Erosion of forestry land: causes and rehabilitation

摘要

Forests cover 70% of the total land area of Japan. Forest lands within Japan are prone to landslides because weakly resistant geological units are eroded by water flowing down steep slopes that are subjected to annual rainfall amounts that are 2.5-times the global average. The environmental effects of deforestation impact upon atmospheric pollution, wildlife, the hydrological cycle, water resources, soil erosion, and the occurrence of landslides. To mitigate disasters that occur upon forestry land, it is important to forecast landslide development and plan for the provision of remedial measures during disaster rehabilitation. This paper describes the causes of the erosion of forestry land and methods of disaster rehabilitation via a case study of the upper reaches of the Tama River, Japan, which is a national park and an important water resource for the Tokyo Metropolitan area. The causes of erosion of forestry land within the upper reaches of the Tama River are classified as one of the following: shallow landslides related to the loss of under-story vegetation, collapse of steep slopes, damage related to the consumption of vegetation by wildlife, and debris flows that occur during periods of torrential rain. In recent times, heavy rains over eroded forestry land within the upper reaches of the Tama River have produced muddy river water due to the erosion and degradation of mountain slopes. In addition, grazing by Japanese deer has destroyed many trees within the upper reaches of the Tama River, and the torrent bed within this area, previously planted with Japanese horseradish, was lost during a debris flow. In this paper, we describe anti-erosion measures undertaken for disaster rehabilitation of wasted forestry land, including timber-thinning methods and the control of wildlife numbers.