COMPARISON OF SPECIES RICHNESS AND TRAITS OF RIPARIAN PLANTS BETWEEN A MAIN RIVER CHANNEL AND ITS TRIBUTARIES

1 We examined differences in species richness and frequencies of vascu lar plants in the riverbank vegetation between the main channel of the Vindel River system and seven of its tributaries which spanned the sa me biogeographic range. 2 Species richness per site was higher in the main channel than in the tributaries, both as a whole and for many spe cies groups. The proportions of woody plants (phanerophytes and chamae phytes), geophytes, and natural species were higher in the tributaries , while the proportions of hemicryptophytes, ruderals, and short-float ing species (i.e. species unable to float > 1 day) were higher in the main channel. Both types of river had species that were more than twic e as frequent there than in the other category. 3 The main channel had a high species richness at intermediate altitudes whereas the tributa ries had least species at intermediate altitudes. Except for the highe st altitudes, the tributaries also had a generally lower mean species richness than the main channel. 4 Stepwise multiple regression analyse s using 15 predictor variables explained statistically up to 85 % of t he floristic variation in the river system. Mean annual discharge and number of substrates explained most of the variation in five equations each, while peat cover explained most of the variation in four equati ons, and altitude and silt cover in one equation each. Mean annual dis charge, peat cover and silt cover differed between the main channel an d the tributaries and could therefore be responsible for the observed differences.