In forests, common mycorrhizal networks (CMNs) often connect the roots of neighbouring plants. Observations of material flows between hosts connected by CMNs have given rise to the hypothesis that CMNs limit the negative effects of competition by overstorey trees on seedlings recruiting underneath them. I conducted an experiment in a temperate forest dominated by ectomycorrhizal conifers and hardwoods to isolate the effects of CMNs on the growth and survival of four tree species that co-occur in the understorey. Ectomycorrhizal networks had strong negative effects on the survival of an arbuscular mycorrhizal species, Acer rubrum, and neutral effects on the survival of three ectomycorrhizal species, Betula allegheniensis, Pinus strobus, and Tsuga canadensis. CMNs had positive effects on the growth of at least one ectomycorrhizal species, P. strobus. Interspecific differences in juvenile responses to CMNs may influence forest community development, promoting coexistence of some tree species while limiting the success of others.