Riparian zones help maintain freshwater and terrestrial biodiversity, and fulfil many functions for human society. They are also rich in plant species and a major contributor to boreal biodiversity. An important factor shaping riparian zones is the flow regime of the waterbody they connect to. Water acts destructively by scouring away vegetation, and constructively as a source of seeds, re-supplementing vegetation. Riparian vegetation is vulnerable to changes in the flow regime and a deeper understanding of its ecology contributes to riparian conservation.

In this thesis, I have examined how community processes shape riparian vegetation. Specifically, I have studied the importance of dispersal and environmental filtering on different spatial scales, in free-flowing and regulated boreal rivers. Water transports seeds year-round but peak floods are crucial for seed dispersal. Seed deposition varies with geomorphology, but this does not always result in clearly different plant communities. A range of human activities affect boreal riparian vegetation, with flow regulation being the most prominent one. Including landscape features such as tributaries and processes such as dispersal in flow management may however help mitigate its negative effects on riparian zones. 

Riparian zones fulfil many ecosystem functions and are biodiversity hotspots, as they are rich in plant species and facilitate many other species. They are largely shaped by the flow regime, which scours away vegetation and supplies propagules for regrowth. Water-borne seed dispersal, hydrochory, is an important building block of riparian plant communities. These can become richer throughout catchments, as seeds and species may accumulate with distance travelled. Seeds, however, do not get deposited equally across lakes, slow-flowing reaches and rapids, the main geomorphological domains in northern Swedish streams. Their respective types of hydrogeomorphological stress co-determine which seeds establish and come to form the vegetation. 

In this thesis, I have studied how different human activities affect riparian zones and vegetation, and how dispersal and environmental filtering form plant communities in boreal riparian zones. Human activities affect both the flow regime and the local habitat, but how their combined effects affect riparian vegetation is unclear. I used seed trap data, soil seed bank samples and vegetation inventories to study how flow regime and geomorphology affect vegetation composition. Hydrochory occurs year-round in free-flowing rivers, but most seeds are deposited during peak flow. That difference is larger in slow-flowing reaches and rapids than in lakes, especially at greater distance from the waterline. Riparian seed banks can have quite different composition than vegetation. When I compared the two, neither followed clear species accumulation patterns or was linked to geomorphological variation. In a larger catchment, I found higher species richness with increasing stream order, and differences in vegetation composition between a regulated and a free-flowing river were limited. Degree of regulation and tributary remediation may have played a role there, and could be taken into account when designing management for riparian conservation.