Co-occurring intertidal ecosystem engineers with opposing growth strategies show opposite responses to environmental gradients during establishment Version 4

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 vs 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 8 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 appears to be more susceptible to stress from desiccation and silty sediments.