Effects of increased flooding on riparian vegetation: Field experiments simulating climate change along five European lowland streams.

In many parts of the world, the magnitude and frequency of cold-season precipitation are expected to increase in the near future. This will result in an increased magnitude and duration of winter and spring flooding by rain-fed streams and rivers. Such climate-driven increases in flooding are likely to affect riparian plant communities, but future vegetation changes are hard to predict due to current lack of data. To fill this knowledge gap, we experimentally modified the hydrology of five streams across three countries in north-western Europe during late winter/early spring over a period of 3 years. We assessed the responses in riparian plant species richness, biomass, plant-available nitrogen and phosphorus and seed deposition to increased flooding depth (+18 cm on average at the lowest positions along the riparian gradient) and prolonged flooding duration (6 weeks on average). After 3 years of increased flooding, there was an overall decline in riparian species richness, while riparian plant biomass increased. Extractable soil nitrogen and phosphorus also increased and are likely to have contributed to the increased biomass. Increased flooding resulted in the arrival of more seeds of additional species to the riparian zone, thereby potentially facilitating the shifts in riparian plant species composition we observed. The results of our concerted experimental effort demonstrate that changes in stream riparian plant communities can occur rapidly following increased winter flooding, leading to strong reductions in plant species diversity.

Can a priori defined reference criteria be used to select reference sites in Danish streams? Implications for implementing the Water Framework Directive.

An important step in the implementation of the Water Framework Directive is to define and characterize the natural status, designated as the reference condition (RC). Here we present the results of a type-specific screening for reference stream sites in Denmark using two different approaches. First, we performed a screening applying physicochemical, hydro-morphological and pressure criteria at the catchment, reach and site level of a total of 128 sites a priori selected by the regional water authorities as representing the best sites in Denmark. Second, we performed a GIS screening of all mapped streams in Denmark (26,000 km representing app. 90% of all Danish streams) using solely land use characteristics in the catchment area to target the search for larger stream sites to comply with the WFD requirements of type-specificity. Among the 128 sites we did not find any that fulfilled all criteria applied at the catchment, reach and site level using recommended RC threshold values and only three sites using threshold values that were less strict. Similarly very few km (<1%) of the GIS screened streams fulfilled catchment land use criteria, suggesting that the potential of identifying RC sites in Denmark is very limited. The lack of success in the screening process clearly demonstrates a need for alternative methods to establish RC for Danish streams. We propose a combined approach that includes the development of a guiding image for RC for all the stream biota needed to evaluate the ecological quality. This guiding image should be based on historical data, expert knowledge and investigations in streams situated in countries that are subjected to less intense land use and, at the same time, share both topographical and climatic similarities with Denmark, e.g. the Baltic countries.