Vertical spatial distribution characteristics of roots in different plant configurations of Lespedeza bicolor and Vitex negundo

In this study, three plant configurations, namely " Vitex negundo + V. negundo”, " Lespedeza bicolor + L. bicolor ”, " V. negundo + L. bicolor ”, were selected from the same rock slope (35 degrees) condition in Jiufeng Forest Farm, Haidian District, Beijing. Complete stratification (0 – 5 cm, 5 – 10 cm, 10 – 15 cm, 15 – 20 cm, 20 – 25 cm, 25 – 30 cm) excavation method and root system diameter classification were adopted. (D ≤ 2 mm, 2 mm < D ≤ 5 mm, D > 5 mm) root scanning method was used to study the vertical spatial distribution characteristics of the roots and pull-out resistance characteristics, such as root length percentage, root dry weight percentage and root density, in order to provide a theoretical basis for evaluating the root vertical spatial distribution characteristics and its impact on slope protection performance under different plant allocation modes. The results showed that the vertical spatial distribution of roots of these three plants showed a decreasing trend with increase in soil depth. In soil depth of 0 – 5 cm, the largest plant allocation of root length percentage and root dry weight percentage were exhibited by " V. negundo + V. negundo” and " L. bicolor + L. bicolor”, which accounted for more than half of the total soil layer; the highest root length density was by " V. negundo + V. negundo”; the highest root length and root weakening coefficient was by " V. negundo + L. bicolor”. In 5 – 20 cm soil depth, the percentages of root length, root dry weight, root density, specific root length and root weakening coefficient were found to be the best, and these were significantly different from the other two plant configurations (P < 0.05). In the depth of 20 – 25 cm soil layer, the percentage of root length was the largest for " L. bicolor + L. bicolor”, and the percentage of root dry weight and root density was the largest for " V. negundo + L. bicolor”. " V. negundo + L. bicolor” has the greatest pull-out resistance and the strongest slope stability.