Populationsentwicklung und Gefährdung von Asplenium septentrionale auf Findlingen im Schweizer Mittelland und Jura
The calcifuge rock-dwelling fern Asplenium septentrionale is rare on the
Swiss Plateau and in the Jura region, where it exclusively inhabits siliceous erratic boulders. A historical scientific debate on the origin of A. septentrionale on erratic boulders – transportation during the ice ages by glaciers on erratic boulders vs. post-glacial long-distance colonization – underlines the importance of those populations for science history. In the present study, a census of A. septentrionale on erratic boulders on the Swiss Plateau and in the Jura region was conducted, since only little is known about the current distribution of these biogeographically remarkable populations. We reviewed herbaria and literature and revisited locations. In total, historical records from 17 populations were found. Of these, five were confirmed, four were extinct, and the status of eight further populations remained uncertain, because the records were not accurate enough to allow the exact erratic boulders to be found. Two well-documented recent populations in the canton of Zurich showed a decline in population size over the last 100 years. Only 72 individuals were found in total. Destruction of erratic boulders for construction and collection for herbaria were significant historical threats, while shading by vegetation and sport climbing (bouldering) are contemporary threats today. On the Swiss Plateau and in the Jura region, regionally rare lichen and bryophyte species unique to siliceous erratic boulders would also profit from conservation measures for A. septentrionale.