Summary 1.One of the main targets of river regulation with dams and dykes is the stabilization of highly fluctuating water tables. While there is information about the overall impact of such regulation measures on plant species composition and richness, far less is known about specific species' response patterns to reduced water level fluctuations. 2. The response of 30 common grassland species to soil moisture and water level fluctuations was assessed. Floristic data were collected from the floodplain of the Elbe River, Germany, from 182 plots, 99 within the recent floodplain and 83 in an older floodplain, separated from one another by dykes. Hydrological data were collected weekly over 2 years at 37 water level wells. Using logistic regression, the patterns of species' responses to hydrological regulation were predicted. 3. The majority of species responded significantly to water level fluctuations. Species of high elevation habitats occurred at lower elevations where water level fluctuations were reduced, indicating increasing drought at high elevation habitats. However, species that occurred in floodplain depressions tended to shift from lower to higher elevations to avoid permanent inundation. 4. Almost half of the species showed a significant preference for either highly fluctuating water tables, characteristic of the recent floodplain, or for stable water tables, characteristic of the older floodplain. The probability of their occurrence was either reduced or increased along a gradient of reduced fluctuations. These species' responses could be partly explained by altered flooding regimes, although other factors, such as disturbance and dispersal processes, were also involved. 5. Synthesis and applications . This study demonstrates that reduced water level fluctuations caused by the construction of dams and dykes lead to substantial changes in the spatial distribution of floodplain plant species and in species composition. The methodology reported here allows accurate prediction of shifts in floodplain vegetation in response to human-induced alterations in floodplain landscapes. This can be used as a tool to assess river regulation measures and for floodplain restoration purposes, such as dyke relocations.

It is known that various poplar species and cultivated poplar hybrids have the potential to interbreed and produce fertile offspring. Conservation strategies for the genetic resources of the endangered Eurasian black poplar (Populus nigra L.) thus rely on a monitoring which enables the identification and verification of the pure species status. At the same time, the risk of hybrid dispersal and introgressive gene flow has to be estimated. In the present study a combination of two molecular markers, one from chloroplast DNA and the other from nuclear DNA, was applied to evaluate a large P. nigra population on the Elbe River. Hybrid clones of P. · canadensis are scattered within this population and also occur as plantations in the surrounding landscape. By means of the DNA markers the taxonomic status of 208 adult trees in the population and 140 young poplars along the riverbank was monitored. From the analysed young poplars, almost 20 percent were found to exhibit at least one of the two P. deltoides or P. · canadensis diagnostic alleles or genotypes, respectively. Possible vegetative spreads of F1 hybrids and precedent mating scenarios are discussed. Most interestingly we found clear evidence for a small number of backcross hybrids where P. · canadensis acted as pollen donor. This case had long been debated and thought to be less probable, so far.

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The effects of floodplain fragmentation by dykes on grassland vegetation were evaluated through field studies along the Middle Elbe River (federal states Saxony-Anhalt and Brandenburg, Germany). Plant species composition was examined in 206 sites between 1996 and 1998 in the entire floodplain, which can be divided into the floodplain types 'recent floodplain', 'older floodplain' (which is separated from the recent one by dykes) and the 'margin of the floodplain' (which is the part of the older floodplain that forms the boundary and is furthest from the river). Dynamics in hydrology were examined weekly between November 1996 and February 1999 with the help of 40 water level wells which were installed near the studied sites. The hydrological parameters 'average water level', 'average groundwater level', 'flooding duration', 'flooding depth higher 50 cm above soil surface' and 'standard deviation of the water level line' were calculated to characterize the considered floodplain types and to relate species composition to hydrology. Furthermore several parameters of current management of the vegetation were recorded to evaluate the importance of land use versus hydrology for floodplain grasslands.Detrended and canonical correspondence analysis (DCA, CCA) were used to identify major environmental gradients governing the vegetation and to determine if there is a relationship between the different locations within the floodplain, variation in species composition, and gradients of measured environmental variables.The results indicate that the vegetation is closely related to a combination of water level fluctuations, which are different among the floodplain types, and soil moisture, while type and intensity of current management are not important in this context.The results of contingency tables underline the significance of dykes for the occurrence and absence of individual species among the floodplain types. The observed patterns can also be explained by the different hydrological properties of the recent and older floodplain as the results of logistic regression reveal. Furthermore, disturbance and dispersal processes and their alteration by dykes have to be taken into account to explain the pattern of species occurrence.Partial ordination detected residual differences in vegetation among the different floodplain types after accounting for the effects of the measured environmental variables. Grain size distribution is discussed as a further factor that might influence species composition.