left on the hillslopes captured sediment moving downslope. In addition, the growth of understories after harvesting possibly reduced surface erosion. Consequently, removal of the forest canopy by forest harvesting directly impacts the microclimate conditions (i.e., diurnal range of ground temperature and net radiation, duration of snow cover) and increases frequency and velocity of periglacial soil creep, while sediment flux on hillslopes is decreased by branches left on the hillslopes and recovery of understories. The impact of forest harvesting on sediment transport activity is seasonally variable in humid periglacial areas, because microclimate conditions relevant to both freezethaw processes and precipitation-induced processes control sediment transport.

This review paper synthesizes geomorphic dynamics, sediment transport and resulting natural hazards in mountains of the southern Japanese Alps and their drainage basins, where climatic and geological situations produce highly active landform dynamics. In alpine areas above the timber line, shallow diurnal freeze-thaw action operating in the thin topsoil produces smallscale periglacial forms, and gravitational spreading leads to numerous sackung features where snow-melt and heavy rain in places promote rockslides. In subalpine and montane areas, deepseated landslides originate from fractured sedimentary rocks, deep V-shaped valleys, and heavy rain, while shallow landslides continue with historical forest clearance. Continuous slope failures prevent vegetation recovery and maintain debris input to valleys. Steep valleys contribute to high-density debris flows. Frequent or repetitive occurrences of these mass movements promote continuous denudation of slopes, rockfall accidents along hiking trails, and sedimentation at artificial dams. They occasionally cause significant hazards to villages further downstream. Predicting and mitigating slope hazards require distinguishing among annual, low-magnitude processes, episodic high-magnitude processes and geomorphic changes associated with long-term climate change.