Tree roots with an underground lateral network play a very important role in slope stability. In studies such as those by Tsukamotoi‚P‚X‚W‚Vjand Abei‚P‚X‚X‚Vj, the authors focused on roots that have a vertical connection between the surface soil layer and below, but they did not place much emphasis on the lateral spread of roots. Because slope failures occur in three dimensions, horizontal roots fully demonstrate their resistance against the failure. This paper aims to describe the resistance force ƒąC that lateral roots provide. Two types of field investigations were made. One was the root pulling test to monitor the strength of a root pulled by a clamp. The other surveyed the lateral root distribution exposed in vertical sections excavated in the plots of Japanese cypress and cedar forests. The observed root distribution was used to calculate the resistance force ƒąC. The field data show that as the distance between two trees increases, the ƒąC value gets smaller. Their relationship can be described by a negative power function. It has also been proven that as the age of a tree increases so does the ƒąC, but its increment gets smaller in the later stages of the tree's lifecycle. Intensive work with the field data has shown that forest thinning operations might decrease the root volume after felling and bring about an increase of ƒąC that lasts for several decades. Moreover, thinning operations produce good results in slope stability when lateral roots are effectively increased. However, if the increase is insufficient, it might not contribute to slope stability.

Key wordsFslope failure, tree root, lateral root, forest thinning operation