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Coastal vegetated ecosystems including mangroves, seagrasses, and salt marshes are often shaped by positive plant–environment feedbacks. Plants improve their own living conditions with increasing patch size and density by attenuating hydrodynamics and stabilizing sediments. As these habitat modifications are critical for survival and growth, the positive density-dependent nature of these feedbacks can lead to establishment thresholds for young plants in absence of mature conspecifics. Although feedback strength is known to depend on hydrodynamic exposure and plant traits (e.g. stiff versus flexible stems), it remains unclear how 1) opposing morphological plant traits affect establishment in contrasting environments, and 2) whether trait plasticity influences establishment success. Here, we investigate this by transplanting two tidal species with opposing growing strategies – Spartina anglica forms tussocks of stiff stems while Zostera noltii forms patches of stress-avoiding flexible shoots – from two different donor sites in eight experimental locations. Results show that the survival and growth of both species was most successful at field locations with diverging environmental characteristics, while overall survival was highest for Z. noltii. Mainly, S. anglica survival was highest at locations with high organic matter and silt content and higher elevation relative to the tidal amplitude. In contrast, Z. noltii survival was highest at locations with larger grainsize and lower relative elevations. Furthermore, despite initial differences in plant traits between the two donor sites of Z. noltii, we found no effects of donor origin. Contrastingly, we found a significant effect of donor origin on S. anglica growth, even though transplants from the two donor sites showed no initial trait differences. Collectively, these results suggest that the stress-tolerance strategy of S. anglica hampers establishment in exposed conditions, whereas the stress-avoiding Z. noltii a

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.