Verlängertes sommerliches Trockenfallen von Bächen gefährdet die Vorkommen der reliktischen Moose Tetrodontium repandum, Geocalyx graveolens und Harpanthus scutatus in Nordbayern

Schanz, M., Meve, U. & Feulner, M. 2021. Prolonged summer drying of streams threatens the relict mosses Tetrodontium repandum, Geocalyx graveolens and Harpanthus scutatus in northern Bavaria. – Herzogia 34: 173 –188. The deeply incised gorges and rock formations of the Rhätium in the west of Bayreuth are an outstanding habitat for bryophyte species with montane distributions such as Tetrodontium repandum, Geocalyx graveolens and Harpanthus scutatus. The special site requirements such as constantly wet microclimate (cold air congestion, high humidity, mostly sites along streams) and the high water capacity of the sandstone rocks are prerequisites for the occurrence of these relict mosses. Due to rapid climate change and its consequences such as increasing dry periods in the summer, prolonged drying of streams and rising temperatures, the question arises to which extent the survival of the investigated species is endangered. To investigate drought damage of the mosses, the population sizes and percentage of dead moss layers as well as exposition, slope, distance to the river and the accompanying species were examined. In addition, the microclimate was measured at three sites using data loggers that recorded temperature and humidity hourly over a period of five months. On average, moss mats of Tetrodontium repandum were reduced by drought damage to 31 % of their former size. Geocalyx graveolens was reduced by 9 %. Desiccation damage to Harpanthus scutatus could not be detected, probably due to rapid die-off after heat periods. The relative humidity at the investigated sites of Tetrodontium repandum was close to 100 % and the temperature was with about 3°C significantly below the comparative values of a reference site outside the gorge. Both parameters showed significantly lower fluctuations inside the gorge than outside. Geocalyx graveolens exhibited a significant increase in drought damage with increasing distance to the stream. Tetrodontium repandum also showed a significant negative correlation between population size and distance to the stream. In conclusion, this study identifies the prolongation of drought periods of streams as the main threat for the persistence of relict moss species. Furthermore, it outlines the importance of reducing the drainage of the environment for achieving both higher water storage in the landscape and shorter drought periods in the streams.