Plant diversity is hypothesised to increase soil stability by increasing the diversity of root types. To test this hypothesis, we took soil cores from machine-graded ski slopes and from the adjacent undisturbed vegetation as a control. We quantified aggregate stability as an indicator for soil stability in relation to (1) abiotic soil properties, (2) above-ground vegetation characteristics and (3) root parameters. From the three groups of variables, the number of plant species, root density (RD) and sand content showed the highest correlation with soil aggregate stability and explained 54% of its variance. In variance partitioning, the number of plant species was the most relevant factor explaining 19% of the variance in aggregate stability. Further, it explained another 11% through shared effects with RD and sand content. An additional 8% was explained through the shared influence with sand content. Plant species showing the highest correlation with overall diversity were from different functional groups (grasses, forbs and shrubs), meaning that beneficial effects can not only be assigned to one specific functional group, but to the combination of several groups. Our data demonstrate the positive effect of plant diversity on aggregate stability. We suggest that high plant diversity is one of the most relevant factors for enhancing soil stability at disturbed sites at high elevation.