Influence of rocky slope gradient on root anchorage of Vitex negundo L.

Abstract

Root anchorage adds stability to rocky slopes because the roots penetrate the shallow soil and extend into fractures in the bedrock. The influence of slope gradient on root anchorage has, however, rarely been investigated. Direct observation and pull-out tests in the field were used to investigate several root anchorage characteristics in 11-year-old Vitex negundo L. growing in three rocky slope conditions with varying gradients: low (29°), moderate (41°), and high gradient (53°). The results show that high- and moderate-gradient slopes in particular affect root anchorage resistance by increasing the tensile strength as well as the number and diameter of secondary upslope lateral roots that grow into the fractures. The upslope lateral roots contributed to anchorage to a greater extent than the downslope lateral roots. The contribution of upslope lateral roots to anchorage was 83% on high-gradient slopes, approximately 78% on moderate-gradient slopes, and 72% on low-gradient slopes. The conclusion of the study is that slope gradient is positively related to the distribution and anchorage of V. negundo L. on rocky slopes.