| Summary: | River regulation cause unnatural water flow patterns which disrupt succession, survival and dispersal of riverine plant communities. Riparian zones in regulated rivers are generally more species poor and have a lower vegetation cover compared to free-flowing rivers. Tributary mouths within the impoundment however, are not only affected by processes in the main channel, but also affected by processes such as sediment dynamics and flooding regime from the tributary which may more reflect a natural regime and may therefore not be affected by hydropeaking to the same extent. Hence, tributary sites and the accompanied tributary mouths may represent hotspots for high diversity plant communities in regulated rivers. The riparian habitat is unique in its interaction with adjacent ecological systems and is therefore considered important for the riverine ecosystem’s ecological functioning. The aim of the study was to evaluate the significance of tributary sites and accompanied tributary mouths as a hotspot for diverse plant communities in regulated rivers. Additionally, geomorphological features were analyzed to access the impact of hydropeaking in sheltered and non-sheltered tributary sites. Data was acquired from sampling sites within the Umeå and Luleå rivers, representing both tributary mouths and non-tributary mouth reaches. Inventories included presence of riparian and aquatic vascular plant species, vegetation cover and soil composition. The results indicate a less extensive impact of hydropeaking in tributary mouths compared to non-tributary mouth reaches. The tributary mouths had a higher species richness, diversity and vegetation cover when compared to that of the non-tributary mouth reaches. This supports the concept of tributary mouths being hotspots for plant diversity. Moreover, the sheltered tributary mouths had a higher species richness than the non-sheltered tributary sites, suggesting tributary shelter as a contributing counter of hydropeaking effects.
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