We performed a pot experiment to study the effects of varying levels of vegetation shade and humus fertility, simulating natural conditions of the boreal forest, on growth, interspecific competition, and ectomycorrhizal colonization of seedlings of three coexisting tree species: Pinus sylvestris L., Picea abies (L.) Karst., and Betula pendula Roth. In contrast to late-successional Picea abies, early-successional species Pinus sylvestris and particularly B. pendula responded to shade with an increase in stem height and with some changes in biomass, especially in the high-fertility humus. Humus fertility had greater effects on biomass of seedlings than did shade treatments. Interspecific competition among pairwise combinations of the three species was greatest in the high-fertility humus, but was not affected by shade. Betula pendula was the strongest competitor among species; it was not affected by the presence of coexisting species and strongly suppressed coniferous seedlings, especially in the high-fertility humus. Generally, ectomycorrhizal colonization was not affected by shade treatments, but was highest in the low-fertility humus. These results show that effects of light, humus fertility, and presence of neighbours are species specific, and these differences are important for the competitive and recruitment abilities of seedlings of coexisting tree species and ultimately for the species composition of developing forests.