Die-back of Phragmites australis in European wetlands: an overview of the European Research Programme on Reed Die-Back and Progression (1993-1994)

Reed (Phragmites australis (Cav.) Trin. ex Steudel) is one of the dominant plant species in European land-water ecotones. During the past decades reed belts have died back, especially in central and eastern Europe. The aim of the European Research Programme on Reed Die-back and Progression (EUREED), was to examine how increasing eutrophication, changed water table management, temperature, reduced genetic variation and their interactions may contribute to reed die-back. Eutrophication appeared to be a key factor, but the effects on P. australis were indirect, via the accumulation and decay of litter and allogenous organic matter, rather than acting directly via disturbed carbohydrate cycling or reduced porosity of the aerenchymous plant tissue. The formation of toxic byproducts of decomposing litter in anoxic environment, such as acetic acid, may reduce reed vigour. Sulphide may act as a principal toxin especially at brackish sites, such as Lake Fert, Hungary. There were large differences in genetic variability. However, populations from eutrophic sites did not grow faster and were not more plastic than populations from oligotrophic sites. Variation within populations could be large as compared to variation between populations when exposed to nitrogen, liquid manure and litter. DNA-fingerprinting showed differences between land and water reeds. This may be due to a differential selective force during establishment, e.g., when the water table recovers after a drawdown. Selection by water table during establishment could affect the susceptibility of clones in the mature stage for indirect effects of eutrophication. It was concluded that in stagnant water bodies the present water table management enforces the effects of eutrophication on the reed die-back. In addition, local disturbances, such as the mechanical mowing, of reeds may enhance reed die-back. [KEYWORDS: wetland ecosystem dynamics; biogeochemical cycles; environmental change; assimilate and nutrient partitioning; gas exchange; genetic fingerprinting;genetic variability;bacterial nitrogen transformation Rhizomes; dieback; phytotoxins; nitrogen; decline; sites; lakes; flow