A functional characterization of bioengineered plant communities along riverbanks

The extent to which soil bioengineering preserves the ecological functions of riverbank vegetation beyond erosion control remains largely unexplored. This study compared the functional response of plant communities on soil-bioengineered riverbanks with those on natural and hard-engineered (ripraps) riverbanks across 124 sites in Quebec, Canada. We hypothesized that the Community Weighted Means (CWM) of 10 functional traits and three ecological attributes relevant in a riparian context and based on 356 plant taxa would be similar between bioengineered and natural sites, while functional diversity measured as richness (FRic) and dispersion (FDis) would be higher in bioengineered sites due to the diverse conditions (substrate, microhabitats). Contrary to expectations, we found trait-specific results. Compared to ripraps, natural sites were dominated by plants that were taller and had lighter seeds, as well as higher dispersion in specific leaf area, height, and seed mass, higher FRic, more dispersion of propagules by water and less by animals, and more plants with vegetative reproduction, shade tolerance, wetland-affinity, native, and fewer ruderal species. Bioengineered sites had intermediate mean trait values and dispersion, and only small differences among the three techniques applied (pure, keyed, and mixed) regarding the proportion of living and mineral material. Consistent with a previous study that used the same set of sites, we found higher plant cover, species richness, and diversity in natural sites compared to ripraps, with bioengineered sites falling in between the two. Bioengineering can partially reproduce natural riparian plant communities both taxonomically and functionally. Practitioners should consider functional diversity in their designs.