Humus Forms in the Forest-Alpine Tundra Ecotone at Stillberg (Dischmatal, Switzerland): Spatial Heterogeneity and Classification

In the forest-alpine ecotone at Stillberg (Dischmatal/Switzerland) the morphology of humus forms and the spatial variability of organic layer properties were investigated. At northeast-exposed gully sites mulls with high acidity in the A-horizon occur. They were classified after the Canadian classification of humus forms as Rhizomulls. Mors occur on ridges and on their east- and north-exposed aspects. They can be differentiated by the ratio between the thickness of the F-horizon and the combined thickness of the F- and H-horizon. The relative thickness of the F-horizon increases significantly in the order: east aspects < ridges < north aspect. The humus forms of the east aspects and the ridges were classified as Humimors and those of the north aspects as Hemimors. The Canadian classification was suitable to describe the properties of the horizons and to classify the humus forms. The results of a grid sampling at the study sites and the computation of nonergodic correlograms show that the spatial variability of organic-layer thickness, bulk density, and moisture is high (CV around 50%), with a pronounced small-scale heterogeneity (range usually below 2 m and more than 50% variability occurs within 0.3 m). Only 33% of the variance of organic-layer thickness were explained by site and vegetation structure, but in spite of the low percentage both proved to be a significant factor. In the forest-alpine tundra ecotone about 30 to 35 soil samples per site are needed for a reliable estimation of the mean of the organic-layer thickness.