Mycorrhizal fungi colonize tree seedlings shortly after germination, and the nature of this relationship (mutualistic to parasitic) has been reported to vary as a function of resources; but this transition has rarely been quantified. Using a light gradient, we grew seedlings of eight tree species in soils that were cultivated by several coexisting species of trees. We used data on mycorrhizal fungi on roots to quantify colonization along the gradient of light. We then analyzed plant growth as a function of both the light gradient and extent of mycorrhizae colonization. Mycorrhizal fungi colonization varied among species but was not correlated with the species’ seed sizes or shade tolerances. Within a species, colonization varied among soil sources, but those differences followed neither the conspecific-heterospecific dichotomy, nor the soil host’s arbuscular-ectomycorrhizal associations commonly reported. At high light, seedlings growth increased with increasing levels of colonization for seven species; and at low light the effect of colonization was negative for five species. We also quantified the light threshold at which the plant-mycorrhizal fungi relationship shifted from neutral to positive (four species), negative to neutral (one species), and neutral to negative (one species), documenting differences among species that could exacerbate competitive interactions during recruitment.