From strandline vegetation to embryo dunes: natural sand retention along a heavily urbanized coast

Problem Statement
Sandy beaches along heavily urbanized coasts, such as the Belgian coast, face increasing pressure from coastal armoring (dikes, promenades), intensive recreation, and mechanical beach cleaning. These interventions often constrain natural dune-building processes. While beach nourishment is widely used to compensate for sediment losses, the extent to which this nourished sand is retained on the upper beach versus redistributed by marine and aeolian processes remains uncertain. The study posits that strandline vegetation—pioneer plants growing on the dry beach—may provide an overlooked mechanism for retaining sand and initiating embryo dune development, yet their potential is often suppressed by human disturbance.

Methodology
The authors employed a multi-step approach combining field surveys, remote sensing, and spatial modeling to assess the potential of four pioneer dune plant species (Cakile maritima, Calamagrostis arenaria, Elymus farctus, and Salsola kali) to establish and accumulate sand.

1. Data Collection: Field surveys were conducted annually from 2017 to 2023 along the entire Belgian coastline. Observations focused on the "dry beach" zone (between the springtide high waterline and the foredune toe). Data included species identity and abundance/cover, recorded as georeferenced points.
2. Reference Zones: To isolate abiotic constraints from human disturbance, the analysis was restricted to 10 Regions of Interest (ROIs) characterized by low trampling and absent mechanical cleaning.
3. Environmental Predictors: Two key abiotic drivers were derived from LiDAR data:
   • Elevation relative to Springtide High Water (SHW): Calculated using spring DTMs and interpolated SHW data.
   • Annual Sand Dynamics: Calculated as the difference between fall-winter and spring DTMs to quantify net erosion or accretion.
4. Statistical Modeling: A two-component spatial modeling framework (using R-INLA) was applied:
   • Occurrence Model: A binomial model using presence-pseudo-absence data to estimate the probability of species establishment based on abiotic conditions.
   • Abundance/Cover Model: A zero-inflated negative binomial (ZINB) model using presence-only data to estimate expected vegetation development (cover) across environmental gradients.
5. Sand Accumulation Quantification: Independent field measurements of embryo dunes were taken in 2023–2025 using LiDAR (iPhone 13 Pro) and orthophotos to quantify the relationship between vegetation cover and sand volume. Linear models (constrained through the origin) relating sand volume to the square root of vegetation cover were fitted for each species.
6. Upscaling: The study projected potential occurrence and abundance across the entire Belgian coastline (including urbanized zones) based on the low-disturbance models. These predictions were then combined with the species-specific sand accumulation rates to estimate total potential sand retention under two scenarios: dispersed individuals and aggregated patches.

Key Results

• Abiotic Suitability: Predicted occurrence was widespread for several species, suggesting that abiotic gradients (elevation and sand dynamics) are not the primary constraints on potential establishment across large parts of the coast.
• Abundance vs. Occurrence: While occurrence was relatively insensitive to sand dynamics, expected abundance/cover showed strong species-specific responses. Most species exhibited increased abundance with higher sand accretion, indicating that sediment dynamics primarily influence post-establishment development rather than initial establishment.
• Spatial Patterns: Under low-disturbance assumptions, Cakile maritima showed broad suitability across the coast. Elymus farctus and Calamagrostis arenaria showed higher expected abundance near the upper beach/dike, while Salsola kali showed high but spatially consistent abundance. The contrast between these high potential predictions and the low observed vegetation on urbanized beaches highlights the constraining role of human disturbance.
• Sand Retention: A positive relationship was confirmed between vegetation cover and local sand accumulation for all four species. Cakile maritima was estimated to contribute the largest share of potential sand retention, followed by Salsola kali, Elymus farctus, and Calamagrostis arenaria.
• Volume Estimates: Total potential sand volumes retained by embryo dunes were estimated at approximately 376,000 m³ (patch scenario) to 552,000 m³ (individual scenario). These volumes are of a similar order of magnitude to the sediment volumes annually supplied via beach nourishment.

Significance and Claims
The paper claims that even along a heavily urbanized and nourished coastline, abiotic conditions can support strandline vegetation and embryo dune initiation where disturbance is reduced. The authors argue that management actions—such as limiting trampling, adapting beach cleaning, and protecting strandline vegetation—could enhance the retention of nourished sand and support nature-based coastal defense.

Crucially, the study does not propose replacing engineered interventions. Instead, it suggests that strandline vegetation may increase the efficiency with which available sediment is retained within the beach–dune system. By facilitating the transition from open beach to embryo dunes, these natural processes can complement existing coastal management strategies, potentially reducing sediment loss to the sea and the need for frequent re-nourishment. The authors emphasize that their estimates represent the potential contribution of early strandline vegetation to the initiation of dune formation, rather than predictions of long-term dune growth or permanent foredune structures.