Medium-term biotic interactions in Olea europaea seedlings along an abiotic stress gradient.

Plant-plant interactions are important drivers for the functioning and structuring of dryland plant communities. Stressful conditions, mainly related to water availability, determine whether plant interactions result in net facilitation or competition. Many studies have been conducted on the impacts of mature individuals, trees, shrubs or tussock grasses, on woody seedlings but little is known about the biotic interactions when they all are at the same developmental stage under field conditions. Here we present a spatially replicated study with Olea europaea as the target species and Pistacia lentiscus and Macrochloa tenacissima as neigbors. We also implemented field techniques to increase water availability and, hence, reproduce an abiotic gradient of stress. Wild olive performance was sensitive to neighbors, water availability and site. Improved microcatchments, which in addition to site recreated a gradient of water availability, improved wild olive performance as well as planted neighbors negatively impacted olive seedlings. We observed increasing negative interacting effects as conditions become less stressful. This is in line with the stress-gradient-hypothesis but also provides important knowledge regarding plant interactions in drylands at the same development stage.