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The objective of this study was to increase understanding of the processes structuring and controlling the species richness of riparian plant communities. In particular, I examined the unimodal relationship, found in many rivers, between plant species richness and location along the river corridor. The most important finding was that this pattern is dynamic and varies with time, most likely in response to large-scale flood disturbances. I also found that the sensitivity to flood disturbance varied with the environmental setting of the riparian reaches. Turbulent sections of the river retained high species richness, whereas tranquil reaches had significantly lower species richness in years following high and prolonged flooding, compared to a period without extreme flood events. Riparian soils along turbulent reaches are more resistant to oxygen depletion during floods, a factor which is likely to contribute to the maintenance of species richness.

The finding that the species richness pattern varied with time led me to ask which factors control plant diversity along riparian zones. I addressed this question by formulating three contrasting, although not mutually exclusive, hypotheses: (1) longitudinal patterns in riparian plant species richness are governed by local, river-related processes independent of the regional species richness, (2) riparian plant species richness is controlled by dispersal along the river, i.e., longitudinal control, and (3) the variation in riparian plant species richness mirrors variation in regional richness, i.e., lateral control. I found indications of all three types of control, although local factors seemed to fit most of the criteria. Riparian species richness was not significantly correlated to species richness in the surrounding upland valley. It was however significantly negatively correlated to soil pH, a local habitat factor of the reach. The fact that the species richness pattern varied in time, corresponding to the presence or absence of extreme flood events suggest that it is influenced by local disturbance regimes. The potential for control by longitudinal dispersal was found to be highest in the middle reaches of a river. Here, the similarity between upland and riparian vegetation was lowest, and invasibility (germination ability) was highest. Earlier work has shown that regulated rivers have an inverted species richness pattern compared to free-flowing rivers, with lowest species richness in the middle reaches. One potential mechanism behind this could be varying susceptibility to disturbance along the river. I tested this by experimentally disturbing the vegetation, applying the same level of disturbance along an entire free-flowing river. However, the response to experimental disturbance did not vary with location, likely because of a major flood disturbance preceding the experiment.