Alpenhumus als klimasensitiver C-Speicher und entscheidender Standortfaktor im Bergwald

Bislang war es weitgehend unklar, welche Rolle die Humuspflege in der Bewirtschaftung der Bergwälder in den
Bayerischen Alpen spielt. Eine Umfrage im Rahmen des Projektes «Alpenhumus als klimasensitiver C-Speicher
und entscheidender Standortfaktor im Bergwald» sollte den aktuellen Wissensstand in der forstlichen Praxis klären
und herausfinden, ob und welche unterstützenden Massnahmen notwendig sind, um die Humuspflege und
den Humusaufbau in den Gebirgswäldern der Bayerischen Alpen zu verbessern. Die Rückmeldungen aus 29 Fragebögen
zeigten, dass die Försterinnen und Förster den Anteil der Tangelhumusstandorte mit mehr als 15 cm
Humusmächtigkeit in ihrem Zuständigkeitsgebiet auf knapp 20% schätzen. Hochgerechnet auf die Waldfläche
der Bayerischen Alpen ergibt dies einen Anteil von rund 48 250 ha. Wichtigster Indikator des Forstpersonals für
die Lokalisation mächtiger Humusauflagen ist die Bodenvegetation. Etwa 20% der Befragten geben einen sichtbaren
Rückgang von Tangelhumus an. Als Hauptgefährdung für Tangelhumus nennen die Befragten die Bodenerosion,
die Klimaerwärmung, das Ausbleiben der Verjüngung und die Kronennutzung. Als relevante Massnahmen
zur Humuspflege geben sie das Belassen von Kronenmaterial im Bestand, die Einleitung und/oder
Sicherstellung der Verjüngung sowie die Jagd an. Zur Verbesserung des Humusaufbaus wünschen sich die Befragten
gezieltere Planungen, eine Änderung des Schalenwildmanagements und Schulungen für die Waldbesitzer.
Verbesserte Karten mit Geodaten, eine Förderung der natürlichen Waldentwicklung und ein Leitfaden zur
Humuspflege sollen dabei zusätzlich unterstützen.

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.

Questions: Although thick forest floors overlying unweathered bedrock are important resources for mountain forests' functioning, their actual distribution is poorly known and difficult to delimit in the field. We therefore asked: (a) What is the specific composition of vegetation growing on Folic Histosols; (b) can indicator plants be used to detect Folic Histosols in mountain forests; (c) what do functional traits of plant indicators tell about the ecological properties of Folic Histosols? Location: Northern Calcareous Alps, south Germany. Methods: Based on representative stratified sampling of joint vegetation plots and soil profile descriptions, we estimated the frequency and thickness of Folic Histosols, determined the proportion of compositional variation specifically attributable to forest floor thickness using ordination, applied Indicator Species Analysis and searched for typical traits and ecological requirements of indicator species. Results: The co-existence of acidophilic and calciphytic plants is typical for the tessellated occurrence and the successional origin of Folic Histosols. In the study region, the detection of Folic Histosols on pure limestone or dolomite by ground vegetation works very well. Particularly acidophilic plants are suitable indicators for thick forest floors. The indicator value of bryophytes and Ericaceae for Folic Histosols is likely related to the colonization of rotten wood. Folic Histosol indicator species are widely spread in the allocation to sociology group, which ranges from open landscapes to dark forests and reflects successional origin. Conclusions: In mountain forests on carbonate bedrock, thick humus layers often occur next to bare rock. This tessellated structure can also be detected in the ground vegetation, where acidophilic and calciphytic plants occur side by side. Thick Folic Histosols in late successional forests are dominated by acidophilic plants colonizing rotten wood. Thus, the detection of Folic Histosols by understorey species is an easy and cost-effective possibility and one key to protect these vulnerable forest sites.