River regulation drives shifts in urban riparian vegetation over three decades

Riparian zones are among the ecosystems with the highest susceptibility to human impacts, and the main drivers influencing their function are river regulation and urbanization. In this study we aimed to assess shifts in species composition of riparian vegetation over three decades and discuss effects of river regulation (expressed as limitation of episodic flooding) on species and functional diversity of the understory of riparian plant communities in an urban landscape before and after dam building. We compared riparian vegetation in 59 pre-dam plots assigned to four types of riparian vegetation, initially investigated in the years 1970–1984 with 50 post-dam plots, sampled by us in 2011-2018. We explored changes in the importance of competition in shaping plant species composition, using a set of ecological indicator values, plant functional traits and components of functional diversity. Results of the study suggest that river regulation drove compositional shifts in plant communities surveyed, probably caused by displacement of species with strategies allowing occurrence under high stress levels by plants with higher competitive abilities, including ornamental alien species. We revealed increasing magnitudes of compositional shifts along an environmental gradient of riparian vegetation with the largest changes observed in pioneer forest communities. Implementing natural flows with consideration for relationships between plant strategies and their key life-history traits, and aspects of the flow regime, may be a proper tool for restoration and maintenance of riparian vegetation as buffer zones in urban landscapes.