Differences in relative air humidity affect responses to soil salinity in freshwater and salt marsh populations of the dominant grass species Phragmites australis

Soil salinity diminishes the dominance of species and affects their distribution. Phragmites australis is a dominant ecosystem engineer with broad distribution, high intraspecific variation and great socio-economic importance. Coastal ecosystems inhabited by P. australis are threatened by salinization. Here, we investigated salinity tolerance of freshwater and salt marsh population of the species, grown under two soil salinities and in two common gardens. Salinity significantly affected the growth, biomass, leaf physiological parameters and ion concentration. Climate altered morphology and ion concentration of P. australis . In contrast to our hypothesis, the salt marsh population was not more salt tolerant than the freshwater population, and both showed a low degree of salt stress, maintaining their photosynthesis and chlorophyll concentration, and only showing small decreases in biomass and height when salt-treated. We therefore ruled out local adaptation to soil salinity. Instead, both populations acclimated by phenotypic plasticity of biomass, root: shoot ratio, stomatal conductance and ion content. The salinity tolerance strategy of both populations was ion concentration by tissue desiccation, which was most efficient in the drier climate common garden. Hence, plants utilizing tissue desiccation as salinity tolerance strategy will have an advantage in areas where climate change leads to drier air humidity.