In order to understand the mechanical characteristics of tree roots and their mechanical effects on slopes

the landslide in Wuping high vegetation coverage area of Fujian province was selected as the research site

and the root tensile mechanical properties of typical tree roots in the study area were tested after classification by diameter class. Furthermore

in-situ direct shear tests of root-soil composites under different root cross-sectional area ratios (RAR) and moisture content were conducted at the landslide site

and investigations were made into the distribution characteristics of roots in the profile to explore the mechanical effects of roots on shallow landslides. The results showed as follows: (1) The tensile force of Pinus massoniana and Cunninghamia lanceolata ranged from 12.45−673.09 N in 1−7 diameter class

and the tensile force was positively correlated with the root diameter by power function; The tensile strength ranges from 7.16 MPa to 60.95 MPa

and the tensile strength is negatively correlated with the root diameter as a power function. The average tensile force and tensile strength of Cunninghamia lanceolata root were higher than those of Pinus massoniana. (2) Tree roots significantly improved the shear strength of soil

and the additional cohesion provided by roots to soil was significantly positively correlated with the shear plane RAR. The root structure of Cunninghamia lanceolata is closer to R type

and that of Pinus massoniana is VH type. Under similar RAR

Cunninghamia lanceolata roots has a better reinforcing effect on the soil than Pinus massoniana. (3) With the increase in moisture content

the shear strength of the root-soil composites of Pinus massoniana and Cunninghamia lanceolata significantly decreases

as water infiltration diminishes the additional cohesion provided by the root systems to the soil. (4) Based on the Wu model

considering the influence of moisture content on soil cohesion and additional root cohesion

an estimation model for the shear strength value of root-soil composites considering moisture content was established. Upon verification

the accuracy of this model proved to be higher than that of the Wu model

and the results were reasonable. (5) Although the root system has a reinforcement effect on shallow landslides

its contribution to the stability of shallow landslides under heavy rainfall is limited due to the influence of root distribution depth

density and water infiltration.