Mid Sweden University

Fragmentation and flow regulation of rivers by large dams are known to obstruct the longitudinal dispersal of waterborne plant propagules between impoundments, and to affect plant community composition. However, even several decades after a dam has been built, impoundments may still have a relatively species-rich riparian flora. We hypothesized that free-flowing tributaries act as the major gene pools for such impoundments, thus alleviating the fragmenting effect large dams have on the main channel. The importance of tributaries as seed sources was tested by releasing wooden seed mimics in three different-sized (0.22-6.93 m3 s-1) tributaries of an impoundment in the Ume River in northern Sweden. In each tributary seed mimics were released, during the spring flood peak, from three points approximately 1, 2 and 3 km upstream the outlet in the impoundment. The importance of a tributary as a seed source increased with tributary size. Of the 9000 released seed mimics 1.5 % reached the impoundment; 1.2 % of the 9000 originated from the largest tributary and 0.3 % from the middle-sized one. The smallest tributary retained all its mimics.