Effects of percent cover of Japanese cedar in forests on slope slides in Sichuan, China

Abstract To understand why slopes with Japanese cedar ( Cryptomeria japonica ) forests slid far more frequently between 2008 and 2013 than slopes with a mixed forest of four native species in Yingchanggou valley, western Sichuan province of China, we studied 118 slopes. We examined the relationship between slope stability, degree of slope, slope substrate and percent tree cover of Japanese cedar in various proportions with mixed native tree forest cover, in regard to landslide occurrence with degrees of slope ranging from 20° to 85°. A binomial logistic regression analysis revealed a statistically significant correlation between interaction of degree of slope and >35% Japanese cedar cover in stands on slopes that suffered landslides during the 2008 earthquake and the following four years during heavy precipitation seasons. One big difference between Japanese cedar and the other four species comprising the native mixed forests is root type. Japanese cedar has shallow, thin roots while the others have deep, tap roots. The present study demonstrates that root type and root depth are an important factor influencing the role of tree roots on slope stability and that multiple species forests contribute to slope stability. Vegetation often contributes to slope stability but on substrates such as those common in this region, with 1 m or more of loose soil, rock mixtures at the surface inappropriate tree species can increase slope failure. To avoid increasing slope instability on slopes of 35° or greater, Japanese cedar should not be planted, as it is now, across wide areas in western China; instead the reestablishment of natural forest species should be encouraged.