Insight into the truffle brule: tripartite interactions between the black truffle (Tuber melanosporum), holm oak (Quercus ilex) and arbuscular mycorrhizal plants

Aim Tuber melanosporum is an ectomycorrhizal (ECM) fungus from Mediterranean transitional ecosystems where ECM trees start to dominate among arbuscular-mycorrhizal (AM) shrubs and herbs (companion plants). Its presence entails the development of 'brules', where vegetation is scarce for unknown reasons. Current T. melanosporum production comes from plantations where management often suppresses the understory vegetation, although empirical knowledge advocates a positive role of some companion plants in truffle production. This study aimed at (i) experimentally testing the reciprocal interaction between T. melanosporum and companion plants and (ii) examining T. melanosporum-mediated soil feedback involved in the dynamics of truffle ground vegetation. Methods A three-year experiment was set up with Quercus ilex associated with T. melanosporum (or not, as control), grown in association (or not, as control) with a companion plant. Six companion plant species were chosen based on different empirical criteria including those indicated by local truffle growers' knowledge. A trait-based approach was applied to plants and associated fungi (abundance of T. melanosporum and AM fungi mycelium). Results-conclusion Companion plants promoted the development of truffle mycelium. In the presence of T. melanosporum, companion plant growth and nutrition and AM fungi abundance decreased, while the nutrition status of its host increased. The truffle inhibited germination of weed seeds. These results highlight the role of T. melanosporum in mediating plant-plant interactions, possible mechanisms underlying brule formation and a potential successional role for T. melanosporum.