Tree diversity and mycorrhizal type independently shape multitrophic biodiversity, with stronger effects belowground than aboveground.

Tree diversity declines and shifts in mycorrhizal dominance may have cascading effects on multitrophic diversity. Whether the effects are additive or synergistic and consistent across taxa and functional groups is unclear. Here we test how microorganisms and invertebrates spanning multiple trophic levels, both above- and belowground, respond to experimental manipulation of tree species richness and mycorrhizal type in a young forest stand (the MyDiv experiment in Germany). Tree diversity increased belowground abundance and taxonomic richness of microorganisms and lower-trophic-level nematodes, cascading to meso- and macrofauna predators, whereas aboveground effects were limited to foliar fungi and soil-surface omnivores. Mycorrhizal type strongly structured belowground food webs and influenced aboveground predators, but mixtures showed only additive effects. For most trophic groups, diversity responses were similar across mycorrhizal types. Tree diversity and mycorrhizal type effects were mediated by leaf quality, canopy complexity and nutrient availability. Our results highlight tree diversity and mycorrhizal type as potentially distinct drivers of multitrophic biodiversity, primarily governing belowground bottom-up effects.