Variation in species composition and species richness within Phragmites australis dominated riparian zones

In riparian wetlands total standing crop often fails to account for a signi ficant part of the observed variation in species richness and species compo sition within communities. In this study, we used abundance of the dominant species instead of total standing crop as the biotic predictor variable an d investigated its relationships with species composition and species richn ess in communities dominated by Phragmites australis (Cav.) Trin. ex Steude l. This was done by measuring soil organic matter content, litter cover and elevation, Phragmites abundance (standing crop and stem density) and speci es composition in 78 releves. In addition, we tried to identify the environ mental boundaries of Phragmites communities by sampling releves in neighbou ring communities. Two gradients were related to a decline in Phragmites abundance: one gradie nt, perpendicular to the shoreline, was mainly related to increased elevati on and the second gradient ran parallel to the shoreline and was related to increased amounts of soil organic matter. Within the releves dominated by Phragmites, stem density of Phragmites and litter cover were the only facto rs significantly related to species composition in the RDA solution. Litter cover and standing crop of the dominant accounted for 64% of the variation in species richness within the Phragmites-dominated community. These resul ts show that dead and living biomass of the dominant species may account fo r a substantial part of the variation in species composition and species ri chness within a single community.