Method for evaluating thinning influences on a forest's ability to prevent shallow landslides

Abstract

Thinning is indispensable for keeping planted forests healthy and producing timber of good quality. However, thinning is not being performed in increasingly large areas of planted forests in Japan due to stagnation of the forestry industry, and dense forests of thin trees are increasing. In such forests, trees can not develop roots properly, and their ability to prevent landslides may be adversely affected. This study aims to propose a method for evaluating the ability of thinned forests to prevent shallow landslides, based on past studies.
Planted forests of Cryptomeria japonica D. Don, which is the planted major tree species in Japan, were used for the analysis. Prior to the development of an assessment method, the distribution of roots was compared between a thinned forest where 1, 400 trees grew per hectare and a non-thinned forest where 3, 400 trees grew per hectare. Since no clear differences were observed in the total root volume, distribution of root volume in the depth direction, and maximum root depth, it was decided to use the same method to estimate the amount of root distribution in the thinned and non-thinned forests. The process by which a thinned forest loses the ability to prevent landslide was investigated by root pull resistance, which was found to disappear in about ten years after thinning. Based on these results, a method was developed to estimate changes in the ability of thinned and non-thinned forests to prevent landslides. This method was used for simulation analysis, which showed that intense thinning, especially of matured 30-to 40-year-old trees, would lower the ability.
On the other hand, thinning of trees younger than 20 years, when forests slopes are prone to slides, caused no adverse effects, and thinning usually starts on the 15th year. The simulation also showed that typical thinning of 20-year-old or older trees would not cause the safety factor of the slope to drop below 1.0. These results indicate that thinning should be performed to ensure the growth of healthy forests where trees are resistant to diseases and meteorological damage, and where such healthy forests would naturally help to prevent landslides.