Sensitivity to elevated salinity in coastal dune plants.

Coastal dune plants provide critical ecosystem and cultural services, which are severely threatened by ongoing climate change and sea-level rise. Coastal habitats account for a disproportionate extent of land on islands, emphasizing the importance of these ecosystems for island biodiversity. We investigated salinity tolerance in a diverse pool of 19 native and invasive plant species from Hawai'i's coastal dunes for insights into their vulnerability to sea-level rise. Salinity tolerance was investigated experimentally in seedling and juveniles by treating plants with artificial seawater for 3 weeks, followed by a 2-week freshwater recovery period. For mechanistic insights, stomatal conductance and leaf chlorophyll content were measured before, during, and after seawater treatment. Salinity tolerance was highly variable among species. The least tolerant species experienced 100% mortality and up to 95% reductions in growth in seawater compared to freshwater conditions. The most tolerant species grew more in seawater, consistent with a halophytic strategy. Most species had intermediate salinity tolerance, with generally high survival and variable reductions in growth under seawater treatment. Phenotypic plasticity in stomatal conductance and chlorophyll content was widespread, and species that maintained the highest levels of stomatal conductance and leaf chlorophyll content under seawater had the greatest biomass tolerance. Despite growing in close proximity to the ocean, Hawai'i's coastal dune plants are highly variable in salinity tolerance, and sea-level rise is likely to be a major threat, leading to reduced seedling establishment and population declines in some species, with cascading effects on associating biodiversity, emphasizing the need for urgent conservation actions.