Climate and Competition Effects on Basal Area Growth Vary with Beech–Fir Mixture and Stand Structure

Mixed stands enhance climate resilience and ecosystem service provision through functional diversity, but their productivity depends on intra- and interspecific competition, forest structure, stand density, and site conditions. In this study, we analyzed the effects of competition and aridity on the growth of European beech (Fagus sylvatica L.) in mixed and pure stands, using data from 38 plots of the Italian National Forest Inventory (NFI, 2015). To understand the variables influencing European beech growth, tree-level basal area increment models were applied, incorporating different competition structures (intraspecific, interspecific, size-symmetric, and size-asymmetric) and aridity index (De Martonne). Results showed that size-asymmetric intraspecific competition negatively affected European beech growth, highlighting low self-tolerance, especially in pure stands where growth was lower than in mixed stands. In mixed stands, European beech growth was shaped by size-dependent competition and the relative dominance of coexisting species, benefiting from size-asymmetric and hindered by size-symmetric interactions. Additionally, European beech growth was shaped by aridity and stand structure (Gini coefficient and density), with drought sensitivity mitigated in mixed stands and enhanced growth in structurally diverse, low-density stands. This study highlights how species interactions, aridity, and stand structure jointly shape tree growth, underscoring their importance for climate-adaptive forest management.