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Coastal dunes occur worldwide along sandy shorelines where they provide essential services such as flood protection, drinking water supply, and recreation. In my thesis, I explored the dynamics of dune formation and development, focusing on the interactions between ecosystem engineering dune grasses, environmental factors, and landscape morphology. First, I compared the clonal expansion strategy of two co-occurring dune grass species (marram grass and sand couch) along the Northwestern European coast, which determines their dune building capacity. I discovered that sand couch grows dispersed, resulting in low and broad dunes, while marram grass generally grows patchier and adapts flexibly to environmental conditions. Next, I investigated how established dune grasses influence the establishment of other dune grasses through the formation of embryonic dunes. Firstly, by examining how marram grass affects the establishment of conspecifics, revealing a negative effect of dune formation due to increased sediment dynamics and elevation. Then, I determined the relationship between sand couch, its landscape characteristics, and the establishment of marram grass, uncovering indirect negative effects of sand couch on marram grass survival through landscape formation. Finally, I determined how extreme conditions, simulated as heat and drought, influences the interaction between sand couch and marram grass. Despite the expected impact of extreme conditions, the presence of sand couch emerged as the predominant factor reducing marram grass growth. My research emphasizes the importance of biophysical interactions at different scales in dune formation. Understanding these interactions is crucial for effective management and restoration efforts.

Vegetated coastal ecosystems such as salt marshes, dunes and seagrass meadows occur at the land–sea interface—a dynamic environment typified by harsh growing conditions. These ecosystems are known as biogeomorphic landscapes because their functioning depends on biophysical interactions by which organisms engineer landforms to their own benefit. The strength of such biogeomorphic feedbacks depends on plant traits, such as stem flexibility and shoot density. Recent work demonstrated that dune grasses with similar morphological traits can build contrasting landscapes due to differences in their spatial shoot organization. However, in contrast to dune grasses that trap and stabilize sand particles in aeolian landscapes, flow attenuation in aquatic environments can generate scouring around plant stems and cause uprooting, leading to establishment thresholds for young plants. Yet, it remains unknown how findings from aeolian landscapes translate to aquatic systems and how young clonally expanding plants in hydrodynamically exposed conditions overcome these establishment thresholds by optimizing shoot placement. Here, we measured shoot patterns of 90 establishing cordgrass patches Spartina anglica at 18 European field sites that cover a broad range of hydrodynamic conditions. Next, we carried out a field experiment to investigate how observed spatial shoot patterns affect plant–sediment feedbacks. Surprisingly, field survey analyses reveal highly consistent clustered shoot patterns, regardless of environmental conditions. Experimental results demonstrate that this clustered pattern minimizes scouring compared to densely clumped organizations typically observed in established patches. Synthesis. In contrast to earlier findings highlighting that establishing dune grasses optimize their landscape engineering capacity via a flexible shoot placement strategy, we find that cordgrass instead follows a fixed strategy that minimizes engineering effects in its early life stages.

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In biogeomorphic landscapes, plant traits can steer landscape development through plant‐mediated feedback interactions. Interspecific differences in clonal expansion strategy can therefore lead to the emergence of different landscape organisations. Yet, whether landscape‐forming plants adopt different clonal expansion strategies depending on their physical environment remains to be tested. Here, we use a field survey and a complementary mesocosm approach to investigate whether sediment deposition affects the clonal expansion strategy employed by dune‐building marram grass individuals. Our results reveal a consistent shift in expansion pattern from more clumped, Brownian‐like, movement in sediment‐poor conditions, to patchier, Lévy‐like, movement under high sediment supply rates. Additional model simulations illustrate that the sediment‐dependent shift in movement strategies induces a shift in optimisation of the cost–benefit relation between landscape engineering (i.e. dune formation) and expansion. Plasticity in expansion strategy may therefore allow landscape‐forming plants to optimise their engineering ability depending on their physical landscape.