Facilitation and constraint: Wave exposure and intraspecific interactions influence mangrove seedling morphology and resistance to dislodgement

Abstract Understanding the factors controlling mangrove seedling establishment is essential for maintaining forests and restoration under changing environmental conditions. While adaptive strategies and plasticity in seedling biomass allocation are expected along hydrodynamic gradients, there is little information on how this is modulated by the interplay between intraspecific facilitation (e.g. wave attenuation, anchoring by roots of adult trees) and competition (e.g. light limitation, space competition). We conducted a field study across 12 sites (in nine estuaries) that spanned a gradient in wave exposure, to examine how mangrove seedling abundance, removal force and biomass allocation varied across three habitat types—mangrove forest, pneumatophore zone and unvegetated intertidal flat. Seedling removal force increased with root biomass and decreased with sediment mud content. Results also indicated strong intraspecific facilitation through adult root anchorage for early establishment of seedlings (where their root biomass <0.2–0.5 g ind −1 ). However, the stabilizing effect of adult root biomass may be counterbalanced by increased light limitation, sediment accretion and competition with conspecifics, as indicated by stem etiolation. Under low wave exposure conditions, seedlings in the unvegetated intertidal flats were characterized by the highest leaf and lateral root biomass, while seedlings in the mangrove forest had the highest stem biomass and lowest stem width to stem height ratio, indicating divergent growth strategies shaped by local facilitation and competition dynamics. As wave exposure increased, all seedlings shifted investment towards below‐ground structures, particularly taproots, as well as towards shorter, thicker stems, indicating morphological convergence across habitats under high environmental stress. Synthesis . Our findings highlight the dual influence of intraspecific interactions and environmental constraints in shaping seedling morphology. While facilitation promotes seedling stability in the mangrove habitat, resource (light) limitation likely drives stem etiolation. Ultimately, wave exposure imposes functional convergence in biomass allocation across habitats. This shift underscores the dominant role of physical stress in structuring early biomass allocation strategies. These insights enhance our understanding of mangrove seedling responses to environmental gradients and inform conservation strategies in dynamic coastal systems.