Abstract: Aims Supratidal plant communities fulfil a vital role in coastal protection, but despite an increased likelihood of seawater flooding resulting from anthropogenic climate change, we understand little about how tidal inundation affects these habitats or interactions between their component species. Our aim was to determine how three common coastal grassland species responded to simulated seawater flooding and how subsequent changes to their ecophysiology, growth and survival might affect plant–plant interactions in mixed assemblages.
Methods Seeds of three widely distributed European coastal grassland species (Leontodon autumnalis Asteraceae, Plantago lanceolata Plantaginaceae and Trifolium pratense Fabaceae) were collected from a coastal grassland site in South West England. In Experiment 1, we quantified changes in leaf ion (K +, Na +, Cl ?) concentrations as a response to short-duration (0, 2, 8 or 24h) immersion in seawater of the root-zone before monitoring longer-term effects on plant survival and growth. In a second experiment, we examined community-level responses by subjecting mixed assemblages of all three species to seawater immersion for (0, 12, 24 or 96h).
Important findings When grown individually, one species (Trifolium) had markedly reduced survival with increasing soil immersion time, but a consistent decline in plant growth for all species with flooding duration was most likely linked to osmotic and ionic stresses caused by salt ion accumulation. In mixed assemblages, all species suffered increased mortality and reduced growth following seawater flooding, although the relative contribution of one species (Leontodon) to total biomass increased in flooded microcosms. We thus demonstrate a number of species-specific responses to simulated seawater flooding and show that when grown together, interactions between plants are altered as a consequence. We argue that variation in the responses of component plant species will dictate how coastal plant communities respond to, and recover from, expected changes in sea levels and transient floods following storm surge events. Such information is vital in order to predict future impacts of seawater floods on supratidal vegetation.

Key words: climate change, salinity, sea-level rise, soil waterlogging, storm surge, microcosm