Estimating slope stability by lateral root reinforcement in thinned and unthinned stands of Cryptomeria japonica using ground-penetrating radar

Tree root diameter and root distribution are crucial factors for root reinforcement in forest soils and can be estimated using nondestructive ground-penetrating radar (GPR). However, no studies on estimating root reinforcement have been performed using GPR. The aim of this study was to evaluate root reinforcement using GPR in thinned and unthinned stands of Cryptomeria Japonica in Andisols. Roots in each stand were detected using a 900 MHz GPR and the diameter and horizontal distribution of roots were estimated. Roots were also excavated to compare their GPR estimates. The relationships between root diameter and tensile force were established through field pullout tests. Finally, root reinforcements in each stand were estimated using the recently proposed root bundle model (RBM). There were no significant differences in the diameter frequency distribution of roots estimated between thinned and unthinned stands, determined using either the GPR or excavation methods. The detection frequency of the number of roots > 5 mm in diameter using GPR was 27.2%, but it increased to 58.7% for roots > 10 mm. The waveform indexes, determined using the sum of time intervals between zero crossings (ET) of all reflection waveforms of GPR, had a significant linear relationship with the diameters of excavated roots, indicating that waveform index could be used to estimate root diameter using the GPR index in these stands. Furthermore, the relationship between the root tensile force (F) and- root diameter (d) was estimated as F = 15.69 d(1.75) (R-2 = 0.87). The root reinforcements at the center of gaps between individual trees were estimated to be 23.4-32.9 kPa in the thinned stand and 8.8-23.9 kPa in the unthinned stand using GPR. These values ranged from 31.9% to 170.7% of those obtained by using the excavation method. There were no significant differences in root reinforcement between thinned and unthinned stands, regardless of the method used. This study revealed that GPR, together with diameter-tensile force relationships, can nondestructively estimate root reinforcement in managed forests. This protocol can be applied to manage forests to evaluate the effects of management on belowground roots.