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

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Prepared by Michelangelo Olleck, Birgit Reger & Jörg Ewald

“Black beauty on the white rock”: The soil profile of a Folic Histosol (Wetterstein Mountains, southern Germany, at an altitude of 1,850 m a. s. l.) shows the typical dark brown to black forest floor overlying white calcareous bedrock. All tree roots depend on the humus layer for water and nutrients; these soils store 3-4 times more carbon than average forest soils. Photo credit: Eckart Kolb.

Folic Histosols are thick layers (15 up to 100 cm) of forest floor (humus, derived from dead plant material) overlying poorly weathered limestone and dolomite bedrock in the Alps. In the absence of mineral soils, the forest floor is essential for the ecosystem functioning of mountain forests, as trees use them as their exclusive rooting zone, water and nutrient storage. If the forest floor disappears (through erosion or mineralisation as favoured by climate warming), the bare karstic rock comes to the surface and prevents the growth of trees over long periods. Another significant function of thick forest floors, especially under climate change, is that they store large amounts of carbon. In order to protect these forest floors and their functions, foresters have to find them in the landscape. Our research uses indicator plants of the ground vegetation to detect thick forest floors.

The study of the Bavarian Alps is located in southern Germany and has a cool and very moist climate. Mixed montane forest of beech, fir and spruce, as well as subalpine spruce forest and dwarf pine shrubs, cover ca. 260,000 ha of this region. Based on 811 plots, located on calcareous bedrock and including soil and vegetation records of the same locations, we investigated the relationship between ground vegetation and thick forest floors. We used Indicator Species Analysis to find out, which plant species prefer to grow on thick forest floors.

Typical mountain forest on a calcareous ridge in the Chiemgau Alps (southern Germany) at an altitude of 1,080 m a. s. l.; the dense ground vegetation of blueberries and clubmosses indicates the trees are rooted in a forest floor of 20 cm or thicker; rotting deadwood logs are left to become the next generation of the forest floor. Photo credit: Markus Stimmelmayer.

We found thick forest floors in 85 (10.5%) of our plots. 63 out of 754 plant species were significant positive indicators with a preference for thick forest floors. The strongest indicator species Homogyne alpina, Hylocomium splendens, Lycopodium annotinum, Vaccinium myrtillus and Vaccinium vitis-idaea require acid soils with low pH. Paradoxically, they often grow together with plants that prefer high pH, which can be explained by the co-occurrence of thick acid forest floors and bare calcareous rocks in the same forest stand.

It is striking that the best indicator species belong to the heather (Ericaceae) and clubmoss family (Lycopodiaceae), which both depend on highly specialised mycorrhizal fungi to acquire nutrients from the forest floors. The next important group of indicators are bryophytes (mosses and liverworts) known to grow on deadwood, which points to the importance of rotting logs in forest floor formation.

Thus, in areas of limestone and dolomite bedrock, foresters can detect the presence of vulnerable thick forest floors from the presence of indicator plants. Key measures to protect the forest floors under climate change are the maintenance of a permanent tree canopy (including natural regeneration) and the deliberate leaving of tree crowns and deadwood logs to replenish forest floors for the next generation.

This is a plain language summary for the paper of Michelangelo Olleck et al. published in the Applied Vegetation Science (https://doi.org/10.1111/avsc.12470).