We studied interactions between microfungi and herbivores sharing a host tree. In a series of experiments and field observations over a 3‐year period, we compared phenotypic and genetic correlations of fungal frequencies and performance of invertebrate herbivores growing on mature half‐sib progenies of mountain birches ( Betula pubescens ssp. czerepanovii ) in two environments, a forested river valley and an adjacent higher‐el...