Rock-humus soils are found primarily in mountainous areas, where they are an important part of the soil landscape. Since they often only occur in small areas, their importance is frequently underestimated on large-scale maps. For example, this soil class is not shown at all on the 1:1,000,000 scale soil map of Europe (Panagos 2006), not even in mountainous regions. This is despite the fact that in the Bavarian Alps, rock-humus soils account for around 10% of the landscape (Olleck et al. 2021; cf. Figure 1). In the Polish part of the Tatra Mountains, a proportion of approximately 3.5% has been reported (Stolarczyk et al. 2024). In addition, the pedological classification of rock-humus soils is not straightforward. In the WRB classification system, they are largely classified as Folic Histosols, but can also be found within the group of Leptosols (Leitgeb et al. 2013). National classification systems often vary considerably in how they categorise these soils. In Bavaria, for example, they can be found under the designations Felshumusböden (rock-humus soil) and O/C-Böden (O/C soil; O for organic, C for parent material of soil development) (LfU 2017). What all rock-humus soils have in common is that their ecology is determined exclusively by the humus layer. It serves as the only rooting zone, nutrient store and water store. If the humus layer disappears, bare rock remains. Due to their unique formation conditions, the properties of the humus layers over rock differ significantly from those of typical humus layers of mineral soils. For this reason, they are classified in a separate humus class, which is widely referred to as Tangelhumus (Kolb and Göttlein 2021). In the Bavarian Alps, Tangel-humus preferentially forms over solid or coarse carbonate rocks with low residual clay content and can reach a thickness of more than 100 cm. It is mainly found in the montane to subalpine zone of the Limestone Alps, where a cool and humid climate favours the accumulation of organic matter (Kolb and Kohlpaintner 2018). However, such humus layers can be found not only on calcareous but also on acidic bedrock, showing expected differences in pH and cation composition at the contact zone with the respective bedrock (Kolb and Göttlein 2022; Stolarczyk et al. 2024). In addition to chemical properties, understanding the water storage capacity of Tangelhumus is crucial, both for estimating the water supply of the trees and for assessing the landscape water regimes. Extensive tables are available for the water storage capacity of mineral soil horizons, differentiated by texture, bulk density and humus content (AK Standortskartierung 2016). However, such values are not available for Tangelhumus horizons. Therefore, pF curves were derived for Tangelhumus horizons of the Bavarian Limestone Alps, which allowing for quantification of the storage
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