Recreation impact on establishment of dune building species

Sandy shores serve a multitude of purposes; they protect the inland from flooding, support a high biodiversity, and are recreation hotspots. To what extent these functions can coexist or are mutually exclusive is unclear, especially given increasing stressors such as rising sea levels and urbanization. Knowledge on the trade-offs between these functions is important when designing these areas and nature-based solutions to ensure the desired results. We investigated the effect of recreational pressure on the establishment of two common dune building grass species (Ammophila arenaria and Elytrigia juncea). We conducted a field introduction experiment with seeds and rhizomes of both species along increasing distance to a beach entrance. We established a total of 300 plots, following a randomised block design with 4 factorial treatments (species * type diaspore) and 60 replicates for two beaches on the Dutch barrier Island of Terschelling. Plant material was collected from the wild, using local genetic material. Plant seeds were left in their husk to mimic natural dispersal. Plots were georeferenced by means of Real Time Kinematic and left unmarked to enable undisturbed recreation.  

Recreation pressure was assessed by counting the number of people at different beach sections, confirming that anthropogenic pressure increased with distance to the beach entrance. Establishment success was monitored by counting the number of emerged seeds and sprouted rhizomes per plot at regular intervals across the growing season. To control for drivers other than recreation pressure, we also monitored environmental variables, such as the change in beach level. Preliminary results suggest that environmental factors such as erosion and burial are limiting the establishment success for all treatments. Moreover, a positive effect of distance from the entrance on the establishment success of both species can be observed. This is especially clear within the first 100 meters. The strongest effect seems to be for Ammophila arenaria. These preliminary results indicate that both sediment dynamics and recreational pressure play a role in the new establishment of these species on the upper beach. This means that the impact of both should be considered when designing sandy coastal areas.