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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. [ABSTRACT FROM AUTHOR]

Summary: Submediterranean landscape around Vic includes, scattered over intensively farmed ground, a number of hills holding diverse mosaics of natural and seminatural communities. We chose two contrasting communities of pastures to investigate the soil water regime, assumed to be the cause of noticeable shifts in the landscape mosaics. Brachypodio-Aphyllanthetum is a Mediterranean, xerophilous, open pasture dominated by the chamaephytic grass Brachypodium retusum, whereas Plantagini-Aphyllanthetum is a dense grassland of Brachypodium phoenicoides, similar to Medioeuropean mesoxerophilous grasslands. We recorded the soil water content in three plots for each community in different hills, during two and a half years. From gravimetric water percentages and soil water retention curves in each plot, we evaluated the soil water potential and the available water during the study period at different depths. Both communities showed roughly similar seasonal soil water dynamics, which included three clearly differentiated hydric periods: (i) humid (October–April), with soils not far from field capacity; (ii) dry (July–August), when soils were drier than wilting point; and (iii) transitional (May–June), defined by a dramatic water loss. The summer values of water content and hydric potential were similar in the two analysed communities, thus rejecting the hypothesised better summer conditions in the mesoxerophilous grassland. The only clear difference was in the way in which the spring transition occurred, namely it was faster and showed sharper ups and downs in the xerophilous Mediterranean pasture. The inverse seasonal transition, from dry to humid, was clearly a sudden event, strongly depending on the timing of late summer rainfall, and it seems not to be a key factor in shaping the functioning of the communities. The lower moisture values at deep soil layers during the humid periods suggest a deficient hydrological functioning, perhaps connected with the pasture-type vegetation. [Copyright &y& Elsevier]