What drives riparian plant taxa and assemblages in Mediterranean rivers?

This study focused on floodplains of four rivers flowing into the Tyrrhenian Sea. Our main questions are: what environmental classifications account for variations in riparian plant assemblages along Mediterranean river floodplains? How is classification predictive power affected by taxonomic resolution? What are the environmental features that may be influencing plant taxa composition? We collected riparian vascular plant and environmental GIS data (morphology and land-use) from 189 river segments. Hierarchical clustering was used to derive environmental and biological groups of segments. We applied a measure of classification strength (CS) to account for variations in riparian plant assemblages, considering CS with respect to species and higher taxonomic levels. We used non-metric multidimensional scaling (NMS) ordination to identify major environmental features influencing plant assemblages. Our results showed the correspondence’s strength between classifications and plant taxa was significant, albeit low, for all data. The hybrid classification, based on morphology plus land use, gave the best CS values for all taxonomic levels and emerged as the best determinant for discriminating differences in riparian plant composition. The correspondence between the classification criteria and plant assemblages depended on the taxonomic level analysed and genus level has best CS results. NMS for taxonomic groups showed that only a few environmental features could be identified as major factors influencing plant composition in floodplains, among which cover of mixed woodlands was the best. A local-scale approach and incorporation of more detailed variables into the classification scheme may therefore improve the match between environmental classification and plant community composition.