Sediment chemistry and flooding exposure: a fatal cocktail for Phragmites australis in the Mediterranean basin?

The common reed die-back syndrome was formerly reported mostly in Central Europe and more recently in the Mediterranean basin. This study investigates a case of reed die-back in Central Italy through a Geographic Information System (GIS) diachronic survey and the analysis of macromorphological and ecological parameters. Data were recorded during field activities on ten plots randomly distributed according to two ecological statuses, permanently and temporarily flooded. Culm density, height and diameter, clumping habit, flowering head and dead apical bud number and rate were measured; for each plot, chemical parameters of sediments were analyzed. Information on interstitial waters for the flooded plots was also provided. Floristic-vegetational features of the reed-bed community such as the total vegetation cover, the species composition and abundance were estimated. Univariate and multivariate analyses were used to demonstrate differences between morphological traits of flooded and emerged stands and their relationships with the chemical features of sediment. Macromorphological traits differed according to the ecological status, with flooded stands showing patterns related to poor health status of Phragmites australis, such as high rates of clumping habit and dead apical bud rate, high culm density and, to a lesser extent, low culm diameter and flowering head rate. Sulfates were relatively abundant in the sediment of flooded stands and, together with some heavy metals, resulted in some of the mentioned traits. Our results showed a relationship between reed die-back and prolonged flooding and highlighted the potential role of some chemical parameters in affecting the growth of permanently flooded reeds.