Chemical interactions in forested ecosystems play a role in driving biodiversity and ecosystem dynamics. Plant phenolics released by leaching can influence surrounding plants and soil organisms such as bacteria, fungi or arthropods. However, our knowledge about such chemically-mediated biotic interactions in Mediterranean oak forests is still limited, in particular whether they play a role in the limited forest regeneration. In this study, we analyzed how phenolics of Cotinus coggygria, a dominant shrub of Mediterranean downy oak (Quercus pubescens) forests, influence understory herbaceous plant species, downy oak regeneration and soil organisms in order to obtain a more integrative view of possible direct and indirect interactions triggered by this shrub species. We performed a series of experiments testing the effect of aqueous extracts of C. coggygria, mimicking natural leachates, on these organisms. Cotinus coggygria contained a high quantity of phenolics in green and senescent leaves but much less in leaf litter. Extracts from C. coggygria leaves stimulated bacterial communities, exhibited few effects on both saprophytic and symbiotic fungi, and negatively affected Collembola. Herbaceous species growth was particularly impaired by extracts from green and senescent leaves, although these effects were alleviated in the presence of soil microorganisms. In both greenhouse and field experiments, C. coggygria affected early oak seedling establishment in particular through a reduced root growth, but exhibited no effect on later seedling and sapling growth. We discussed the implication of these results for the balance between competition and facilitation in oak forests and concluded that C. coggygria has the potential to strongly alter biotic interactions, understory plant diversity and oak forest dynamics.