Thick forest floors in the Calcareous Alps – Distribution, ecological functions and carbon storage potential

The thickness and composition of forest floors plays an essential role for the efficiency and resilience of mountain forests to store carbon, water and nutrients. Up to now, the distribution of particularly thick organic forest floors (TOFF) in the Bavarian Calcareous Alps is poorly known and their ecosystem services deserve increased consideration under climate change. We wanted to improve the knowledge of the TOFF-distribution and to investigate the forcing processes and ecological functions of TOFF. We aimed to quantify their carbon storage potential and to model areas in which humus management is mandatory for sustainable forest use. We drew a stratified sample of soil profiles. Through the combination of relief and soil parameters, we identified crucial control variables and modelled actual and potential (without human disturbance) forest floor thickness in the Bavarian Calcareous Alps based on quantile regression and Generalized Additive Models (GAM). TOFF were predicted to occur on approximately 10% of the forest area of the Bavarian Alps. A decisive condition for the development of TOFF was the absence or only shallow development of mineral fine soil. Contrary to conventional wisdom, these TOFF were found across a wide range of (montane to subalpine) elevations. C-storage of TOFF amounts to ca. 6.9 t C/ha per cm of humus depth and ca. 5.2 Mt C in the study area, resulting in C accumulations comparable to peatlands. TOFF are decisive for the delivery of ecosystem services, especially in the protection forests of the Bavarian Calcareous Alps. Due to the absence or ephemeral depth of mineral soil, all ecological functions depend solely on the forest floor. Therefore, the careful handling of the humus stock is mandatory for a sustainable management in these forests.

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