Effects of increased nutrient supply on bryophytes in montane calcareous fens

A fertilization experiment was carried out in 18 nutrient-poor calcareous fens in Switzerland in order to investigate the effects of nutrient enrichment on bryophytes. Plots were fertilized with N or NPK, to simulate enhanced atmospheric nitrogen deposition and the influx of a nutrient cocktail from adjacent farmland, respectively. Despite large floristic differences between sites, the response of the bryophyte layer to fertilization was remarkably uniform. After 1.5 years, bryophyte biomass, as well as bryophyte species density, were significantly reduced as a result of the fertilization treatments. Fertilized plots contained 39% (N-plots) and 53% (NPK-plots) less bryophyte biomass than the controls, and 9% (N-plots) and 18% (NPK-plots) fewer bryophyte species. Differences between N- and NPK-plots were not significant for either bryophyte biomass or species density. The decline of bryophyte biomass and species density can be attributed only partly to the strong increase of vascular plant biomass in the fertilized plots, leading presumably to very low light intensities at the bryophyte layer. Some bryophytes tended to have lower frequencies in the fertilized plots, but only the rather shade-intolerant Bryum pseudotriquetrum declined significantly. It is concluded that enhanced N-influx is sufficient to cause significant changes in the bryophyte layer of calcareous fens, and that species with a high demand for light and rather low morphological plasticity are at particular risk of being ousted.