Interactions between transplants of Phragmites australis and Juncus acutus in Mediterranean coastal marshes: The modulating role of environmental gradients

• Variable dominance of Phragmites australis and Juncus acutus along environmental gradients.
• Interactions change from competition toward facilitation depending on stress combinations.
• P. australis can suppress J. acutus in waterlogged and non-saline conditions.
• J. acutus can suppress P. australis along a wide range of saline conditions when water table is depth.
• Microgradients should be taken into account when considering the stress-gradient hypothesis.

Interactions between two coastal marsh plants (Phragmites australis and Juncus acutus) were investigated along three natural gradients of salinity, water table and soil texture, variously combined in a microtidal Mediterranean coastal marsh. Our aim was to clarify to what extent plant interactions explain the occurrence of stands of both species, since they are not solely due to their tolerance to environmental conditions. We used a replacement series design of field transplants. Mortality and relative yield index of aerial biomass were analyzed to assess the outcome of interactions using Generalized Linear Mixed Models. Results show that mortality was entirely driven by natural gradients. Specifically, higher salinity and soil clay contents increased mortality in both species, whereas high water table increased survival only in P. australis. Growth was controlled by plant interactions and by natural gradients. According to these results P. australis can suppress J. acutus in waterlogged and non-saline conditions. Where water table is deeper, J. acutus can suppress P. australis along a wide range of saline conditions. In the driest situations both species exhibited mutual interference and, in some cases, P. australis can again suppress J. acutus. Facilitation occurred along the salinity gradient in situations that correspond to low, medium or high stress for the beneficiary species, pointing that exceptions to the stress gradient hypothesis may occur in systems including multiple stress gradients. Our results suggest that competitive interactions and environmental gradients are not fully responsible for plant distribution in microtidal coastal marshes.