Dune establishment drivers on the beach: narrowing down the window of opportunity

Abstract. Coastal dune development is typically initiated by the interaction between recently established dune-building vegetation and sediment transport processes. Narrowing down biotic and environmental conditions needed for vegetation establishment could improve predictions of dune initiation, yet obtaining such data on a meaningful spatial scale has proven to be challenging.

We investigated grass establishment and dune initiation across a range in environmental conditions at four beach sections in the Netherlands. To understand spatial patterns of spontaneous establishment, we mapped grass seedling occurrence in July 2021 in 1899 plots. To explore the role of environmental drivers on grass establishment and ensuing dune initiation, we conducted an establishment experiment using 750 plots. We introduced seeds and rhizome pieces of Elytrigia juncea and Ammophila arenaria and monitored establishment success (shoot emergence, survival and shoot density), dune initiation and environmental conditions (soil moisture, salinity and beach bed level change) between February 2022 and March 2023.

Field observations in 2021 showed that 69 – 84 % of seedlings occurred close to adult dune-building grasses, suggesting limited dispersal of diaspores, or alternatively, strong positive biotic feedback during the seedling stage. Establishment of introduced seeds and rhizomes peaked at locations with high soil moisture (at 15 – 20 %), low salinity ( < 340 mS/m) and low sediment dynamics ( –2 - +5 cm bed level change). Here, the highest dune initiation probabilities were found, with the highest probability being associated with substantial shoot emergence (330 shoots/m²). Moreover, dune initiation was associated with the middle section of the beach, characterised by moderate slopes and elevation and sufficient beach width.

Our findings indicate that the window of opportunity for dune initiation is smaller than for plant establishment, as it not only depends on the arrival of plant material but also favorable environmental growing conditions. Our results can be applied to better predict dune initiation and development on the beach.