Riparian zones are species-rich and functionally important ecotones that sustain physical, chemical and ecological balance of ecosystems. While scientific, governmental and public attention for riparian zones has increased over the past decades, knowledge on the effects of the majority of anthropogenic disturbances is still lacking. Given the increasing expansion and intensity of these disturbances, the need to understand simultaneously occurring pressures grows. We have conducted a literature review on the potential effects of anthropogenic pressures on boreal riparian zones and the main processes that shape their vegetation composition. We visualised the observed and potential consequences of flow regulation for hydropower generation, flow regulation through channelisation, the climate crisis, forestry, land use change and non-native species in a conceptual model. The model shows how these pressures change different aspects of the flow regime and plant habitats, and we describe how these changes affect the extent of the riparian zone and dispersal, germination, growth and competition of plants. Main consequences of the pressures we studied are the decrease of the extent of the riparian zone and a poorer state of the area that remains. This already results in a loss of riparian plant species and riparian functionality, and thus also threatens aquatic systems and the organisms that depend on them. We also found that the impact of a pressure does not linearly reflect its degree of ubiquity and the scale on which it operates. Hydropower and the climate crisis stand out as major threats to boreal riparian zones and will continue to be so if no appropriate measures are taken. Other pressures, such as forestry and different types of land uses, can have severe effects but have more local and regional consequences. Many pressures, such as non-native species and the climate crisis, interact with each other and can limit or, more often, amplify each other's effects. However, we found that there are very few studies that describe the effects of simultaneously occurring and, thus, potentially interacting pressures. While our model shows where they may interact, the extent of the interactions thus remains largely unknown.

Question

Riparian vegetation and seed banks are the foundation of functional riparian zones, yet insight in the processes that explain their composition is limited. We tested three theories fundamental to dispersal and environmental filtering of riparian seed banks and vegetation. Combining these theories, we expected hydrochory to lead to increased species richness downstream in both soil seed bank and vegetation with accumulation restarting after a lake section, and geomorphological filtering to lead to higher similarity of seed bank and vegetation composition locally and within lakes, slow-flowing sections and rapids, respectively, than between them.

Location

Svartån, a free-flowing river in central Sweden.

Methods

We surveyed riparian vegetation and conducted a germination experiment on riparian soil seed bank cores from lakes, slow-flowing sections and rapids. We combined these with trait data on seed dispersal syndromes, floating capacity of seeds and seed longevity. We analysed differences throughout the system with linear models and between process domains with Kruskal–Wallis tests and similarity in community composition with the Horn–Morisita similarity index.

Results

Our results indicated that species richness did not increase downstream and was relatively similar throughout lakes, slow-flowing sections and rapids for both riparian seed banks and vegetation. Seed floating capacity was similar throughout these river sections, too, and seed longevity was higher in seed banks than in vegetation.

Conclusions

Geomorphology and hydrochory were not as important drivers in this area for riparian seed bank and vegetation composition as expected. In the area and on the scale studied here, other local and regional environmental factors and dispersal syndromes are more likely to be determinants of riparian dynamics and composition. Continued studies of riparian seed banks can help improve our understanding of riparian composition and functioning in the future.