Shading by Phragmites australis: a threat for species‐rich fen meadows?

Abstract

Abstract. Common reed (Phragmites australis Trin.) has spread in fen meadows on the Swiss Plateau during the last decades. An increased dominance of this tall grass may reduce the plant species richness and displace rare or endangered species because of the additional shade. To investigate whether this has actually happened and whether shading by Phragmites was likely to be responsible for the reduction, the plant species composition was surveyed in 241 plots (4 m2) with differing above‐ground biomass of Phragmites (ABP). Species richness and the occurrence of characteristic fen species were negatively related to site productivity (total above‐ground biomass), but correlations with ABP were generally weaker. The main change associated with increasing ABP within species‐rich fens (alliances Molinion and Caricion davallianae) was an increase of species characteristic of the more species‐poor alliance Filipendulion. Thus, Phragmites did apparently not play a disproportionate role in the reduction of species richness in the plant communities studied. Selective clipping experiments and light measurements also indicated that shading by Phragmites does not have a strong effect upon the performance of other species in the community, at least in the short term. The reason appeared to lie in the later seasonal growth of Phragmites compared with the other species. These results suggest that the direct impact of shading by Phragmites on the species richness of fen meadows is probably lower than has been assumed. However, possible long‐term or indirect effects still need to be investigated.