Relationship between tensile properties and microstructures of three different broadleaf tree roots in North China

Abstract: Forest vegetation is known to increase slope stability by reinforcing soil shear resistance, and root tensile properties are important factors to consider when choosing suitable species for reinforcing soil on an unstable slope. The macroscopic properties are determined by the microscopic structure; as a result, the microscopic structure of roots needs to be further observed to help us understand the relationship between the soil's mechanical properties and the microstructure of roots. We chose three common broadleaf species (Betula platyphylla, Quercus mongolica, Ulmus pumila) in North China and carried out several experiments, including single-root tensile tests, root anatomical-structure tests, and wood-fiber segregation experiments. To analyze data from those experiments, we used multiple regression and analysis of variance (ANOVA) in order to determine the relationship between root tensile properties and selected anatomical structure. Results showed that in all species a significant power relationship existed between tensile properties and root diameter (R2>0.8). The root tensile force increased with increasing root diameter, but root tensile strength increased with decreasing root diameter. According to the results of root tensile tests, we found that Ulmus pumila was the most resistant to failure, followed by Betula platyphylla and Quercus mongolica. Through the observation of microscopic sections, the root anatomical features of three kinds of broadleaf species were the same as those of their trunk, and wood fiber was the main anatomical feature in the root. According to the multiple regression models, the area percentage of wood fiber in the root cross-section (short for the area percentage of wood fiber), the area percentage of phloem in root cross section (short for the area percentage of phloem), and fiber dimensions affected single roots' tensile properties. The percentage of wood fiber area was the most important impact factor on root tensile force, exhibiting positive correlation. The percentage of phloem area had the biggest influence on root tensile strength. Wood fiber length, the length-width ratio of wood fiber, and the ratio of cell wall to cell cavity also affected tensile properties, but their quantitative influence was less than other factors. Although the area percentage of phloem affected root tensile strength, within a given species there was no significant difference in different root diameter. From the measurement of wood fiber in roots, we found that wood fiber size and type were different in the three selected species. The quantity of wood fiber increased with increasing root diameter, but the size of wood fiber varied only slightly. In three different broadleaf species' roots, the related index (including the percentage of wood fiber area, the percentage of phloem area, and the fiber dimensions) of Ulmus pumila was higher than that of Betula platyphylla and Quercus mongolica in the same diameter; that is to say, Ulmus pumila had better tensile properties than the other species. In accordance with the root anatomical features, fiber dimension, and tensile properties, Ulmus pumila, the roots of which have a better ability to reinforce soil on unstable slopes, is an appropriate broadleaf species for soil and water conservation.