Patterns in vegetation, hydrology, and nutrient availability in an undisturbed river floodplain in Poland

In the undisturbed floodplain of the Biebrza river (N.E. Poland) we compared vegetation composition, standing crop and the nutrients in standing crop to site factors such as flood duration and inundation depth during spring floods, summer water levels and concentrations of chemical constituents in water and nutrient release rates from peat. Our analysis shows a number of clear spatial patterns of biotic and abiotic variables in the ca. 1 km wide river marginal wetland. The distribution of vegetation types follows a certain pattern: Glycerietum maximae close to the river, followed by respectively Caricetum gracilis and Caricetum elatae and finally Calamagrostietum strictae at the margin of the river plain. Species richness increases and standing crop decreases from the river towards the margin. The elevation of the ground surface gently rises with increasing distance from the river; flood duration and flooding depths in spring decrease in the same direction. Groundwater tables in summer are less correlated to the elevation gradient but tend to be closer to the ground surface at the valley margin. These differences also lead to a higher amplitude close to the river and a fairly stable water table far away from the river. Concentrations of major ions and ammonium increase towards the river. Nutrient release rates are also higher closer to the river. Absence and presence of species and the variation in species composition of the vegetation was explained best by flood variables; variables from groundwater explained much less of the variance. Variations in standing crop and nutrients in standing crop corresponded better to the rates of nutrient release from the organic soil than to nutrient concentrations in the soil water. We concluded that river hydrology and nutrient release from the soil are clearly related to vegetation composition, species richness and productivity of the vegetation.