Vulnerability analysis of the rare and endangered woodland fern Polystichum braunii in Germany: three possible causes of population decline

Abstract

Background: Various rare and endangered temperate ferns are being threatened by their recent population decline, but there is limited understanding of the causes behind it.Aims: This study attempted to identify the possible drivers of regional population decline and extinction in the globally distributed woodland fern Polystichum braunii.Methods: A comparison was undertaken of the climatic, edaphic and phytosociological characteristics of sites with increasing, decreasing or recently extinct populations in Germany.Results: A significantly higher frequency of episodes of low relative air humidity (<60%) was found at sites with decreasing or extinct populations compared to habitats with population increases. Sites with decreasing or extinct populations were also characterised as having less summer precipitation (<500 mm year−1) and a shorter duration of snow cover (<110 days year−1) than sites with increasing populations. The latter had significantly higher moss cover (56% of the forest floor), but less cover by a tree litter layer (23%) compared to decreasing (36% and 38%) or recently extinct populations (22% and 52%). All increasing populations were located in intact Tilia – Acer ravine forests, while those suffering population decline were mostly located in Fagus-dominated forests.Conclusions: It was concluded that the probable causes of the recent decline in German P. braunii populations are reduced air humidity levels, decreasing snow duration or a shift from moss-covered to tree litter-covered forest floors due to climate warming or altered forest management.