Ectomycorrhizal diversity alters growth and nutrient acquisition of grey birch (Betula populifolia) seedlings in host-symbiont culture conditions.

•  The influence of ectomycorrhizal fungal diversity on plant performance was investigated by establishing a gradient of ectomycorrhizal diversity on Betula populifolia (grey birch) seedlings. •  We measured growth, as well as N and P uptake, of individual B. populifolia seedlings inoculated with replicate one, two and four species 'communities' of ectomycorrhizal fungi simultaneously and without mycorrhizas in axenic culture. •  Colonization of B. populifolia by individual species of ectomycorrhizal fungi decreased with increasing fungal diversity although total colonization increased. Shoot biomass decreased with increasing ectomycorrhizal diversity and mycorrhizal root biomass increased. Plant biomass did not differ with individual mycorrhizal species or composition. Shoot N concentration showed a small increase with increasing ectomycorrhizal diversity. Whole plant P content and concentration increased across the ectomycorrhizal diversity gradient. Despite higher mycorrhizal colonization rates with increasing fungal diversity, plant growth and nutrient responses were best explained by changes in ectomycorrhizal diversity. •  Greater ectomycorrhizal diversity per se, rather than colonization or composition, increased mycorrhizal root biomass at the expense of shoot biomass and increased P uptake of B. populifolia seedlings.