Hydraulic and physical properties of alpine soils in relation to terrain and vegetation

Abstract

High-mountain areas are hotspots of biodiversity and provide critical ecosystem services. These high-mountain areas are very sensitive to climate change. This causes temperature rise and strong changes in precipitation patterns, which leads to glacier retreat and snow cover reduction. Another consequence of climate change is greening, which is an increase in productivity of vegetation. This causes enhanced growth and spreading of vegetation. Besides the climatic conditions, greening depends on interactions between soil, vegetation and topography. This research aims to improve the insights in the relationships between soil, vegetation and topography in an alpine environment. Data was collected on a field research in the Meretschibach catchment in Valais, Switzerland. Five different vegetation classes (bare, pioneer, grass, shrub and forest) were defined previously, from which 42 locations were randomly selected based on spectral characteristics using NDVI images. At these 42 locations plots of 2x2 m were installed. In these plots, various measurements of the surface cover, topography and soil characteristics were executed. Also, soil samples were collected, which were used to derive additional soil characteristics using experiments in the laboratory. To find out whether there are relationships between the measured soil, vegetation and topographic parameters a statistical analysis was executed, which consist of a correlation, boxplot, cluster, PCA and NMDS analysis. In the PCA and NMDS ordinations, the topographic and surface cover parameters were plotted passively as explanatory variables and the soil parameters as response variables. One of the main findings is that the predefined vegetation classes are no good predictor for the soil characteristics. However, the correlation and boxplot analyses show that several soil characteristics can be related to vegetation. Moreover, vegetation cover has a large explanatory power for the variance in the soil parameters. This is also the case for rock cover, which has a negative association with vegetation cover. From the different topographic parameters (elevation, slope angle, aspect and slope form), only elevation has a strong explanatory power on the variance of the soil data. This is mainly caused by climatic conditions which vary with elevation. Generally, slope angle and aspect cause an mosaic of microclimates that result in a strong variation in soil characteristics. However, the outcomes of the statistical analysis did not show the impacts of slope angle and aspect. The elevation dependent climatic conditions influence plant growth and consequently, several soil characteristics like the organic matter content and the soil depth. Besides, the soil microbial activity depends soil temperature and moisture, which is also influenced by the elevation dependent climatic conditions. This controls the soil nutrient cycle and therefore, the soil development. All together, they cause elevation to have a large explanatory power on the variance in soil data. It is expected that greening will intensify in the Meretschibach catchment due to climate change. Upward migration of vegetation is likely to occur at locations where the snow cover duration shrinks due to climate change. Furthermore, stable soils that are relatively developed are the most favorable conditions for plant colonization. Greening causes increased evapotranspiration rates in the catchment. Therefore, decreased precipitation together with increased evapotranspiration during summer in the future, will cause a reduction in runoff from the catchment. This may result in browning, which is a decrease in productivity of vegetation. Greening is also beneficial for increasing soil depth and the water holding capacity of soils. Consequently, it will reduce the runoff from the catchment and therefore, gr