Evaluating the semi-mangrove traits of coastal shrubs co-occurring with Hibiscus hamabo under salinity–waterlogging stress

Coastal plants often experience salinity and waterlogging simultaneously, yet their combined effects are rarely tested. In a 35-day greenhouse experiment on Jeju Island, we exposed Hibiscus hamabo , Paliurus ramosissimus , Litsea japonica , and Vitex rotundifolia to salinity, waterlogging, and salinity–waterlogging. We quantified growth, deterioration (mortality, survival time, defoliation, visible damage), leaf gas exchange (net CO₂ assimilation, Aₙ; intrinsic water-use efficiency), and Na⁺ and K⁺ contents. Due to the small size of individuals, Paliurus ramosissimus did not yield sufficient material and was excluded from gas-exchange and ion analyses. Salinity–waterlogging caused the strongest deterioration in all species and generally elevated Na⁺/K⁺ ratios relative to single stress. H. hamabo initiated decline earliest but sustained lower mortality and longer survival, maintained Aₙ near pre-treatment despite reduced stomatal conductance, and retained K⁺, particularly in roots. By contrast, L. japonica and V. rotundifolia showed near-cessation of photosynthesis under salinity; L. japonica exhibited root-centric dysfunction under waterlogging with concurrent declines in root Na⁺ and K⁺. We infer that tolerance in H. hamabo reflects coordinated ion homeostasis (notably K⁺ retention) and stomatal–nonstomatal adjustments that stabilize Aₙ, likely aided by an abscission-mediated “salt storage and disposal” strategy. Co-occurring shrubs display partial or organ-specific responses insufficient to withstand the combined stress. These results identify H. hamabo as a plausible semi-mangrove candidate for future range expansion and coastal planting under warming climates and highlight the need to consider differential sensitivities to hypoxia and ionic toxicity in species selection.