Along the river: Longitudinal patterns of functional and taxonomic diversity of plants in riparian forests

Abstract

Questions: The River Continuum Concept (RCC) predicts a gradual shift of functional adaptations of organisms 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 also be observed in riparian plant communities. Location: The study was conducted in the upper catchments of the San and Wisłoka Rivers in southeastern Poland. In both catchments, very well preserved riparian forests occur continuously along both rivers. Methods: 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' positions in the catchments. Results: 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. 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 height of herbaceous plants in communities along a longitudinal gradient. There was also a shift in the dispersal syndromes, toward a higher importance of zoochory in the lower reaches. Conclusions: The study shows that plant communities in natural riparian forests show continuous species turnover along the longitudinal gradient that, along with the longitudinal shift of abiotic factors, creates patterns that are known from theoretical predictions.