In recent years, plantations of fast-growing tree species have emerged as a possible way to meet the increasing demand for biomass for renewable energy in Europe. Agroforestry plantations including fast-growing tree species could be an attractive option because they reduce land competition for biomass and food production while providing forest benefits. Today, the species interactions that determine whether a given mixture will be more productive than the corresponding monocultures are still poorly understood. Our objective was to assess the performance of fast-growing poplar and alder trees in an experimental plantation set up in 2014 including different treatments, i.e. poplar and alder (1) in monocultures, (2) in association together (forest mixture) or (3) in association with herbaceous species (agroforestry) either N2-fixing species (succession alfalfa and clover) or graminoids (succession of wheat, triticale and temporary grassland) respectively. During eight growing seasons, tree growth was monitored monthly, while leaf (eco)physiological traits (e.g. photosynthetic capacities) were investigated since 2017. During the early stages, negative interspecific competition between crops and poplars predominated and required management actions. A facilitation process in the poplar agroforestry treatment, due to a significant enrichment in soil nitrogen by the N2-fixing crops, led to a higher stem height in agroforestry poplars than in the forest mixture and the monoculture at the end of the 6th growing season*. For alders, tree growth did not appear to be affected by either an association with graminoids or with poplar. No overall significant treatment effect was observed on leaf traits but intra-annual dynamics revealed different tree responses to specific seasonal conditions, with for instance agroforestry poplars maintaining higher growth rates and photosynthetic performances during the 2018 summer drought compared to poplars in forest treatments. This could highlight a better acclimation capacity to potential constraining conditions for poplars in agroforestry stands.