A regional assessment of changes in plant-plant interactions along topography gradients in Tunisian sebkhas

Facilitation among plants in dry ecosystems is crucial for diversity and ecosystem functioning and stability. However, the importance of facilitation in extremely stressful conditions is highly debated.  We attempt to separate the effects of direct non-resource and resource stress factors on plant-plant interactions by assessing changes in facilitation and competition along salinity gradients at two contrasting levels of aridity. We selected eight saline depressions (hereafter sebkhas) in the wet and dry Mediterranean arid climate of North-Africa, from central Tunisia to the Libyan border 500 km south-eastward. In each sebkha, we transplanted at four positions along the salinity gradient induced by topography, both in open areas and below dominant shrubs, three target species with contrasting tolerances to salinity stress. Target plant survival, soil electrical conductivity and moisture were recorded before and after the dry summer season in all treatments. Shrubs decreased salinity and...