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The production of vigorous and die-back sites of the same reed belt and nutrient content of reed were studied to detect possible causes of reed die-back. Shoot density, total aboveground and belowground biomass and their C, N, P and S contents were also determined. Samples were collected from three vigorous and three die-back sites of Lake Fertő/Neusiedler See in 1996 and 1997. Reed stands were compared when the biomass was maximal in both years. Reed shoots were significantly (p = 5%) shorter, thinner and had less internodes at the die-back sites than at the vigorous sites, where the aboveground biomass was 1.5-2.0 times and the leaf area index (LAI) 1.7-2.5 times higher. The belowground biomass was nearly the same at the vigorous and die-back sites. The amount of decaying belowground biomass was less than 50% of the total belowground biomass at the vigorous sites, while it reached 75% at the die-back sites. Different N, C, P and S concentrations were found in the reed organs studied (leaves, culm, rhizome and roots). The N concentration in leaves, culm and rhizome was higher at the die-back sites, where the P concentration was the lowest. There were no significant differences in the S concentration of the examined reed organs as comparing the vigorous and degraded reeds. The C concentration of culms were higher, while that of the living rhizomes were lower at healthy than at the die-back sites. The N, P, C and S standing stocks in the aboveground biomass were significantly lower at the die-back sites than at the vigorous sites. In the belowground biomass the P standing stock was significantly lower, while the N standing stock was significantly higher at the die-back sites than at the vigorous sites. Not more than 17% of the total dry mass m −2 , 17% of the C, 24% of the N, 14% of the S and 27% of the P standing stock was found in the aboveground phytomass produced in the actual year.

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