In the context of ongoing climate and biodiversity crises, mixed forest stands are increasingly considered as a sustainable management alternative to mono-cultures. We developed a new individual-based and process-based forest growth model, PDG-Arena, to simulate mixed forest functioning and test ecophysiological interactions among trees in mixed stands. The model builds upon of a validated ecophysiological stand-scale model and integrates tree competition for light and water. We evaluated the simulation performance of PDG-Arena using annual growth data from 39 common beech and silver fir monospecific and mixed plots in the French Alps. PDG-Arena showed similar performance as the validated stand-scale model when simulating even-age and monospecific forests, and significantly better performance when using structure-diverse and species-diverse inventories. It also showed a significant positive effect of species mixing on gross primary production, canopy absorbance and transpiration. Our results thus show that tree-level process-based models such as PDG-Arena, formally simulating interspecific interactions, are needed to better understand and simulate the functioning of mixed stands.