Climate change is a global threat. For island plants with narrow distributions and limited habitat to track suitable conditions, climate change may be particularly devastating, contributing to already high rates of endangerment and extinction. Coastal dune plants face increasingly high salinity due to sea level rise and reduced precipitation as a consequence of global climate change. Although coastal dune plants are predicted to have evolved some tolerance to salinity, whether they can tolerate elevated levels beyond historical levels is largely uncertain, particularly on islands. Using experimental approaches, I investigated the survival and growth of more than 20 coastal dune plants under freshwater and artificial seawater conditions. Seawater effects varied dramatically among species, with some showing elevated growth as expected for halophytes, and other species experiencing 100% mortality. Across all species, salinity tolerance increased from the seedling to juvenile stage, highlighting the sensitivity of establishing plants to salinity stress. Functional trait analyses provided mechanistic insights to species variability. While all species expressed a decrease in stomatal conductance during seawater exposure, the recovery of stomatal conductance to levels comparable to freshwater conditions significantly predicted tolerance in terms of growth. Specifically, species with persistent reductions in stomatal conductance, even following a three-week freshwater recovery period, had the greatest reductions in growth, while species that recovered full stomatal conductance achieved comparable growth to control plants. These results illustrate the importance of phenotypic plasticity in physiological function as a mechanism of salinity stress tolerance in coastal dune plants. Species with weak phenotypic plasticity are at greatest risk of decline due to sea level rise, particularly during seedling establishment.
