DIEBACK OF PHRAGMITES-AUSTRALIS IN EUROPEAN WETLANDS - AN OVERVIEW OFTHE EUROPEAN RESEARCH-PROGRAM ON REED DIEBACK AND PROGRESSION (1993-1994)

Reed (Phragmites australis (Cav.) Trin. ex Steudel) is one of the domi nant plant species in European land-water ecotones. During the past de cades reed belts have died back, especially in central and eastern Eur ope. The aim of the European Research Programme on Reed Die-back and P rogression (EUREED), was to examine how increasing eutrophication, cha nged water table management, temperature, reduced genetic variation an d their interactions may contribute to reed die-back. Eutrophication a ppeared to be a key factor, but the effects on P. australis were indir ect, via the accumulation and decay of litter and allogenous organic m atter, 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 fr om eutrophic sites did not grow faster and were not more plastic than populations from oligotrophic sites. Variation within populations coul d be large as compared to variation between populations when exposed t o nitrogen, liquid manure and litter. DNA-fingerprinting showed differ ences between land and water reeds. This may be due to a differential selective force during establishment, e.g., when the water table recov ers after a drawdown. Selection by water table during establishment co uld affect the susceptibility of clones in the mature stage for indire ct effects of eutrophication. It was concluded that in stagnant water bodies the present water table management enforces the effects of eutr ophication on the reed die-back. In addition, local disturbances, such as the mechanical mowing, of reeds may enhance reed die-back. (C) 199 7 Elsevier Science B.V.