Key message : In a mixed poplar/black locust plantation in central France, adverse conditions have led to a prevalence of interspecific competition, resulting in a poorer performance than monocultures. Context : In mixed tree plantations, the presence of woody N-2-fixing species is thought to reduce N needs by fertilization. However, benefits associated to soil nitrogen enrichment have to outweigh the negative effects of interspecific competition. To do so, co-occurring tree species have to be chosen carefully to promote niche sharing between species and reduce competition. Black locust and poplar mixtures therefore seem promising since both species are fast growing and have potentially complementary crown shapes. Aims : Our objective was to evaluate the impact of the poplar/black locust mixture on the growth, above-and belowground biomass production, and nitrogen allocation of the two species, as compared to their respective monocultures. Methods : An experimental plantation mixing poplar and black locust was set up in central France. For five growing seasons, growth, nitrogen allocation, and carbon allocation were monitored for the two species growing either in mixture or in monoculture. Results : After a couple of promising growing seasons, black locust growth and survival slowly declined, mainly in the mixture. At the stand level, biomass production in the mixed plots was nearly 50% below the most productive monoculture (poplar) by age 5 years. Conclusion : Under adverse conditions, interspecific competition in the mixture was the preponderant interaction, resulting in higher mortality and lower biomass production than the two monocultures.