Temporal Changes in Erosion and Collapse Types and Sediment Yields of Yarding Roads in an Abandoned Clear-cut Plantation Forest, Kumamoto Prefecture, Japan

Abstract

  We conducted field investigations to study temporal changes in erosion and collapse types and sediment yields of yarding roads in an abandoned clear-cut plantation forest. The study site in Kumamoto Prefecture, Japan, is 0.89 km², and was clear-cut in September 2002. Moreover, to study sediment yields before replantation was abandoned in the study site, we interpreted aerial photographs taken in 1963, 1968, 1977, 1983, 1988, and 1995. Many of the shallow landslides that occurred on replanted slopes in the study site before replantation was abandoned were smaller than 400 m², and were on slopes with gradients greater than 30 degrees. Sediment yields from these landslides were greater when the tree age was 20 years or less than when the tree age was 20 years or more. The annual average sediment yield/km² between September 2002 and 2007 from yarding roads in the abandoned clear-cut forest was～17 times greater than the maximum annual sediment yield/km² from shallow landslides on replanted slopes prior to abandonment of replantation. The annual sediment yield/km² from September 2007 to 2008 from those yarding roads was～7 % of the annual average yield during that period from yarding roads. The decrease in annual sediment yield/km² from the yarding roads was due to washout of slope collapse material from filled slopes toward lower parts of the slopes and an increase in the rate of water infiltration caused by vegetation spread into cuttings in grass tracks for cutting and filling slopes.