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The hydrology of riparian areas changes rapidly these years because of climate change‐mediated alterations in precipitation patterns. In this study, we used a large‐scale in situ experimental approach to explore effects of drought and flooding on plant taxonomic diversity and functional trait composition in riparian areas in temperate Europe. We found significant effects of flooding and drought in all study areas, the effects being most pronounced under flooded conditions. In near‐stream areas, taxonomic diversity initially declined in response to both drought and flooding (although not significantly so in all years) and remained stable under drought conditions, whereas the decline continued under flooded conditions. For most traits, we found clear indications that the functional diversity also declined under flooded conditions, particularly in near‐stream areas, indicating that fewer strategies succeeded under flooded conditions. Consistent changes in community mean trait values were also identified, but fewer than expected. This can have several, not mutually exclusive, explanations. First, different adaptive strategies may coexist in a community. Second, intraspecific variability was not considered for any of the traits. For example, many species can elongate shoots and petioles that enable them to survive shallow, prolonged flooding but such abilities will not be captured when applying mean trait values. Third, we only followed the communities for 3 years. Flooding excludes species intolerant of the altered hydrology, whereas the establishment of new species relies on time‐dependent processes, for instance the dispersal and establishment of species within the areas. We expect that altered precipitation patterns will have profound consequences for riparian vegetation in temperate Europe. Riparian areas will experience loss of taxonomic and functional diversity and, over time, possibly also alterations in community trait responses that may have cascading effects on ecosystem functioning.

Plant-environment relationships can be assessed through functional traits, but we have little understanding of how they vary on larger scales due to limited sampling. Using a fine-grained digital elevation model and vegetation survey data from a national monitoring program, we now have the chance to investigate the importance of topographically determined water availability in shaping the functional structure of vegetation of different habitat types across Denmark. Plant community responses to hydrology were detected through community-weighted Ellenberg F values and six community-weighted functional traits. We used mixed-effect models to account for the variability related to unknown site-specific factors such as management regime and regional species pool. Additionally, we evaluated whether we can trust a remote-sensing-based topographically determined water availability index (TWI) that calculates how water accumulates on the surface of the landscape to represent actual hydrology. Remote-sensing-based topographically determined water availability represented actual local water availability as indicated by a positive correlation with community-weighted Ellenberg F values (P \ 0.001), showing that this is an effective method of measuring water availability at large scale. The strength and direction of vegetation-TWI relationships differed between habitat types. Functional responses were also habitat dependent and to a certain degree explained by non-considered site-specific factors which presumably include historical land use and current management. This study contributes to the understanding of plant-water relationships which is highly relevant, as the hydrological regime might change rapidly in the near future with potential prevalence of extremes in the hydrological environment.

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