The role of vegetation in Building with Nature pilot sandy foreshore Houtribdijk

The natural situation of foreshores in shallow lakes with a gradual slope, sometimes a sandy beach and a well-developed riparian zone (submerged macrophytes and helophytes) is quite rare in the Netherlands. Still, the various ecosystem services that natural foreshores offer, e.g. attenuation of incoming wave action and habitat for fauna, gives inspiration to innovative solutions. Especially the wave attenuation service has received attention as a viable alternative to conventional dike reinforcement. To increase our understanding of a constructed foreshore, we applied 70.000 m3 of sand along a 400 m long stretch of the Houtribdijk (Markermeer) during the summer of 2014, creating a foreshore with a gradual slope (1:30). The pilot was divided into four sections, two of which have an additional top layer of a sand-clay mixture to promote vegetation development. Each section was partially planted with selected wetland species, related to the possibilities and limitations to construct the desired abiotic environment. The vegetation is expected to deliver two ecosystem services related to flood protection: roots will prevent erosion of the sand, and aboveground biomass will dissipate wave energy. One of the chosen species is Common reed (Phragmites australis). To further enhance our understanding of the success of reed, we study the genetic diversity of Common reed in the Netherlands to identify if specific genotypes or phenotypes can be associated with its success at a given habitat. We present the first results of this genetic analysis and give advice on how this species can be promoted in newly constructed foreshores.