Abstract. The above‐ground standing crop and nutrient concentrations in plant material were examined in 45 stands of mire vegetation in the Biebrza peatland, Poland. The stands included flood‐plains, rich fens, transitional fens and bogs. The pattern in nutrient concentrations in the above‐ground plant material resembled the pattern in nutrient concentrations in peatwater and peat which had been investigated in an earlier study. Concentrations of N were quite uniform along the gradient. P‐concentrations were highest in the transitional fen. Critical nutrient concentrations were defined on the basis of a review of nutrient concentrations in plant material from peatlands in which a fertilization experiment had been carried out. Defined critical values for phanerogams were: 13‐14 and 0.7 mg/g dry wt for N and P respectively. Concentrations lower than these values indicate deficiency. P/N ratios ≥ 0.07 indicate N‐deficiency and P/N ratios ≤ 0.04 — 0.05 indicate P‐deficiency. According to these values the Biebrza fens and bogs appear to be primarily deficient in N. The growth of the flood‐plain vegetation does not appear to be restricted by nutrients.

We estimated, using logistic regression techniques, the realized niches of the four dominant species in an experimental marsh complex located in the Delta Marsh, Manitoba, Canada. These models were then used to predict the probability of occurrence of these species in selected elevation ranges when water levels were raised in 1985 either 0, 30 or 60 cm above the long-term normal water level. These realized-niche models were calculated using elevation and species data collected in 1980. After having been eliminated by two years of deep flooding, the emergent vegetation in this complex had been re-established during a drawdown beginning in either 1983 or 1984. Our hypothesis was that from 1985 to 1989 the frequencies of occurrence of species in selected elevation ranges would converge to their probabilities predicted from the 1980 logistic models.This was not borne out by our results. Actual frequencies and predicted probabilities of occurrence of a species were similar at best less than 40 % and then mostly in the control (0 cm) treatment. The realized-niche models were not adequate to predict the distribution of emergents after an increase in water level in the short term because the emergent species did not migrate upslope. Emergent species in the medium and high treatments either (1) died out -Scolochloa festucacea and Scirpus lacustris -after 3 yr because they could not survive permanent flooding, (2) stayed where they were -Phragmites australis -because they were unable to move upslope through clonal growth, or (3) became more widespread -Typha glauca -only because of the expansion of small local populations already established in 1985 in areas dominated formerly by other species.