Abstract: Plant roots could enhance the shear strength of soil. To understand the root characteristics and their impacts on shear strength at the initial stage of highway slope greening, the mixed grassshrub samples were prepared by spraying seeds. Seven kinds of grassshrub combinations were artificially simulated in the slope greening. The insitu shear test was carried out three months after plant growth. Plant roots were excavated, with root characteristics such as root diameter, root length density, total surface area, total volume, number of root tips, number of branches, root density and inclination angle being measured. The effects of root characteristics on the shear strength of soil were analyzed by grey correlation method. The results showed that there were differences of root characteristics in various grass-shrub combinations with the changes of soil depth. At the initial stage of greening, fine roots of 0-2 mm accounted for 96%-99% of the whole root system. The horizontal roots, lateral roots and vertical roots constituted 7%-16%, 44%-57% and 33%-47% of the total root system, respectively. The roots of different grassshrub combinations increased the shear strength of soil by 6-17 kPa, and the shear strength of the different ratios of shrubs and grasses was in the order of 1∶1 > 1∶4 > 1∶5 > 3∶1. The results of grey correlation analysis showed that the effects of root characteristics on shear strength were as follows: number of lateral roots > number of horizontal roots > depth of root distribution > root density > root length density above shear plane > root length density > number of fine roots (0-2 mm) > root length density below shear plane > total surface area> total volume > number of root tips > number of vertical roots> number of branches > number of roots (2-10 mm). Our results provide a basis for evaluating the stability of the slope at the initial stage of highway slope greening and guiding vegetation restoration of the slope.

Key words: ecological and environmental changes, remote sensing, the Three Gorges Reservoir area, ecosystem assessment, ecological process.