From competition to complementarity: growth mechanisms and structural drivers in pure and mixed silver fir–beech stands of southern Italy

Mixed-species forests are increasingly promoted to improve forest functioning, yet their growth responses depend strongly on site conditions, stand structure, and management legacies, particularly in Mediterranean mountain environments. This study investigates how species mixture, stand structure, competition, and site conditions jointly influence tree growth and productivity in silver fir–beech forests of southern Italy. We combined stand-level indicators (current annual increment, recruitment period, structural diversity indices) with tree-level growth analyses based on basal area increment and linear mixed-effects models, comparing pure and mixed stands across three montane sites characterized by contrasting environmental conditions and management histories. Mixed stands showed higher productivity than monospecific stands, with increased current annual increment, shorter recruitment periods, and greater structural complexity. Linear mixed-effects modelling identified tree size, height, competition intensity, species identity, and stand mixture as key drivers of growth. Both species benefited from mixture, although silver fir showed a stronger positive response, with 12–15% higher Basal Area Increment (BAI) in mixed stands compared to pure stands, while beech gains were more moderate (5–8%). Structural diversity, quantified by Shannon and Gini indices, was markedly higher in mixed stands, indicating enhanced size differentiation and resource-use complementarity. Overall, the results demonstrate that productivity gains in silver fir–beech forests arise from the interaction between species mixture, stand structure, competition, and site-specific conditions. Species mixture enhances individual-tree growth primarily by modulating competitive interactions, with the magnitude of benefits varying across sites according to structural and management-related constraints. These findings provide quantitative support for site-adapted management strategies that promote mixed and structural diverse forests in Mediterranean mountain regions.