Interactive effects of leaf pathogens and plant mycorrhizal type on plant diversity–productivity relationships

Diversity–productivity relationships can differ between forests dominated by different mycorrhizal types and be modulated by specialist and generalist pathogens. However, little is known about how these factors interact to modulate biodiversity effects. We addressed this knowledge gap with a 2-year experiment combining the manipulation of plant richness (1, 2, 4, 8 species) and mycorrhizal type (arbuscular mycorrhizal [AM] tree-dominated; ecto-mycorrhizal [ECM] tree-dominated) with fungicide application (added or control). Biodiversity effects were quantified for community productivity and its two components (shoots and roots). We observed non-linear diversity-productivity relationships, with productivity of ECM-tree dominated communities increasing at low to intermediate diversity and declining at the highest species richness. Fungicide application reduced positive complementarity effects and increased productivity in both ECM tree monocultures as well as 8-species mixtures. This finding suggests that dilution effects of specialised pathogens may dominate at low diversity, while the spillover effects of generalist pathogens may become dominant at high diversity, resulting in unimodal diversity-productivity relationships. In AM tree-dominated communities, aboveground productivity strongly increased in response to leaf pathogen suppression in 8-species mixtures, and the release from leaf pathogens benefited most the species that were most productive in fungicide-treated monocultures. This agrees with the prediction that spillover effects of generalist pathogens in diverse plant communities could differentially supress highly productive species due to the trade-off between growth and defence. In addition, positive biodiversity effects on root production were significantly stronger in AM-tree that ECM-tree dominated communities. Our results demonstrate that relationships between plant diversity and productivity can be non-linear due to combined effects of specialised and generalised plant-fungal interactions, depend on plant mycorrhizal type and differ between aboveground and belowground compartments.