Environmental control of plant species abundance in a microtidal Mediterranean saltmarsh

Question: To what extent are environmental factors the main determinants of species abundance in Mediterranean coastal marshlands? Location: The Llobregat delta, Barcelona, Spain. Methods: Vegetation relevés were performed and a set of water table and soil variables were periodically monitored in 43 sampling points randomly distributed in four marsh areas (sites) along a coastal-inland gradient. A canonical correspondence analysis (CCA) was performed to identify the primary water and soil correlates of species cover, after considering the effect of site and point spatial location. The realized niches of dominant species were modeled through GLMs performed on the first two axes of CCA. Niche overlapping among these species was compared with their coexistence, assessed through pairwise correlations of relative species cover in each sampling point. Results: Water and soil variables explained more of the variation in species' abundance than site and spatial position. Mean water table level, maximum water conductivity and sodium adsorption ratio (SAR), summarized in the two first CCA axes, explained 23.8% of the variability in species' cover. Arthrocnemum fruticosum, Phragmites australis subsp. australis, Juncus acutus, Spartina versicolor and Juncus maritimus dominated the vegetation stands. Niches obtained from GLM response curves showed moderate overlapping among all these species except for A. fruticosum. However, pairwise correlations were mainly negative or non-significant, indicating low coincidence, and even segregation, between species' cover. Conclusions: The abundance of dominant plants in Mediterranean marshes is only partly explained by the environmental gradients summarized in niche models. The role of other factors such as facilitation or competition between species and random recruitment should be explored.