•The overstorey coniferous trees and understorey ericaceous dwarf shrubs of northern temperate and boreal forests have previously been considered to form mycorrhizas with taxonomically and functionally distinct groups of fungi.

•Here, we tested the hypothesis that Meliniomyces variabilis and Meliniomyces bicolor, isolated from Piceirhiza bicolorata ectomycorrhizas of pine, can function as ericoid mycorrhizal symbionts with Vaccinium vitis-idaea. We used split-compartment microcosms to measure the reciprocal exchange of 13C and 15N between V. vitis-idaea and three fungal isolates in the Hymenoscyphus ericae aggregate isolated from Scots pine ectomycorrhizas (M. variabilis and M. bicolor) or Vaccinium roots (M. variabilis).

•The extramatrical fungal mycelium of labelled mycorrhizal plants was significantly enriched in 13C, and the leaves were significantly enriched in 15N, compared with nonmycorrhizal and nonlabelled controls.

•These findings show for the first time that fungi in the H. ericae aggregate, isolated from pine ectomycorrhizas, can transfer C and N and can thus form functional ericoid mycorrhizas in an understorey ericaceous shrub.