Understanding common reed die-back: A phytocoenotic approach to explore the decline of palustrine ecosystems

It is well known that since more than half a century, in Europe, Phragmites australis is suffering a process of decline, known in literature as ‘common reed die-back’. Several hypotheses have been formulated but the actual causes of the phenomenon have been only partially understood. The several studies produced on this topic generally focused on the population approach and took seldom into account the floristic and vegetational features of the reed-dominated plant communities involved in die-back processes. The present study tries to fill this knowledge gap. Starting from a phytosociological approach, supplemented by the results of a recent three-year-long research project focused on morphological and ecological traits of dying-back reed beds, we analyzed the floristic and vegetational differences between declining and non-declining stands, based on a data set constituted by 80 relevés. Data refer to reed-dominated stands along the shores of five freshwater ecosystems in central Italy: the Lakes Trasimeno, Chiusi and Vico, the Fucecchio and Colfiorito Marshes. The statistical process, including cluster analysis and PCA, allowed to refer all the relevés to the association Phragmitetum australis Savič 1926, with eight variants differentiated from an ecological and floristic point of view. The indicator species analysis pointed out the taxa playing a diagnostic and/or differential role in each group, and provided useful information to understand pattern and processes occurring in the declining and non-declining reed-dominated phytocoenoses. As a general outcome, a clear inverse relation between number of species per relevé and intensity of the die-back process was showed. This supports the idea that the aquatic monospecific reedbeds are the most suffering ones, while the nitrophilous species-rich phytocoenoses, colonizing drier sediments and often in contact with disturbed areas, are the ones where common reed grows most healthily.