The reed die-back syndrome and its implications for floristic and vegetational traits of Phragmitetum australis

Common reed [Phragmites australis (Cav.) Steud.] die-back is a well known phenomenon in Central Europe, recently detected also in Italy. It has been described as a complex syndrome affecting reed populations, including multiple symptoms of retreat and decline, with significant impacts on important wetland ecosystem services. The present study takes into account, for the first time, floristic and vegetational traits in a declining reed bed and tries to search for possible relations between die-back and plant community features. 45 phytosociological relevés were carried out in the years 2006, 2007 and 2009 in 19 permanent plots at Lake Trasimeno (Central Italy), where reed die-back has been detected and monitored in former studies. Relevés have been clustered into 3 groups: dying-back, suboptimal and optimal stands, on the ground of the level of decline of the corresponding reed plots. The considered traits are: number of species, total and specific cover values, diversity indexes, Ellenberg’s ecologic indicator values, number of nitrophilous species. The differential species in the three groups are also pointed out. Results show that both floristic and vegetational traits in stands affected by reed decline are significantly different from optimal and suboptimal stands. Number of species, total cover value and biodiversity indexes are significantly lower in the declining stands, while Ellenberg’s indicators for nitrogen (N) and moisture (M) point out interesting correlations. It appears that reed tends to form monospecific stands in permanently flooded areas rich in autogenous litter, where the symptoms of decline are more severe. Species rich stands develop in terrestrial areas where reed shows vigorous growth and healthy condition, however this floristic richness is largely due to invasive and nitrophilous taxa which originate a ‘pseudo-reed bed’, often lacking in typical palustrine species, referred to as a variant with Rubus ulmifolius of Phragmitetum australis Savič 1926. The results draw attention to the risk of loss of an ecosystem which plays an important role in biodiversity conservation.