Narrowing down dune establishment drivers on the beach

Coastal dune development is typically initiated by the interaction between recently established dune-building vegetation and sediment transport processes. Narrowing down the biotic and environmental conditions needed for vegetation establishment could improve predictions of dune initiation, but obtaining such data on a meaningful spatial scale has proven to be challenging. We investigated the establishment of grasses and the initiation of dunes across a range of environmental conditions on four beach sections in the Netherlands. To understand spatial patterns of spontaneous establishment, we mapped the occurrence of grass seedlings in 1899 plots in July 2021. To explore the role of environmental drivers of grass establishment and the 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, dune initiation, and environmental conditions (soil moisture, salinity, and the change in beach bed level) between March 2022 and February 2023. Field observations in 2021 showed that 69 %-84 % of the seedlings occurred close to adult dune-building grasses, suggesting limited dispersal of diaspores or, alternatively, strong positive biotic feedback during the seedling stage. The establishment of introduced seeds and rhizomes peaked in locations with high soil moisture (at 15 %-20 %), low salinity (<340 mS m^-1), and low sediment dynamics (-2 to +5 cm change in bed level). Here, also the highest probabilities of dune initiation were found, with the highest probability associated with substantial shoot emergence (330 shoots m^-2). In addition, dune initiation was associated with the middle section of the beach, characterized by moderate slopes and elevation and sufficient beach width. Our findings indicate that the conditions required for the initiation of dunes are more restrictive than those for plant establishment, as they depend not only on the arrival of plant material but also on favorable environmental growing conditions. Our results can be applied to better predict the onset and development of coastal dunes.