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.
The results of the study illustrate the remarkable differences between root traits of alpine plants, some of which cannot be assessed from simple morphological inspection, e.g. tensile strength. PFT classification based on root traits seems useful to categorize plant traits, even though some patterns are better explained at the individual species level.
Aims Plant diversity has been shown to significantly increase topsoil aggregate stability of machine-graded ski slopes. We hypothesise that this effect is specific for these disturbed sites and that at sites of low and no disturbance the effect decreases. Methods We determined plant species richness, cover percentage of five functional groups, root (length) density, and biomass as well as soil aggregate stability, gravimetric soil moisture, soil density, and particle size distribution at different levels of disturbance (i.e. graded and un-graded ski slopes and the surrounding area). Results Plant species richness, vegetation cover, aggregate stability and soil moisture were significantly reduced on machine-graded slopes compared to control plots but hardly on un-graded slopes. On the contrary, machine-grading increased soil density and friction angle compared to un-graded ski slopes. The influence of species richness on aggregate stability was only positive on gravely soils and graded ski slopes. Aggregate stability increased linearly up to approximately eight plant species, 70% vegetation cover and 0.006 gcm −3 root density. Conclusions Our study showed that the relationship between plant diversity and aggregate stability was strongest on slopes with high disturbance and relatively low species numbers. We suggest that high plant diversity, vegetation cover and root density need to be established after major human disturbance such as grading.
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