Abstract
Abstract. 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 freeze–thaw processes and precipitation-induced processes control sediment transport.
Highlights
Sediment transport activities in the periglacial environment are controlled by a hillslope’s microclimate conditions, which are highly affected by vegetation cover (Matsuoka, 2001; Boelhouwers et al, 2000; Harris et al, 2008b; Ueno et al, 2015)
We studied seasonal changes in the impact of forest harvesting on microclimate conditions and sediment transport by intensive and comprehensive monitoring of harvested and non-harvested forests in the southern Japanese Alps
Clear changes in the winter microclimate conditions, including ground surface temperature, net radiation, and snow depth, following forest harvesting were monitored in site CC (Figs. 4, 5)
Summary
Sediment transport activities in the periglacial environment are controlled by a hillslope’s microclimate conditions (i.e., air and ground temperatures, throughfall), which are highly affected by vegetation cover (Matsuoka, 2001; Boelhouwers et al, 2000; Harris et al, 2008b; Ueno et al, 2015). Tree crowns intercept precipitation (Xiao et al, 2000; Fan et al, 2014) and control net radiation and ground surface temperature (Ueno et al, 2010, 2015). Sediment transport triggers, such as changes in soil moisture, generation of overland flow, and freeze–thaw of groundwater, are affected by these microclimate conditions (Wainwright et al, 2000; Gray et al, 2002; Ueno et al, 2015). Removal of the canopy by forest harvesting alters sediment transport opportunities via changes to the microclimate conditions
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