Establishment of dune-building grasses in relation to beach moisture, burial and erosion

Globally, sandy shorelines are among the ecosystems most affected by climate change, driven by rising sea levels and increased frequencies of extreme events. More frequent extreme storm surges increase the risk of flooding, potentially affecting the livelihood of coastal communities and putting pressure on flood defenses. For coastlines relying on sand dunes, flood safety partly depends on how fast dunes can re-establish on a bare beach. In temperate regions, initial development of coastal dunes oftentimes starts with the establishment of perennial grasses such as marram grass (Ammophila arenaria) and sand couch (Elytrigia juncea).   We assessed the establishment probabilities of dune-building grasses in relation to beach soil moisture and bed level change (burial and erosion) under dynamic field conditions. We selected four beaches with a wide range in environmental conditions and sufficient space for vegetation establishment (widths greater than 200 m). 180 blocks within a randomized block design were created, and locally collected plant material was introduced within small 50 x 50 cm quadratic plots. We randomly introduced seeds and rhizome pieces from marram grass and sand couch, leaving one plot untreated to account for natural establishment. All of the plots were re-visited three times throughout the growing season of 2022. Each time, soil moisture (WET-2 sensor, Delta-T Devices), the height of the plots (real-time kinematic positioning system, Topcon), and the number of emerged shoots were recorded. We found that natural establishment occurred throughout the entire growing season, with environmental conditions directly after arrival playing an important role. The establishment probability was significantly influenced by soil moisture. We also found a positive effect of bed level change on establishment when erosion was less than –3 cm and burial was no more than 11 cm. Moreover, we found that locations independent from groundwater (heights > 2.81 m, mean moisture = 5.2%) exhibited significantly lower establishment probabilities during periods of drought. Our results suggest that the recovery and resilience of coastal dunes depend not only on the nature of the extreme event but also on the environmental conditions following a disturbance by storms. While establishment does not appear to be temporally constraint, coastal dunes likely recover quicker when dune building grass establishment is promoted by conditions of moderate burial and sufficient beach moisture. On the other hand, extreme bed level changes and extremely dry conditions following storms likely lead to slower recovery. This might pose a challenge for coastal communities, considering these conditions could arise more frequently in the future due to climate change.