Processes and Rates of Turf Scarp Denudation on the Summit Area of Mt. Tairappyo, the Mikuni Range, Central Japan

Abstract

Mt. Tairappyo (1, 983. 7m a. s. l.) is a middle mountain located in the Mikuni range, central Japan. The summit of this mountain has a saddle-like ridge extending to east. Along this gentle ridge many patch barelands which stretch from south to north are distributed. They often form steps similar to turf-banked terrasses. Though surfaces of barelands are almost flat, there exist turf scarps or notches about 20cm high at the northern margins. This study was made to clarify the processes and rates of turf scarp denudation in patch barelands. We measured the rates of turf scarp retreat in seven barelands and gained average recessional rate of 1.8cm/year. This value exceeds the velosity of 0.69cm/year reported from high mountain in Benezuelan Andes by Perez (1992). Barelands are considered to have been enlarged in consequence of turf scarp denudation at their northern margins.
Erosional processes on turf scarps are as follows. In April and May rubbles on turf scarps become loose by melting of frozen ground and soils on turf scarps are loosened by needle ice activity. Soils are dried up from May to July, and soil particles are denudated by strong south wind which constricted at narrow cols on mountain ridge. In August and September wind erosion becomes weak and turf scarp retreat is little. In October denudation becomes active on account of strong winds and severe rains. Though freeze-thaw cycle starts again in November, snow cover and protection of withered grass reduce turf scarp retreat. In winter erosion is very little because of ground freezing. And snow covers protect earth surface. As upper part of turf scarps are tightened by root systems, denudation of turf scarps accelerates at their bases and as a result of erosion turf scarps change to notches with eaves. After a while eaves collapse and new turf scarps are formed.
In Japan, patch bareland on high mountains are considered to be formed mainly by prevailing northwestern winter monsoon. However, in Mt. Tairappyo barelands are formed by south wind of spring and autum. It is extremely characteristic.