Effects of increased drought and flooding episodes on stream riparian vegetation

Stream riparian wetlands, the temporarily flooded zones along streams, form a unique type of habitat for many plant and animal species. These wetlands are globally under threat. This PhD study was conducted as part of the European research project REFRESH and aimed at identifying the main short-term effects of increased summer drought and winter flooding episodes on stream riparian vegetation. Along a climate gradient ranging from Denmark to Spain, I studied the hydrological changes and the vegetation response in stream riparian zones. Increased droughts can be expected to narrow the riparian wetland zone with characteristic species and lead to species losses in the near future. A meta-analysis showed that a drought lasting longer than approximately one month strongly reduces riparian plant biomass and seedling survival. Increased flooding episodes are expected to result in species shifts, leading to an increase or decrease in riparian species richness depending on the nutrient, climatic and hydrological status of the catchment. Adaptations to drought and flooding were shown to be crucial for the survival of riparian plants. Strong reductions in riparian plant species diversity have been observed in only a three-year period. Also, losses of functional diversity in adaptation strategies in riparian plant communities are expected. In order to mitigate the effects of extreme episodes on stream riparian wetlands, preservation or restoration of wide riparian zones along the entire stream corridor should be a priority. A focus on the reduction of nutrients is often needed in order to reach a good water quality for a robust, species-rich riparian system.