<p>Forest stand composition influences ecosystem structure and functioning, yet its integrated effects on canopy architecture, competition intensity, and soil processes are rarely evaluated under undisturbed conditions that provide essential silvicultural reference states. The virgin oriental beech forests of the Hyrcanian region in northern Iran represent one of the last remaining temperate broadleaf forest systems worldwide that have developed without logging, grazing, or silvicultural intervention, providing a rare natural reference for examining coupled above- and belowground processes. We compared pure and mixed beech stands by quantifying canopy structural attributes, competition indices, and key organic and mineral soil properties. Pure stands exhibited larger trees, stronger vertical canopy organization, and significantly higher competition intensity, whereas mixed stands supported greater microbial biomass and respiration in the mineral soil despite lower competitive pressure. Correlation and redundancy analyses revealed strong linkages between canopy structure, competition intensity, and soil properties, with clear multivariate separation between pure and mixed stands. These findings demonstrate that stand composition reorganizes the coupling between canopy structure, competition intensity, and soil processes, highlighting the importance of integrating above- and belowground components to understand ecosystem functioning in undisturbed forests. By leveraging one of the few remaining virgin temperate beech forest landscapes, this study provides a basis for targeted silvicultural and restoration strategies by identifying when mixed-species stands enhance soil microbial functioning and when pure beech stands promote larger tree development and greater vertical canopy stratification.</p>