Salt tolerance during germination identifies native intertidal plant species at risk under increasing salinity with sea level rise

Climate change and sea level rise are causing an increase in salinity in coastal salt marshes around the world. These changes may alter the germination dynamics of some halophytes, threatening their conservation. Our study area (Atlantic coast of the southwestern Iberian Peninsula) is currently being affected by sea level rise, and increases in sediment salinity have been recorded. We studied the germination responses during and after a wide range of imposed salinities (0-600 mM) of 5 Asteraceae species; 3 colonizing salt marshes (Artemisia caerulescens, Aster tripolium and Inula crithmoides) and 2 (Dittrichia spp.) inhabiting adjacent non-tidal environments. In addition, habitat characteristics (plant cover, sediment redox potential, electrical conductivity, pH, water content and bulk density) associated with each marsh species were measured and compared with the spatial distribution of the Asteraceae species in the field and their capacity to germinate at different salinities. The germination syndrome of Aster tripolium and Inula crithmoides was characterized as germinating in salinity ranging from fresh water to 600 mM salinity (36 ppt). The rare halophyte A. caerulescens and the 2 non-tidal Dittrichia species presented a germination syndrome based on no germination at high salinities and high recovery germination (55-80%). Considering the high sensitivity of A. caerulescens to salinity during germination, active conservation measures are recommended to promote its preservation in the Mediterranean Basin. These include creating new populations and promoting salt marsh connectivity to facilitate the processes of species mobility and marsh migration to uplands where sediment salinity is lower.