Functional and taxonomic diversity indices of riparian plants in river networks

1. The River Continuum Concept (RCC) predicts a gradual shift of organisms' functional adaptations along the longitudinal (upstream-downstream) gradient, as well as the maximization of the biotic diversity in mid-reaches. Although this theoretical framework was originally developed for stream macroinvertebrates, we tested whether such a pattern can be also observed in riparian plant communities.

2. We studied the cover of plant species in riparian forests across two river networks. We analyzed the taxonomic and functional diversity indices, as well as community-weighted means of functional traits in relation to the plots' position in the catchments.

3. The observed patterns were largely in line with the predictions of RCC. We discovered a significant decrease in the specific leaf area and an increase in the herbaceous plants' height in communities along a river gradient. There was also a shift in the dispersal syndromes, towards a higher importance of zoochory in the lower reaches.

4. The functional richness and divergence displayed unimodal patterns of increasing values in the mid-reaches. The patterns of taxonomic diversity were similar, but some plots in the lowest reaches were more diverse than expected, forming an additional increase in diversity.

5. The study shows that plant communities in natural riparian forests show high connectivity along the longitudinal gradient, which along with the environmental gradients creates patterns that are known from theoretical predictions.