Abundance of common reed (Phragmites australis), site conditions and conservation value of fen meadows in Switzerland

The abundance of Phragmites australis (Trin.) has increased in many fen meadows of the Swiss Plateau. The vegetation of 241 4-m super(2) plots was surveyed to determine how the abundance of Phragmites australis was related to site conditions and to the botanical conservation value. The aboveground biomass of P. australis (ABP) ranged from 0 g to 1270 g dry weight m super(-2). ABP was positively correlated to the nitrogen indicator value of the vegetation, but uncorrelated to the soil moisture indicator value. The abundance of 55 of 85 plant species was significantly related to ABP. High ABP was, on an average, associated with low conservation value in terms of species richness, number of rare or endangered species, and proportion of typical mire species. Strong differences were, however, found between plots with tall-herb vegetation (alliance Filipendulion) and plots dominated by heathgrass or small sedges (alliances Molinion and Caricion davallianae). The conservation value of Filipendulion plots was low and decreased significantly with increasing ABP. The conservation value of Molinion and Caricion plots was generally high and not related to ABP. Nevertheless, the current spread of P. australis might reduce the conservation value of Molinion and Caricion meadows in three ways: (1) a direct impact of shading is likely to become apparent only after several years; (2) if P. australis spreads into a site together with tall herbs, the loss of conservation value will be greater than if only tall herbs invade; and (3) if the presence of P. australis increases nutrient availability, it may indirectly cause the spread of tall herbs and the resulting loss of conservation value. Thus, spread of P. australis, spread of tall herbs and increased productivity are strongly related to each other and must be considered together when changes in the vegetation of fen meadows are evaluated.