Plant indicators for Folic Histosols in mountain forests of the Calcareous Alps

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.