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.

Plant species cover was studied in 70 rectangular plots (150m^2) in river networks of two watersheds: San and Wisloka Rivers in southern Poland. To analyze diversity and trait shift patterns along the longitudinal (upstream-downstream) gradient, a synthetic variable "PC1_Longitudinal_gradient" was calculated by takig the first principal component of PCA using environmental variables (Catchment area, Flow length and Strahler river order). For each plot, species cover was calculated and functional and taxonomic diversity indices were calculated using R package FD. The dataset inclue also community weighted means of each plant trait (SLA, plant height, seed mass, Anemochory, Hydrochory, Zoochory, Nitrogen-fixing, Insect pollination, Wind pollination) in the plots, calculated separately for whole communities and for herb layer only.