Many studies have highlighted the value of mixed plantations and their advantages over monocultures. The success of mixed plantations is usually assessed by measuring the increase in biomass and/or plant production compared to the corresponding monocultures. Among the structural determinants of growth, the vertical distribution of branches and crown shape are important to take into account because they directly impact access to light, which conditions tree growth. We evaluated the effect of two types of species mixtures in northeastern France (agroforestry and a forest mixture) on the crown architecture of poplars (Populus deltoides × P. nigra) and alder (Alnus glutinosa) after seven growing seasons and compared the mixtures to their respective onocultures. Four tree architecture variables (height, crown depth, crown projection area, crown volume) were evaluated. Our study shows that the poplars in the agroforestry plot altered their crown morphology through a true mixture effect due to the presence of clover, an N2-fixing species, as well as to reduced competition for light due to larger spacing compared to the monoculture. In the forest mixture, despite a stratification of the canopy suggesting an optimized sharing of the aerial niche, thus possibly creating an additive effect, poplar crown morphology was not different compared to the monoculture. Finally, the different types of mixtures did not affect alder crown morphology. From an agronomic perspective, the more important crown development that occurred when the poplar was associated with an N2-fixing crop makes this type of mixture a very promising way to increase the contribution of biomass to the renewable energy mix in Europe.