Differences in Riparian Flora between Riverbanks and River Lakeshores Explained by Dispersal Traits

Rivers and river lakes, i.e., lakes that are part of river systems, provide distinctive geomorphic and hydrologic conditions for riparian plants. This variation between lotic and lentic water bodies results in various environments for establishment and growth of plants, but also presents a range of conditions under which plant propagules are transported and deposited along riverbanks and lakeshores. Propagules may be differentially deposited in specific types of fluvial settings depending on their buoyancy. Among riverbanks and lakeshores, we predicted that lakeshores would capture the highest proportion of long-floating seeds, because short floaters will sink before reaching the shoreline. We also predicted that turbulent reaches would receive the highest proportion of short-floating seeds, because these are the sections where buoyancy is least important. Tranquil reaches of rivers were predicted to be intermediate between turbulent reaches and lakes with respect to their efficiency in capturing of long- and short-floating seeds. We tested whether these differences were mirrored in the floras of the different reach types, using reaches of a range of current velocities in 67 sites along a free-flowing river system in northern Sweden. We also related floristic differences to environmental conditions in each of the three reach types (turbulent sections, tranquil sections, and river lakes). The proportions of species with long-floating propagules, herbs, and aquatic species were higher along river lakeshores and tranquil reaches than along turbulent reaches, and the opposite was true for species with short-floating propagules, dwarf shrubs, graminoids, and terrestrial species. The patterns remained when the largest species groups (herbs and graminoids) were tested for floating ability, i.e., the highest proportions of long-floating herbs and graminoids were found in lakes, and the lowest in turbulent reaches; tranquil reaches were intermediate or similar to lakes. Exposure to waves and currents and peat cover explained most of the variation in proportions of species with different floating abilities. We suggest that reach type is a good indicator for predicting the composition of riparian vegetation, especially the proportionate representation of species with various dispersal traits. These results suggest that there is a functional relationship between dispersal traits, channel characteristics, and plant community composition.