Wood ants (Formica rufa group) are considered as key species in forest ecosystems where they live in mutualistic relationships with canopy-dwelling aphids. These aphids excrete honeydew which the ants use as a food source, and as a trade-off the ants protect the aphids against predators. This mutualism can have cascading effects on third parties, e.g. by inducing carbon (C) and nitrogen (N) fluxes from their host trees. Wood ants are strong competitors and can affect the functional biodiversity of boreal forests. However, none of the earlier studies have considered the conceptually complex interactions between host tree choice, the effects of ant-aphid mutualism on the growth of trees, and competitive interactions among trees within a stand. Our aim was to find out how tree properties (such as tree species, height or diameter) affect host tree choice and how the ant-aphid mutualism affects tree growth and the abundance of the other invertebrate fauna. We test three hypotheses: 1) trees with faster growth rates are favoured as host trees, 2) the mutualism increases throughfall C flux onto the forest floor and reduces that of N and 3) ant-aphid mutualism lowers the species biodiversity of the invertebrate fauna. We conducted manipulative experiments both at tree and stand level (ant-aphid mutualism eliminated or not). Preliminary results are discussed.