Within-species variation of seed traits of dune engineering species across a European climatic gradient

Recreation impact on the early establishment of dune-building grasses Elytrigia juncea and Ammophila arenaria on the beach

Sandy shores serve multiple ecosystem services, including recreation. To what extent these services can coexist is unclear, especially given increasing stressors such as rising sea levels and urbanization. We investigated the effect of recreational pressure on the establishment of two dune building grass species representative for European beaches (Ammophila arenaria and Elytrigia juncea). We conducted a field introduction experiment with seeds and rhizomes (diaspores) of both species along an anthropogenic pressure gradient on the upper beach of the Dutch barrier Island of Terschelling. Across two beach sites 300 plots were set out following a randomised block design with 4 factorial treatments (species*diaspore). Local plant material was collected. Plots were left unmarked to enable undisturbed recreation. Establishment success was monitored by counting the number of emerged shoots per plot at regular intervals across the growing season of 2022. To control for environmental drivers, we included the environmental variables: soil moisture, bed level change, and distance to the sea. We found that establishment success increased significantly with longshore distance from the beach entrance, irrespective of species or diaspore type. This effect was especially strong within the first 100 m, where plants did emerge from seeds or rhizomes but progressively died over the summer. Establishment success was further constrained by changes to the beach bed level and distance to the sea. Our results indicate that recreational pressure can constrain dune development on the upper beach. This implies trade-offs between beach functions, that should be considered when designing sandy coastal areas.

Habitat modification by marram grass negatively affects recruitment of conspecifics

Ecosystem engineers alter their environment often benefiting their own survival and growth yielding self-reinforcing feedbacks. Moreover, these habitat modifications have been found to facilitate recruitment of conspecifics for some species, while for others engineering inhibits recruitment. Whether dune grasses facilitate or inhibit recruitment of conspecifics is yet unknown. Here, we investigated how habitat modification by European marram grass (Ammophila arenaria) through embryonic dune development affects recruitment from seeds and marine dispersed rhizome fragments. Specifically, we tested at three locations with different dune morphologies how habitat modification affected natural seed and rhizome presence and shoot emergence from plots in which seeds or rhizome fragments were added. In addition, we investigated how sediment burial (i.e., the main effect of habitat modification by dune grasses) affected germination and emergence in a controlled experiment. Results show that regardless of habitat modification or beach width, seeds and rhizomes were absent in natural conditions. Habitat modification negatively affected shoot emergence from seeds (8 × less) and rhizomes (4 × less) and was negatively related to sediment dynamics. Furthermore, fewer seedlings were found with higher elevations. In controlled laboratory conditions, the highest seedling emergence was found with slight burial (0.5-3 cm); both germination and seedling emergence decreased as seeds were buried deeper or shallower. Overall, habitat modification by marram grass negatively affects recruitment of conspecifics through increased sediment dynamics and elevation. Consequently, storm events or eradication programs that include removal of adult vegetation-which leads to an unmodified system-might benefit new recruitment from seeds or clonal fragments.