FLORISTIC DIFFERENCES BETWEEN RIPARIAN CORRIDORS OF REGULATED AND FREE-FLOWING BOREAL RIVERS

Riparian ecosystems are central elements in many landscapes because of their shape, diversity and function as filters and corridors. They are also among the environments most disturbed and threatened by humans. Human-induced changes in riparian vegetation and flora were assessed by comparing free-flowing and regulated rivers in northern Sweden. Although riparian vegetation structure is rather uniform along free-flowing rivers, it varies distinctly along regulated rivers because of different water-level fluctuations in storage reservoirs, run of the river impoundments and unimpounded but regulated reaches. The total species richness of vascular plants per river in the riparian zone was similar between four free-flowing and four regulated rivers in northern Sweden. However, species richness per 200 m long site was considerably lower, and almost all groups of species were more species-poor per site in the regulated rivers due to perturbations caused by regulation. Both free-flowing and regulated rivers showed an increase in the species richness of ruderal plants towards the coast. In contrast, the species richness of natural plants showed different longitudinal patterns in the two types of rivers, and differences were largest along the middle reaches of the rivers. The reasons why responses in vegetation and flora to human perturbation varied downstream along regulated rivers are not known, but factors such as different disturbance patterns, irregular distribution of remnants of former riparian soils and vegetation and differences in regional plant species richness and plant dispersal along the river corridor may be important.