Differences in species richness and site preference of the vascular plant flora of river shores in northern Sweden in relation to the degree of natural habitat disturbance

Samples from sixty-two geolittoral sites along two rivers in northern Sweden were analysed in order to quantify possible differences in the vascular plant flora related to differences in the type of natural disturbance of the habitat (more disturbed: rapids (water flow) and lakes (wave action); less disturbed: slow-flowing stretches). We found that the mean species richness was lower along the slow-flowing stretches, than along the rapids and lakes, which were quite similar. When the geolittoral was subdivided into the three vertical belts, the floras of both the upper and middle belts exhibited this same pattern, whereas the species richness of the lower geolittoral belt was also low along the stretches of rapids. Along both rivers, the steepest gradient in mean species richness was recorded on the shores bordering rapids. The highest numbers of 'constant' species (frequency (F) > 60%); and of those which showed a statistically significant (probability (p) > 0.05) preference for a certain type of stretch in at least one of the three geolittoral belts, or in the entire geolittoral, were found along the rapids, in comparison to the flora of the slow-flowing stretches. The flora of the lake shores also comprised a large number of specific species compared to that along the slow-flowing stretches. The predominant species along the above-mentioned stretches reflect both phytogeographical and habitat differences.