River environments are complex and dynamic ecosystems, and provide valuable ecosystem services such as clean water. The species rich riparian vegetation performs many important ecosystem functions such as reducing erosion and filter inputs from upland areas. Regulated flow regimes have decreased riparian plant species richness, cover and plant performance. To restore the integrity of riparian ecosystems, mitigation measures such as re-regulation of water-level regimes toward more natural seasonal fluctuations may be needed. The aim of this study was to assess potential responses of riparian plants to changes in water-level regulation in run-of-river impoundments to better match natural flow regimes. The elevational extent of plant species on riverbanks of two run-of-river impoundments in the Ume River were surveyed and their probability of occurrence along the gradient of inundation duration was modelled and compared to their distribution in the free-flowing Vindel River. Most species showed similar tolerance to flooding in the Ume and Vindel Rivers. Changes in elevational extent in response to three simulated environmental flow regimes were predicted by using the relationship between plant occurrence and inundation duration. A simulated spring flood and low water levels during the latter part of the growing season is predicted to result in the largest increase in elevational extent, with increases of 70-80% for several riparian species. However, only 47% of the riverbanks along run-of-river impoundments in the Ume River is deemed to be suitable for plant establishment, since many riverbanks are steep and devoid of fine-grained substrate as a result of erosion. / Älvmiljöer utgör komplexa och dynamiska ekosystem som tillhandahåller värdefulla ekosystemtjänster så som rent vatten. Den artrika strandvegetation bidrar till många viktiga ekosystemsfunktioner som närings- och giftupptag och till minskad erosion. Vattenregleringen med förändrade flödesregimer har minskat artrikedom, täckningsgrad och tillväxt av strandväxter. För restaurering av strandekosystemen kan omreglering till mer naturliga säsongsvariationer i vattenståndet vara nödvändigt. Den här studien syftade till att förutsäga hur utbredningen av strandväxter längs stränder i vattenkraftsmagasin potentiellt skulle förändras vid användande av miljöanpassade flöden för att mer likna naturliga flödesregimer i outbyggda älvar. Jag undersökte växternas utbredning i höjdled på stranden längs två magasin i Umeälven och beräknade sannolikheten för varje arts förekomst längs strandens översvämningsgradient. Av de arter som förekom i både Umeälven och den närliggande, outbyggda Vindelälven jämfördes växternas utbredningsgränser i respektive älv. De flesta arterna uppvisade liknande översvämningstolerans i Umeälven och Vindelälven. För att förutsäga förändringar i utbredning som respons på tre olika simulerade miljöanpassade vattenståndsregimer, jämfördes arternas översvämningstolerans vid nuvarade vattenstånd med simulerade vattenståndsregimer. En simulerad vårflod och lågt vattenstånd under sensommaren förväntas ge de största responserna i artutbredning med ökningar på 70-80% för ett flertal strandväxter. Endast 47% av älvstränderna i magasinen i Umeälven bedöms vara lämpliga för växtetablering eftersom stora delar av strandsträckorna är branta och saknar finkornigt substrat till följd av erosion.

Models of climate change predict that temperature will increase during the 21th century and the largest warming will take place at high northern latitudes. In addition to warming, predictions for northern Europe include increased annual precipitation and a higher proportion of the precipitation during winter falling as rain instead of snow. These changes will substantially alter the hydrology of rivers and streams and change the conditions for riverine communities. The warming is also expected to result in species adjusting their geographic ranges to stay within their climatic tolerances. Riparian zones and wetlands are areas where excess water determines the community composition. It is therefore likely that these systems will be highly responsive to alterations in precipitation and temperature patterns. In this thesis we have tested the predicted responses of riparian vegetation to climate-driven hydrologic change with a six year long transplant experiment (I). Turfs of vegetation were moved to a new elevation with shorter or longer flood durations. The results demonstrate that riparian species will respond to hydrologic changes, and that without rare events such as unusually large floods or droughts, full adjustment to the new hydrological regime may take at least 10 years. Moreover, we quantified potential effects of a changed hydrology on riparian plant species richness (II) and individual species responses (III) under different climate scenarios along the Vindel River in northern Sweden. Despite relatively small changes in hydrology, the results imply that many species will become less frequent than today, with stochastic extinctions along some reaches. Climate change may threaten riparian vegetation along some of the last pristine or near-natural river ecosystems in Europe. More extensive loss of species than predicted for the Vindel River is expected along rivers in the southern boreal zone, where snow-melt fed hydrographs are expected to be largely replaced by rain-fed ones. With a seed sowing experiment, we tested the differences in invasibility between open wetlands, forested wetlands and riparian zones (IV). All six species introduced were able to germinate and survive in all habitats and disturbance levels, indicating that the tested wetlands are generally invisible. Germination was highest in open wetlands and riparian zones. Increasing seed sowing density increased invasion success, but the disturbance treatments had little effect. The fact that seeds germinated and survived for 2 to 3 years in all wetland habitats indicates that wetland species with sufficiently high dispersal capacity and propagule pressure would be able to germinate and establish here in their respective wetland type. Our results clearly demonstrate that a changed climate will result in substantial changes to functioning, structure and diversity of boreal wetland and riparian ecosystems. To preserve species rich habitats still unaffected by dams and other human stressors, additional protection and management actions may have to be considered.