Functional diversity turnover in the western Mediterranean saltmarshes: Effects of edaphic features and biotic processes on the plant functional structure

Salinity and soil moisture are considered main drivers of the plant zonation in Mediterranean saltmarshes. Therefore, both factors could have a remarkable effect on the plant functional structure of these habitats. The aim of this study was to identify the effects of abiotic and biotic factors on the plant functional structure of western Mediterranean saltmarshes. A total of 20 saltmarshes were assessed, and seven plant traits were considered. Community weighted mean and Rao index were used to measure the functional structure of the plant communities. Redundancy analysis was used to estimate the effects of soil variables on the community-weighted mean trait and functional diversity, and standardised effect size was used to assess the effect of biotic interactions. The functional traits showed a clear zonation along the salinity gradient in Mediterranean saltmarshes, mainly related to the electrical conductivity, and mainly gathered in trait syndromes. The succulent nanophanerophytes grew at the highest salinity zones. Salt excretors, both chamaephytes and mesophanerophytes, appeared in the intermediate and lowest salinity zones, respectively. Finally, geophytes with both selective cation root uptake and rhizome were mostly located in the lowest salinity zones. The abiotic factors strongly modulated the biotic interactions, and some convergence patterns were observed. The highest functional diversity was observed in the lowest salinity zones, a marked turnover. These findings indicate that multiple assembly processes determine the plant structure of Mediterranean saltmarshes, yet abiotic environmental filters strongly shape the local species assemblages and functional diversity turnover. Our results support that the whole salinity gradient should be protected to conserve the widest range of functional traits.