Soil fungal communities contribute to the positive diversity–productivity relationship of tree communities under contrasting water availability

Abstract

Abstract Plant diversity has often been linked to increased productivity; however, this apparent diversity–productivity relationship may rely on intertrophic interactions such as those between plants and soil microbes. Soil fungi can create complementarity between plant species via altered plant resource partitioning, facilitation via fungal networks or biotic feedbacks, thereby promoting plant diversity–productivity relationships. Furthermore, these relationships are likely to be context dependent in response to resource availability. We used a biodiversity–ecosystem function experiment with trees exposed to high and low water availability treatments to determine the contribution of soil fungal communities to the diversity–productivity relationship in tree communities. We used amplicon sequencing of soil fungi to assess fungal richness, community composition and richness of functional guilds. We then applied structural equation modelling to determine relationships between tree diversity, fungal communities and tree productivity and the role of water availability in these relationships. Tree species richness and functional diversity both increased above‐ground tree productivity and influenced soil fungal community composition. Fungal community composition had a direct impact on tree productivity and enhanced net diversity effects on productivity. Therefore, fungal communities mediated a positive, indirect effect of tree richness on productivity. While total fungal richness was not associated with tree diversity, pathogen richness decreased and mycorrhizal richness increased with tree richness. Pathogen and mycorrhizal richness had either no impact or a weak negative effect on productivity. Tree species traits strongly affected fungal communities and these changes promoted productivity. Finally, water availability greatly influenced fungal communities but did not interact with tree diversity to affect productivity; indicating possible resilience of tree communities to altered precipitation regimes and associated changes in fungal communities. Synthesis: Our study highlights the crucial role that fungal communities play in shaping the relationship between tree diversity, traits and productivity, and resilience to altered water availability.