COS 135-1
What trees help stabilize slopes in mountains of western China?
To understand what type of tree species increase slope stability, we studied 118 slopes in
Sichuan province of southwestern China. We examined the relationship between slope stability, degree
of slope, slope substrate and % tree cover of Japanese cedar (Cryptomeria japonica) in various
proportions with mixed native tree forest cover, in regard to landslide occurrence in a site of the 2008
Sichuan earthquake, with degrees of slope ranging from 20-85 degrees. Japanese cedar is a naturalized
species that is planted extensively in the area as a timber source and for revegetation. Its main roots
bend horizontally about 10cm below the surface with a dense mass of fine, thin (<2mm) roots emerging
from it which extend no more than 1.2 m maximum depth at tree maturity.
Results/Conclusions
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. The present study demonstrates that root depth is an important factor
influencing the role of tree roots on slope stability. Trees with shallow root systems and increasing
height increase load on slopes. For trees with weak anchoring force, the bigger the trees, the more load
is added to the slope, and the higher the risk of the slope sliding. To avoid increasing slope instability on
slopes of 35 degrees 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.