Fungal diversity dominates the response of multifunctionality to the conversion of pure plantations into two-aged mixed plantations.

Plantation forests are essential in driving global biogeochemical cycling and mitigating climate change. Biodiversity and environmental factors can shape multiple forest ecosystem functions simultaneously (i.e., multifunctionality). However, their effect on multifunctionality when pure plantations are converted into two-aged plantations remains underexplored. Therefore, we assessed above- and below-ground biodiversity and environmental factors and 11 ecosystem functions in different plantation types in subtropical China. The two-aged mixed plantations exhibited higher multifunctionality than did a pure plantation, primarily due to soil fungal diversity and secondarily due to tree diversity, based on the coefficient of variation for tree diameter at breast height (CVD) and community-weighted specific leaf area (CWMSLA). Further analysis revealed saprotrophy as the key soil fungal trophic mode in maintaining multifunctionality. Moreover, structural equation modeling confirmed that soil environmental factors, namely the soil water content and pH, had no direct association with multifunctionality, but were indirectly related to multifunctionality via elevated CVD and CWMSLA, respectively. Our results indicate that the tree and soil fungal diversity, as well as soil environmental factors, resulting from the conversion of pure plantations to two-aged mixed plantations, can enhance multifunctionality, and provide a better comprehensive understanding of the driving mechanisms of multifunctionality, leading to the sustainable management of subtropical plantation forest ecosystems.