How Does Plant Diversity Affect Soil Aggregate Stability and Erosion Processes in Disturbed Alpine Ecosystems?

Plant species diversity is hypothesized to be among the most relevant factors for enhancing soil stability in disturbed sites at high elevation. Because a more diverse plant community may comprise a high number of plant species of different growth forms, plant functional groups, and root characteristics, the chance of including a species that fulfills a specific key function may increase, thereby ensuring ecosystem integrity. Although plausible, the contribution of plant diversity to soil stabilisation in general, and to erosion control on alpine ski slopes in particular, has scarcely been demonstrated. This thesis investigated the relationship between plant diversity and soil stability in disturbed alpine ecosystems. The main objectives were (i) to determine root traits of 13 alpine pioneer species growing on a machine-graded ski slope without the in uence of neighbors, in order to address mechanisms of belowground diversity effects (Chapter 2), (ii) to quantify the contribution of plant diversity to soil stability by testing the aggregate stability of the soil from machine-graded ski slopes and from adjacent undisturbed vegetation (Chapter 3), (iii) to compare aggregate stability data from both graded and un-graded ski slopes and the associated control sites (Chapter 4), and (iv) to investigate the relationship between interrill erosion, vegetation cover, and plant functional diversity by applying rainfall simulations (Chapter 5). Both field and laboratory experiments were conducted. Chapter 2. Belowground diversity in root characteristics of alpine plants: key traits for soil restoration. Alpine plant species were highly diverse in functional root traits. Three of the 13 studied species were significantly different in root length and spreading, plant age and tensile strength. The results show that belowground diversity can be enhanced substantially by employing a few plant species with specific root traits. This study suggests that restoration attempts should aim at selecting species with a high belowground diversity to achieve a high diversity in functional root types which can positively affect slope stability. Chapter 3. Higher plant diversity enhances soil stability in disturbed alpine ecosystems. Plant diversity, vegetation cover and root density positively influenced aggregate stability, which was significantly lower on ski slopes compared to control sites. Out of all the tested variables, plant diversity explained considerably more variance than abiotic soil parameters. Plant species associated with more diverse communities were from various functional groups, i.e. grasses, forbs and shrubs. A higher density of fine roots and, to a lesser extent, coarse roots positively influenced aggregate stability. This study showed that, in addition to re-establishment of a functional vegetation cover with a dense root system, a high number of plant species with different growth forms is important. Chapter 4. Soil aggregate stability on alpine ski slopes. Aggregate stability was lower on graded ski slopes, but not on ungraded ski slopes compared with control plots . Root length density, number of plant species and vegetation cover were all positively correlated with aggregate stability. However, more than 13 plant species, 80% vegetation cover and 0.011 g cm-3 root density did not cause a further increase in aggregate stability. In multiple regression analysis, we determined that the effect of the number of plant species on aggregate stability was more pronounced on graded and gravelly ski slopes than on un-graded and sandy slopes. This study provided evidence that high plant diversity plays an important role in soil aggregate stability at disturbed alpine sites. Chapter 5. Interrill erosion at disturbed alpine sites: effects of plant functional diversity & vegetation cover. Interrill erosion was reduced the most when vegetation cover was high: 60% vegetation cover reduced the sediment yield by 83% compared to unvegetated ground. At 60% vegetation cover, sediment yield was significantly reduced in the presence of three functional groups compared to one plant functional group. Furthermore, combinations of plant functional groups including grasses reduced the sediment yield more than combinations without grasses. This study supports the view that, besides the re-establishment of a closed vegetation cover, high plant functional diversity is relevant for reducing interrill erosion at disturbed alpine sites. In conclusion, the resistance of topsoil to water-induced erosion at disturbed alpine sites depends not only on the degree of vegetation cover, but also on the presence of a high number of plant species that are highly diverse in functional traits above and below ground.