Objective  Pinus tabuliformis, Betula platyphylla, Quercus mongolica, Larix principis-rupprechtii and Ulmus pumila are widely grown and planted in the North China. Their roots play an important role in anchoring soil and preventing landslides. Therefore, it is of scientific significance to study their root soil interface friction performance.

  Method  In this paper, the roots of five tree species dug from the Beigou Forest Farm of Mulan Forest Administration Bureau in Mulan Weichang County of Hebei Province, northern China were divided into several small sections according to different diameters, and the roots were buried in the soil specimen box with a dry density of 1.52 g/cm³ and a moisture content of 12.72%. The root pulling test machine developed by the plant mechanics laboratory of Beijing Forestry University was used to pull out the roots embedded in the soil, to explore the mechanism of root pulling out and analyze the factors affecting the friction force from different tree species and varied diameters.

  Result  (1) There were two failure modes in the process of root pulling out. When the maximum tensile strength of root was less than the maximum friction between root and soil, the root pulling off failure will occur, otherwise, the root pulling out failure will occur. (2) The maximum friction of root system increased as a power function with the diameter increasing and the fitting degree was very high. (3) The maximum shear stress of root soil interface of Q. mongolica and U. pumila decreased with the increase of diameter, while that of B. platyphylla, L. principis-rupprechtii and P. tabuliformis increased with the increase of diameter. (4) The shear stress and displacement curve of the root soil interface of five tree species can be fitted by the improved hyperbolic model, and the shear stress of each tree species reached the peak value when the displacement was 6% of the total value.

  Conclusion  The results show that different vegetation types and diameters have great influence on the soil consolidation effect of roots. The traditional prediction method of soil consolidation effect overestimates the soil consolidation effect of roots. Therefore, the influence of roots should be considered comprehensively in the construction of artificial protective forest.