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

Plant diversity has often been linked to increased productivity; however,
this apparent diversity-productivity relationship may rely on
inter-trophic 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.